The MASSIVE Survey VI: The Spatial Distribution and Kinematics of Warm Ionized Gas in the Most Massive Local Early-type Galaxies

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by  Viraj Pandya

Draft version February 1, 2017
PreprinttypesetusingLATEXstyleemulateapjv.01/23/15
THE MASSIVE SURVEY VI: THE SPATIAL DISTRIBUTION AND KINEMATICS OF WARM IONIZED GAS
IN THE MOST MASSIVE LOCAL EARLY-TYPE GALAXIES
Viraj Pandya1,2, Jenny E. Greene1, Chung-Pei Ma3, Melanie Veale3, Irina Ene3, Timothy A. Davis4,5, John P.
Blakeslee6, Andy D. Goulding1, Nicholas J. McConnell6, Kristina Nyland7, Jens Thomas8
1DepartmentofAstrophysicalSciences,PeytonHall,PrincetonUniversity,Princeton,NJ08540,USA
2UCO/LickObservatory,DepartmentofAstronomyandAstrophysics,UniversityofCalifornia,SantaCruz,CA95064,USA
3DepartmentofAstronomy,UniversityofCalifornia,Berkeley,CA94720,USA
4CentreforAstrophysicsResearch,UniversityofHertfordshire,Hatfield,HertsAL109AB,UK
5SchoolofPhysics&Astronomy,CardiffUniversity,QueensBuildings,TheParade,Cardiff,CF243AA,UK
7 6DominionAstrophysicalObservatory,NRCHerzbergInstituteofAstrophysics,Victoria,BCV9E2E7,Canada
7NationalRadioAstronomyObservatory,Charlottesville,VA22903,USAand
1 8MaxPlanck-InstituteforExtraterrestrialPhysics,Giessenbachstr. 1,D-85741Garching,Germany
0 (Dated: February 1, 2017)
2 Draft version February 1, 2017

n
ABSTRACT
a
J We present the first systematic investigation of the existence, spatial distribution, and kinematics of
0 warm ionized gas as traced by the [O II] 3727˚Aemission line in 74 of the most massive galaxies in
3 the local Universe. All of our galaxies have deep integral field spectroscopy from the volume- and
magnitude-limited MASSIVE survey of early-type galaxies with stellar mass log(M∗/M(cid:12)) > 11.5
] (MK < −25.3 mag) and distance D < 108 Mpc. Of the 74 galaxies in our sample, we detect warm
A ionizedgasin28,whichyieldsaglobaldetectionfractionof38±6%downtoatypical[OII]equivalent
G width limit of 2˚A. MASSIVE fast rotators are more likely to have gas than MASSIVE slow rotators
with detection fractions of 80±10% and 28±6%, respectively. The spatial extents span a wide range
.
h ofradii(0.6−18.2kpc; 0.1−4R ), andthegasmorphologiesarediverse, with17/28≈61±9%being
e
p centrallyconcentrated,8/28≈29±9%exhibitingclearrotationouttoseveralkpc,and3/28≈11±6%
-
being extended but patchy. Three out of four fast rotators show kinematic alignment between the
o
r stars and gas, whereas the two slow rotators with robust kinematic measurements available exhibit
t kinematic misalignment. Our inferred warm ionized gas masses are roughly ∼105M . The emission
s (cid:12)
a line ratios and radial equivalent width profiles are generally consistent with excitation of the gas by
[ theoldunderlyingstellarpopulation. WeexploredifferentgasoriginscenariosforMASSIVEgalaxies

andfindthatavarietyofphysicalprocessesarelikelyatplay,includinginternalgasrecycling,cooling
1
out of the hot gaseous halo, and gas acquired via mergers.
v
Keywords: galaxies: elliptical and lenticular, galaxies: kinematics and dynamics, galaxies: evolution,
2
galaxies: ISM, ISM: kinematics and dynamics, ISM: lines and bands
7
7
8
1. INTRODUCTION gas (e.g., McDonald et al. 2010; Werner et al. 2014).
0
Despite more than thirty years of studying warm
. Massiveearly-typegalaxiestypicallyhaveveryoldstel-
1 ionized gas in early-type galaxies (e.g., Caldwell 1984;
lar populations. Given the lack of ongoing star forma-
0 Demoulin-Ulrich et al. 1984; Phillips et al. 1986; Buson
tion, we expect a corresponding lack of an interstellar
7 et al. 1993; Goudfrooij et al. 1994; Zeilinger et al. 1996;
medium (ISM). However, a significant fraction of the
1 Macchetto et al. 1996; Caon et al. 2000; Martel et al.
massive early-typepopulationhas been observedto con-
:
v tain a complex and multi-phase ISM (e.g., Knapp et al. 2004; Sarzi et al. 2006, 2010), we still do not have a
i 1985; Phillips et al. 1986; Knapp et al. 1996; Goudfrooij clearunderstandingofthedifferentoriginandexcitation
X
mechanisms of the ionized gas. We do not know if the
1999; Sarzi et al. 2006, 2010; Davis et al. 2011, 2016).
r mainsourcesareinternal(e.g.,stellarmassloss)orexter-
a Roughly107−109M(cid:12) worthofmoleculargasisdetected nal (e.g., mergers). The total ISM masses that we infer
in∼25%oftypicalearly-typegalaxies(Knapp&Rupen
are in general lower than predictions from stellar mass
1996; Combes et al. 2007; Young et al. 2011, and refer-
lossmodels,suggestingthattherecycledgasisheatedor
ences therein). The warm ionized gas traces the dust in
expelledinatimelymanner(althoughourunderstanding
these galaxies quite well in terms of both mass fraction
of stellar mass loss is not complete; e.g., see Athey et al.
and morphology (roughly 103−105M ; e.g., Kim 1989;
(cid:12) 2002; Temi et al. 2007; Martini et al. 2013; Simonian &
Buson et al. 1993; Goudfrooij et al. 1994), and is of-
Martini 2016). Given the typically low star formation
ten accompanied by the detection of molecular gas (e.g.,
ratesofearly-typegalaxies, itisinterestingtoaskwhere
Youngetal.2011;Davisetal.2011;Alataloetal.2013).
the gas comes from, whether it is long-lived or recently
The relationship between the warm ionized and hot X-
accreted/cooled, and what excites the gas.
