Spectro-polarimetry confirms central powering of a Ly$\alpha$ nebula at z=3.09

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by  Melanie Beck

Draft version January 27, 2016
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
SPECTRO-POLARIMETRY CONFIRMS CENTRAL POWERING IN A Lyα NEBULA AT z = 3.09
Melanie Beck1, Claudia Scarlata1, Matthew Hayes2,3,4, Mark Dijkstra5,6, and Terry J. Jones1
Draft version January 27, 2016
ABSTRACT
Wepresentafollow-upstudytotheimagingpolarimetryperformedbyHayesetal.(2011)onLAB1
in the SSA22 protocluster region. Arguably the most well-known Lyman-α “blob”, this radio-quiet
emission-linenebulalikelyhostsagalaxywhichiseitherundergoingsignificantstarformationorhosts
6 an AGN, or both. We obtain deep, spatially resolved spectro-polarimetry of the Lyα emission and
1 detect integrated linear polarization of 9-13%±2-3% at a distance of approximately 15 kpc north
0 and south of the peak of the Lyα surface brightness with polarization vectors lying tangential to
2 the galactic central source. In these same regions, we also detect a wavelength dependence in the
n polarization which is low at the center of the Lyα line profile and rises substantially in the wings of
a theprofile. Thesepolarizationsignaturesareeasilyexplainedbyaweakout-flowingshellmodel. The
J spectral dependence of the polarization presented here provide a framework for future observations
5 andinterpretationsofthesouthernportionofLAB1inthatanymodelforthissystemmustbeableto
2 reproduce this particular spectral dependence. However, questions still remain for the northern-most
spur of LAB1. In this region we detect total linear polarizatin of between 3 and 20% at the 5%

] significance level. Simulations predict that polarization should increase with radius for a symmetric
A geometry. That the northern spur does not suggests either that this region is not symmetric (which
G is likely) and exhibits variations in columns density, or that it is kinematically distinct from the rest
of LAB1 and powered by another mechanism altogether.
.
h Subject headings: galaxies: evolution – galaxies: formation – galaxies: high-redshift
p
-
o
1. INTRODUCTION et al. 2004), luminous infrared and submillimeter galax-
r
t First discovered over a decade ago during the course ies (SMGs) (Geach et al. 2005, 2007; Yang et al. 2012),
s unobscured and obscured quasars (QSOs) (Bunker et al.
a of deep optical narrowband imaging (Francis et al. 1996;
2003; Weidinger et al. 2004; Basu-Zych & Scharf 2004;
[ Steidel et al. 2000), Lyman-α “blobs” (LABs) are large,
Smithetal.2009),aswellasstarburstinggalaxies(Scar-
rare, gaseous nebulae in the high-redshift Universe de-
1 lata et al. 2009; Colbert et al. 2011).
tectable by their extensive Lyα luminosity. Found pre-
v Though most LABs seem to have in common a host
dominantly in regions of galaxy overdensities (Palunas
6 galaxy or galaxies, the debate over the powering mecha-
et al. 2004; Matsuda et al. 2004; Prescott et al. 2008;
8 nismoftheextendedLyαemissionremainsunresolvedin
Yang et al. 2009), these objects are some of the most
7
partduetothefactthatmanyofthesegalaxiesseemun-
6 promisingcandidatesforthestudyofongoinggalaxyfor-
abletoproducesufficientionizingfluxtolightupthesur-
0 mation (Mori & Umemura 2006). Displaying a range of
rounding medium (Matsuda et al. 2004; Smith & Jarvis
. sizes from tens to hundreds of kiloparsecs and luminosi-
1 tiesspanning∼1043−44 ergs−1,LABsarereminiscentof 2007). In addition to photoionization from luminous
0 AGN and/or young stars as a power source (Haiman &
high-redshift radio galaxies, yet most are not associated
6 Rees 2001; Jimenez & Haiman 2006; Geach et al. 2009;
with strong radio sources (Saito et al. 2006). Instead, it
1 Cantalupo et al. 2012), other possible mechanisms in-
seems that LABs are singularly associated with galaxies
: clude mechanical energy injected by supernovae winds
v ofonevarietyoranotheraseventhefamedNilsson’sBlob
during powerful starbursts (Taniguchi & Shioya 2000;
i (Nilsson et al. 2006), widely cited as the most overt ex-
X Scarlataetal.2009),andradiativecooling(Haimanetal.
ampleofahost-lessLAB,isnowbelievedtobeassociated
r with an AGN (Prescott et al. 2015). Other LABs have 2000; Fardal et al. 2001; Dijkstra & Loeb 2009; Faucher-
a Gigu`ere et al. 2010; Rosdahl & Blaizot 2012). In re-
been associated with an assortment of galaxy popula-
ality, it is more than likely that LABs are powered by
tions including Lyman break galaxies (LBGs) (Matsuda
multiple mechanisms simulataneously (Furlanetto et al.
1MinnesotaInstituteforAstrophysics,SchoolofPhysicsand 2005). Theoretical studies have shown that polarization
Astronomy,UniversityofMinnesota,116ChurchSt.,Minneapo- ofLyαphotonscanbeinducedbyscatteringthusprovid-
lis,MN55455,USA,[email protected] ing a potential diagnostic to probe these various power-
2Department of Astronomy, Oskar Klein Centre, Stockholm ingmechanisms(Lee&Ahn1998;Rybicki&Loeb1999;
University, AlbaNova University Centre, SE-106 91 Stockholm,
Loeb & Rybicki 1999; Dijkstra & Loeb 2008).
Sweden
3Universit´e de Toulouse, UPS-OMP; IRAP, F-31000 In partiuclar, we focus our attention on a giant LAB
Toulouse,France (dubbed LAB1) in the SSA22 protocluster region first
4CNRS,IRAP,14AvenueEdouardBelin,F-31400Toulouse,
discovered by Steidel et al. (2000). This nebula is one of
France
5Institute of Theoretical Astrophysics, University of Oslo, themostwell-studiedwithobservationsrangingfromop-
Postboks1029,0858Oslo,Norway tical to X-ray. LAB1 is known to be loosely associated
6MPI fuer Astrophysik, Karl-Schwarzschild-Str. 1, 85741 with an LBG (C11, Steidel et al. 2000; Matsuda et al.
