Table Of ContentMon.Not.R.Astron.Soc.000,1–20(2016) Printed24January2017 (MNLATEXstylefilev2.2)
OMEGA – OSIRIS Mapping of Emission-line Galaxies in
A901/2: II. – Environmental influence on integrated star
formation properties and AGN activity
7
1 Bruno Rodr´ıguez del Pino1,2(cid:63), Alfonso Arago´n-Salamanca2, Ana L. Chies-Santos2,3,4,
0
2 Tim Weinzirl2, Steven P. Bamford2, Meghan E. Gray2, Asmus Bo¨hm5,
n Christian Wolf6, David T. Maltby2
a
1Centro de Astrobiolog´ıa, INTA-CSIC, Torrejo´n de Ardoz, 28850, Madrid, Spain
J
2School of Physics and Astronomy, The University of Nottingham, University Park, Nottingham, NG7 2RD, UK
3 3Departamento de Astronomia, Instituto de F´ısica, Universidade Federal do Rio Grande do Sul, Porto Alegre, R.S, Brazil
2 4Instituto de Astronomia, Geof´ısica e Ciˆencias Atmosf´ericas, Universidade de Sa˜o Paulo, S˜ao Paulo, SP, Brazil
5Institute for Astro- and Particle Physics, University of Innsbruck, Technikerstr. 25/8, 6020 Innsbruck, Austria
] 6Research School of Astronomy and Astrophysics, Australian National University, Cotter Road, Weston Creek, ACT 2611, Australia
A
G
.
h
AcceptedforpublicationinMNRAS.
p
-
o
r ABSTRACT
t
s
a We present a study of the star formation and AGN activity for galaxies in the
[
Abell 901/2 multi-cluster system at z ∼ 0.167 as part of the OMEGA survey. Using
1 Tuneable Filter data obtained with the OSIRIS instrument at the GTC we produce
v spectra covering the Hα and [Nii] spectral lines for more than 400 galaxies. Using
3 optical emission-line diagnostics, we identify a significant number of galaxies hosting
8 AGN, which tend to have high masses and a broad range of morphologies. Moreover,
4 within the environmental densities probed by our study, we find no environmental
6 dependence on the fraction of galaxies hosting AGN. The analysis of the integrated
0
Hα emission shows that the specific star formation rates (SSFRs) of a majority of the
.
1 cluster galaxies are below the field values for a given stellar mass. We interpret this
0 result as evidence for a slow decrease in the star formation activity of star-forming
7 galaxies as they fall into higher-density regions, contrary to some previous studies
1 which suggested a rapid truncation of star formation. We find that most of the
:
v intermediate- and high-mass spiral galaxies go through a phase in which their star
i formation is suppressed but still retain significant star-formation activity. During this
X
phase,thesegalaxiestendtoretaintheirspiralmorphologywhiletheircoloursbecome
r redder.Thepresenceofthistypeofgalaxiesinhighdensityregionsindicatesthatthe
a
physical mechanism responsible for suppressing star-formation affects mainly the gas
component of the galaxies, suggesting that ram-pressure stripping or starvation are
potentially responsible.
Key words: Clusters – Galaxies: AGN, star-formation
1 INTRODUCTION and environment act as strong drivers of galaxy evolution
throughcosmictimeandassuchtheyareconnectedtomany
Despitesignificantadvancesinourunderstandingofgalaxy
galaxy properties.
evolution there is still ongoing debate on to what extent
the fate of galaxies is set by their intrinsic properties,
The morphologies, colours and star formation rates
such as their mass (‘nature’), or by the environment where
(SFRs)ofgalaxieschangeasafunctionofbothstellarmass
they reside (‘nurture’). It is clear now that both mass
and environmental density (Dressler 1980; Go´mez et al.
2003; Goto et al. 2003; Kauffmann et al. 2003b; Blanton
et al. 2005; Bamford et al. 2009; Vulcani et al. 2011).
(cid:63) E-mail:[email protected] Of particular interest is the correlation between SFR and