ray-emitting gas is far less clear (e.g., Sparks et al. 1989;
One way to proceed is to ask whether the presence of
Macchetto et al. 1996; Goudfrooij 1999), and may well
ionized gas correlates with other properties of the galax-
depend on the temperature profile of the X-ray-emitting
ies. In particular, early-type galaxies are typically di-
vided into two classes that are thought to be related to
Electronicaddress: [email protected]
2 V. Pandya et al.
their formation history (e.g., Kormendy & Bender 2009; Since the MASSIVE sample is thoroughly described
Cappellari 2016). Less massive early-type galaxies tend in Ma et al. (2014), here we will only briefly review
to be flattened, have younger stellar populations, and the selection criteria. The MASSIVE survey targets
show regular rotation around the major axis (fast rota- the most massive local early-type galaxies by requiring
tors; e.g., Emsellem et al. 2007). In contrast, the most M <−25.3mag(basedonthe2MASSRedshiftSurvey;
K
massiveearly-typegalaxiestendtoberoundandoldand Huchra et al. 2012) and applying a morphological cut to
have very little net rotation (slow rotators). Slow rota- removelargespiralandinteractinggalaxies(basedonthe
tors show a number of indirect signs of past merging Hyperleda database; Paturel et al. 2003). The survey is
activity, including kinematically decoupled components volume-limited out to a distance of 108 Mpc, which re-
(e.g., Krajnovi´c et al. 2011; Naab et al. 2014). sultsinafactoroftengreatervolumecomparedtoprevi-
An excellent data set to address these questions is oussimilarIFUsurveys(e.g.,ATLAS3D;Cappellarietal.
provided by the integral-field ATLAS3D survey (Cappel- 2011) and allows for the inclusion of many more massive
lari et al. 2011), a volume-limited sample of early-type early-type galaxies such as those in the Coma cluster.1
galaxieswithlog(M∗/M(cid:12))∼> 9.8(MK <−21.5mag)out The full MASSIVE sample consists of 116 galaxies, but
to a distance of 42 Mpc. The ATLAS3D survey (and the sample size for this paper is 74 galaxies (these con-
its predecessor SAURON; de Zeeuw et al. 2002) find a stitute, with a few exceptions, the brighter two-thirds of
wide range of kinematic misalignment angles between the survey with MK <−25.5 mag).
the stars and gas in slow rotators, suggesting that in This paper is based on integral field spectra obtained
generalthegasinthesesystemshasbeenaccreted(Sarzi with the Mitchell Spectrograph (formerly VIRUS-P) on
et al. 2006; Davis et al. 2011; Sarzi et al. 2013). On the2.7mHarlanJ.SmithTelescopeatMcDonaldObser-
the other hand, more than half of fast rotators are ob- vatory. TheMitchellIFUspectrographhasa107(cid:48)(cid:48)×107(cid:48)(cid:48)
served to host warm ionized gas kinematically aligned field of view with 246 4(cid:48)(cid:48)-diameter fibers per pointing.
relative to the stars, which at least suggests that in a The spectra cover the wavelength range from ∼ 3500˚A
significant fraction of fast rotators, the gas may arise to ∼ 5800˚A. Data reduction details are given in Greene
from internal processes such as stellar mass loss (Sarzi etal.(2012,2015),butseealsoVealeetal.(2016)forad-
et al. 2006; Davis et al. 2011), although kinematic align- ditional details. We generally have three dithered point-
ment can occur in merger remnants as well (van de ings for a total of 738 spectra per galaxy. We work with
Voort et al. 2015). Galaxies in richer environments are the single-fiber IFU spectra as well as a radially and az-
more likely to show kinematic alignment between the imuthallybinnedIFUdatasetwhereinfibersaregrouped
stars and gas compared to more isolated galaxies (Davis together to ensure continuum S/N ≥ 20 in each spa-
et al. 2011). In addition, the most massive fast rotators tial bin. Fibers with spectra that have abnormally high
in ATLAS3D, with log(M∗/M(cid:12))∼> 10.9, uniformly show median fluxes, exceptionally low continuum S/N, or are
kinematic alignment between the gas and stars. How- near foreground stars are masked. The spatial bins are
ever, because of the small volume limit of the ATLAS3D constructed from the individual fibers (remaining after
survey, they have poor coverage of the most massive masking) from the center outward, leaving central fibers
early-type galaxies (only 14% of their galaxies are slow that already meet the S/N threshold unbinned. Begin-
rotators; Emsellem et al. 2011). ningatthefirstfiberbelowtheS/Nthreshold,wedivide
The MASSIVE survey (Ma et al. 2014) of early-type fibersintocircularradialbinsandchooseazimuthalbins
galaxies within 108 Mpc (including the Coma cluster) (of equal size within each annulus but not across an-
providesanidealsampletostudythewarmISMingalax- nuli) that minimize the aspect ratio to avoid thin rings
iesthatareafactoroftenmoremassivethanthetypical or wedges. The radius of each annulus is then increased,
galaxies included in previous systematic surveys such as from center outward (reducing the number of azimuthal
ATLAS3D, and to probe a wider range of slow rotator bins when needed to maintain the proper aspect ratio),
properties. Sincethegalaxieswewillconsiderinthispa- until the S/N threshold is reached (for more details, see
perallhavelog(M /M )>11.5(M <−25.3mag),our Veale et al. 2016).
∗ (cid:12) K
sampleisdominatedbyslowrotators(Vealeetal.2016). Thispaperrepresentspartofourcomprehensiveeffort
Using integral field unit (IFU) spectroscopy obtained as to study gas of all phases in the MASSIVE sample, in-
part of the MASSIVE survey (Ma et al. 2014), we will cluding using CO to trace molecular gas (Davis et al.
presentthefirstsystematicinvestigationoftheexistence, 2016) as well as hot X-ray gas (Goulding et al. 2016).
spatial distribution, and kinematics of warm ionized gas
in the most massive galaxies in the local Universe. 3. METHODS
The structure of this paper is as follows. In section 2,
Here we describe our methods for detecting warm ion-
we present our sample of MASSIVE galaxies. In sec-
ized gas emission in MASSIVE galaxies.
tion 3, we describe our methods for detecting warm ion-
ized gas with our IFU observations. In section 4, we
presentthedetectionfractions,spatialextent,lineratios 3.1. Stellar Continuum Subtraction
and kinematics of the gas. In section 5, we consider the
For robust detection of faint lines, it is crucial that we
implications of our results for the origins of excitation
modelandsubtractthestellarcontinuum. Wemustalso
mechanisms of the gas. We summarize our results and
ensure that each spectrum is at rest with respect to the
discuss future work in section 6.
1 We have removed NGC 7681 from our sample even though it
hosts warm ionized gas near its nucleus because it is actually a
closepairandeachindividualgalaxyisunlikelytomakeitintothe
2. SAMPLEANDDATADESCRIPTION
MASSIVEsample(seealsoVealeetal.2016).
