First spectroscopic measurements of [OIII] emission from Lyman-alpha selected field galaxies at z ~ 3.1

0.29 MB·English

by  Emily M. McLinden

Draftversion January25,2011
PreprinttypesetusingLATEXstyleemulateapjv.11/10/09
FIRST SPECTROSCOPIC MEASUREMENTS OF [OIII] EMISSION FROM LYMAN - ALPHA SELECTED
FIELD GALAXIES AT Z ∼ 3.11,2
Emily M. McLinden3, Steven L. Finkelstein4, James E. Rhoads3, Sangeeta Malhotra3, Pascale Hibon3, Mark L.
A. Richardson3, Giovanni Cresci5, Andreas Quirrenbach6, Anna Pasquali7, Fuyan Bian8, Xiaohui Fan8, Charles
E. Woodward9
Draft version January 25, 2011
1 ABSTRACT
1 We present the first spectroscopic measurements of the [Oiii] 5007˚A line in two z ∼ 3.1 Lyman-
0
alphaemittinggalaxies(LAEs)usingthenewnear-infraredinstrumentLUCIFER1onthe8.4mLarge
2
BinocularTelescope(LBT).Wealsodescribetheopticalimagingandspectroscopicobservationsused

n to identify these Ly-α emitting galaxies. Using the [Oiii] line we have measured accurate systemic
a redshifts for these two galaxies, and discovered a velocity offset between the [Oiii] and Ly-α lines
J in both, with the Ly-α line peaking 342 and 125kms−1 redward of the systemic velocity. These

4 velocity offsets imply that there are powerful outflows in high-redshift LAEs. They also ease the
2 transmission of Ly-α photons through the interstellar medium and intergalactic medium around the
galaxies. By measuring these offsets directly, we can refine both Ly-α-based tests for reionization,
]
and Ly-α luminosity function measurements where the Ly-α forest affects the blue wing of the line.
O
Our work also provides the first direct constraints on the strength of the [Oiii] line in high-redshift
C LAEs. We find [Oiii] fluxes of 7 and 36 ×10−17 erg s−1 cm−2 in two z ∼ 3.1 LAEs. These lines
. are strong enough to dominate broad-band flux measurements that include the line (in this case, K
h s
band photometry). Spectral energy distribution fits that do not accountfor the lines would therefore
p
- overestimatethe 4000˚A(and/orBalmer)breakstrengthinsuchgalaxies,andhencealsothe agesand
o stellar masses of such high-z galaxies.
r
t Subject headings: galaxies: high redshift — intergalactic medium
s
a
[
1. INTRODUCTION for 2 . z . 7. However, the transmission of Ly-α emis-

2 sionis complicatedby its resonantscatteringinteraction
The Lyman-α emission line is a highly efficient tool
v with neutral hydrogen, both within the galaxy emitting
for identifying and studying star forming galaxies at
5 the line and in the surrounding intergalactic medium
high redshifts. This line can carry up to 6% of the
9 (IGM). The Ly-α line is observed in about 25% of z ∼
bolometric luminosity of a young stellar population
8 3 – 5 Lyman-break galaxies (LBGs) (e.g., Steidel et al.
(Partridge & Peebles 1967) and is conveniently placed
1
2000; Dow-Hygelund et al. 2007; Rhoads et al. 2009), a
. for observations by ground-based optical observatories
6 percentage that may increase with increasing redshift
0 1The LBT is an international collaboration among institu- (Shimasaku et al. 2006; Stark et al. 2010). This trend is
0 tionsintheUnitedStates,ItalyandGermany. LBTCorporation supported by work at redshifts less than two, where the
1 partnersare: TheUniversityofArizonaonbehalfoftheArizona fraction of galaxies exhibiting Ly-α emission decreases
: university system; Istituto Nazionale di Astrofisica, Italy; LBT at these lower redshifts (Reddy et al. 2008; Hayes et al.
v Beteiligungsgesellschaft,Germany,representingtheMax-Planck
2010; Cowie et al. 2010). The Ly-α line is observed to
i Society, the Astrophysical Institute Potsdam, and Heidelberg
X University; The Ohio State University, and The Research Cor- haveacharacteristicallyasymmetricprofile,withasharp
poration,onbehalfofTheUniversityofNotreDame,University cutoff onthe blue side and a more extended wing on the
r
a ofMinnesotaandUniversityofVirginia. red side (e.g., Rhoads et al. 2003). In Lyman-break se-
2Observations reportedherewereobtainedattheMMTOb-
lected galaxies, the peak of the Ly-α line is typically
servatory, a joint facility of the University of Arizona and the
SmithsonianInstitution. redshiftedbyseveralhundred kms−1 withrespecttoin-
3School of Earth andSpace Exploration, ArizonaState Uni- terstellar absorption lines and/or nebular emission lines
versity,Tempe,AZ85287
(Shapley et al. 2003; Steidel et al. 2010), whereas this
4George P. and Cynthia Woods Mitchell Institute for Fun-
damental Physics and Astronomy, Department of Physics and measurementhas not been made in Ly-α selected galax-
Astronomy, Texas A&M University, 4242 TAMU, College Sta- ies until this paper.
tion,TX77843 Besidesbeingausefultoolforstudyinggalaxyproper-
5INAF-OsservatorioAstrofisicodiArcetri,LargoE.Fermi5,
ties, Ly-αgalaxiesalsooffer unique and powerfulprobes
I-50125Firenze,Italy;MaxPlanckInstitutfu¨rextraterrestrische
Physik,Postfach1312, Garching85741,Germany of cosmological reionization (e.g., Rhoads & Malhotra
6ZAH, Landessternwarte, Universit¨at Heidelberg, Knigstuhl 2001; Malhotra & Rhoads 2004, 2006; Kashikawa et al.
12,D-69117Heidelberg,Germany 2006; McCandliss 2009; Dayal et al. 2010) The de-
7Max-Planck-Institut fu¨r Astronomie, K¨onigstuhl 17, D-
tailed interpretation of these tests can be substan-
69117Heidelberg,Germany
8Steward Observatory, The University of Arizona, 933 N tially affected by velocity offsets between Ly-α and
CherryAve.,Tucson,AZ85721 the systemic redshift, because a redshifted line is
9Department ofAstronomy,351 TateLaboratoryofPhysics, less affected by the damping wing of Ly-α absorption
116ChurchStreet,S.E.,UniversityofMinnesota,Minneapolis,
from the IGM (Santos 2004; Malhotra & Rhoads 2006;
MN55455, USA
2
Dijkstra & Wyithe 2010). The data was obtained through time allocated by Stew-
It is not sufficient to assume that the velocity off- ard Observatory in February 2007 (PI Finkelstein) and
sets seen in LBG samples hold for Ly-α selected sam- February and March 2009 (PI McLinden). We have cre-
ples. LAEs are typically less massive than presently ated a 1.96 deg2 image, representing a total integration
available Lyman-break selected samples at similar time of 16.67 hours. The complete details of this survey
redshifts (Venemans et al. 2005; Gawiser et al. 2006; and the data reduction process will be highlighted in a
Finkelstein et al. 2007; Pirzkal et al. 2007; Nilsson et al. forthcoming paper.