(cid:13)c 2016RAS
2 Rodr´ıguez del Pino et. al.
stellarmass,usuallyreferredtoasthe‘starformationmain influencing galaxy evolution requires detailed observations
sequence’ (Salim et al. 2007; Whitaker et al. 2012; Lee of a large sample of galaxies with a broad range of
et al. 2015). Stellar mass also imposes a limit above which properties residing in different environments. Aiming
internal processes lead to the cessation of star formation at providing new clues on our understanding of these
(Baldryetal.2006).Ingeneral,quenchingofstar-formation processes, the OMEGA survey (OSIRIS Mapping of
can be produced by the establishment of a virialized hot Emission-line Galaxies in A901/2) was designed to study
halo (e.g., Rees & Ostriker 1977), leading to longer cooling the star formation properties and AGN activity of the
times; star formation can be further prevented by feedback galaxies in the evolving multi-cluster system A901/2 at
from supernovae and active galactic nuclei (AGN; Booth z ∼ 0.167. Based on tuneable filter observations with
& Schaye 2009, Newton & Kay 2013). The formation and the OSIRIS instrument at the GTC, the survey maps
sustaining of AGN are subject to the availability of gas the Hα (λ6563˚A) and [Nii] (λ6548˚A, λ6584˚A) emission
acting as fuel. The amount of available gas to fuel the lines for a large sample of galaxies in an area of the sky
AGN might also be affected by the environment (Chung of ∼ 0.51 × 0.42 deg2, equivalent to ∼ 5.2 × 4.3 Mpc2
etal.2009;Jaff´eetal.2015),whichcanstripordepletethe at z ∼ 0.167. Our observations are complemented by a
gas content of the galaxies. In fact, the presence of AGN wide range of multi-wavelength data already available for
in galaxies has been observed to be somehow related to A901/2 (XMM-Newton, HST imaging, Spitzer, GALEX,
the environment: while low-luminosity AGN do not show COMBO-17), that has also been subject of study by the
particular preference for any kind of environment (Martini STAGES collaboration (Gray et al. 2009).
etal.2002;Milleretal.2003;Martinietal.2006),luminous In Chies-Santos et al. (2015, hereafter Paper I) we
AGN are preferentially foundin thefield (Kauffmannet al. presented a detailed description of the survey, the design of
2004) and groups (Popesso & Biviano 2006) rather than in the observations and the results obtained from the study
clusters. In a more recent work (de Souza et al. 2016), the of the two densest regions of A901/2. In this paper, we
lattertrendhasbeenfoundtoholdforellipticalgalaxiesbut extendouranalysistoamuchlargerareacoveringtheentire
notforspiralones.Thiswholepictureisfurthercomplicated structure, and explore star formation and AGN activity in
by the existent link between mass and environment, as a significantly larger sample of galaxies distributed over a
moremassivegalaxiestendtoresideinhigh-densityregions wider range of environments.
(Bolzonellaetal.2010),althoughPengetal.(2010)showed This Paper is structured as follows: in Section 2 we
that their effects can be separated up to z∼1. describe the data products available from previous work
As a result of these correlations there is a clear that are used here; in Section 3 we explain the methods
bimodality in the observed properties of galaxies. Galaxies employedforthereductionandanalysisofthedata;Section
living in low-density regions tend to have low masses, 4 contains the criteria chosen to select our galaxy samples;
blue colours, late-type morphologies and are actively in Section 5 we explore the properties of AGN galaxies; in
forming stars, populating what is called the blue cloud in Section6weanalyzetheintegratedstarformationproperties
the colour-magnitude diagram (Strateva et al. 2001). In oftheclustergalaxiesandinSection7wepresentasummary
contrast,high-densityregionstendtobepopulatedbymore and the conclusions of our work. Throughout this paper we
massive,redobjectswithearly-typemorphologiesandlowor adopt an H0 = 70 kms−1Mpc−1, Ωm = 0.3 and ΩΛ = 0.7
absent star formation, occupying the so-called red sequence cosmology.
in the colour-magnitude diagram.
The existence of this bimodality indicates that objects
falling into higher density regions must experience a
2 AVAILABLE DATA FOR THE A901/2
suppression of their star formation, accompanied by a
SYSTEM
change in their colours and, eventually, their morphologies.
The low number of objects inhabiting the green valley Our study benefits from the many data products already
(region between the blue cloud and the red sequence in the available from a number of previous studies of the A901/2
colour-magnitudediagram)impliesthatthetransitionfrom multi-cluster system. Photometric redshifts, stellar masses,
blue to red must be quite rapid. Such a fast transition is visualmorphologies,andspectral-energy-distribution(SED)
supportedbythesubstantialchangeinthefractionsofblue types of the galaxies were obtained within the STAGES
and red galaxies as a function of environment, whereas the collaboration (Gray et al. 2009) and are described in
colours and Hα equivalent widths of star-forming objects Paper I. We note here that the mass estimates of the
remain relatively invariant (Balogh et al. 2004b; Bamford mostmassivespirals(logM /M >11)sufferfromaperture
(cid:63) (cid:12)
et al. 2008). However, the timescales associated with the effects (Gray et al. 2009) and therefore these objects are
change in SFR and the morphological transformation seem not considered when galaxy mass is used in our analyses.