Warm Ionized Gas in MASSIVE Early-type Galaxies 3
stars. For each IFU spectrum, we therefore fit the stel- detected, we allow V and σ to be free parameters
gas gas
lar continuum using the penalized pixel-fitting method of the shared line profile for [OIII] and Hβ (with similar
(pPXF;Cappellari&Emsellem2004). Weonlysolvefor constraints as for [O II]). We require S ≥ 3 for signifi-
the first two moments (V ,σ ) of the full Gauss- cant[OIII]detections,butS ≥2.5forHβ fits. Itismore
stars stars
Hermitedecomposition(seevanderMarel&Franx1993; difficult to constrain the presence of Hβ because of the
Gerhard 1993). We adopt the Bruzual & Charlot (2003) strong stellar absorption, but after we subtract the ex-
library of stellar population templates that span a grid pected absorption contribution from stars based on our
of ages (5 Myr to 12 Gyr) and metallicities (Z = 0.004, best-fit pPXF composite templates, excess Hβ emission
0.02, and 0.05). To help account for flux calibration and can be revealed. When necessary, upper limits on [OIII]
sky subtraction errors, we fit with sixth order additive and Hβ are derived similarly to what would be done for
and sixth order multiplicative polynomials. We found [OII].
that our Hβ, [OII] and [OIII] equivalent widths did not The [N I] emission line falls in the Mgb wavelength
change significantly if we instead used first-order addi- region where the underlying stellar continuum is only
tive and twelfth-order multiplicative polynomials. This poorly modeled by current stellar population templates,
is an important check because additive polynomials, un- asdescribedbySarzietal.(2006). Therefore,werequire
like multiplicative polynomials, can change the equiva- more stringent significance criteria for it, following Sarzi
lent widths of stellar absorption lines (e.g., see Koleva et al. (2006, 2010). Both [O III] and Hβ must be signif-
et al. 2009). icantly detected in order for the [N I] fit to be trusted.
Each best-fit composite template spectrum is sub- If both [O III] and Hβ are indeed significantly detected,
tracted from its corresponding observed spectrum. The and [N I] itself has S ≥ 4, then [N I] is considered to be
residual spectrum is used to measure the emission lines. significantly detected. Otherwise, an upper limit is de-
Note that Sarzi et al. (2006) advocate an iterative ap- rived for [N I] as described above. We detect [N I] only
proach; we do not implement such an algorithm here. in three spectra, and these are the fibers closest to the
We verified that the RMS of the residual spectrum is photometric centers of NGC 0708, NGC 1167, and NGC
roughly comparable to the pixel-by-pixel uncertainties 1453.
of the original spectrum, which means that we can use TheresolvingpowerofourspectraisR≈830(roughly
the RMS of the local continuum around an emission line 150 km s−1) near [O II], and R ≈ 1060 (roughly 120
todefinethesignificanceofourfits(seenextsubsection). km s−1) near Hβ, [O III] and [N I]. We subtract these
All gas and stellar velocities presented in this paper are instrumental broadening values in quadrature from our
relative to the galaxy systemic velocity (see also Veale measured line widths. However, the procedure for [OII]
et al. 2016). is a bit more involved because it is actually an unre-
solved doublet with a line separation of only 2.8˚A.2 Be-
3.2. Emission Line Detection Algorithm
fore correcting for instrumental broadening of [OII], we
Since our spectra span the wavelength range from apply an empirical deblending calibration to its mea-
∼ 3500˚A to ∼ 5800˚A, we constrain the presence of sured line width, which is derived as follows (see also
the [O II] 3727˚A, [O III] 4959,5007˚A, Hβ and [N I] Salviander et al. 2007). For a grid of input velocity dis-
5199˚Aemissionlinesasfollows. Webeginbyfitting[OII] persions (50 to 300 km s−1), we create and sum two
with a single Gaussian plus zero-slope additive contin- Gaussians with the same amplitude, fixed line separa-
uumoffset. TheGaussianisconstrainedtohavepositive tion, and input line width. We fix the amplitude ratio
amplitude, 1.0˚A < σ < 4.0˚A (80 to 320 km s−1 ve- [O II]λ3728.8/[O II]λ3726.0 to one because this corre-
gas
locitydispersion),andamaximumcentralwavelengthof spondstoelectronnumberdensityne =100cm−3,which
µ=3727˚A±10˚A. The constraint on σ prevents very issensibleforthediffuseISMofearly-typegalaxieswhere
gas theactualdensitiesaremuchlowerthanthecriticalden-
narrowGaussiansfrombeingfittonoisespikesandvery
sity for both doublet lines, n ∼103 cm−3 (see, e.g., Os-
wide Gaussians from being fit to broad continuum resid- e
terbrock 1989). We then fit a single Gaussian to each of
uals. The constraint on µ ensures that we allow only a
reasonable range of gas velocities (±800 km s−1). Al- thesummedGaussiansandobtainaone-to-onemapping
between intrinsic line width and blended line width. We
though it is of course possible that emission lines can
use a quadratic polynomial fit to the empirical mapping
exist with velocities beyond that which our range of µ
to convert measured line widths to intrinsic line widths.
allows, we have confirmed through visual inspection of
If the deblending calibration results in an intrinsic line
the spectra that no such cases exist in our sample.
widththatisbelowourresolutionlimit,wesimplysetthe
Wedefinethesignificanceofafitastheratiooftheline
line width to the resolution limit; otherwise, we further
amplitude to the RMS of the local continuum residuals
subtract the instrumental broadening in quadrature.
(i.e.,afterbest-fittemplatesubtraction),asinSarzietal.
Figure1showsthecentralfiberspectrumofNGC1453
(2006): S ≡ A/N. [O II] fits with S ≥ 3 are considered
to be significant. For fits with S < 3, we derive the in which [OII], [OIII], Hβ and [NI] are all significantly
detected. Figure2showsexampletwo-dimensionalmaps
3σ upper limit on the integrated flux of [O II] using the
of various quantities for NGC 1453.
knownrelationbetweenGaussianamplitude, width, and
integrated area while assuming A = 3N and σ =200
gas 2 [N I] is also an unresolved doublet with a line separation of
km s−1.
only2.4˚A,andinprincipleweshouldalsocorrectitsmeasuredline
The fit to [O III] is similar to that for [O II], but we width for blending. However, [NI] is in a poorly modeled part of
fix V and σ to the values derived for [O II]. Hβ is thestellarcontinuumandwerarelydetectit, asdescribedabove.
gas gas
simultaneously fit with the [O III] doublet by fixing its Therefore, we only correct the measured line width of [N I] for
instrumentalbroadening.
kinematics to those of [OII]. In cases where [OII] is not
4 V. Pandya et al.
Figure 1. Example of detecting [O II], [O III], Hβ and [N I] emission lines in the central fiber of NGC 1453. The original spectrum is
shown in gray, best-fit pPXF composite template in gold, residual spectrum in black, error spectrum in pink, and best-fit Gaussians to
emissionlinesinorange. Verticaldashedlinesmarktherest-framecentralwavelengthsofthevariousemissionlines.