2007). They should have correspondingly lower escape
speeds,providedthatstellarmasscorrelatesbroadlywith 2.2. Broadband Data
dark matter halo mass. Such a trend is discussed by
We obtained publicly available broadband imaging
Gawiser et al. (2007), where their sample of z ∼ 3.1
data in CFHT u∗ and SDSS g+ bands from the
LAEs have typical stellar masses of 1 × 109 M⊙ and NASA/IPAC Infrared Science Archive11 to complement
median halo masses of 7.9 × 10(10) M⊙. Gawiser et our narrowband survey. The g+ imaging data (v2.0)
al. point out that these values are significantly smaller comes from Suprime-Cam (Miyazaki et al. 2002) on the
than those values for LBGs at z ∼ 3.1, which have Subaru8.3mtelescope. Theu∗ bandimagingdata(v5.0)
stellar masses of ∼ 2 × 1010 M⊙ and halo masses of comesfromtheMegaPrime/MegaCam12ontheCanada-
∼ 3 × 1011 M⊙ (Shapley et al. 2001; Adelberger et al. France-Hawaii 3.6m Telescope. The 5σ depth in a 3′′
2005). Galactic winds (and indeed many other astro- aperturein eachbandis 26.4and27.0for the u∗ andg+
physical outflows) typically have flow speeds near the
bands, respectively (Capak et al. 2007). These broad-
escape speed for the object, and the observed velocity
band images were registered to our narrowband image
offset of a Ly-α line is roughly double the wind speed
usingtheIRAFtasksWCSMAPandGEOTRAN,which
(Verhamme et al. 2006). Additionally, the velocity off-
resamplesthebroadbandimagestomatchthecoordinate
sets in Lyman-break samples are inversely correlated system of the narrowband image (0.45 ′′/pixel).
with the Ly-α emission strength, as characterized by
equivalent width (Shapley et al. 2001), and the equiva-
2.3. Candidate Selection from Narrowband and
lentwidths ofthe Ly-αselectedsamples aremuchlarger
Broadband Data
on average than those of LBG samples. Finally, Ly-
α selected galaxies are typically small in physical size We used the SExtractor package (Bertin & Arnouts
(Bond et al. 2009, 2010; Malhotra et al. 2010). 1996)toperformsourcedetectioninthenarrowbandand
We present here the first direct measurements of the broadband images. SExtractor was run in dual-image
velocity offset between Ly-α and nebular emission lines mode, first with narrowband image as both the detec-
for typical Ly-α selected galaxies. Our measurements tion and measurement image, and a second time with
arebasedonacombinationofnear-infraredspectroscopy the narrowband as the detection image and broadband
withthenewLUCIFERinstrumentontheLargeBinocu- imageasthemeasurementimage. WeselectedLAEcan-
lar Telescope, and optical spectroscopy using Hectospec didatesbasedonthestrengthoftheirnarrowbandversus
on the MMT. We selected targets for the study from broadband excess as well as their colors as outlined in
a large area narrowband survey conducted with the Rhoads & Malhotra (2001).
90Prime camera on the 2.3m Bok telescope of the Stew- Namely, LAE candidates must be detected in the nar-
ard Observatory. rowband at the 6σ level, their flux in the narrowband
In section 2 we describe our observations and data mustexceedthatinthebroadband(g+ band)byatleast
analysis methods. We present our observational results afactorof2andtheirnarrowbandfluxmustexceedtheir
in section 3, and discuss their implications in section 4. g+ band flux at the 4σ level. Finally, candidates must
Where relevant, we adopt H = 70 km s−1 Mpc−1, have flux in the filter bluward of of the Ly-α line (u∗
0
Ω = 0.3, Ω = 0.7 (Spergel et al. 2007). Also we band) consistent with expected Ly-α forest absorption
usme the followΛing vacuum wavelengths, 1215.67 ˚A for blueward of the Ly-α line and consistent with a u∗ - g+
color≥2. Selectioncriteriaareshownbelowinequations
Ly-α, 3729.875 ˚A for [Oii], 4862.683 ˚A for Hβ and
1 – 4.
4960.295/5008.240 for [Oiii] from the Atomic Line List
v2.0410. All magnitudes quoted are AB magnitudes un- f /δf ≥6 (1)
nb nb
less otherwise specified. f /f ≥2 (2)
nb g
2. OBSERVATIONSANDDATAPROCESSING fnb−fg≥4(cid:0)δfn2b+δfg2(cid:1)1/2 (3)
2.1. Narrowband Survey - Observations and Data
Reduction fu≤10−4/5fg+(3×δfu) (4)
We completed a deep narrowband survey for LAEs at Heref isthe narrowbandflux, f isthegbandflux, f
nb g u
z∼3.1usingthe90PrimeCameraonthe2.3mBoktele- is the u band flux, δf is the flux error in the narrow-
nb
scopeattheStewardObservatory(Williams et al.2004). band, δf is the flux error in the g band, and δf is the
g u
The survey was completed in the COSMOS field cen-
tered at RA 10:00:28.6 and DEC +02:12:21.0 (J2000) 11 http://irsa.ipac.caltech.edu/data/COSMOS/datasets.html
(Capak et al. 2007). The KPNO MOSAIC[Oiii] filter, 12 Basedonobservations obtainedwithMegaPrime/MegaCam,
centeredat 5025˚A, with a bandwidth of 55 ˚A, was used a joint project of CFHT and CEA/DAPNIA, at the Canada-
France-Hawaii Telescope (CFHT) which is operated by the Na-
to select Lyman alpha emission at redshifts z ∼ 3.1. tional Research Council (NRC) of Canada, the Institut National
des Science de l’Univers of the Centre National de la Recherche
10 http://www.pa.uky.edu/∼peter/atomic/index.html Scientifique(CNRS)ofFrance,andtheUniversityofHawaii.
3
flux error in the u band. The second criterion requires
that objects have Ly-α equivalent widths ≥ 57.5 ˚A. The
decisiontorequirea6σ detectioninthenarrowbandand
to require a 3σ non-detection blueward of the Lyman-
break indicates that these are stringent selection crite-
ria, meant to exclude false detections and low redshift
interlopers. We used isophotal magnitudes (MAG ISO)
fromSExtractortomeasurethemagnitudesandfluxesof
each object. Isophotal magnitudes were chosen because
theyhavebeenfoundtoproducethemostaccuratecolors
when SExtractor is run in dual-image mode (Holwerda
2005). Isophotal magnitudes are not measured within a
fixedapertureforeachobject,butratherdeterminesthe
magnitudefromthenumberofcountsinpixelsabovethe
userdefinedthresholdandhenceeachobjecthasaunique Fig.1.— Hectospec optical spectra - used for confirmation of
the objects as Ly-α emitting galaxies at z ∼ 3.1. LAE40844 has
‘aperture’inwhichitsfluxismeasured. Forinstance,the an additional feature, a ‘blue bump’, at λ ∼ 4990 ˚A, which is
two LAEs with detected [Oiii] emission have extracted discussed in section 3.4. The spike seen near 4982 ˚A is noise and
narrowbandisophotalareasof20.86arcsecond2 and8.71 notanadditionalfeature.
arcsecond2 (later referred to as objects LAE40844 and
ThreeofourbrightestconfirmedLAEsafterHectospec
LAE27878, respectively). Similar selection criteria used
observationswereobservedinthenear-infrared(NIR)us-
at z=4.5 have typically yielded a spectroscopic success
ing the new near-infrared instrument LUCIFER (LBT
rate of 80% (Dawson et al. 2004, 2007, Rhoads et al.
NIR Spectrograph Utility with Camera and Integral-
2003, 2005,Wang et al. 2009).
Field Unit for Extragalactic Research) on the 8.4m
LBT(Seifert et al.2002;Ageorges et al.2010)TheLy-α
2.4. Optical Spectroscopy - Observations and Data
line flux of these objects chosen for NIR followup, de-
Reduction
rivedfromtheirnarrowbandandbroadbandmagnitudes,
We obtained optical spectra of LAE candidates in rangesfrom0.94–3.6×10−16 ergs−1 cm−2. LUCIFER1
January, February and April 2009, using the Hectospec is the first of two planned NIR instruments for the two
multi-fiber spectrograph at the 6.5m MMT Observatory 8.4mmirrorsofthe LBT.LUCIFER1currentlyoperates
(a joint facility of the Smithsonian AstrophysicalObser- onone mirrorofthe LBT andiscapable ofspectroscopy
vatoryandtheUniversityofArizona). Hectospechasa1 and imaging in the wavelength range 0.85µm – 2.5µm.
deg2 fieldofviewandspectralcoveragefrom3650-9200 Our observations were performed in queue mode during
˚A. The resolution of the instrument is ∼ 6 ˚A. Optical LUCIFER’s Science Demonstration Time in December
spectra are crucial for confirming that candidates are in 2009 and continued during the Arizona Queue in Jan-
fact LAEs at the correct redshift and not lower redshift uary and February 2010.