Garching,Germany
2 Beck, M. et al.
2004), though the peak Lyα surface brightness (SB) is an isotropized photon. Scattering “near” this doublet is
more likely associated with an 850 µm source (Geach called resonant or core scacttering and has been shown
et al. 2014) with a weak radio counterpart (Chapman to be a superposition of Rayleigh and isotropic scatter-
et al. 2004) as well as associated detections in the near ing producing a minimum level of polarization (Brandt
infrared (Geach et al. 2007), all suggestive of a dust- & Chamberlain 1959; Brasken & Kyrola 1998). In most
obscuredstar-forminggalaxyleakingLyαphotonswhich astrophysical circumstances Lyα undergoes this type of
interact with the surrounding medium. Deep integral- scattering and the Lyα photons are repeatedly absorbed
field spectroscopy of the Lyα emission has been pre- and re-emitted until they are either destroyed by dust
sentedbyWeijmansetal.(2010)supportingthisconclu- or escape the surrounding neutral medium. However,
sion for the brightest regions of LAB1 though they sug- thermal motions within the gas cause ‘partially’ coher-
gest this mechanism is less promising in regions of lower ent scattering where the absorbed and emitted photons
Lyα SB. Additionally, McLinden et al. (2013) perform are equal only in the rest-frame of the scattering atom.
longslit NIR spectroscopy of portions of LAB1 detecting To the outside observer, the Lyα photons are Doppler
[OIII] emission in the LBGs C15 and C11 thus deter- boosted with respect to the scattering atom and thus
mining their systemic velocity. Hayes et al. (2011, here- perform a random walk in both frequency and physical
after H11) perform narrow-band imaging polarimetry space (Neufeld 1990; Loeb & Rybicki 1999). This can
and report low polarization in the central region rising causetheLyαphotonstoscatterinthewingoftheprofile
to P=11.9±2% within a radius of 7(cid:48)(cid:48) (45 kpc physical). whereithasbeenshownthatthephasefunctionandde-
Coupled with tangential polarization vectors around the greeofpolarizationarequalitativelyconsistentwithpure
central region, they conclude their observations are con- Rayleigh scattering (Stenflo 1980). Furthermore, Stenflo
sistent with powering from an obscured galaxy resulting (1980)hasshownthatwingscatteringcanproducethree
in scattered Lyα photons by HI. times more polarization than resonant scattering. Thus,
Due to the observational expense involved, polariza- photons scattering in the wing of the profile are those
tion measurements of spatially extended Lyα emission which are most highly polarized and which see the low-
have so far been attempted only three times. In addi- est optical depth in the surrounding medium, enabling
tion to the work of H11, Prescott et al. (2011) present them to escape preferentially.
narrow-band imaging polarimetry of a LAB associated Theoretical predictions have been made by Dijkstra &
with a radio-quiet galaxy at z=2.66 though polarization Loeb (2008) for the expected amount of polarization in
was not detected. Humphrey et al. (2013) present the the Lyα line for various astrophysical situations. They
spectro-polarimetry of the gas surrounding the z=2.34 explore both an expanding shell and a collapsing cloud.
radio galaxy TXS 0211-122 and report low polarization The expanding shell is a simple model of backscattering
centrally, rising to P=16.4±4.6% in some parts of the offagalacticoutflowandpredictspolarizationtoincrease
nebulaandconcludethatatleastaportionofthenebula with radial distance from the central source with total
ispoweredbythescatteringofLyαphotonsproducedby Lyα polarization as high as 40%, depending on the as-
thegalaxywithin. Inthispaperwepresentafollow-upto sumed column density and velocity of the outflow. Such
H11 with the first spectropolarimetric measurement of a large values of polarization can be understood due to
radio-quietLAB.In§2wediscussLyαradiativetransfer, the kinematics of the gas. Photons scattering off the
scattering, and polarization basics. In §3 we discuss the “back” of the expanding shell are quickly shifted out of
observations and data analysis. Our methods and initial resonancewiththegasandintothewingofthelinepro-
resultsarepresentedin§4,andin§5wepresentadiscus- file thus allowing many to escape after a single wing-
sion of our results in the context of recent observational scattering. Similar levels of total polarization (p∼ 35%)
work. Finally, in §6 we discuss the future of polarization are expected in the case of cooling radiation from a col-
as a diagnostic tool in relation to upcoming space-based lapsing, optically thick gas cloud with the polarization
polarimeters. again increasing as a function of radius from the central
source due to photons emitted over a spatially extended
2. LyαPOLARIZATIONBASICS region within the cloud. .
Lyα polarization requires photons be scattered imbu- In both cases, detecting a high level of polarization
ing them with a preferential direction or impact angle. throughnarrow-bandimagingpolarimetrywouldbeable
Localized (in situ) production of Lyα photons, either to rule out in situ production of Lyα photons. However,
from stars or gas, is not expected to have significant po- imaging polarimetry alone can not distinguish between
larization as these photons will either not scatter suffi- outflowsorinflowsasbothpredictsimilarlevelsofpolar-
cientlyorhavenopreferentialorientation. Inthissection izationandincreasingpolarizationasafunctionofradius
webrieflyreviewthenecessaryphysicsbehindgenerating from the central source. Instead, the frequency depen-
a significant Lyα polarization signal. dence of Lyα polarization is required. For the case of
The detection of signifiant polarization fraction of an outflowing thin shell, Dijkstra & Loeb (2008) predict
Lyαemissiondependsontwocrucialfactors: wingvsres- that Lyα polarization will increase redwards of the line
onantscatteringandDopplerboostingbythermalatoms center. This is because the redder Lyα photons appear
in the surrounding medium. Lyα is the transition be- fartherfromresonanceintheframeofthegasandscatter
tween the first excited and ground states of hydrogen less thus achieving higher levels of polarization. In fact,
and is a resonant transition, a doublet consisting of two Dijkstra & Loeb (2008) state that this frequency depen-
fine-structurelines: 1S −2P and1S −2P . The dence can be interpreted as a “fingerprint” for outflows
1/2 1/2 1/2 3/2
latter transistion can exhibit polarization while the for- and predict that Lyα polarization could be as high as
mercannotasscatteringthroughthistransitiondoesnot ∼ 60% in the reddest part of the line profile. In stark
retaininformationonthescatteringanglethusproducing contrast, polarization increases blueward of line center
3
foracollapsingcloud. Thusthefrequencydependenceof
Lyα polarization can also constrain the kinematic struc-
16
ture of the surrounding gas.