3.3. Sensitivity Limits EW sensitivity, although stacking only works well when
thegasfillsmuchofabin. Giventhatthestellarcontin-
In Appendix A, we discuss simulations that assess our
uum level is three to four times fainter near [O II] than
integrated flux completeness limits and uncertainties for
near [OIII], we have comparable detection limits for the
[O II], Hβ and [O III]. Here we will only briefly com-
twolinesintermsofintegratedflux. ThehigherEWde-
ment on the results of those simulations. Uncertainties
tection limit of [OII] is due to the lower continuum S/N
on the measurements of detected emission lines (notably
at blue wavelengths. While we use [OII] as our primary
the integrated flux) were assumed to be 10% in all cases
detectionline,noneofourconclusionswouldchangehad
based on the simulations. We can reliably detect the
we used [OIII] instead.
[O II] emission line down to a minimum integrated flux
Since we will compare our results to those of the
of ∼3×10−16 erg s−1 cm−2.
SAURON (de Zeeuw et al. 2002) and ATLAS3D (Cap-
We reach an EW limit of ∼ 2˚A and ∼ 0.5˚A for [O II]
pellari et al. 2011) surveys, it is useful to mention their
and [O III], respectively, in the central regions of galax-
sensitivities. They both use the SAURON instrument (Ba-
ies where we work with individual fibers. In the binned
conetal.2001)andutilize[OIII]forlinedetection([OII]
data, we reach nearly an order of magnitude deeper in
is not in their observing band). The SAURON survey
Warm Ionized Gas in MASSIVE Early-type Galaxies 5
Figure 2. Two-dimensionalmapsofvariousquantitiesforNGC1453. Fromlefttoright: [OII]integratedflux,gasandstellarvelocity,
gasandstellarvelocitydispersion,and[OIII]/Hβ integratedfluxratio. Thepinkcrossineachsubpanelmarksthephotometriccenterof
the galaxy. The pink ellipse in the integrated flux subplot shows the stellar effective radius, photometric position angle, and axis ratio,
whereas the red dashed circle in each panel marks the maximum radial extent of the gas, Rgas. The black line in the velocity subplot
shows the kinematic position angle of the rotating gas. In the velocity and velocity dispersion subplots, the star marker symbols in the
background represent the velocities and velocity dispersions of the stars in each fiber as derived with pPXF. In all panels, circle marker
symbols represent measurements from the individual fibers and squares represent measurements from the bins (shown where the single-
fibermeasurementsareinsignificantorhavelargeuncertainties). Intheintegratedfluxsubplot,downwardfacingtrianglesrepresentupper
limits, whereas in the excitation ratio subplot, upward facing triangles represent lower limits (shown where [OIII] is detected but Hβ is
not, leading to an Hβ upper limit in the denominator). The number of kpc corresponding to four arcsec at the redshift of NGC 1453 is
writtenintherightmostsubplot. Similartwo-dimensionalmapsfortheother27galaxiesareshowninAppendixB.
quotes limits in [O III] and Hβ of ∼ 0.3˚A and ∼ 0.2˚A, 2016), which afford us significantly greater continuum
respectively (Sarzi et al. 2006). S/N atlargegalactocentricradii. Inouroutermostbins,
we typically have [O II]-local continuum S/N > 4 and
4. WARMIONIZEDGASPROPERTIES [OIII]-localcontinuumS/N >30. Ifthegasisuniformly
4.1. Detection Fractions distributed rather than patchy, binning can increase the
significance of a detection. In every case where [O II] is
With the emission line measurements in hand, we now
detected in our binned spectra with S/N ∼ 20 at large
address whether a galaxy hosts warm ionized gas using
galactocentric radii, we also detect [O II] in the central
[O II] as our primary tracer. This is complicated by
fibers. When individual fibers at large galactocentric
false positive [O II] detections that can crop up in the
radii do not reveal significant emission, sometimes the
outer parts of an IFU dataset (typically at the ∼< 10% corresponding binned IFU spectra do reveal real emis-
level). We thus take two approaches to identify which
sion.
galaxies host warm ionized gas. First, for the purpose
Wefindthat28/74≈38±6%oftheMASSIVEgalax-
of assigning a detection flag to each galaxy, we apply an
ies in our sample host a warm ionized medium.3 The
additional [O II]-local continuum S/N ≥ 3 cut. This
properties of these 28 galaxies are given in Table 1, or-
additional S/N cut removes most fibers within an IFU
dered according to whether the gas is extended and ro-
datasetexceptthosewithin∼15arcsecofthephotomet-
tating, extended and non-rotating, or centrally concen-
ric center, which generally have [O II]-local continuum
trated. Two-dimensional maps of various quantities for
S/N ∼> 10 and [O III]-local continuum S/N ∼> 100. The these galaxies are shown in Appendix B using the same
muchhigherlocalcontinuumS/N ratiosofthesecentral
orderasinTable1(NGC1453isshownseparatelyinFig-
fibers drastically reduces the chance of identifying noisy
ure2). Mostofthesegalaxieshostwarmionizedgasthat
residual spikes as false positive [OII] detections.
is detected only in the central fibers: 17/28 = 61±9%.
If, after applying this continuum S/N cut, there are
In3/28≈11±6%galaxies, thewarmionizedmediumis
≥ 3 fibers with [O II] detections remaining in the IFU
spatially extended but patchy and non-rotating, and in
data, then the galaxy is flagged as hosting warm ionized
8/28≈29±9%oftheMASSIVEgalaxies,thewarmion-
gas. If there are no fibers with [O II] detections, then
izedgasisspatiallyextendedandshowsregularrotation.
the galaxy is flagged as hosting no detectable warm ion-
IfwesplitthefullMASSIVEsamplebystellarkinematic
izedgas. Incaseswithonlyoneortwofiberswithdetec-
classification, then we find that 12/15 = 80 ± 10% of
tions,wefindthatonlyfibersclosetothecenterandwith
MASSIVEfastrotatorshostwarmionizedgascompared
S > 3.5 are reliable (in such cases we flag the galaxy as
toonly16/58=28±6%MASSIVEslowrotators. Atwo-
hosting a nuclear warm ionized gas reservoir). Although
proportionz-testconfirmsthatthedetectionfractionsfor
one may worry that these criteria discard galaxies that
MASSIVE slow and fast rotators are indeed statistically
only have off-center [O II] emission, we verified through
visualinspectionthattherearenosuchcasesinoursam-
3 Unless otherwise noted, the uncertainties that accompany all
ple. detectionfractionsarecomputedassumingabinomialdistribution:
The second complementary approach involves using δf =(cid:112)f(1−f)/N,wheref andN arethedetectionfractionand
the binned IFU spectra (see section 2 and Veale et al. samplesize,respectively.