interlopersandfor determining the exactwavelengthfor We used the longslit mode of LUCIFER with a 1′′
the Ly-α line. slit utilizing the H+K grating with 200 lines/mm and
WereducedtheHectospecdataandextracted1Dspec- the N1.8 camera. The image scale of the N1.8 camera
trausingtheExternalSPECROAD13pipelinedeveloped is 0.25′′/pixel. We obtained 10 two-minute integrations
by Juan Cabanela. The External SPECROAD pipeline for our brightest LAE (henceforth LAE40844). Our sec-
applies bias, dark and flat field corrections as well as ondobject(henceforthLAE27878)wasobservedover20
wavelength calibration using He-Ne-Ar arc lamps. Typ- four-minute integrations. Our final object (henceforth
ical residuals from the wavelength calibration are 0.15 LAE14310) was observed over 25 four-minute integra-
˚A. tions. The central wavelength in this setup is 1.93 µm,
The optical spectra of the three objects chosen for andthespectralcoveragespansessentiallythefullHand
near-infrared followup are shown in Figure 1. These K band windows. The spectral resolving power with the
optical spectra confirm that these objects are in fact 4 pixel slit ranges from 940 near 1.6µm to 1286 near
LAEs. The spectra show strong Ly-α emission at the 2.2µm or a resolution of ∼ 4.3 ˚A/pixel.
expected wavelength and the line displays the charac- We utilized the DOSLIT routine in IRAF (Valdes
teristic asymmetry expected for this line when emitted 1993) to reduce the 2D spectra. To simplify reduction,
from a high-z source (Rhoads et al. 2003; Dawson et al. our observations were designed so that a bright (R ∼ 12
2004; Kashikawa et al. 2006). See section 3.1 for further – 18) continuum source also shared the slit with each
discussion of this asymmetry. Finally the spectra were LAE. This allows for a trace to be created using the
checkedforthepresenceofanyotheropticallines. Noad- bright continuum object. The trace was then shifted
ditional emission lines were observedat the wavelengths along the spatial axis to extract the much fainter LAE,
where they might be expected for foreground [Oii] or whose continuum emission is undetectably faint in indi-
[Oiii] emission line objects. vidual exposures. We performed flat fielding and dark
correction before aperture extraction. An aperture of 6
pixels was used for extraction. Wavelength calibration,
2.5. Near-Infrared Spectroscopy - Observations and
alsoperformedaspartoftheDOSLITtask,wasdoneus-
Data Reduction
ing an argonlamp spectrum observed in the same setup
as our science observations. After reduction, we aver-
13http://iparrizar.mnstate.edu/∼juan/research/ESPECROAD/index.apghepdindividualframesusingtheIRAFtaskSCOMBINE
4
to produce a single averaged spectrum for each object. spectra—belowthetypicalratiofx/fLy-α ∼8fortypeI
Average RMS uncertainties from wavelengthcalibration quasars, and overlapping the range (fx/fLy-α ∼ 3 – 4)
for the two spectra with [Oiii] detection were 0.64 ˚A observed for type II quasars (e.g., Zheng et al. 2010).
and 0.48 ˚A for LAE40844 and LAE27878, respectively. Thus the X-ray observations suggest that the Ly-α in
Residualbrightnightsky lines, a problemwhenextract- these objects is indeed poweredby starformationrather
ing faint sources, were interpolated over in each night’s than AGN activity, though the present X-ray data are
averaged spectrum using the SKYINTERP task found not deep enough to prove this case by themselves. Also
in the WMKONSPEC package designed for Keck NIR- we note that the modest [Oiii] velocity widths of ∼ 200
SPEC reduction14. –300 kms−1 seeninourtwoLAEsaremuchlowerthan
Flux calibration proceeded using the bright contin- the typical velocity widths of around 1000 kms−1 ex-
uum sources that shared the slit with our LAEs as de- pected for Type 1 AGN.
scribed in the paragraph above. Henceforth these con-
3. RESULTS
tinuumobjectswillbecalledcalibrationstars. Thispro-
cesscorrectedfortelluricabsorptionandtransformedour 3.1. Gaussian Fits to the [Oiii] and Ly-α Lines
flux to Fλ units. LAE40844 was calibrated using SDSS To determine the central wavelength and line flux of
J100126.08+021902.2and LAE27878 was calibrated us- each emission line we fit a Gaussian plus constant to
ingSDSSJ100025.10+022552.0. Wefluxcalibratedeach eachemission line. In the case of the [Oiii] line, we fit a
night’s calibration star spectrum using an appropriate symmetric Gaussian to the line using the MPFITEXPR
Pickles model spectrum (Pickles 1998), scaled in flux to IDL routine,whichis partofthe MPFIT package.16 For
match the apparent V magnitude of the observed cali- the Ly-α line we fit an asymmetric Gaussian by modi-
bration star. The appropriate Pickles model was chosen fying the ARM ASYMGAUSSFIT IDL routine17, which
based on the spectral type of the calibration star and alsoutilizestheMPFITEXPRroutine. Initsunmodified
spectraltypewasdeterminedfromSDSSu-gandg-rcol- form,ARM ASYMGAUSSFIT basically fits the left and
ors of the calibrationstars as outlined in Fukugita et al. rightsidesofthecentralwavelengthwithdifferentGaus-
(2010). The SDSS u, g and r magnitudes come from sians and then requires that in the final fit the left and
SDSSDR7. TheVmagnitudeoftheobservedofthecal- rightcurves must have the same center and same ampli-
ibration star was determined from its SDSS colors and tude where they meet, meaning there are eight parame-
the Lupton (2005) color transformation from SDSS g-r ters, four for each side of the curve (amplitude, center,
color to V magnitude15. We then created a sensitivity sigma, constant) but only six of these are free parame-
curve by dividing the scaled down Pickles model by the ters. Thisallowsforasinglecurvetobefit,butthecurve
calibration star’s stellar spectrum in counts. We then canhavedifferentsigmavaluesfortherightandleftsides
multipliedeachnight’sLAEspectrum(incounts)bythat of curve, making it ideal for fitting a Ly-α line with a
night’ssensitivitycurvetoproduceafinalflux-calibrated truncatedbluesideandextendedredwing. Wemodified
LAE spectrum. ARM ASYMGAUSSFIT by fixing the constant on the
left side of the Ly-α emission line to a pre-determined
2.6. Cross check of photometric redshift constant measured as the average continuum level from
WecrosscheckedthecoordinatesofeachLAEwiththe 4000– 5000˚A. This reduces the number of free parame-
sources in the COSMOS Photometric Redshift Catalog tersfrom6to5whenfittingtheLy-αline. Theconstant
Version 1.5 (Ilbert, Capak & Salvato et al. 2009). We on the right side of the Ly-α line is allowed to vary, as
found a unique match for each object, within 1′′ in all onecanexpectaslightlyhighercontinuumlevelredward
cases. The photometric redshift of LAE40844 is z = of rest-frame Ly-α.
phot
3.094, with a 68% confidence range of 3.08 < z < We quantified the asymmetry of the Ly-α peaks by
phot
3.11. The photometric redshift of LAE27898 is 3.086, definingtheratiooftheredsidebest-fitsigmatotheblue
with a 68% confidence range of 3.02 < zphot < 3.12. side best-fit sigma, or asymmetry= σred/σblue. Using
Finally, the photometric redshift of LAE14310 is 3.035, this definition, any line with an asymmetry measure >
with a 68% confidence range of 2.98<z <3.11. 1.0 is considered asymmetric. From this definition of we
phot
findasymmetrymeasurementsof1.1±0.1,2.1±0.2and
2.7. Cross check with Chandra COSMOS X-ray Sources 1.0 ± 0.1 for LAE14310, LAE40844 and LAE27878, re-
spectively, meaning the Ly-α line in LAE40844is highly
We also compared the locations of our LAEs with
asymmetric, whereas the Ly-α lines in LAE14310 and
the Chandra COSMOS Survey Point Source Catalog
LAE27878appeartobesymmetricwithintheerrors. For
(Elvis et al.2009)toexcludecontaminationfromAGNs.