to be different, as suggested by the existence of galaxies Based on their SED type, galaxies are divided into normal
with late-type morphologies but lower SFRs and redder star-forming galaxies or blue cloud, passive ones or old
optical colours that would place them, apparently, on the red and dust-reddened star-forming galaxies or dusty reds.
red sequence (van den Bergh 1976; Bamford et al. 2009; The term dusty red was introduced by Wolf, Gray &
Wolf et al. 2009). In contrast, other works have also found Meisenheimer (2005) and it refers to galaxies that show
some evidence of a gradual decline of the star formation in red colours but still host star formation typically ∼ 4
star-forming objects (Poggianti et al. 2008; Vulcani et al. times lower than that in blue spirals at fixed stellar mass
2010; Paccagnella et al. 2016) towards denser regions in (Wolf et al. 2009). The term dusty is perhaps misleading
clusters, so the situation is still somewhat unclear. because these objects do not have more dust than normal
Exploring the internal and external processes star-forminggalaxies.However,becausetheirstarformation
(cid:13)c 2016RAS,MNRAS000,1–20
OMEGAII.–EnvironmentalinfluenceonintegratedstarformationpropertiesandAGNactivity 3
is suppressed, their dust content appears relatively high to generate a wavelength-dependent zero-point function for
comparedwiththeirstarformation.Perhaps,abettername each image, in the way described in Section 3.3 of Paper I.
would be red star-forming, but we keep the term dusty In Paper I the wavelength calibration for two of the
red throughout the paper for consistency with previous fields (numbers 21 and 22) was carried out using the
published works. solution proposed by Gonza´lez et al. (2014), which is
We also benefit from the availability of spectroscopic aimed at achieving accuracies of 1–2˚A. The precision of
redshifts for 356 bright cluster galaxies obtained with this crucial calibration could only be tested with our data
the AAT 2dF spectrograph and for 182 disk galaxies if an object is observed in pointings centred at different
obtained with the VIMOS instrument at the VLT positions. Unfortunately, fields 21 and 22 do not overlap
(Bo¨sch et al. 2013a,b). Moreover, we have low-resolution with each other (see Figure 1 in Paper I), thus preventing
prism spectroscopy from the PRIsm MUlti-object Survey us from testing the accuracy of the wavelength calibration
(PRIMUS,Coiletal.2011).WereferthereadertoWeinzirl at the time. However, the data from the 20 observed fields
et al. (in prep.) for a thorough explanation of the redshift contain a large number of galaxies that were observed in
surveys and a study of the properties of galaxies with twooreventhreepointings(seeFigure1inPaperI).When
spectroscopic redshifts. comparing the spectra of these galaxies, we find offsets
as high as 7˚A, much larger than expected. These offests
are primarily due to inaccuracies in the determination of
the central wavelength of each pointing. Although these
uncertainties in the wavelength calibration do not affect
3 OSIRIS DATA REDUCTION AND ANALYSIS significantly the estimation of the fluxes in Hα and [Nii],
In Paper I we described the methods adopted for the they compromise the calculation of the redshifts of the
reduction and analysis of the OSIRIS data from the two galaxiesandconsequentlythestudyoftheclusterdynamics,
densest regions. These methods were accurate enough to which are also being analysed (Weinzirl et al. in prep).
probe the potential of our observations, one of the primary To overcome these difficulties we adopt the improved
goalsofthatpaper.However,furtherworkwiththedatahas wavelength calibration developed by Weinzirl et al. (2015),
led to the development of significant improvements in the which uses the known wavelength of several sky emission
skysubtractionandwavelengthcalibrationthatincreasethe lines to obtain wavelength solutions accurate to ±1˚A for
qualityofourdataproductsandconsequentlytheaccuracy our data.
of our measurements. These modifications are based on the Thenextstepinthedataanalysisisthemeasurementof
methods described in Weinzirl et al. (2015). For the benefit the aperture photometry for all the galaxies in each image.
ofthereader,webrieflydescribeherethedatareductionand As mentioned above, one of the main goals of the OMEGA
analysis processes. More detailed descriptions can be found survey is to study star formation and AGN activity in the
in Paper I and Weinzirl et al. (2015). cluster galaxies. Since the radiation from an AGN host
The first steps of the data reduction (bias subtraction, comesmainlyfromthenuclearparts,weprobethepresence
overscan removal and flat-fielding) were performed using ofAGNbymeasuringthegalaxyfluxwithinaPSF-matched
the OSIRIS Offline Pipeline software (OOPs, Ederoclite aperture of radius 1.2 arcsec (RPSF). Integrated fluxes,
2012). This pipeline was also used in Paper I to subtract necessarytomeasuretotalSFRs,areobtainedusingalarger
the sky emission, which consists of a series of sky rings aperture that encompasses the full extent of the galaxies
around the optical centre that are a consequence of the (Rtot). Thus, for every galaxy we produce one integrated
wavelength radial dependence. However, further work with andonenuclearspectrum,whicharethesameforunresolved
the data showed that the sky subtraction performed by objects.Moredetailsontheaperturephotometryprocedure
OOPs introduces a significant number of negative pixels can be found in Section 3.4 in Paper I.