3. OBSERVATIONS,REDUCTION,ANDCALCULATIONS ds12
n
We now turn our attention to the spectro-polarimetry o
c
of LAB1. Hayes et al. (2011) present imaging polarime- se 8
c
try of this nebula in which they find significant polar- Ar
ization increasing as a function of radius from the point 4
of brightest Lyα SB as calculated in Voronoi bins. Fur-
thermoretheyfindpolarizationvectorswhichlietangen-
0
tially around this central point and conclude that LAB1 4950 4975 5000 5025
is indeed powered by a bright central galaxy obscured Wavelength [Å]
from our line of sight. In this portion of the paper we Fig. 1.—Spatialandwavelengthdistributionofthemastertotal
present follow-up spectro-polarimetry in order to con- intensity Lyα spectrum. Co-added Lyα spectrum of 77 science
firm and further probe the kinematics of this enigmatic spectrasmoothedbyaGaussianwithFWHM=0(cid:48).(cid:48)5. Duetolight
lossattheedgesoftheslit,wepresenthereonlythecentral18(cid:48)(cid:48).
object. In this section we discuss the observations and
data reduction methods, as well as the polarization and
tively. Thus at each retarder position angle we obtain a
error calculations performed.
total integration time of 18,000 seconds.
The entirety of the observing time was classified as
3.1. Observations
clearorphotometricwithnocloudspresentonanygiven
We choose as our target one of the largest known night. Since the ord and ext beams are obtained simul-
Lyα blobs located in the SSA22 protocluster region at taneously,deviationsfromphotometricitywouldanyway
z=3.09 (see Steidel et al. (2000)). Dubbed LAB1, this have no impact on the determination of the Stokes pa-
object was observed over the course of five consecutive rameters. Observationsweretakenaroundthenewmoon
half nights from 5-9 October 2010, using the FOcal Re- in order to minimize the sky background at bluer wave-
ducer and low dispersion Spectrograph (FORS2) (Ap- lengths with moonrise not occurring until after obser-
penzeller et al. 1998) instrument mounted on the Antu vations were complete each night. Because we observe
(UT1) node of the Very Large Telescope (VLT) Euro- LAB1 with constant position angle, atmospheric disper-
peanSouthernObservatory(ESO).Thefirststageofthe sion will vary with airmass over the course of the expo-
dedicated dual-beam polarization optics is the introduc- suretime. Airmassrangedfrom1.1to1.53andwasthus
tion of a strip mask designed to avoid overlapping on well within the FORS instrument’s atmospheric disper-
the CCD of the two beams of polarized light. Six MOS sioncorrectortocompensate. Thebulkofthe20hoursof
slitlets, each 1(cid:48)(cid:48) wide and 20(cid:48)(cid:48) long are then positioned observation time experienced astronomical seeing which
over the objects of interest. The light is passed through variedbetween0.5and1.0arcsecondwithmedianseeing
a super-achromatic half-wave plate (HWP) retarder mo- at ∼0.(cid:48)(cid:48)75. There were three observations which experi-
saic (RETA2+5), which rotates the angle of the polarized enced seeing as high as 1.7(cid:48)(cid:48). These were excluded from
light. We adopt the standard four angles for unambigu- the following analysis although their inclusion does not
ous recovery of the Q and U Stokes parameters: 0◦, significantlyalterourresults–polarizationfractionsvar-
22.5◦, 45◦, and 67.5◦. The rotated beam is subsequently ied only by a few per cent. We thus obtain a total of 37
passed through a Wollaston prism (WOLL 34+13), which individual observations for a total of 12,600 seconds of
splits the randomly polarized and unpolarized light into integration.
twoorthogonal,linearlypolarizedoutgoingbeams,arbri-
trarily denoted the ‘ordinary’ (ord) and ‘extraordinary’ 3.2. Data Reduction
(ext) beams. Finally, the beams are passed through a
The intial steps of data reduction were carried out
grism dispersion element (GRISM 1400V+18) with a cen-
in the standard manner for spectroscopic observations.
tralwavelengthof5200˚A(∼1271˚Arestframe),spectral Individual frames were biased subtracted. Master flat
rangeof4560-5860˚A,anddispersionof.63˚A/pixel. The frames were created from several dome flats using Es-
spectralresolutionisapproximately2100withour1(cid:48)(cid:48) slit oRex7, an ESO recipe execution tool, and applied to in-
width. This produces simultaneous ord and ext spectra dividual frames to correct for pixel-to-pixel variations.
whicharethenprojectedontotheCCD.Thuseachframe Eachframewasthennormalizedbyexposuretime. Cos-
consisted of six spectra: three slits contained only sky, mic rays were throroughly removed using L.A. Cosmic8
two contained stars which were used in the alignment (van Dokkum 2001). To account for small variations
process (see below), and one was positioned over LAB1. in the spatial direction during observing, frames were
We observed LAB1 at a position angle of −3.09◦ in the aligned using the shift sub function in IDL where the
standard N=0◦– E=90◦ reference frame. pixel shift was calculated as the average of fitted Gaus-
Throughout the course of each night the sequence of sians of each stellar continua in the slits above and be-
four HWP position angles was observed repeatedly with low the science spectra. At this stage all the individual
two full sequences being completed each night. Integra- frames were split into their ord and ext beams (37 ob-
tion times were 1,800 seconds for each HWP position servations × 2 beams = 74 spectra). Only slits 3 (sky)
with the exception of the first night where each expo-
sure had 1,600 and 2,000 seconds of integration time per 7 http://www.eso.org/sci/software/cpl/esorex.html
HWP position for the first and second sequences respec- 8 http://www.astro.yale.edu/dokkum/lacosmic/
4 Beck, M. et al.
Fig. 2.—SlitpositionoverLAB1ascomparedwithresultsfromH11. IntheleftpanelweshowthecombinedLyαintensityframefrom
H11adaptivelysmoothedtoshowdetailincludingemissionfromLAB1andnearbyLBGsC11andC15. Overlaidinwhiteareboxesfrom
Weijmans et al. (2010), regions in which they performed integral-field spectroscopy on the Lyα emission. As before, the slit shown here
spans∼18(cid:48)(cid:48). Contoursdenotearbitraryfluxlevels. IntherightpanelweshowfractionalpolarizationresultsfromH11forthosebinswhich
weredetectedatorabove2σ (SeeH11Fig. 2,panele). OurslitpassesovertheregionofbrightestLyαemissioninthesouthernportion
oftheslitaswellasadimmerregioninthenorth.