comparison with other asymmetry measurements in the
The Chandra COSMOS Survey Point Source Catalog
literature we also calculatedasymmetry using a anda
contains1761X-raysourcesin the full 0.5– 10keVband λ f
with a limiting depth of 5.7×10−16 erg s−1 cm−2. The (Rhoads et al.2003) fromthe best fit asymmetric Gaus-
sians. a is 1.2,2.2 and1.2 and a is 1.2,1.8 and 1.1for
survey covers the central ∼ 0.9 deg2 of the COSMOS λ f
LAE14310,LAE40844and LAE27878,respectively.
field. We find no X-ray sources matching the coordi-
nates of any of our LAEs within 12.8′′, which is much We defined the redshift of the emission line using the
central wavelengths determined from these fits (from z
larger than the combined positional uncertainties of the
=(λ /λ ) −1 where λ is the rest-frame vacuum
narrowbandand X-ray catalogs. This gives upper limits obs em em
fx/fLy-α .1.6–6.0forthethreesourceswithLUCIFER 16 developed by Craig Markwardt
http://www.physics.wisc.edu/∼craigm/idl/idl.html
14 http://www2.keck.hawaii.edu/inst/nirspec/wmkonspec.html 17 developed by Andrew Marble
15http://www.sdss.org/dr6/algorithms/sdssUBVRITransform.html http://hubble.as.arizona.edu/idl/arm/
5
wavelengthandλ isthecentralwavelengthofthebest based on the redshift measured from the [Oiii] line did
obs
fit). Linefluxforthe[Oiii]linewasdeterminedfromthe not yield a significant upper limit.
areaunderthebestfitsymmetricGaussian. Linefluxfor
the Ly-α line was determined from the narrowband line 3.3. Systemic Redshifts and Velocity Offsets Between
flux. The area under the best fit asymmetric Gaussian [Oiii] and Ly-α
was scaled to match this flux. The passband of the fil-
Using the Gaussian fits described above, we measured
ter transmission curve was taken into account to assign
systemic redshifts from the strongest [Oiii] line in the
theappropriateamountoffluxtothemainLy-αpeakin
two objects with detections, finding redshifts of 3.11170
LAE40844 and this object’s secondary ’blue bump’ dis-
± 0.00014 and 3.11879 ± 0.00011 for LAE40844 and
cussed in more detail in section 3.4. Errors on the area
LAE27878,respectively.
weredetermineddirectlyfromthePERRORoutputfrom
Measuring the redshift of each object using the Ly-
the MPFITEXPR routine, which returns the one-sigma
α line instead of the [Oiii], yields redshifts of 3.11639
erroronfittedparameters. PERRORoutputvalueswere
± 0.00021 and 3.12051 ± 0.00021 for LAE40844 and
also used to quantify the error on the best fit central
LAE27878,respectively,aftercorrectionsforthe Earth’s
wavelength, but an additional error term was included
motion. To correctfor the Earth’s motion we calculated
here to account for wavelength calibration errors from
topocentric radial velocities18 for the two observing lo-
the Hectospec and LUCIFER spectra. Errors on calcu-
cations(MMT andLBT) for the nights the objects were
lated values for redshift and velocity offsets between the
observedat each location. The generally accepted inter-
Ly-α line and the [Oiii] line were derived from best fit
pretation of this discrepancy in redshift measurements
central wavelength and its associated error as described
from interstellar emission lines and Ly-α is that there is
directlyabove. Bestfitsarecalculatedfromunsmoothed
a kinematic offset between the lines caused by a large
spectra, while the spectra in figures 2 and 3 are plotted
scale outflow, an outflow likely driven by active star for-
after 3-pixel boxcar smoothing.
mation.
3.2. [Oiii] Detection with LUCIFER Assuming that the emission lines originate from a sin-
gle redshift, we pin the lines to the redshift of the [Oiii]
We detect the [Oiii] 5008.240/4960.295 ˚A doublet in
line and use this frame to calculate a velocity offset be-
two of the three LAEs, LAE40844 and LAE27878. For
tween the two lines. We justify using the nebular emis-
the stronger [Oiii] line (rest frame vacuum wavelength
siontodefinethesystemicredshiftofthegalaxysincethe
of 5008.240 ˚A), we measure a line flux of 35.48 ± 1.15
[Oiii] emission originates from Hii regions surrounding
× 10−17 erg s−1 cm−2 in LAE40844 and 6.96 ± 0.33 × ionizingstars. Theseregionsoughttobe atthe systemic
10−17 erg s−1 cm−2 in LAE27878. redshift of the galaxy. While the Ly-α initially departs
The second strongest [Oiii] line (rest frame vacuum fromthe sameregions,resonantscattering,whicheffects
wavelengthof4960.295˚A)wasalsofoundinthesametwo Ly-α and not [Oiii], changes the observed location of
LAEs. The line fluxes measured for this line from best Ly-α emission.
fit Gaussians were 14.82 ± 2.24 × 10−17erg s−1 cm−2 We derived velocity offset between the 5008.240 ˚A
and 1.47 ± 0.37 × 10−17erg s−1 cm−2 for LAE40844, [Oiii]lineandLy-αlinebasedonthecentralwavelength
LAE27878, respectively. The ratio of this secondary of each line, determined by the best fit asymmetric and
[Oiii] line to the stronger [Oiii] line is within 2σ of the symmetric Gaussians for the Ly-α and [Oiii] lines, re-
theoretical value (1/3) in both galaxies. This provides a spectively. Wefindvelocityoffsetsof+342±18.3kms−1
check of the data analysis and increases confidence that and +125 ± 17.3 km s−1 for LAE40844 and LAE27878,
thisisthe4960.295˚Aline. Table1summarizesthe[Oiii] respectively. The velocity offsets between the [Oiii] and
and Ly-α line fluxes for each LAE, along with relevant Ly-α lines are shown in Figure 2.
broadband and narrowbandcharacteristics. Steidel et al. (2010) note that their redshift determi-
LAE14310showed no detectable [Oiii] emission. This nationsforz≃2–3galaxiesbasedonNIRHαmeasure-
could be explained if LAE14310 was in fact a lower red- mentshaveaninherentuncertaintyof∼60 kms−1. This
shift interloper, but a visual inspection of the optical estimate is based on repeated observations of the same
spectrum has ruled out the object as an [Oii] emitter at galaxywiththeir0.76′′slitindifferentpositions. Theun-
z ∼ 0.34 or an [Oiii] emitter at z ∼ 0. In the case of an certaintyisexplainedasarisingfromthefactthatineach
[Oii] emitter at z ∼ 0.34 we would expect to see [Oiii] measurementtheyareonlymeasuringthevelocityofthe
at λ ∼ 6710 ˚A, which we do not see. If the object were fractionofthefluxthatenteredtheslit. Wefindthatour
a z ∼ 0 [Oiii] emitter, we would expect to see the λ = [Oiii] measurements for our z ∼ 3.1 galaxies should not
4960.295 ˚A[Oiii] line with a line flux of ∼ 1.8 × 10−16, besubjecttosuchalargeuncertaintyfromthiseffectdue
which we also don’t see. The more likely scenarios are to our larger slit width (1′′) and small galaxy sizes. The
then that the [Oiii] emission in this object is weak, or sizesofourgalaxies,fromACSi-bandhalflightradii,are
the [Oiii] line is being covered by strong OH lines/H O 1.1 and 1.3 kpc, for LAE40844, LAE27878, respectively
2
absorption in this region. (Malhotra et al. 2010). The corresponding half-light an-
We are unable to detect Hβ and/or determine upper gulardiametersarestilllessthanhalftheslitwidth. The
limitsforHβ emission,likelybecausetheredshiftofeach 1.5′′ diameter fibers should also minimize flux losses for
object has placed the Hβ line under strong OH lines our Ly-α observations,however,we concede that our er-
and/or under H O absorption features. We do not de- ror bars may be lower limits due to such systematics we
2
tect the [Oii] (3729.875 ˚A) line by visual inspection in may not be fully taking into account.