6 V. Pandya et al.
significantly different, with a z-score of 3.7 correspond- 4.3. Comparison to SAURON and ATLAS3D
ing to a p-value (cid:28) 0.01. The distance and stellar mass
Our ∼39±6% global detection fraction is lower than
distributionsofMASSIVEfastandslowrotatorsarenot
the 36/48 = 75±6% detection fraction for SAURON’s
significantly different from each other, as confirmed by
lessmassiveearly-typegalaxies(Sarzietal.2006). While
non-parametricstatisticaltests. Thismeansthatthesig-
the Sarzi et al. (2006) work has not been extended to
nificantly different detection fractions for the two kine-
the full ATLAS3D sample, Davis et al. (2011) report
matic classes cannot be attributed to an underlying bias
that 67±3% of fast rotators and 86±6% of slow ro-
in their distance and stellar mass distributions.
tators in ATLAS3D have detectable warm ionized gas
We are clearly more likely to detect warm ionized gas
emission. Similarly, Nyland et al. (2016) report a warm
in MASSIVE fast rotators than slow rotators. We now
ionized gas detection fraction of 54/101 ≈ 53±5% for
ask whether there are other defining characteristics of
thesubsetofATLAS3D galaxieswithradioobservations.
the galaxies with warm ionized gas since that may pro-
Both of these quoted ATLAS3D detection fractions are
vide indirect clues about the origin of the gas. For ex-
higher than ours. In this section, we will explore the de-
ample, if cooling flows represent an important gas ori-
tectability and resolvability of ATLAS3D and SAURON
gin scenario for MASSIVE galaxies, then we may ex-
gasreservoirsiftheyinsteadexistedinthemoreluminous
pect different parent populations of halo masses (e.g.,
and distant MASSIVE galaxies.4 We compare with an
Kereˇs et al. 2005). If instead the dominant factor is
ATLAS3Dcatalogofionizedgasmeasurements(M.Sarzi,
whether a galaxy can recycle and retain the mass lost
by its old stellar population, then we might expect dif- private communication). We only consider ATLAS3D
ferent underlying distributions of stellar velocity disper- galaxies that have detected warm ionized gas by requir-
sion. We thus investigate the following physical proper- ing that the spatially integrated [O III] EW > 0.02˚A as
ties: stellar mass, halo mass, and central stellar velocity in Davis et al. (2011); there are 183 such objects.
dispersion, the distributions of which are shown in Fig- First, if the ATLAS3D gas reservoirs were relocated to
ure3. Weusedthreedifferentnon-parametrictests(two- 80 Mpc (the median distance of the MASSIVE sample),
sample Kolmogorov-Smirnov, Mann-Whitney-Wilcoxon, would they be spatially resolved with our observations?
and Anderson-Darling) to check for statistically signif- Consider first that the 183 ATLAS3D galaxies with de-
icant differences between the distributions of the three tected warm ionized gas span a range of physical radii
physical properties for the MASSIVE sample with and from0.2−4.4kpc,withamedianof1.6kpc. Withafew
without gas. All three tests returned p-values > 0.01 exceptions on the smaller end, these values are consis-
in every case, which suggests no statistically significant tentwiththephysicalradiiofMASSIVEcompact(unre-
differences between the underlying parent populations. solved) gas reservoirs, whose upper limits on R of five
gas
arcsec correspond to physical radii of 0.6−2.5 kpc, with
4.2. Spatial Extent
amedianof1.8kpc. Indeed,wefindthat121/183≈66%
Becauseoftheroughly2×2arcmin2fieldofviewofthe of ATLAS3D gas reservoirs would have a radial gas ex-
Mitchell Spectrograph, we are able to study the spatial tent of less than five arcsec if they were relocated to 80
extentofthegasouttogenerally2−4×Re. Weusethe Mpc, which means that the majority of the ATLAS3D
binned IFU datasets described in section 2 to determine sample would be classified as spatially unresolved in our
via visual inspection the radially furthest bin in which observations.
an [OII] emission line exists. Second, would our observations be sensitive enough to
Of the 28 MASSIVE galaxies with warm ionized gas, detect emission from ATLAS3D gas reservoirs, if they
11 galaxies have spatially extended gas and the remain- were relocated to the median MASSIVE distance of 80
ing 17 galaxies have [O II] emission only in a few fibers Mpc? Only 101/183 ≈ 55% of ATLAS3D gas reser-
near their photometric centers. For the 11 spatially ex- voirs shifted to 80 Mpc would have [O III] EW above
tendedgalaxies,weadoptedthemidpointofthefurthest 0.5˚AwithinasingleMitchellfiber. Ifwecrudelyconvert
bin with reliable [O II] emission as the maximum radial the ATLAS3D [O III] EW to [O II] EW by multiplying
extent of the warm ionized gas (Rgas), and for the 17 the former by a factor of six, then 123/183 ≈ 67% of
compact cases, we assumed an upper limit on R of 5
gas ATLAS3D gas reservoirs relocated to 80 Mpc have [OII]
arcsec (recall that the fiber radii are 2 arcsec, but there
EW above 2˚A within a single Mitchell fiber.5 This exer-
are usually three dithers). The adopted R values are
gas cise suggests that the approximately factor of two lower
given in Table 1. The maximum physical extents span
warm gas detection fraction for MASSIVE galaxies, as
a wide range from only 0.6 kpc up to 18.2 kpc, with a
comparedtoboththeSAURONandATLAS3D samples,
median of 2.3 kpc; likewise, the R /R fractions run
gas e might simply be attributable to sensitivity differences.
from 0.12 up to 3.71, with a median of 0.25.
Therefore, while the warm ionized gas detection frac-
The furthest radial bin with detectable [OII] emission
tion might indeed drop as a function of mass, we do not
contains on the order of ∼100 stacked individual fibers.