and 4 (LAB1) were considered for the remainder of the total intensity frames. Finally, a master total intensity
analysis. frameiscreatedbyaveragingthesefourframestogether,
Each beam of the sky and LAB1 spectra was wave- as shown in Figure 1.
length calibrated individually via a He-Ar arc lamp
spectrum using NOAO/IRAF9 onedspec and twodspec 3.3. Polarization and Error Calculations
packages. The identify - reidentify - fitcoords
Polarization of Lyα is expected to be linear, thus the
- transformsequencewasusedonthe2Dspectrayield-
decomposition of polarized light falls only into Q and U
ing a fit r.m.s. typically between 0.05 and 0.09 ˚A.
normalized Stokes parameters. The V parameter repre-
Sky subtraction was performed using the sky spectra
sentscircularpolarizationandisnotexpectedforLyαra-
in slit 3. For each ord and ext beam, the sky spec-
diation. The fourth parameter I is the total intensity
trum was median extracted, normalized to the spatial
whichisequaltothesumoftheordandextbeams. For
dimensionofthe2DLAB1spectra, andsubtractedfrom
eachHWPposition,θ,thenormalizedfluxdifference,F ,
θ
the corresponding LAB1 beam. The residual sky back-
is defined as:
ground in the wavelength direction was modeled with
a linear fit after masking the Lyα line. This was then ford−fext
subtracted from the spectra. This step was appropri- F = θ θ (1)
θ ford+fext
ate as there is no evidence of UV continuum according θ θ
to our preliminary analysis of recently acquired MUSE
where ford is the flux in the ordinary beam for a given θ
datawhichwillbepublishedinHayesetal.,inprep. At- θ
and likewise of fext for the extraordinary beam.
mosphericcorrectionwasappliedusinganextinctionco- θ
Once the four HWP angles have been obtained, Q, U,
efficientasafunctionofwavelengthobtainedfromPatat
and I relate to the observables by
et al. (2011) and the airmass at the midpoint of each
observation. Spectra were then co-added using a mean Q 1 1
combination with simple minmax rejection of the highest q = I = 2F0.0− 2F45.0
and lowest value at each pixel using IRAF’s imcombine. (2)
U 1 1
Asmentionedpreviously,weexclude3observationsfrom u= = F − F
further analysis. These include one 45◦ observation and I 2 22.5 2 67.5
two 67.5◦ observations whose delivered seeing was well From these, the polarization fraction, p, and the polar-
above 1.0(cid:48)(cid:48). We combine eight groups of 10 spectra to ization angle, χ, can be calculated by
produce science frames for polarimetry (ord and ext at
(cid:112)
each of the four angles, see Appendix). We sum each p= q2+u2
individual frame’s ord and ext beams for a total inten- 1 u (3)
sity spectrum for that frame yeilding 40 frames. These χ= arctan
2 q
are then combined by HWP angle to create four science
However, we actually desire an estimate of the “true”
9 IRAF is distributed by the National Optical Astronomy Ob-
polarization, p . When it is assumed that the Stokes
servatories, which are operated by the Association of Universities 0
forResearchinAstronomy,Inc.,undercooperativeagreementwith parameters q and u are drawn from Gaussian distribu-
theNationalScienceFoundation tions centered around the true values (q and u ) each
0 0
5
withvarianceσ, itcanbeshown(e.g.Plaszczynskietal. one must instead rely on confidence intervals (CIs). As
2014)thatthedistributionofthepolarizationfollowsthe we show below, different binning techniques yield differ-
Rice distribution: ent SNR and thus in some cases we report point es-
p
timates of the fractional polarization while in others we
fp(p)= σp2e−p22+σ2p20I0(pσp20) (4) petroavl.id(e20th1e4)9.5%CIsaccordingtoEqn.26inPlaszczynski
where p is the true amplitude of the polarization and Finally, we consider the measurement and error esti-
0
I is the modified Bessel function of the zeroth order. matesforthepolarizationangle. Ithasbeenshown(e.g.
0
Equation3isconsideredthenaiveestimatorforthisdis- Wardle & Kronberg 1974; Vinokur 1965) that the dis-
tribution and is known to be strongly biased at low po- tribution of χ is symmetric about the true value of the
larizationsignal-to-noiseratio(SNR )inpartbecause,in angle and thus the estimator presented in Equation 3 is
p
thisregime,theRicedistributioncanbeapproximatedas already unbiased. At large SNRp this distribution also
a Rayleigh distribution which is highly skewed to larger tends towards a Gaussian with a standard deviation of
values of polarization. Additionally, the naive estima- σχ ≈ σp/2p. However, at low SNRp this approximation
torcannottakeexperimentalnoiseintoaccount. Several underestimates the error. In this work we follow War-
attempts have been made to produce an unbiased esti- dle & Kronberg (1974) and approximate the error by
matorforp (seeSimmons&Stewart1985,forareview) their Eqn. A6 (see also their Figure 3) which provides
0
but most have other undesirable qualities such as being the most conservative error estimate for measurements
unphysical at very low signal-to-noise or containing dis- with SNRp >0.5.