either LAE27878 or LAE40844. Determining an upper
limitforthislinebyfixingtheexpected[Oii]wavelength 18 http://fuse.pha.jhu.edu/support/tools/vlsr.html
6
TABLE 1
GalaxyCharacteristics LAE14310 LAE27878 LAE40844
u∗ Magnitude 25.46±0.29 26.54±0.50 25.56±0.32
Narrowband KPNO MOSAIC[Oiii]Magnitude 22.56±0.11 23.34±0.15 21.82±0.06
g+ Magnitude 24.49±0.13 25.47±0.19 23.66±0.06
zLyα1 3.11043±0.00021 3.12051±0.00021 3.11639±0.00021
zOIII 3.11879±0.00011 3.11170±0.00014
Ly-αEquivalent Width2,6 89+−1270 118+−3440 78+−88
Ly-αLine Flux from Narrowband3,4 18.7+−22..2551 9.41+−11..4623 36.1+−22..3457
Upperlimit on xray / Ly-αFlux Ratio 3.0 6.1 1.6
Oiiiline flux (λ = 5008.240 ˚A)3 6.96±0.33 35.48±1.15
Oiiiline flux (λ = 4960.295 ˚A)3 1.47±0.37 14.82±2.24
Oiiivelocity width(λ = 5008.240 ˚A)5 189.3±10.3 281.1±9.8
OiiiFWHM (λ = 5008.240 ˚A)6 13.0±0.7 19.3±0.7
voffset of Oiiifrom Lyα5 +125±17.3 +342±18.3
1 correctedforEarth’smotion
2 Rest Frame, from Narrowband flux, calculated as (FNB−Fg)/(Fg/55˚A−FNB/1265˚A) where 55 ˚A is bandpass
ofKPNOMOSAIC[Oiii]filterand1265˚AisbandpassofSubarug+ filter,FNB isfluxinnarrowband,Fg isfluxin
g+ band.
3 10−17 ergs−1cm−2
4Calculatedas(FNB−Fg)(c/λ2c)dλwhereλccentralwavelengthanddλisbandpassofKPNOMOSAIC[Oiii]filter
5 kms−1
6 ˚A
Fig. 2.— [Oiii] line and Ly-α lines with their corresponding
best-fit Gaussians. Velocity offset between [Oiii] and Ly-α line
for LAE40844 is 342 km s−1. Velocity offset between [Oiii] and Fig.3.—LAE40844,bestfitasymmetricGaussiantoredshifted
Ly-αlineforLAE27878is125kms−1. Opticalspectrahavebeen Ly-αemissioninred,bestfitasymmetricGaussiantoblueshifted
calibrated using the Ly-α line flux determined from narrowband Ly-α emission in blue, optical spectrum in black. Velocity offset
imaging. Thefeaturenear+1400 kms−1inLAE27878isaresidual betweentheLy-αtwopeaksis+796.2±53.9kms−1.
nightskylineat∼20728.17/20729.859˚A(Rousselotetal.2000).
Additionally,usingthefluxcalibrationwederivedfrom
3.4. ‘Blue bump’ in LAE40844 - Velocity Offset of the narrowband line flux, we find that this blue Ly-α
Secondary Ly-α Feature peak has a line flux of ∼ 1.08 ×10−16 erg s−1 cm−2.
When determining this calibration we found that based
LAE40844hasanotherfeatureofinterestinits optical
on the transmission curve of the narrowband filter, the
spectrum, namely a smaller, secondary Ly-α peak blue-
blue bump contributed ∼ 9.4 % of the total narrowband
wardofthe systemic velocity ofthe object. See Figure 3
line flux (3.61 ×10−16 erg s−1 cm−2). Comparing this
foradetailedviewofthisfeature. Thisfeatureisfitwith
to a line flux of ∼ 3.27 ×10−16 erg s−1 cm−2 for the red
an asymmetric Gaussian as described for the main Ly-α
Ly-α peak we find an approximate flux ratio, red:blue,
lineinsection3.1,buttheconstantsontheleftandright
for the two lines of 3.0. In other words, the strength of
sidesofthe Gaussianarerequiredtobe equal(andtobe
secondary (blue) peak is roughly 33% that of the main
equaltothepre-determinedconstantlevelalsodescribed
(red) peak. In section 4 we discuss a scenario that can
insection3.1)toensurethatthemainLy-αpeakdidnot
giveriseto this blue bump andcomparethe velocityoff-
interferwithourbestfitmeasurementsofthissecondary
setwe findbetweenthe twoLy-αlines tovelocityoffsets
peak. This essentially reduces the number of fitted pa-
thathavebeenpresentedintheliteratureonLy-αradia-
rameters for the blue bump from 5 to 4, meaning that
tive transfer.
when both the main Ly-α line and the blue bump are
fit,atotalof9parametersarereturned(5formainLy-α
3.5. Effect of the [Oiii] Emission Line on Mass and
peak, 4 for blue bump). Our method yields a velocity
Age Estimates
offset from the [Oiii] line of -453.7 ± 50.7 kms−1, after
correction for the Earth’s motion. From this measure- The COSMOS field has a deep Ks-band (centered at
ment we determine that the two Ly-α peaks are offset 21460˚A,∆λ = 3250 ˚A) coverage fromCFHT WIRCAM,
from one another by +796.2 ± 53.9 km s−1. with a 5σ depth of ∼ 23.8 mag in a 3′′ aperture on a
7
PSF matched image. For our z ∼ 3.1 LAEs, this band ing this to our range of luminosities for the 5007 ˚A line
will encompass both the continuum and [Oiii] emission. for two z ∼ 3.1 LAEs of 6.1 – 31.0 × 1042 erg s−1, it
The 2008 COSMOS Intermediate and Broad Band Pho- appearsthatourfainter5007˚Ameasurementfallsinthe
tometryCatalog(Capak et al.2007)hasKsbandmagni- P01 range, while the stronger of our two 5007 ˚A lines
tudesof22.61±0.07forLAE40844,and24.91±0.62for is almost double that of the brightest luminosity in the
LAE27878(MAG AUTO measurements from a 3′′ aper- P01sample. Luminosities for the 4959˚A line in the P01
ture). Ourmeasured[Oiii]linefluxescanaccountforthe sampleare1.4–6.5×1042 ergs−1. Thisyieldsthesame
entireKsbandfluxes,wherewefindKs-bandmagnitudes trend we see in the 5007 ˚A line; where our fainter 4959
of 23.09 ± 0.036 and 24.79 ± 0.053 for LAE40844 and
˚A measurement falls in the P01 range, and our brighter
LAE27878,respectively, using just the [Oiii] line fluxes.
4959 ˚A line is approximately twice that of the brightest
This shows that our detected [Oiii] emission lines alone
can be responsible for all or nearly all the flux measured 4959 ˚A measurement in the P01 sample.
in the K band for both of these LAEs. [Oiii] luminosities from lensed galaxies around z ∼ 3
s
This has important implications for mass and age es- have also been documented. Fosbury et al. (2003) mea-
timates of high redshift galaxies. These estimates typi- sured the [Oiii] line in a lensed Hii galaxy, also known
cally rely on the size of the Balmer break to determine asthe Lynxarc(Holden et al.2001), ataredshiftofz∼
the age of the galaxy. If, as is the case for our two 3.36. UsingNIRSPECK-bandspectratheyfindan[Oiii]
LAEs, there is a dominant emission line polluting the luminosity of 28.3 ± 0.3 × 1042 erg s−1 for the 5007 ˚A
locationredwardoftheBalmer/4000˚Abreak,thenthe line and 9.8 ± 0.3 × 1042 erg s−1 for the 4959 ˚A line.
size of the break may be overestimated and the subse- These luminosities (which have been corrected for mag-
quently derived ages and stellar masses may be overes- nification)arequitecomparabletoourmeasurementsfor
timated. Schaerer & de Barros (2009) found that when the 5007 ˚A and 4959 ˚A lines. Finkelstein et al. (2009b)
nebular emission lines were included, SED fitting of the measuredthe [Oiii] line in a lensed ultraviolet-luminous
Eyles et al. (2007) sample of 10 z ∼ 6 galaxies yielded z=2.73galaxyknownasthe8o’clockarc. Forthe 4959
an average age ∼ 4 times younger than what was found ˚Aline they find a luminosity of 8.9± 0.4× 1042 erg s−1
without the emission lines included. The average stellar (aftercorrectionformagnification). Thisagainfallsright
mass estimate also decreased from 1.2×1010 M⊙ with- intherangereportedforourtwoLAEs. The5007˚Aline
outemissionlinesto7.8×109M⊙ whennebularemission was not reported by Finkelstein et al. (2009b) because
was included. Some studies of Ly-α emission in Lyman-
it fell in an area of low atmospheric transmission which
breakselectedpopulationshavefoundthatthestrongest
required a correspondingly large tellluric correction and
Ly-αemittershaveblueUVspectralslopesbutredopti-
led to large uncertainties in any measurements from the
cal slopes (Shapley et al. 2003; Kornei et al. 2010), and
line.