around the galaxies, a pattern that is more apparent when The last steps of the analysis are building the spectra
the images are stacked. This is due to the relatively small fromthephotometricmeasurementsandestimatingtheline
“dithering”offsetsusedbyOOPswhencreatingthemedian fluxes and positions, and the continuum levels. These steps
skyimage.Thesenegativevaluesforindividualpixelsareof are described in detail in Sections 3 and 4 of Paper I.
the order of (cid:46)5% of the total flux. Given the strong radial The line fluxes (FHα, F[Nii]), equivalent widths (WHα) and
dependenceoftheskybackground,largerXYoffsetscannot redshiftofthegalaxies(z)thatweusethroughoutthispaper
beused.Anewskysubtractionmethodthattakesadvantage correspondtothemedianvaluesoftheirprobabilitydensity
of the slow azimuthal variation of the sky background was distributions obtained from Markov chain Monte Carlo
developed by Weinzirl et al. (2015). Using this method, the (MCMC) spectral fitting techniques. These probability
sky subtraction was significantly improved, resulting in an density distributions are also used to estimate the errors
increase in the mean emission-line fluxes of ∼ 40%. See associated with these parameters. We refer the reader to
Weinzirl et al. (2015) for details. section 4.1 of Paper I for a thorough explanation of the
Aftersubtractingthesky,thebestastrometricsolutions spectral fitting process.
for the images are found using the IRAF routines CCMAP
and WCSCTRAN. Since the observations were taken
3.1 Correcting for underlying stellar absorption
during nights with different atmospheric seeing conditions,
the images of all observed fields are convolved with an The emission lines in our spectra originate from the gas
appropriateGaussianfunctiontomatchtheworstseeing(∼ ionised by either hot stars or AGN. However, the stellar
1.2arcsec,5pixels).Thefluxcalibrationisperformedusing populations inhabiting these galaxies also leave an imprint
a set of stars (between 15 and 20 per image) that are used onthegalaxies’spectrabyabsorbingpartofthelightintheir
(cid:13)c 2016RAS,MNRAS000,1–20
4 Rodr´ıguez del Pino et. al.
stellaratmospheres.Asaconsequenceofthisabsorption,the continuum. We consider that the Hα line is detected in
totalfluxinemissionmightbeunderestimated.Wetherefore emission if the following criteria are fulfilled:
need to account for the absorption from the underlying
• F (cid:62)3×10−17erg−1cm−2s−1;
stellar population in order to obtain an estimate of the Hα
• W (cid:62)3.0˚A;
true total Hα fluxes and SFRs. To do that, we use the Hα
• the Hα line is inside the observed wavelength range, with
PEGASE models (Fioc & Rocca-Volmerange 1997) that
its peak more than half the instrumental resolution (i.e.,
were employed by Wolf, Gray & Meisenheimer (2005) to
3.75˚A) from the edges of this range.
fit the COMBO-17 SEDs. These models give an estimate
of the expected absorption in Hα for a given age of the ThesecriteriaensurethedetectionofHα,andareused
SED templates. We use the Hα absorption of the model to build the main sample. We note here that the selection
corresponding to the age of the best-fitting SED template criteria have slightly changed from Paper I, following the
(available in the main STAGES catalogue of Gray et al. improved sky subtraction and the application of a more
2009) to correct the Hα fluxes and equivalent widths from robust and systematic visual inspection to avoid unreliable
the stellar absorption. or spurious detections. Since in some of the scientific
Inyoungobjects(∼0.5–1Gyr),theexpectedabsorption applications of these data we also need to analyse the [Nii]
is typically ∼ 6˚A, whereas in old objects ((cid:62) 3Gyr) the line (e.g., when studying AGN activity), we also build a
absorption becomes less than 2˚A. Because less massive sub-sample of the main sample by requiring that [Nii] is
galaxies tend to host younger stellar populations, the containedintheobservedwavelengthrange.Specifically,for
average expected Hα absorption correlates with galaxy thissub-samplewerequirethatthelastcriterionisalsomet
stellar mass, changing from ∼ 5.5˚A at 109M to ∼ 4˚A at by the [Nii]λ6584˚A line.