continuities. Plaszczynski et al. (2014) develop a polar-
ization estimator dubbed the Modifed Asymptotic Esti- 4. POLARIZATIONOFLyαINLAB1
mator(MAS)whichislessbiasedinbothlow(Rayleigh)
4.1. Polarization integrated over the line profile
andhigh(Gaussian)SNR regimesandiscontinuousbe-
p
tween these regions. Furthermore, this estimator also Because our data have low signal-to-noise per pixel
takes into account measurement error in q and u. For (SNRp (cid:46)1), binning of the science frames is a necessity.
these reasons we adopt this estimator for the polariza- However, any Lyα polarization signal will result from
tion which goes as: the particular geometry inherent in the HI gas with a
unique set of Stokes parameters and polarization angle.
pˆMAS =pi−b2i1−ex2pp−p2i/b2i (5) Iofvetrhleaspepirneggioenacsharreegnioont’sazpiomlaurtihzaaltliyonreasnoglvleesd,thounsearvisekrs-
i aging the polarization signal and potentially washing it
where p is given by Equation 3 and b2 is the noise bias outentirely(Dijkstra&Loeb2008). Thus,somebinning
i
of the estimator given by is necessary but overbinning will make it unmeasurable.
To aid in the determination of appropriate bins we
q2σ2 +u2σ2 examine the slit position over LAB1 as shown in Fig-
b2 = i u i q (6)
i q2+u2 ure 2. LAB1 fills the slit and contains regions of varying
i i Lyα surface brightness (SB) as denoted by the set of
where each i is an individual binned measurement of the arbitrary contours. Also shown in this figure are white
quantity of interest. boxes corresponding to Lyα emission integral-field spec-
In practice, the quantities p , q and u are calculated troscopy as presented by Weijmans et al. (2010). Our
i i i
asgiveninequations1,2,and3ineachbin(seebelowfor slit overlaps their regions R1 and R3 and we adopt
adiscussiononourbinningstrategy). Theσ andσ are this nomenclature throughout. R3 is situated over the
q u
computedfromMonteCarlosimulationswherebyeachof brightestpeakoftheLyαSBwhileR1isassociatedwith
the eight science frames is allowed to deviate according a somewhat dimmer region. Between these two features
to a Gaussian spread wherein the deviates are simply there exists a distinct gap that can also clearly be seen
computed as the standard deviation of a large portion in the Lyα emission shown in Figure 1. We determine
of the background of each individual science ord and to bin these regions separately as they can exhibit dif-
extframe. Tenthousandrealizationsareperformedand ferent polarization fractions as shown in the right panel
foreachrealizationq anduarecomputed. Theresulting of Figure 2. Altogether, we bin the slit into 8 individ-
probability distributions of q and u are Gaussian as ex- ual spatial regions, each spanning 2(cid:48)(cid:48)–3(cid:48)(cid:48), as this is large
pectedandfromtheseweobtainσ andσ bymeasuring enoughtoachieveadequateSNR insomebinsyetsmall
q u p
the spread of the distributions. enough that we do not wash out any polarization signal.
We consider the polarization SNR by computing These spatial elements are labelled A through H, with
p
A being the southern-most portion of the slit and H the
p
SNR = MAS , (7) northern-most. These spatial bins are shown explicitly
p (cid:113)
1(σ2+σ2) in Figure 3.
2 q u
With these considerations in mind we first spectrally
where again we allow for measurement error by incor- integrate over the Lyα emission to calculate the total
porating both σ and σ . Following the prescription of p for comparison with H11. Integration is carried out
q u
Plaszczynskietal.(2014), valuesofSNR >3.8indicate over the wavelength range 4965–5000 ˚A. We note that
p
that the Rice distribution is sufficiently Gaussian and though the range of the Lyα emission varies within each
thus unbiased. In this regime one may compute a point aperture, varying the integration range only changes the
estimate along with the estimator variance. Values less fractional polarization by a few percent difference for all
than this, however, fall in the Rayleigh regime wherein but bin H which has an increase in p of 10%. However,
6 Beck, M. et al.
16 H
G
12 F
s R1
d
n E
o
c
e
s 8 D
c
r
A
C
R3
4 B
A
0
4970 4980 4990 5000 0.0 0.1 0.2 0.3
Wavelength [Å] Polarization Fraction
Fig. 3.—PolarizationofspectrallyintegratedLyα. IntheleftpanelweshowthespatialbinningoftheLyαemissionspectrumwithbins
rangingfrom2-3(cid:48)(cid:48)overlaidinred. Lyαemissionisintegratedover4965-5000˚A.ThebluelinesindicatethespatialextentofWeijmansetal.
(2010)IFU boxes. Depictedin theright panelare ourpolarization measurements inorange. Polarization pointestimates aredenotedby
orangeboxeswith1σ errorbars;95%CIsareshownasclosedbrackets;andupperlimitsaredenotedwithorangearrowswherewedefine
ourupperlimitsastheupper95%confidenceboundforthatspatialbin. BlacksquaresshowfractionalpolarizationfromH11asmeasured
inVoronoibinswhichoverlapwithourslit. Seetextfordiscussionondifferencesandlimitationsbetweendatasets.
partially overlaps our slit and we include in Figure 3 all
TABLE 1
Polarizationsignal-to-noiseandfractionalpolarizationmeasurements suchbins. Thelargestdisparitybetweendatasetsoccurs
forspatialbins. ForthosebinswithsufficientSNRp wereportthe inBinF.Inthisregion,H11measurethefractionalpolar-
polarizationpointestimateandcorresonding1σp error. Otherwise ization to be p∼20% whereas we find, at most, ∼12%.
wereport95%confidenceintervalsforbinsinwhichthelower95%
The reason for this discrepancy is not fully understood.
confidenceboundisgreaterthanzero.
In Table 1 we report the 95% confidence intervals for
Bin SNR p p p σ those spatial bins whose lower 95% confidence bound is
p min max p
greater than zero. We see polarization in spatial bin C
B 4.6 8.59 1.9 which is on the order of a few per-cent though not quite
C 2.6 1.00 5.92 consistentwithzero. Thefractionalpolarizationthenin-
creases to 13.6% north and 8.6% south of this region as
D 4.9 13.6 2.7
seen in spatial bins B and D. The distance between bin
F 2.8 2.78 12.4 C and these bins is roughly 15 kpc in either direction.