Since we are binning so many fibers, a few of which can
4ThemediandistancesoftheSAURONandATLAS3Dsamples
have detected emission (occasionally false positives), we
are16Mpcand24Mpc,respectively(Sarzietal.2006;Cappellari
ask whether the individual fiber detections might domi- et al. 2011), substantially lower than the median distance of 80
natethesignaldetectedinthebinnedspectrum. Infact, MpcfortheMASSIVEsample(Maetal.2014).
the individual fibers constitute < 10% of the integrated 5 The factor of six assumes [O II]/[O III]= 2, which is typi-
cal for our MASSIVE galaxies as well as stacked SDSS spectra of
flux of the line detected in the binned spectrum. Thus,
extended LINER-like objects (without reddening corrections; Jo-
thebinneddetectionsuggeststhatthereisextendedlow- hansson et al. 2016). We also assume that the stellar continuum
surface-brightness emission filling a large fraction of the is a factor of three fainter near [OII] as compared to near [OIII]
bin. (seesubsection3.3).
Warm Ionized Gas in MASSIVE Early-type Galaxies 7
Table1SampleofMASSIVEGalaxieswithWarmIonizedGas GalaxyDRRMMMMLσ[O]/HβPAPAPAFR/SRExtendedRotatingIIIegas∗gasgasstarsgas−starsKhHβstars,central−1−1−[Mpc][kpc][kpc][mag][logM][logM][logM][logergs][kms]−[deg][deg][deg]−−−(cid:12)(cid:12)(cid:12)(1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17) NGC070869.016.324.09-25.6511.7514.55.5239.1248.032.14356.8±1.6126.0±17.550.8±17.6SRYesYesNGC116770.29.9718.21-25.6411.7413.05.7339.3231.815.71284.3±15.3254.0±3.830.3±15.8FRYesYesNGC145356.47.638.41-25.6711.7513.65.8239.4341.042.59312.5±3.135.0±3.297.5±4.4FRYesYes†NGC220884.110.918.86-25.6311.7410.05.5539.1280.342.25241.8±13.495.0±9.5−SRYesYesNGC232089.48.165.65-25.9311.8714.15.5739.1345.862.66148.2±16.5147.0±4.01.2±17.0FRYesYesNGC2783101.418.242.85-25.7211.7812.35.5439.1274.76>1.18148.5±5.0269.5±11.059.0±12.1SRYesYesNGC742680.07.265.74-25.7411.7913.75.7139.3306.752.66261.7±3.1251.0±1.210.7±3.3FRYesYes†NGC755072.79.642.35-25.4311.6511.85.3238.9270.733.31238.7±10.4155.2±15.5−SRYesYesNGC170054.46.0612.27-25.6011.7212.64.8238.4262.74>0.63−268.0±1.8−FRYesNoNGC648261.44.4416.46-25.6011.7213.05.6139.2330.441.26−63.5±2.0−SRYesNoNGC726582.810.937.41-25.9311.8714.65.0838.6256.21>1.20−259.5±9.5−SRYesNoNGC031570.38.421.68-26.3012.0313.44.7438.3353.32>1.06−222.0±7.2−SRNoNoNGC038371.37.091.73-25.8111.8214.34.6038.1304.02>1.56−140.5±2.8−FRNoNoNGC041071.311.301.79-25.9011.8614.34.6638.2313.07>0.97−161.0±18.5−SRNoNo†NGC053377.915.291.88-26.0511.9213.84.9638.5304.811.44−51.2±5.1−SRNoNoNGC054774.07.131.81-25.8311.83−4.5838.1277.65>2.14−176.5±21.8−SRNoNo†NGC106067.412.981.76-26.0011.9014.04.1937.7325.910.88−74.0±7.4−SRNoNoNGC380599.47.342.22-25.6911.7614.74.5838.1280.801.25−61.5±1.5−FRNoNo†NGC384299.410.202.11-25.9111.8614.75.3838.9272.951.50−1.6±0.2−SRNoNoNGC5129107.510.112.32-25.9211.8610.04.6638.2277.81>1.21−3.0±2.5−FRNoNoNGC5208105.05.411.48-25.6111.7313.04.2737.8274.842.09−343.0±1.5−FRNoNoNGC532234.22.420.60-25.5111.6813.54.0837.6258.94>1.13−267.5±7.8−SRNoNoNGC622386.77.852.05-25.5911.7213.55.1438.7294.97>0.72−288.0±2.2−FRNoNoNGC637595.89.082.26-25.5311.6910.04.7638.3251.26>2.99−322.0±7.0−FRNoNoNGC705269.38.981.65-25.6711.7510.03.6337.2314.09>1.26−61.5±2.2−SRNoNoNGC7436106.612.392.48-26.1611.9714.24.5938.1311.13>1.07−197.5±10.2−SRNoNo†NGC762654.06.211.16-25.6511.7513.93.5237.1305.02>0.94−10.5±1.1−SRNoNoUGC0226170.65.321.68-25.6111.7311.84.0937.6308.33>0.65−66.0±2.5−FRNoNo Note.—PropertiesoftheMASSIVEsamplewithwarmionizedgas.(1)Galaxyname.(2)DistancetakenfromMaetal.(2014)[Mpc].(3)Stellareffectiveradius[kpc].(4)Maximumgasradius[kpc].(5)AbsoluteK-bandmagnitude[ABmag].(6)Logarithmofstellarmass.(7)Logarithmofthedarkmatterhalomass.(8)Logarithmofthewarmionized−1gasmass,whichisderivedusingtheHβluminosityaccordingtoEquation1.(9)LogarithmofHβluminosity.(10)Centralstellarvelocitydispersion[kms].(11)[O]/HβfluxIIIratiointhecentralspectrum(lowerlimitifHβisnon-detected).(12)Kinematicpositionangleofrotatingwarmionizedgasmeasuredcounterclockwisefromnorthtotheredshiftedemissionside[deg].(13)Kinematicpositionangleofthestarsmeasuredcounterclockwisefromnorthtotheredshiftedemissionside[deg].(14)Kinematicmisalignmentangleofthewarmionizedgasrelativetothestars[deg].(15)KinematicclassificationasfastorslowrotatorfromVealeetal.(2016)basedonangularmomentumwithinR.(16)Whethere†thewarmionizedgasisextendedbeyondthecentralthreeditheredfibers.(17)Whetherthewarmionizedgasshowsregularrotation.Incolumn(12),aidentifies“non-rotators”forwhichwasunabletodetermineakinematicstellarpositionangle;forthesegalaxies,theirphotometricstellarpositionanglesaregiveninsteadwithanassumed10%kinemetryfractionaluncertainty(seesubsection4.4fordetails).