have concluded that Ly-α emission is strongestin LBGs Lookingatoursampleoftwogalaxies,the[Oiii]lumi-
that are older but relatively dust-free. If strong nebular
nosities in LAE27878 are most similar to the P01 LBGs
line emission contributes to the observations of red rest- while the more luminous [Oiii] lines in LAE40844 are
optical slope, it might be possible to reinterpret such more comparable to more luminous [Oiii] lines found in
observations in terms of young, strongly line emitting
galaxiesstudiedvialensing. AlargersampleofNIRspec-
galaxies, although Kornei et al. (2010) argue that their tra with [Oiii] line measurements for LAEs will help us
observed correlations between Ly-α strength and stel- understand the range of [Oiii] luminosity in LAEs and
lar population age are unchanged when they select ob-
its implications.
jectsonlyatredshiftswheretheopticalcontinuumfilters
are line-free. Previous work on stellar populations of
4.2. Ly-α Line Profiles and Outflow Models
Ly-α-selected galaxies has found that nebular line emis-
sion is required to explain observed rest-optical colors Inadditiontotheinformationthatcanbegleanedfrom
(Finkelstein et al. 2008). Recently, at the highest red- the line fluxes of nebular emission lines such as [Oiii],
shifts, Ono et al. (2010) have shown that either old stel- theasymmetricprofilesofLy-αemissionlinesthemselves
lar populations or young ones with strong nebular emis- carry information on the physical conditions and pro-
sioncan reproduce the composite SEDs ofLy-α selected cesses in these objects. Resonant scattering can lead to
galaxies. The older models have correspondingly higher asymmetric profiles through radiative transfer processes
stellarmasses,sincemasstolightratioincreasesstrongly operating either within the Ly-α emitting galaxy, or in
with age. Our observations provide direct observational the surrounding intergalactic medium. Establishing the
evidencethatnebularlinefluxdominatestherestoptical systemic velocity with the [Oiii] line sheds new light on
in analogous objects at z ≈ 3, and hence supports the those processes and conditions.
interpretation of the high redshift Ly-α selected popula- We find that the Ly-α line profiles seen in LAE40844
tions as young and low-mass objects. and LAE27878, and the velocity offset of the Ly-α line
fromsystemic in both objects is in goodagreementwith
4. DISCUSSION what is predicted by outflow models, where the Ly-α
is redshifted through interaction with receding gas on
4.1. [Oiii] Luminosities
the far side of the galaxy, and transmitted through ap-
The [Oiii] line has been measured in other objects proaching gas on the near side due to the line’s kine-
at similar redshifts. Pettini et al. (2001, henceforth matic redshift. Two particular types of outflow models
P01) find [Oiii] (5007 ˚A) luminosities of 3.5 – 15.6 × are discussed here due to their apparentagreementwith
1042 erg s−1 in four individual z ∼ 3.1 LBGs observed our results - the case where the outflow is in a coher-
withVLT1/ISAACand/orKeckII/NIRSPEC.Compar- ent shell (Tenorio-Tagle et al. 1999; Dawson et al. 2002;
8
Zheng et al. 2010; Verhamme et al. 2006, 2008) and the Doppler parameter, V , the neutral hydrogen column
exp
case of a clumpy outflow (Neufeld 1991; Hansen & Oh density N , the dust opticaldepthτ , andthe intrinisc
HI a
2006; Steidel et al. 2010). EW and FWHM of the input Ly-α line. Nine of the 11
Understanding our results in the context of an out- LBGs investigatedwere found to have V ∼ 150 – 200
exp
flow is justified in that high-redshift LAEs are typically kms−1. Two LBGs, which resemble our LAE40844 in
young (age ≃ 107 years) with vigorous star formation thatthey havea bluebump, canbe fiteither withlarger
(e.g., Finkelstein et al. 2007, 2009; Pirzkal et al. 2007; velocities, of order 300 – 400 kms−1 or a with a quasi-
Gawiser et al. 2007; Lai et al. 2008). Their typical star static medium with V of order 10 – 25 kms−1. V08
exp
formation intensities are well above the threshold re- prefer the quasi-static explanation. We note that our
quired to drive galactic winds (Malhotra et al. 2010). resultsofv =125and342 kms−1isfullyconsistent
offset
Similar winds are seen in nearby starbursting galaxies, with the V08 results. However, we emphasize that we
with velocities of order 102-103 km s−1 (Heckman et al. measure the velocity offset between the Ly-α line and
1990;Heckman2002),numbersthatencompassourmea- the systemic redshift as defined by [Oiii], whereas V06
sured velocity offsets. and V08 are quantifying V , which is the velocity of
exp
In the model with a single expanding shell, with a a spherically expanding shell around the central Ly-α
central monochromatic source (Verhamme et al. 2006, source in their model. We also note that LAE40844,
henceforth V06), the redshifted Ly-α line is built up of with its blue bump and v of 342 kms−1, appears
offset
photons that underwent one or more backscatterings off toshowthataquasi-staticmodelis notalwayspreferred
the expanding shell. The more backscatterings the pho- for objects with a blue bump.
ton undergoes, the further it is redshifted, giving rise to Steidel et al. (2010, henceforth S10) consider out-
theprominentredwingthatisseenintheredshiftedLy- flows to be an important component of the mechanism
α line. Photons that are emitted from the blue wing of that shapes the observed Ly-α profile, but they prefer
the Ly-α line in the part ofthe shellthat is approaching a scenario in which the structure and kinematics of the
the observer can give rise to the blue bump we see in circum-galactic medium can produce our observed pro-
LAE40844. See Figure 12 of V06 for a detailed descrip- files, instead of relying on Ly-α radiative transfer in an
tion of photons escaping from an expanding shell. expanding shell to create the profiles we see. In the S10
For the parameter space examined in the V06 simula- scenario, a clumpy outflowing gas will allow some Ly-α
tions, velocity offsets of a few 100 kms−1 are predicted photonstoescapefromagalaxyproducingtheredshifted
for the redshifted Ly-α line. The magnitude of the Ly-α Ly-α line we have observed. This same scenario is also
velocity offset depends on the velocity of the expanding capable of producing the faint ‘blue bump’ we have dis-
shell, the column density of neutral hydrogen and the cussed in section 3.4. S10 find an average velocity offset
Dopplerparameter(seeV06fordetailsontheseparame- of 445 kms−1 in a sample of 42 z ≃ 2 - 3 LBGs with
ters). Ourvelocityoffsetsof125 kms−1 and342 kms−1 Ly-α emission. The velocity offset measurement in this
appear to be at the low end of this range. When the case was made with respect to the redshift defined by
blue bump is seen in these simulations it is offset from Hα emission. As is the case when we compare our re-
theredshiftedLy-αlineby∼1000 kms−1,inagreement sults to the Verhamme model, our velocity offsets of 125
with our measured offset in LAE40844 between the red kms−1 and342 kms−1 forourobservedLAEsarebelow
and blue peaks of 796 kms−1. Thus there is good gen- these values. This conclusion includes consideration of
eral agreement between our observations and the V06 the fact that S10 measures the centroid of a single func-
models,whichhavenotbeen tuned specificallyto fit our tion whereas we measure the peak of Ly-alpha through
data. Itseemslikelythatanexpandingshellmodelcould an asymmetric function composed of two Gauss func-
fit our data well with some adjustment of the input pa- tions; which inherently causes the S10 measurements to
rameters. Additionally, while our total offset between behigher. Thiscausesa10–15%differenceintheveloc-
the blue and red Ly-α peaks in LAE40844 agree with ity offsets calculated, and hence we stil find our velocity
the total offsets seen in their simulations, we find that offset values to be lower than those in the S10 sample
v ∼-2v inLAE40844. InV06thevelocity evn after this consideration.