G 2.8 4.51 20.0
4.2. Polarization across the line profile
We next explore p as a function of wavelength. Us-
since we are unable to place reasonable constraints on ing the same spatial elements, we further bin each into
the polarization fraction in this bin we consider this to 5 ˚A increments in the wavlength direction (see the Ap-
bemoot. OnlytwoofthesespatialbinshaveSNR >3.8 pendix for comparison of q, u and p). In Figure 4 we
p
and for these we report the measured polarization and presenttheextracted1Dspectraforeachspatialelement
1σ error. The remaining bins have SNR < 3.8 and along with the wavelength response of p in orange. For
p p
for these we report 95% CIs for those bins in which the those bins with sufficient SNRp (as shown in the mid-
95% lower confidence bound is greater than zero. Our dle panel of Figure 5), we show the polarization point
results are summarized in Table 1 as well as in Figure 3 estimate along with 1σp errors. For those bins which
along with the spatial binning pattern and the polariza- have lower 95% confidence bounds greater than zero, we
tionmeasurementsfromH11forthosebinswhichourslit show the CI as orange closed brackets. Otherwise, we
overlapped. The fractional polarization values roughly show upper limits defined as the upper 95% confidence
agree when one takes into account the distinct methods bound for that bin. In general we see a trend of high
used between our two analyses. H11 utilize a Voronoi (low) polarization corresponding to lower (higher) rela-
binning technique whereby the size of each bin is deter- tive Lyα intensity. In particular, bin B displays p which
minedbytheachievedSNRwithinthatbin. Thisallows is consistent with zero in near 4985˚A but which rises
themtohavebinsofvarioussizes. Eachoftheirbinsonly substantially in the wings of the profile, reaching up to
7
0.8 H D 0.6
0.4 0.3
0.0 0.0
]]]]]]]]
y Unitsy Unitsy Unitsy Unitsy Unitsy Unitsy Unitsy Units 00..48 G C 00..36 PolarPolarPolarPolarPolarPolarPolarPolar
bitrarbitrarbitrarbitrarbitrarbitrarbitrarbitrar 0.0 0.0 izatioizatioizatioizatioizatioizatioizatioizatio
rrrrrrrr nnnnnnnn
y [Ay [Ay [Ay [Ay [Ay [Ay [Ay [A 0.8 F B 0.6 Fra Fra Fra Fra Fra Fra Fra Fra
nsitnsitnsitnsitnsitnsitnsitnsit 0.4 0.3 ctioctioctioctioctioctioctioctio
eeeeeeee nnnnnnnn
ntntntntntntntnt 0.0 0.0
IIIIIIII
0.8 E A 0.6
0.4 0.3
0.0 0.0
4950 4975 5000 5025 4950 4975 5000 5025
Fig. 4.— Polarization of Lyα as a function of wavelength. Figures A through H contain the extracted 1D Lyα spectrum of the
corresponding aperture from Figure 3. All spectra have been scaled to show their relative intensity. Overplotted in orange we show the
polarization fraction as a function of wavelength in bins of 5˚A. Polarization point estimates are denoted by orange boxes with 1σ error
bars;95%CIsareshownasclosedbrackets;andupperlimitsaredenotedwithorangearrowswherewedefineourupperlimitsastheupper
95%confidenceboundforthatbin. Thedashedbluelinerepresentstheaveragesystemicvelocityasmeasuredfrom[OIII]offourgalaxies
whichareassociatedwithLAB1. Thedash-dottedlinesaretheminimumandmaximumofthoseobjects. InbinsB-E,weseeatrendof
lowpolarizationassociatedwiththecoreoftheLyαemissionandhigherpolarizationtowardthewingsofthelineprofile. Thistrendisless
apparentinotherbins, thoughthelineprofilesarenotaswelldefinedandmanydisplayadoublepeak. Ingeneral, pistypicallyhighest
atthosewavelengthsinwhichtheLyαintensityisrelativelylow.
45% bluewards and with upper limits as high as 65% that many areas of high intensity have very low polar-
redwards. InspatialbinsCandDweseethesuggestion izationsignal-to-noiseindicatingthattheseregionsmost
of similar behavior with lower polarization in the core of likelyhaveverylowornofractionalpolarizationforusto
the line and potentially higher polarization in the wings detect with the current data. However, portions of the
of the profile though the data do not allow us to furher spectrum exhibiting relatively less intensity have much
constrain the trend. moresignificantSNR . Thefractionofpolarizedlightin
p
It is important to recall that these measurements in- these bins is relatively more substantial. We also point
form us as to the fraction of the total intensity which is outthatinthisSNR mapweseearangeofvaluesfrom
p
polarized. In the case of box B, for example, the wave- ∼ 2 to ∼ 5 which indicates that for some bins we re-
length bin at 4970˚A is 45% polarized. The intensity in port point estimates of the fractional polarization while
this portion of the line is quite low, however, relative to for others we instead provide 95% CIs. In the bottom
the peak at 4990˚A. It is instructive to compare this to panel, we show p in individual bins in which we either
the integrated polarization in Figure 3. In that figure, have sufficiently high SNRp or in which we calculate a
box B has fractional polarization of ∼9%. Thus we see lower 95% confidence bound greater than zero. In gen-
that spectropolarimetry gives us more information than eral we see higher values of p in the reddest part of the
whatcanbegainedsolelythroughimagingorintegrated line though we also note some substantial polarization
polarimetry. Highlypolarizedindividualwavelengthbins in the blue wing as well. In most cases we see that the
are ‘washed out’ by the relative strength of the core of centralemissionregionischaracterizedbylowSNRp and
theprofilewhichistypicallynotstronglypolarized. The low fractional polarization.
overall fraction of polarized photons decreases when in- Finally, in Figure 6 we present the direction of the
tegrating over the entire line and it is impossible to re- polarization vectors, χ, for those spatial elements (B-D,
construct from imaging alone which wavelength regimes F, G) which have SNRp >2. Errors on the polarization
are the most highly polarized. angles range from ∼6–15◦. In this figure we also plot
In Figure 5 we present 2D maps of the spectrally polarization vectors from H11 for comparison, including
binned intensity, SNR , and polarization of LAB1. In only those which they measured at or above 2σ. We see
p
the middle panel we show the SNR where we stress that both sets are generally consistent. Like H11, our
p
that the “noise” in this equation is not the equivalent angles lie tangentially around the peak of Lyα SB.