8 V. Pandya et al.
Figure 3. The distributions of stellar mass (left), halo mass (middle), and central stellar velocity dispersion (right) for the MASSIVE
galaxieswith(orange)andwithout(black)warmionizedgasdetections. Afterrequiringmeasurementsofthethreephysicalpropertiesto
beavailable,thereare26galaxieswithgasand36galaxieswithoutgasthatcomprisethedistributionsshown. Non-parametrictestsreveal
thateachsetoftwodistributionsisconsistentwithbeingdrawnfromthesameunderlyingparentpopulation(seetext). Notethatahalo
massof1010M(cid:12) isanarbitraryvalueforgalaxieswithnoidentifiedfriends(Crooketal.2007,2008).
Warm Ionized Gas in MASSIVE Early-type Galaxies 9
have good enough statistics to robustly constrain that PA values are well-aligned.
expected trend. In Figure 4, we show the kinematic misalignment an-
Wealsonotethatthewarmionizedgasdetectionfrac- gle between the stars and warm ionized gas for MAS-
tions of fast rotators and slow rotators in ATLAS3D are SIVE slow and fast rotators as compared to ATLAS3D
comparable, which is not the case for MASSIVE. In ad- slowandfastrotators. KinematicclassificationsofMAS-
dition,theslowrotatordetectionfractionismuchhigher SIVEgalaxiesintoslowandfastrotatorsarepresentedin
for ATLAS3D (∼ 86%) than for MASSIVE (∼ 28%). Veale et al. (2016) whereas the kinematic misalignment
These differences between the two surveys appear to be angles and kinematic classifications of ATLAS3D galax-
significant even though there are relatively few fast ro- ies are taken from Krajnovi´c et al. (2011), Davis et al.
tators in MASSIVE and relatively few slow rotators in (2011) and Emsellem et al. (2011). Misalignment an-
ATLAS3D. While the distributions of [O III] EW for gles for MASSIVE galaxies are defined as PA ≡
gas−stars
slowandfastrotatorsinATLAS3D arestatisticallyindis- PA −PA , where PA is the kinematic position
gas stars gas
tinguishable (according to three non-parametric tests), angleofthewarmionizedgasandPA istheposition
stars
there is a longer tail toward high EWs for the fast rota- angle of the stars (kinematic when available, else pho-
tors. Since we are not as sensitive, it is possible that a tometric). The misalignment angles for all ATLAS3D
large fraction of ATLAS3D slow rotators with relatively galaxies are based on kinematic stellar PAs. All mis-
low [O III] EW would not be detected by us. Alterna- alignment angles are defined to lie between 0 and 180
tively, itmaybethattheslowrotatorshavepatchiergas degrees.
distributions than the fast rotators, and thus our larger In general, misalignment between stars and gas points
spaxels hurt us disproportionately. towards an external origin of the gas, while alignment
between gas disks and stars suggests that the gas orig-
4.4. Kinematics: Gas versus Stars inated from the stars. However, see Serra et al. (2014)
A comparison of the kinematics of the stars and the and Lagos et al. (2015) for the expected misalignment
warm ionized gas can reveal clues about the origin of when gas cools out of the halo, and van de Voort et al.
the gas. For this reason, we run the fit kinematic pa (2015) for the timescales on which externally accreted
routinedescribedinKrajnovi´cetal.(2006)todetermine gas is expected to become aligned with the stars due to
the kinematic position angle (PA) of the warm ionized gravitational torques. The MASSIVE fast rotators are
gasfortheeightgalaxiesinwhichthegasisrotatingand predominantlyaligned(3/4=75±22%),whichisconsis-
extended.6 We define the kinematic gas PA as the angle tent with the results from ATLAS3D that more massive
between north and the line that bisects the gas veloc- fast rotators tend to host the warm ionized gas kine-
ity field, measured counterclockwise from north to the matically aligned with the stars (Davis et al. 2011). In
redshifted emission side. In order to alleviate the im- contrast, the two MASSIVE slow rotators are consistent
pact of false positive fibers at large galactocentric radius withmisalignmentbetweenthestarsandgas,whichisin
and explore how well the PA can be constrained, we run agreementwiththeinterpretationthatslowrotatorspre-
fit kinematic pa three times, applying different fiber dominantly get their gas via external means (Sarzi et al.
selection criteria each time and then taking the mean 2006; Davis et al. 2011). Our total number of rotating
and standard deviation of the resulting values. The re- and spatially extended gas reservoirs is too small to say
sultingkinematicgasPAsaregiveninTable1. Notethat anything definitive about the distribution of misalign-
it is difficult to constrain the kinematic position angle of ment angles. Nevertheless, the MASSIVE fast rotator
the gas in NGC 1167 because rotation is observed in the NGC 1453 is especially interesting because its gas and
gas only out to ∼1R and because NGC 1167 harbors a stellar PAs are nearly perpendicular to each other; we
e
nearly face-on disk (see also subsection 5.3). will return to this galaxy in section 5.
WehavealsomeasuredkinematicstellarPAsforMAS-
SIVE galaxies using kinemetry (Krajnovi´c et al. 2006). 4.5. Line Ratios and Radial Profiles
These values are listed in Table 1; additional details and
We now turn to the sources of ionization for the warm
measurements for the full MASSIVE sample will be pre-
ionized gas in MASSIVE galaxies. There are five ioniz-
sented in Ene et al. (in preparation). To ensure consis-
ingsourcestypicallyconsidered(seeSarzietal.2010,and
tency with the kinematic gas PAs, our kinematic stellar
references therein): photoionization by an active galac-
PAs are also measured counterclockwise from north to
tic nucleus (AGN; e.g., Ho et al. 1997; Ho 2008), pho-
the redshifted emission side. MASSIVE is dominated by
toionization by evolved stars (e.g., Binette et al. 1994;
slow rotators, and in some cases, there simply is not a
Stasin´skaetal.2008),photoionizationbyayoungstellar
preferredaxisofrotation. Ofthefourslowrotatorswith
component(e.g.,Gravesetal.2007;Kavirajetal.2007),
extended and rotating warm ionized gas, NGC 2208 and
excitationbyshocks(e.g.,Koski&Osterbrock1976;Do-
NGC7550donothavemeasurablekinematicstellarPAs
pita & Sutherland 1995; Allen et al. 2008), and thermal
andwethereforedonotconsiderthemfurtherinthissec-
conduction via the hot X-ray-emitting gas (e.g., Sparks
tion. The kinematic and photometric stellar PAs of the
etal.1989;Mathews&Brighenti2003;Sarzietal.2013).
two other slow rotators, NGC 0708 and NGC 2783, are
Sincewedonothaveaccesstostandardexcitationdia-
misaligned. In contrast, all four of the MASSIVE fast
grams that make use of the [NII]λλ6548,6584/Hα ratio
rotators that we consider here (NGC 1167, NGC 1453,
(e.g., Baldwin et al. 1981; Veilleux & Osterbrock 1987),
NGC2320,andNGC7426)havemeasurablestellarkine-
itisdifficulttodistinguishbetweenthefirstfoursources
matic PAs, and their kinematic and photometric stellar
ofionizationforoursample. Nevertheless,wecanspecu-
6 Thefit kinematic paPythonroutineispubliclyavailableat lateusingtwoalternativemethods: (1)usenon-standard
http://www.ascl.net/1601.016. excitation diagrams to find extraordinary cases where a
10 V. Pandya et al.
higher[OIII]/Hβ andlower[NII]/Hαvaluesasmetallic-
itydecreasesandtheionizationparameterincreases(e.g.,
see Kennicutt, R. 1998; Kewley & Dopita 2002; Kauff-
mann et al. 2003; Tremonti et al. 2004; Kewley et al.