bluepeak redpeak
offsetbetweenthesetwopeaksisnearlysymmetricinthe At present, our sample of two z ∼ 3.1 LAEs reported
caseswherebothpeaksarepresentandtheexpansionve- in this paper does not allow us to distinguish between
locity of the shell is small (< 200 kms−1). Where the theexpandingshellscenarioortheS10interpretation,as
expansionvelocityoftheshellislarge(300–400 kms−1) both are able to produce profiles and velocity offsets in
the blue peak is nearly -1/2 the velocity shift of the red reasonable agreement with our observations. Whichever
peak. In either case, these predictions do not directly of the scenarios discussed here (V06 or S10) is pro-
match our observations presented here. Further work is ducing the observed velocity offsets, we emphasize that
needed to understand the discrepancy between the ve- winds/outflows are important in either case.
locitiesofthe redshiftedandblueshiftedpeaksinmodels The additional observational samples to which we can
and observations. Deviations from spherical symmetry best compare our current results are z ∼ 3 LBGs. P01
of the expanding shell model could help account for this andShapleyetal. (2003,henceforthS03)havebothmea-
difference,aswasnotedinSchaerer & Verhamme(2008). sured the velocity offset of the Ly-α line from systemic
These same authors further investigate their model in in a population of z ∼ 3 LBGs. P01 find velocity offsets
alaterpaper(Verhammeetal. 2008,henceforthV08)by for the Ly-α line of 200 - 1100 kms−1 in a sample of
applyingittofitthe observedLy-αemissionlineprofiles 13 LBGs that also show Ly-α in emission. The velocity
of the LBGs from Tapken et al. (2007), which lie at offset is measured compared to the redshift of nebular
2.7 ≤ z ≤ 5. Varying model parameters include the Hii emission. S03 find a velocity offset of 360 kms−1
9
from a composite spectrum of 811 z ∼ 3 LBGs. The works by noting that substantial transmission of Ly-α
Ly-α velocity offset measurement in S03 was made with through a generally neutral IGM requires a locally
respect to interstellar absorption lines. (Tapken et al. ionized region around each observed LAE. The product
2007) measured a velocity offset between LIS lines and of the bubble volume V and the LAE number density
Ly-αinsevenLBGswithLy-αemissionatredshiftsof2.7 n is then a filling factor of ionized gas, from which the
– 5 (the samesample of galaxiesusedin V08)finding an volume fraction of the ionized phase is ≈1−exp(−nV)
average offset of 580 kms−1. Including the 445 kms−1 (MR06). The relevant bubble volume V is sensitive
offsetmeasurementfromtheS10samplediscussedabove, to the velocity offset, since Ly-α photons that are
we note that in all cases our observed velocity offsets of already redshifted before leaving the emitting galaxy
125 - 342 kms−1 in two z ∼ 3.1 LAEs are at the low are less strongly scattered by the damping wing of
end of the values reported for the various LBG samples, neutral hydrogen in the surrounding intergalactic gas.
although the velocity offset measurements have thus far The effect is explored in detail in Figure 1 of MR06.
been made via different methods. Replacing the range 0 ≤ ∆v ≤ 360 kms−1from that
An alternative explanation of the observed Ly-α pro- paper with our measurements, ∆v = 125 kms−1and
files, not based on galactic scale outflows, is that they ∆v =342 kms−1, would narrow the range of permitted
arisethroughresonantscatteringinthe intergalacticgas volume ionized fractions from the 20% – 50% range
surrounding an LAE. Zheng et al. (2010) have explored derivedin MR06 to ∼ 20%– 40%. While this discussion
such a mechanism in detail for redshift z = 5.7. Their is subject to refinement as the sample of LAEs with a
models can produce Ly-α lines that qualitatively resem- measured velocity offset grows, it shows the importance
ble our observations both in the line asymmetry and in of measuring ∆v for studying reionization with Ly-α
the redshift of the Ly-α line. However, some caution is lines.
needed in applying these results to our data set, given Finally, even at redshifts where the IGM is predom-
that the IGM density at redshift z = 3.14 is ∼ 1/4 that inantly ionized and affects the Ly-α line only through
at z = 5.7, and the ratio of neutral gas density between the Ly-α forest, our systemic redshift measurements
thesetworedshiftsisstillmoreextreme. Overall,wecon- haveimportantimplications. Severalgroupshaveshown
sider it more likely that winds play an important role in that the observed Ly-α luminosity function is largely
Ly-α escape, given that winds are generically expected unchanged from z ≈ 3 to z ≈ 6 (e.g., Dawson et al.
for galaxies with the high specific star formation rates 2004;Stern et al.2005;Ouichi et al.2008;Cassata et al.
typical of LAEs. 2011). Recently, Cassata et al. (2011) have combined
this observation with the expected optical depth evolu-
4.3. Implications of Detected Outflows
tion of the Ly-α forest (Madau 1995; Fan et al. 2008) to
Detection and characterization of galaxy scale out- infer that the Ly-α luminosity function is in fact evolv-
flows at high redshift is important because these out- ing towards higher luminosities at higher redshifts. The
flows have important consequences for the evolution keyassumptionin this argumentis thatthe line emitted
of individual galaxies as well as the evolution of the by the LAE is symmetric and centered on the systemic
IGM. Large scale galactic outflows are capable of driv- velocity, so that the fraction scattered by the IGM ap-
ing materials out of the galaxy and may therefore con- proaches 50% by z ≈ 6. This implies that the fraction
tribute to metal enrichment of the IGM at high-z by of Ly-α flux observed would decline by a factor of ≈0.6
introducing materials produced from starbursts into the between redshift z ≈ 3 and z ≈ 6. However, for the
IGM. In addition, galactic winds likely provide a cru- two objects where we observe [Oiii], we know that only
cial channel by which ionizing photons can escape from thosephotonsobservedbluewardofthesystemicvelocity
agalaxy(Steidel et al.2001;Heckman et al.2001). This wouldbe subjectto additionalLy-αforestabsorptionat
has important implications for the contribution of high- higherredshift. ForLAE40844,the blue bump wouldbe
z galaxies to the reionization of the IGM. In terms progressively obscured by the forest at higher redshift,
of shaping an individual galaxy, superwinds are re- resulting in a flux loss of a factor ≈ 3.0/4.0 = 0.75 or
sponsible for driving dust from a starbursting galaxy so at most. For LAE27878, there is no significant flux
(Ferrara et al. 1991; Heckman et al. 2000; Shapley et al. blueward of the systemic velocity, and the fraction of
2001; Bianchi & Ferrara 2005) and the mass loss from a its Ly-α emission that we can see should remain nearly
galaxy due to an outflow may be capable of suppressing unchanged from z = 3.1 until the IGM neutral fraction
star formation (Somerville & Primack 1999; Heckman becomes so large that the red damping wing of the IGM
2002). While our work presented in this paper has now becomes optically thick— i.e., until we reachthe central
demonstrated that that our sample of two Ly-α selected stages of reionization.
galaxies at z ∼ 3.1 are driving winds, further character- To recap, our objects show relatively little Ly-α flux
ization of these winds from a larger sample will help us blueward of the systemic velocity. This either requires
understand and test some of the broader implications of dropping the assumption that Ly-α is symmetric and
winds detailed above. centered at the systematic velocity, or else implies that
The observed velocity offsets between the Ly-α line we happen to have observed two sources where the Ly-
and systemic velocity also have important implications α forest at z ≈3.1 is unusually optically thick. We now
forLy-αbasedtestsofreionization(Malhotra & Rhoads proceedtoevaluatethelikelihoodofthesecondscenario.