(cid:12)
1010M and ∼ 2˚A at 1011M . All our Hα estimates and
(cid:12) (cid:12)
thereforetheSFRsarecorrectedforstellarabsorptioninthis
4.2 Photometric redshifts criteria
way, and the corresponding uncertainties are propagated
when estimating the relevant errors. On average, this Apart from selecting galaxies with clear emission lines in
correctionreducestheSFRofthegalaxysampleby∼20%, their spectra, it is crucial to ensure that these lines are
being proportionally less important at high galaxy masses. indeed Hα and [Nii]. Although our program is designed
These corrections are only applied to the sources that to map these lines in galaxies residing in the multi-cluster
satisfied the selection criteria defined in Section 4. system, in particular with redshifts in the [0.150, 0.176]
interval, we also obtain spectra of objects in front and
behind the system for which any detected emission line
will not be Hα or [Nii]. Fortunately, the COMBO-17
4 SAMPLE DEFINITION
photometric redshifts are accurate enough to remove any
Our parent sample is drawn from the STAGES catalogue, possible interlopers from our sample (see Section 2). For
where we select all the objects with R-band magnitude instance, contamination from higher-redshift [Oiii]λ5007˚A
(m ) (cid:54) 23 that are detected in both COMBO-17 emissionatz∼0.53orHβatz∼0.57caneasilyberejected.
R
and the STAGES HST imaging (combo flag >1 and With this in mind, emission-line objects selected
stages flag >1 in Gray et al. 2009). The objects are applying the criteria described in Section 4.1 are only
also required to lie within the footprint of the 20 observed accepted in our sample if their photometric redshifts obey
OMEGAfields(PaperI),yielding∼7600objects.Aperture the condition 0.150−3∆z (cid:54) zphot (cid:54) 0.176+3∆z, where
photometryiscarriedoutonalltheOMEGAimagesatthe ∆z = 0.008 is the photometric redshift uncertainty at
position of these sources, and spectra are constructed and mR =20.5,themedianapparentmagnitudeoftheemission
analysed as described in Section 3. line galaxy sample (see eq. 5 of Gray et al. 2009). Based on
To construct a sample containing only emission-line this, only emission-line galaxies with photometric redshifts
galaxies from the A901/2 multi-cluster system we intheinterval[0.126,0.200]areacceptedinoursample.The
investigate the presence of emission lines in the galaxy availablespectroscopicdata(cf.Section2)allowsustocheck
spectra and assess whether they belong to this system the effectiveness of the photo-z selection. There are 149
basedontheCOMBO-17photometricredshifts.Thissection emission-linesourcesinourOMEGAsamplehavingspectra
describes in some detail the selection process we follow. from2dF/VIMOS.Ofthese,134/149aretruememberswith
z inthe[0.150,0.176]interval.Thez criterionselects
spec phot
136/149 sources. Only 4/136 of these are false positives
(meeting the z criterion but not the z one). This
4.1 Spectral fitting criteria phot spec
means the sample is over 97% pure. There are two false
The presence of emission lines in the spectra is evaluated negatives(meetingthez criterionbutnotthez one),
spec phot
through the spectral fitting process. Since the integrated so the sample is more than 98% complete. Of course, this
(R ) and nuclear (R ) spectra can vary significantly in test is only representative of the galaxies with magnitudes
tot PSF
galaxies hosting an AGN and those with non-homogeneous comparable to those in the spectroscopic redshift sample
distributions of star formation, in the following we define (brighter than m ∼ 22), while the OMEGA emission-line
R
different samples for each set of spectra (nuclear and galaxysamplereaches∼1magdeeper.Weexpectthepurity
integrated). and completeness of the full sample to be somewhat worse,
As explained above, we use MCMC techniques to fit but given the quality of the COMBO-17 redshifts, we are
each observed spectrum using a model containing three confident that this issue will not significantly affect our
emission lines (Hα and the two [Nii] lines) on top of a results.
(cid:13)c 2016RAS,MNRAS000,1–20
OMEGAII.–EnvironmentalinfluenceonintegratedstarformationpropertiesandAGNactivity 5
200 1000
100
100
N N
1
1
-16.5 -15.5 -14.5 0.0 0.5 1.0 1.5 2.0 2.5
log(FluxHα) log(WHα)
Figure 1. The distribution of the Hα √flux for the 439 ELGs in Figure 2. The distribution of WHα√for the 439 ELGs in our
oursample,wheretheerrorbarsare1/ N. sample, where the error bars are 1/ N. For equivalent widths
larger than ∼ 30˚A the distribution declines steeply following a
powerlowwithslope−2.3andintercept5.9(solidline).
The final step in the selection of the sample is a
visual inspection of the OSIRIS spectra to ensure that
all the selected galaxies exhibit bona-fide emission lines.