of a polarization error. This figure instead serves to give
5. DISCUSSIONANDCONCLUSIONS
the reader a feeling for the relative quality of our mea-
surements. Comparingthemiddleandtoppanelswesee We have presented deep spectro-polarimetry of the
LAB1 Lyα emission nebula. The data allow us to probe
8 Beck, M. et al.
.35
16
12 .25
8 .15
4
Total Intensity .05
0 P=1
s16 5
d
n12
o 4
c
e 8
s 3
c 4
Ar Pol SNR 2
0
16 0.55
12 0.45
0.35
8
0.25
4 0.15
Pol Fraction
0.05
0
4950 4975 5000 5025
Wavelength [Å]
Fig. 5.—Top.Totalintensityofthe2DLyαspectruminbinsof Fig. 6.—DirectionofLyαpolarizationvectors. Smoothedtotal
5˚A by 2-3(cid:48)(cid:48). Middle. SNRp map which demonstrates the relative Lyα intensity from H11 overlaid with polarization vectors from
quality of our polarization measurements. SNRp>2 is generally H11 (black) and from this analysis (cyan) for those spatial bins
enough for us to discriminate p statistically signicant from zero from Figure 3 which have SNRp > 2. Because we measure small
with 95% confidence and we show CIs for such bins in Figure 4. polarization amplitude in our bins we use a larger scaling for our
SNRp>3.8indicatestheRicedistributionissufficientlyGaussian vectors in order for the reader to more easily compare the angles
and we measure polarization with standard errors, also shown in wemeasurewithwiththosepresentedinH11. Bothsetsofangles
Figure4. Thebottompaneldepictsthemapofpdeterminedtobe generallylietangentiallyabouttheregionofhighestLyαSB.
statisticallysignificantfromzerowith95%confidence(lower95%
confidenceboundsgreaterthanzero). Forthosebinswithsufficient
SNRp, the bin color reflects the point estimate of p, otherwise it
representsthemiddlevalueoftheCIassociatedwiththatbin. ward of the line core. The strength of this increase de-
pendsstronglyonthedistancefromthecenterofLyαSB
as well as the column density and outflow velocity. In
the kinematics and distribution of the neutral gas and other words, for a given column density and outflow
reinforce the idea that LAB1 is likely composed of sev- speed, polarization will increase weakly (up to ∼ 10%)
eral smaller, more complex regions instead of one large, in the reddest part of the wing at the peak of Lyα SB,
kinematic structure. In particular, our observations sug- but will increase substantially (up to ∼ 70%) at larger
gest at least a weak outflow in the southern portion of radiifromthisregion. Wecautionthatdirectcomparison
the LAB as we discuss below. of our data with these simulations comes with a caveat
Simulations predict that polarization due to scatter- since we are not integrating over the entire shell with
ing should exhibit a radial dependence on the sky. The our long-slit observations. Nevertheless, the spectrally
region of highest Lyα SB would not be strongly polar- resolved polarization in bins C and B at least suggest
ized but the polarization would rise with increasing ra- this behavior.
dius (Dijkstra & Loeb 2008). The observed polarization To see this trend, we first estimate the systemic veloc-
of Lyα in the southern portion of the slit is consistent ity of the region as the average from four galaxies mea-
with Lyα photons produced by a luminous galaxy (or sured in [O III] and known to be components of LAB1
galaxies) and scattered at large radii by the surrounding (Kubo et al. 2015; McLinden et al. 2013) (see Figure 4).
neutral hydrogen. As in H11, we see this signature here, We note that these measurements are all within ∼ 230
most notably in spatial elements B-D where the peak of km/s. Given the width of the Lyα profile, this uncer-
the Lyα SB has little observable polarization as shown tainty has minimal impact. In this context we can see
in spatial element C. North of this location (D), we find that the polarization in C is well constrained in the red
p=13.6±2.7%whichisalsoconsistentwiththeVoronoi wingtobeatmost∼20%polarized. Thoughweareun-
bins from H11 lying on either side of our slit at approx- abletotightlyconstraintheredwinginbinB,largepo-
imately the same radial distance (see the right side of larizationvaluesaresuggestedbythedetectionat4995˚A,
Figure 2). Similarly to the south (B), we find total p = andarenotruledoutatlongerwavelengths. Thispartic-
8.59±1.9%. Thus we have increasing polarization away ular polarization pattern can be explained easily with a
from the peak Lyα emission with a radius of ∼ 15 kpc. weakoutflowingshellmodelwherebyLyα photonsemit-
However, this general trend cannot determine between ted from an embedded source interact with the expand-
inflowingoroutflowinggasasbotharepredictedtohave ingshell. Thosephotonswhichinteractwiththereceding
this observational signature. portion of the shell (from our point of view) are Doppler
Dijkstra & Loeb (2008) also predict that, for an en- shifted into the red wing of the profile. Being in the
velope of expanding gas, those photons in the bulk of wing of the line profile, these photons see a lower optical
thelineprofileshouldexhibitincreasingpolarizationred- depth and thus preferentially escape the medium having
9
onlyscatteredafewtimeswhichpreservestheirpolariza- Furthermore, the spectrally integrated polarization can
tion. Photons which remain in the core of the profile in stillprovidespatialcluesastoanyexistingradialdepen-
theframeofthegasscattermanymoretimes,effectively dencewithadvantageousslitplacement. Becausethege-
erasing any polarization signal. ometry of the blob is important to the overall detection
The polarization data presented here provide a frame- of a polarization signal, spectropolarimetry should not
work for future observations of the southern portion of be conducted blindly but instead by guided by spatial
LAB1. The imaging polarimetry of H11 coupled with information obtained from the already existing narrow-
a recent strong submillimeter source within 1.5(cid:48)(cid:48) of the band surveys of LABs as well as IFUs.