2004; Santos 2004; Malhotra & Rhoads 2006; Presently, our sample consists of two galaxies, with
Dijkstra & Wyithe 2010). In particular, the ion- measured blue to red flux ratios of 0.33 and 0.08 for
ized volume test proposed by Malhotra & Rhoads objects 40844 and 27878, respectively. We estimate
(2006) (hereafter MR06) is much more sensitive if ∆v the chance of such an occurrence under the assump-
(Ly-α peak vs. systemic) is typically small. That test tion of Cassata et al (i.e., that the intrinsic ratio is 1:1,
10
and deviations indicate absorption by the Ly-α forest). indicate that our measurements are the result of Ly-α
McDonald et al.(2000)giveprobabilitydistributionsfor emission emerging in the presence of a galactic outflow.
transmission through the Lyman α forest at z = 3.00 InadditionwehavemeasuredLy-αfluxbluewardofsys-
and z = 3.89. Interpolating their results to our redshift temic in a ‘blue bump’ in one of our objects. This is
(z = 3.12), we infer probabilities P(T ≤ 0.33) = 0.24 anotherphenomenononecanexpectwhenobservingLy-
andP(T ≤0.08)=0.15. Givenasampleof2,andtreat- α emission in the presence of anoutflow. We find that a
ing their Ly-α forest transmissions as independent ran- scenarioin which radiativetransfer effects of Ly-α emis-
domvariables,the probabilitythatatleastonewillhave sion emerging from an expanding shell (V06) is able to
T ≤ 0.08 while the second has T ≤ 0.33 becomes 0.050. reproducereasonablywellourobservedLy-αprofilesand
So,our presentresultsdisfavorthis assumptionofanin- velocityoffsets. WealsofindthatascenarioinwhichLy-
trinsic1:1ratioofblue:redflux,suggestingthatthelumi- αphotons escapefroma circumgalacticgasasdescribed
nosityfunctionevolutioninferredinCassata et al.(2011) by Steidel et al. (2010) is capable of reproducing our re-
isaconsequenceoftheirimplicitassumption∆v =0and sults reasonably well.
notastrongconclusionaboutthe true evolutionofLy-α Alargersampleofmeasuredvelocityoffsetswillbetter
galaxy populations. For now, this is a 2σ result. Ob- constrain the range of velocity offsets we can expect for
servations of a few more Ly-α emitters with systemic z ∼ 3.1 LAEs and will allow for better understanding of
velocitymeasurementscouldresolvethisquestionfirmly. how these offsets compare to those observed in Lyman-
break selected samples. These comparisons should shed
light on the relationship between crucial characteristics
5. CONCLUSIONS likegalaxymass,starformationratesandthemagnitude
of the observed velocity offsets. Finally, larger samples
We have detected [Oiii] emission in two Ly-α selected
of velocity offsets will further improve our ability to in-
galaxiesatz ∼ 3.1 using the new NIR spectrographLU-
fer constraints on cosmological reionization from obser-
CIFER on the LBT. This is a successful demonstration
vations of Ly-α galaxies at high redshifts.
that the [Oiii] line can be detected in high-z Ly-α se-
lected galaxies and that this line can be used to investi-
gate the characteristics of these galaxies. WethanktheLUCIFERInstrumentteamandtheLU-
In both LAEs we measured a velocity offset between CIFER SDT team for their hard work and for exciting
theLy-αemissionandthesystemicredshiftofthegalaxy science with this new instrument. This work was sup-
as defined by the [Oiii] emission. These velocity offsets portedbyNSFgrantAST-0808165. Inadditionwethank
range from 125 - 342 kms−1. We find that these veloc- the referree for insightful comments that have improved
ity offsets and the observedprofile of the Ly-α line both this paper.
REFERENCES
Ageorges,N.etal.2010,“LUCIFER1commissioningatthe Fukugita,M.,Yasuda,M.,Doi,M.,Gunn,J.E.&York,D.G.
LBT”,SPIEProc.Vol.7735-56, tobepublished 2010,submittedtoAJ
Bertin,E.&Arnouts,S.1996, A&AS,117,393 Gawiser,E.etal.2006,ApJ,642,L13
Adelberger,K.L.,Steidel,C.C.,Pettini,M.,Shapley, A.E., Gawiser,E.etal.2007,ApJ,671,278
Reddy, N.A.&Erb,D.K.2005,ApJ,619,697 Hansen,M.&Oh,S.P.2006, MNRAS,367,979
Bianchi,S.&Ferrara,A.2005,MNRAS,358,379 Hayes,M.etal.2010, Nature,464,562
Bond,N.A.,Gawiser,E.,Gronwall,C.,Ciardullo,R.,Altmann, Heckman,T.M.,Armus,L.,Miley,G.,K.1990,ApJS,74,833
M.&Schawinski,K.2009,ApJ,705,639 Heckman,T.M.,Lehnert, M.,D.,Strickland,D.,K.,M.,Armus,
Bond,N.A.,Feldmeier,J.J.,Matkovich,A.,Gronwall,C., L.2000, ApJS,129,493
Ciardullo,R.,&Gawiser,E.2010,ApJ,accepted Heckman,T.M.,Semback, K.,Meurer,G.,Leitherer,C.,
Bremer,M.N.&Johnstone, R.M.1995,MNRAS,277,L51 Calzetti,D.&Martin,C.2001,ApJ,558,56
Capak,P.etal.2007,ApJS,172,99 Heckman,T.M.2002,ASPCS,254,292
Cassata,P.etal.2011, A&A,525,143 Holden,B.P.,etal.2001, AJ,122,629
Cowie,L.L.etal.2010,ApJ,711,928 Holwerda,B.W.2005, astro-ph/0512139
Dawson,S.,Spinrad,H.,Stern,D.,Dey,A.,vanBreugel,W.,de Ilbert,O.,CapakP.,&Salvato,M.etal.2009,ApJ,690,1236
Bries,W.&Reuland,M.,2002,ApJ,570,92 Kornei,K.A.,etal.2010, ApJ,711,693
Dawson,S.,Rhoads,J.E.,Malhotra,S.,Stern,D.,Dey,A., Kashikawa,N.etal.2006,ApJ,648,7
Spinrad,H.,Jannuzi, B.T.,Wang, J.X.&Landes,E.2004, Lai,K.etal.2008,ApJ,674,70
ApJ,617,707 Madau,P.1995,ApJ,441,18
Dayal,P.,Maselli,A.&FerraraA.2010,MNRAS,410,830 Malhotra,S.&Rhoads,J.E.2006,ApJ,647,L95
Dijkstra,M.&Wyithe, S.2010, MNRAS,408,352 Malhotra,S.&Rhoads,J.E.2004,ApJ,617,L5
Dow-Hygelund,C.C.etal.2007,ApJ,660,47 Malhotra,S.etal.2010(inprep)
Elvis,M.etal.2010, APJS,184,158 McCandliss,S.R.2009,AIPCS,1135, 309
Eyles,L.P.etal.2007,MNRAS,374,910 McDonald,P.,Miralda-Escud´e,J.,Rauch,M.,Sargent,W.L.W.,
Fan,X.etal.2006,AJ,132,117 Barlow,T.A.,Cen,R.,&Ostriker,J.P.2000, ApJ543,1
Ferrara,A.,Ferrini,F.,Barsella,B.&Franco,J.1991, ApJ,381, Miyazaki,S.etal.2002,PASJ,54,833
137 Neufeld,D.A.1991,ApJ,370L,85
Finkelstein,S.L.,Rhoads,J.E.,Malhotra,S.,Pirzkal,N.& Nilsson,K.K.etal.2007,A&A,471,71
Wang, J.X.2007,ApJ,660,1023 Ono,Y.etal.2010, ApJ,724,1524
Finkelstein,S.L.,Rhoads,J.E.,Malhotra,S.,Grogin,N.& Ouichi,M.etal.2008,ApJS,176,301
Wang, J.X.2008,ApJ,678,655 Partridge,R.B.&Peebles,P.J.E.1967,ApJ,147,868
Finkelstein,S.L.,Rhoads,J.E.,Malhotra,S.&Grogin,N.2009, Pettini,M.,Shapley,A.E.,Steidel,C.C.,Cuby,J.-G.,Dickinson,
ApJ,691,465 M.,Moorwood,A.F.M.,Adelberger,K.L.,Giavalisco,M.2001,
Finkelstein,S.L.etal.2009, ApJ,700,376 ApJ,554,981
Fosbury,R.A.E.,etal.2003,ApJ,596,797 Pickles,A.J.1998,PASP,110,863
Pirzkal,N.etal.2007,ApJ,667,49