The Hα detection process is complex and it would be
As discussed in Sa´nchez-Portal et al. (2015), this step is
very difficult to model accurately the detection limits and
necessary to reject spurious detections which are due to
the completeness of our sample from first principles. For
instrumental and observational artefacts that cannot be
this reason, in Paper I we devised an empirical procedure
efficiently detected automatically. Three co-authors (AA-S,
to test our ability to detect Hα emission and quantify the
BRdP and TW) carried out this inspection, and galaxies
completeness of the OMEGA survey. This procedure takes
were accepted or rejected according to a majority vote.
advantageofthewealthofdataavailablefromtheSTAGES
Applying the selection criteria explained in this
data base, and is followed here for the whole sample. A
section together with those based on the spectral fitting
detailed description can be found in Paper I, but we repeat
(Section 4.1) our sample contains a total of 439 galaxies
the main points here for completeness.
with Hα detected in the total aperture spectra, of which
Two types of galaxies are expected to host star
321alsohave[Nii]intheobservedwavelengthrange.When
formationandthereforeexhibitHαemission.Thefirsttype
considering the R aperture spectra (used here to study
PSF arebluestar-forminggalaxiesinthebluecloud.Thesecond,
AGN activity) the sample contains 360 galaxies with Hα
perhapslessobvioustype,aretheso-calleddustyred spirals
detected and [Nii] in the observed wavelength range.
which,despitehavingopticalcoloursthatwouldapparently
place them on the optical red sequence, still have a certain
degreeofstarformation(Wolfetal.2009).Wewillcompare
4.3 Detection efficiency and limits
our Hα detections with the numbers of blue-cloud and red
The breadth and depth of our study is constrained by spiral galaxies (all of them star-forming candidates, and
our ability to reliably detect the Hα line. As described in thus expected Hα emitters) in the STAGES catalogue in
PaperI,theprobabilityofdetectingHαinemissiondepends order to evaluate how efficient the OMEGA survey is at
on its total flux (F ) and equivalent width (W ). The detectingstar-forminggalaxiesandthusquantifythesample
Hα Hα
distributions of F and W for the 439 Hα-detected completeness.
Hα Hα
galaxies are plotted in Figures 1 and 2, respectively. UsingtheSTAGESphoto-z’sandSEDclasses(Section
WeareabletodetectgalaxieswithHαfluxesaslowas 2) we find that in the parent sample (Section 4) there
3×10−17ergcm−2s−1,andashighas5×10−15ergcm−2s−1. are ∼ 7600 objects, of which 1244 are classified as cluster
WesamplehigherHαfluxesthaninPaperIasaconsequence members(Section4.2).Ofthese,316aredustyreds and630
of the improved sky subtraction that we apply here (see belongtotheblue cloud.WesecurelydetectHαin69dusty
Section 3). The W distribution is broadly compatible reds and in 330 blue cloud galaxies, yielding preliminary
Hα
withtheonefoundbyBaloghetal.(2004a).Forequivalent Hα-detection efficiencies of ∼ 22% and ∼ 52% for each
widths larger than ∼30˚A, the distribution declines steeply, galaxy class respectively. However, these numbers assume
followingapowerlawwithslope−2.3,andreachingashigh thattheSTAGESstar-forminggalaxysamplesarecomplete
as ∼ 300˚A. At lower equivalent widths, the distribution and uncontaminated, which they are not. Furthermore, we
flattens out, reaching ∼3˚A. Incompleteness probably kicks need to quantify how the detection efficiency depends on
in below ∼5˚A. When interpreting our data it is important galaxy mass/brightness.
to bear in mind that the sensitivity of our survey declines As described in Gray et al. (2009), the cluster
forlowerequivalentwidths.Wediscusstheeffectofourflux membershipwasdeterminedusingCOMBO-17photometric
and equivalent-width detection limits later in the paper. redshifts. Despite the relatively high accuracy of these
(cid:13)c 2016RAS,MNRAS000,1–20
6 Rodr´ıguez del Pino et. al.
1.0 300
200 Photo-z sample
Hα-detected Hα-detected
160 250
blue cloud 0.8
120 dusty red
N
200
80
s0.6
40 on
cti 150N
1.00 a
Fr0.4
0.8 100
0.6
fdet0.4 0.2 50
0.2
0.0 0
0.0 E S0 Sa Sb Sc Sd Irr
24 22 20 18 Morphology
m
R
Figure4.Fraction(filledcirclesandleftscale)andtotalnumber
Figure 3. Top panel: mR distribution of the Hα-detected
of galaxies (histograms and right scale) detected as Hα emitters
galaxies, blue cloud and dusty red galaxies determined to be
with respect to the parent galaxy population using the same
cluster members using the COMBO-17 photometric redshifts
stellarmassandredshiftselectioncriteria(“Photo-zsample”,see
(see Section 4.2). The dashed histograms correspond to blue
Section4.2).Asexpected,irregularandspiralgalaxies,specially
cloud and dusty red samples corrected for completeness and
Sb and Sc types, have much higher Hα detection rates than
contamination as discussed in the text. Bottom panel: The Hα
ellipticalsandS0s.