2006; Moustakas & Kennicutt 2006). If we assume that
thehighstellarmetallicitythatcharacterizesourgalaxies
(Greene et al. 2015) also applies to the gas, then we can
moreeasilymapthe[OIII]/Hβ ratiotosourcesofioniza-
tion(seediscussioninSection6.2ofNylandetal.2016).
High values of [O III]/Hβ (∼> 3) are likely to point to-
wardsAGNactivity,whileverylowvalues(∼< 0.5)would
point towards star formation.
In oursample, NGC 1167has [OIII]/Hβ ≈6, which is
indicative of AGN activity (Groves et al. 2004a,b). The
remaining galaxies have intermediate [O III]/Hβ mea-
surements (or lower limits when Hβ is not detected).
These intermediate [OIII]/Hβ values are a defining fea-
tureoflow-ionizationnuclearemissionregions(LINERs;
e.g., see Heckman 1980; Kewley & Dopita 2002), which
may arise from excitation by evolved stars, shocks, or a
low-luminosity AGN (e.g., Leitherer et al. 1999; Dopita
et al. 2000; Ho 2008; Allen et al. 2008; Eracleous et al.
2010; Groves & Allen 2010). In several of our galaxies,
non-centralfiberstendtoalsoexhibittheseso-called“ex-
Figure 4. Kinematic misalignment angle between the stars and tendedLINER-like”excitationratios(i.e.,lowionization
spatially extended warm ionized gas reservoirs. MASSIVE slow
emissionregionsorLIERs;Belfioreetal.2016;Johansson
rotators are shown as green circles and MASSIVE fast rotators
as orange circles. ATLAS3D fast rotators (black circles) and slow et al. 2016). In principle, the [NI]/Hβ ratio can help us
rotators (magenta circles) are taken from Davis et al. (2011) for distinguish between excitation grids for fast shocks and
comparisonpurposes. TheerrorbarsforMASSIVEgalaxiesarea HIIregions(seedetaileddiscussionbySarzietal.2010),
quadraturesumofthegasandstellarPAs. AswithATLAS3Dfast but the [NI] line is detected in the centers of only three
rotators, MASSIVE fast rotators tend to have the warm ionized
galaxies. One of those galaxies is the aforementioned
gasandstarsaligned. Thedashedhorizontallineat30degisthe
cutoff angle adopted from Davis et al. (2011) for stellar and gas NGC 1167, which is already consistent with central ex-
alignment. citation by an AGN based on its high [O III]/Hβ ratio
alone. For the remaining two galaxies (NGC 0708 and
single source of ionization can be unambiguously identi- NGC 1453), thecombinationof [OIII]/Hβ and [NI]/Hβ
fied(e.g.,seeSarzietal.2006,2010;Annibalietal.2010; ratios is more consistent with excitation grids for fast
Yan & Blanton 2012), and (2) check whether the radial shocks than H II regions, which again broadly suggests
profile of the ionized gas flux follows the radial profile of LINER-like emission.
thestellarcontinuumfluxasprobedbytheemissionline Giventhelimitationsofthelineratioapproachforour
EW.AsweobtainmoreX-rayobservationsofMASSIVE dataset, we now turn to the spatial regime. If the source
galaxies using Chandra (PI: Goulding), we will be in a of ionization was centrally concentrated, then we would
positiontoalsoexaminetrendsbetweenX-rayproperties expect an upturn in emission line EW toward the center
and the presence of warm ionized gas. ofthegalaxyalongwithcentralexcitationratiossugges-
The first clue about the sources of ionization comes tive of, say, an AGN. In the simplest case where a uni-
from emission line ratios.7 Since we can only compute formly distributed old stellar population is responsible
robust line ratios for the central parts of our galaxies, for ionizing the gas, we would expect a roughly constant
we will use only the value in the central fiber for each radialEWprofile. InFigure5,weshowtheradialprofiles
galaxy. However, we should note that our 4 arcsec di- of logarithmic [O II] EW as well as the best-fit straight
ameter central fiber typically covers the inner ∼1.5 kpc lines for the 11 MASSIVE galaxies with extended warm
of our galaxies. In such a large physical aperture, even ionized gas. Note that the radial scale of the EW profile
in cases where an AGN plays some role in the photoion- is different for every galaxy because both R and R
e gas
ization of the gas, there are likely to be contributions aredifferent. Astraightlinemodelforlog (EW /[˚A])
10 OII
from other sources (e.g., Eracleous et al. 2010). There- as a function of galactocentric radius does not always
fore,asidefromrelativelyluminousSeyfertgalaxiessuch capture the complexity of the data, but at least half of
as NGC 1167 (see below), we cannot say much about the cases shown are consistent with a slope of zero.
optical AGN activity from these data. In standard ex- Inseveralcases, thedataunambiguouslyrevealanon-
citation diagrams (e.g., Baldwin et al. 1981; Veilleux & constanttrendintheequivalentwidthradialprofile. An
Osterbrock 1987; Kewley et al. 2006), the [O III]/Hβ example is NGC 1167 in which the [OII] EW rises non-
ratio is combined with another strong line, typically linearlyandrapidlytowardthecenterofthegalaxy,sug-
[N II]/Hα, to distinguish between H II regions, shocks, gestingthatanAGNdominatesasthesourceofionizing
andanAGN.Star-forminggalaxiesfollowapathtoward photons near the center of this galaxy (this upturn to-
ward the center is also seen in the radial EW profile of
7 Duetothelimitedwavelengthcoverageofourspectra,wecan Hα by Gomes et al. 2016b, in the CALIFA survey). A
onlymeasurethe[OIII]/Hβ and[NI]/Hβ ratios.