peakLyαintensity(Geachetal.2014)providethe‘smok- Though it remains to be seen, suggestions have arisen
inggun’thatthereisindeedatleastonepowerfulsource that the next generation of ∼30 m telescopes could ex-
embedded in this region, the photons from which are tend Lyα polarization studies. This is supported in that
likely scattering at large radii. If there are indeed mul- all projected Extremely Large Telescopes (ELT) have
tiple sources within about 30 kpc of this region, obser- proposed polarimetry as a necessary part of their in-
vations must also provide a mechanism by which these strument suite. On the E-ELT, the Exo-Planet Imaging
sourcescanreproducethespectralpolarizationsignature Camera and Spectrograph (EPICS, Kasper et al. 2008)
presented here, namely, a configuration which is consis- includes the EPOL polarimeter (Keller et al. 2010). The
tant with low polarization in the core of the Lyα profile Thirty Meter Telescope has discussed plans to include
and high polarization in the wings. the Second-Earth Imager for TMT (Matsuo & Tamura
However, the picture remains obscure for R1 corre- 2010). And the Giant Magellan Telescope has discussed
sponding to our spatial elements E-F. Were R1 to be spectro-polarimetric capabilities as necessary to meeting
part of the same smooth, kinematic structure as R3 we their science goals (GMTO Corporation 2012). Off the
would expect its total Lyα polarization to increase rela- ground, several probe-scale NASA space missions have
tive to R3 due to its increased radius from the galactic been proposed to study exoplanetary systems such as
center. Instead, we see the total polarization drop and the AFTA and “EXO” missions (Stapelfeldt et al. 2014;
flatten across R1. It’s possible that the gas in this re- Seager et al. 2014), with considerable emphasis given to
gion is clumpier or denser than the southern portion of polarimeterstoenrichthescienceoutput. Whilemanyof
the blob. An increase in the column density of the gas theseprojectsfocusonimagingpolarimetry,theUVMag
will decrease the observed polarization fraction as addi- consortium has proposed the Arago space mission which
tional scatterings tend to isotropize the photons. An- would be devoted to unprecedented spectropolarimetry
otherpossibilityisthatthisregionispoweredbyfloures- from the FUV through the NIR (Pertenais et al. 2014).
cence from ionizing radation eminating from the central This field is currently driven almost exclusively by exo-
source. This would naturally explain the lower polariza- planetary science for studying the scattering off plane-
tion as this type of in situ production of photons is not tary atmospheres and circumstellar disks but it is the
expected to be highly polarized. Weijmans et al. (2010) development of such instrumentation which is of great-
present compelling evidence that suggests this region is est importance. At this stage, we cannot say whether
kinematically distinct from the rest of LAB1 and thus wewillbeabletopointoneoftheseinstrumentsdirectly
a third possibility is that this region is instead powered towards a Lyα blob without slight modification of the
by radiative cooling. Most likely is the possibility that initial design or incorporation of additional settings but
this region is dominated by an embedded souce of its it is optically plausible (Hayes & Scarlata 2011).
own. Though interesting to speculate, the wavelength In the meantime, much can still be accomplished from
dependence of the polarization in these spatial bins is thegroundwith8mclasstelescopes. Todate,onlythree
not sufficient for us to further probe the kinematics and Lyα emitting sources have been studied in depth: LAB1
polarization properties. (Hayes et al. 2011, and this work), LABd05 (Prescott
etal.2011)andradiogalaxyTXS0211–122,knowntobe
6. THEFUTUREOFLyαPOLARIZATION associated with a 100 kpc scale Lyα nebula (Humphrey
With another successful detection of the spectral de- etal.2013). Radio-quietLABswerefirsttargeteddueto
pendence of Lyα polarization the question arises: What theirapparentlycontroversialnatureunlikehighredshift
does the future hold for Lyα polarization? Addition- radio galaxies (HzRGs) which did not pose an energy
ally, should emphasis be placed on imaging or spectral problem. With the discovery that a HzRG is at least
polarimetry? The integration times for either mode are partially polarized due to Lyα scattering, it behooves us
similar in magnitude and require a substantial commit- to test further what relationship, if any, exists between
ment so the choice between methods is not a trivial one. radio-loudand-quietnebulae. CompactLyαsourcesalso
We have explicitly demonstrated that much informa- remainunexploredwiththepolarimeter. Thoughtarget-
tion can be gleaned from the spectral dependence of the ing resolved objects ensures that the Stokes parameters
polarization signal. In particular, features emerge which do not all cancel, symmetry is likely broken in most sys-
narrowband imaging polarimetry simply cannot detect. tems. Thus we may expect a measureable signal from
Not only are we able to detect the wavelength depen- LAEs (Lee & Ahn 1998).
dence for many of our spatial bins but we also find high Additionally, the interpretation of Lyα polarization
polarization upwards of 60% in portions of the Lyα pro- is still a challenging prospect of its own. Most state-
files – information which is completely lost in imaging of-the-art simulations assume density and kinematic
polarimetry. Whileimagingpolarimetrycanconfirmthe structures that are still unrealistic in that variations
presence of scattering and probe the overall geometry of proceed smoothly. What is urgently needed is the
the scattering medium, it cannot probe the kinematics implementation of Lyα polarization in all Lyα radiative
of the system to determine potential outflows or inflows. transport codes to generate predictions for various
10 Beck, M. et al.
applications including clumpy and filimentary media as in tandem.
well as non-spherically symmetric geometries. While
some work has already been done in this area (Dijkstra We thank the anonymous referee for the useful com-
& Kramer 2012), there is still much to be explored ments that significantly improved the analysis and pre-
in terms of predicting observable polarized Lyα line sentation of our results. Additionally, MB and CS are
profiles. With the current limitation on instumentation grateful to J´er´emy Blaizot and Maxime Trebitsch for
coupled with exacting observations, we need dedicated proofing the manuscript and providing feedback which
theoreticalandobservationaldevelopmentsthatproceed helped to clarify the text.
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APPENDIX
In this appendix we demonstrate how our final polarization spectra are created. In Figure 7 our eight ord and
ext beams provide the basis of all measurements. These 2D spectra are obtained by stacking their corresponding