detection rate, expressed as the fraction of Hα-detected cluster
members(greenstars),dustyreds (yellowellipses)andbluecloud
galaxies(bluecircles),asafunctionofmR.Thesefractionstake
intoconsiderationthecompletenessandcontaminationcorrection
estimatedusingEquation1. averagecomparedwithblue cloud galaxiesofsimilarstellar
mass (Wolf et al. 2009). This suppression of star formation
impliesreductionsinbothHαfluxandequivalentwidth.For
redshifts, the cluster sample suffers from a certain degree bothtypesofgalaxies,thecompletenessdropsoffforfainter
on incompleteness and contamination, particularly at magnitudes, as expected. This drop is more pronounced for
faint magnitudes. Therefore, the “true” (corrected for dusty reds fainter than mR ∼ 19.5, where WHα declines
incompleteness and contamination) number of cluster dramatically. In blue cloud galaxies the detection rate drop
galaxies Nc can be estimated as with magnitude is slower because when we move towards
c17
fainter and less massive galaxies, the reduction in SFR (or
N (1−cont )
Nc = cmc17 frac , (1) Hα flux) is partially compensated by an increase in specific
c17 comp
frac SFR and thus W .
Hα
where N is the observed number of cluster members Note that the apparent drop in Hα detection efficiency
cmc17
based on their photometric redshifts (see Section for galaxies brighter than m ∼ 19 is somewhat uncertain
R
4.2). In this equation, comp and cont are the duetolow-numberstatisticsinthebrightestbins.However,
frac frac
magnitude-dependent completeness and contamination more massive galaxies tend to have lower specific star
fractions respectively (see Fig. 14 of Gray et al. 2009). A formation rates and thus low W , making them harder
Hα
similar calculation is done to estimate the ‘true’ number of to detect. The detection rate for all cluster members as a
blue cloud (BC)clustergalaxiesanddusty red (DR)cluster functionofm isrelativelyflatsinceatlowermasses(fainter
R
galaxies (cf. Paper I). magnitudes) a greater fraction of cluster galaxies are star
InthetoppanelofFigure3weshowthem distribution forming, whilethe fraction ofstar-forming galaxies declines
R
oftheHα-detectedgalaxies,bluecloud anddustyred cluster at higher masses.
galaxies. The dashed histograms correspond to the last two As a complement to the study of the global detection
samplescorrectedforcompletenessandcontamination.The efficiency, we also explore which morphological types are
fraction of cluster galaxies (total, blue cloud and dusty red) more frequently detected as Hα emitters in OMEGA.
that are detected in Hα are shown in the bottom panel In Figure 4 we show, for each morphological type, the
of the same figure (green stars, blue circles and orange fraction of galaxies detected in Hα with respect to the
ellipses, respectively). This fraction takes into account the parent galaxy population with the same stellar mass and
completenessandcontaminationcorrectionestimatedusing photometric redshift selection criteria (“Photo-z sample”,
Equation 1. Section 4.2). As expected, spiral and irregular galaxies are
Here we can see a clear difference in our efficiency the morphological types most frequently detected as Hα
of detecting star formation in blue cloud and dusty reds. emitters, whereas we only detect Hα in ∼10% of ellipticals
Whereas we effectively detect Hα for all blue cloud cluster and S0s. The drop in detection rate for Sd and Irregular
members brighter than m ∼20, the fraction of dusty reds galaxies is explained by the fact that these galaxies tend to
R
detected is always lower. This is not surprising since the be less massive and fainter, and, for a given specific SFR,
starformationofdustyreds issuppressedbyafactorof4on their SFRs and Hα luminosities are lower.
(cid:13)c 2016RAS,MNRAS000,1–20
OMEGAII.–EnvironmentalinfluenceonintegratedstarformationpropertiesandAGNactivity 7
4.4 Environment Definition
The data presented in Paper I focused only on the two 3.0
AGN (33)
densest regions of the multi-cluster system, effectively Star-forming
SF (239)
sampling one single environment. Here we cover a much 2.5 X-ray AGN
larger field and therefore a broader range of densities.
Since one of our main interests is to study the influence 2.0 sAGN/Seyfert
of environment on the properties of galaxies, we need to Å))
explicitly define how we measure environment. Previous (α
H1.5
works using the STAGES data, such as Wolf et al. (2009) W
g(
and Maltby et al. (2010), have defined their field and o
l
cluster samples using the photometric redshift distribution. 1.0
OMEGA is designed to only target galaxies that are in the wAGN/
multi-cluster system, avoiding field galaxies, but we need a 0.5 LINERs
measure to account for the different environments in which Retired
these galaxies reside within the system. 0.0
3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.5 1.0
It has been previously shown that, in the A901/2
log([NII]/Hα)
system, galaxy property trends with environment are more
sensitive to local galaxy density than to cluster-centric Figure 5. The WHAN diagram diagnostic plot (Cid Fernandes
distance(Laneetal.2007).Therefore,hereweevaluatethe et al. 2011). The vertical and horizontal lines separate regions
environment using the stellar mass surface density which, withdifferentsourcesofionization.Weonlyshowdetectionsusing
unlike galaxy number surface density, does not depend RPSF aperturemeasurementswherebothHαand[Nii]fallinthe
OMEGA probed wavelength range: a total of 360 ELGs. From
stronglyontheselectionoftheunderlyingsample(eitherby
these we find 33 secure ((cid:62) 3σ confidence) AGN hosts (Seyferts;
luminosityorbystellarmass;Wolfetal.2009).Followingthe
purple circles) and 237 star-forming galaxies (SF; green stars).
procedureofWolfetal.(2009)weuseasourdensitymeasure
Thebluesquaresrepresentgalaxiesthatcontainmixedsourcesof
the parameter ΣM (> 109M ), in units of M /Mpc2.
300kpc (cid:12) (cid:12) ionisation and/or the classification is more uncertain. The black
This parameter measures the stellar mass surface density crosses correspond to the X-ray AGN discussed in Section 5.1
inside a fixed aperture of radius r = 300kpc at the redshift whose OMEGA spectra are shown in Figure 6 (see text for
ofthemulti-clustersystem(z=0.167),includingonlythose details). In the bottom-left corner we show the 25, 50 and 75
galaxies with log M /M > 9. Following Maltby et al. percentilesoftheerrorsforreference.
(cid:63) (cid:12)
(2010), galaxies with ΣM>109M(cid:12) > 12.5 M Mpc−2 will be
300kpc (cid:12)
considered to belong to the cluster cores whereas the rest
of cluster galaxies will be associated with the infall regions
of the system. Stellar mass surface densities are available and [Nii], was proposed by Cid Fernandes et al. (2010).
for all the objects that are classified as cluster members by
The proposed diagnostic diagram, known as the WHAN
Grayetal.(2009),but,sinceourcriteriatoselectOMEGA
diagram, compares the equivalent width of Hα (W ) with
Hα
emission-linegalaxiesasclustermembersisslightlydifferent theratiobetweenthefluxesof[Nii]andHα([Nii]/Hα).This
(cf. Section 4.2), there are a few galaxies without stellar
diagramisdesignedtodetectlowerlevelsofionization.Itis
mass surface density estimates. In total, 381 out of the 439
worth mentioning that in nature there is not such a sharp
Hα-detectedgalaxiesand306outof360galaxieswhereHα
division between systems dominated by star formation and
and [Nii] are detected have stellar mass densities.
those dominated by AGN activity. In fact, the position of
the vertical boundary varies depending on the work used
as a reference (Kewley et al. 2001; Kauffmann et al. 2003a;
Stasin´ska et al. 2006). In our case, we employ the position
5 THE WHAN DIAGNOSTICS DIAGRAM
proposed by Stasin´ska et al. (2006), [Nii]/Hα =0.4, which
AND AGN ACTIVITY
despite being based on photoionization models, it is the
The identification of AGN activity using optical emission one that best follows the distribution of the SF population
lines is based on the fact that an accreting black in the diagram (Cid Fernandes et al. 2010). Furthermore,
hole produces a harder (more energetic) radiation field there is now a growing body of evidence indicating that
than star formation. As a consequence of their different in some cases, high [Nii]/Hα ratios can also be caused
energetic input, the atomic energy levels ionized and by shocks (Ho et al. 2014) or other sources of ionising
the subsequent intensities of the emission lines after radiation not directly caused by AGN activity (Singh et al.
recombination are different, making the ionization source 2013;Belfioreetal.2016),althoughthistendstooccurover
traceable. Historically, the most-widely used emission-line extended regions of the galaxies rather than the nucleus.
diagnosticisbasedontheBPTdiagrams(Baldwin,Phillips In these cases, the extended emission would be more easily
&Terlevich1981),ofwhichthemostcommononeusesfour detected using a large aperture for the flux estimation
strong emission lines [O iii], Hβ, [Nii] and Hα. However, (encompassingmoreregionswithhigh[NII]/Hα).However,
in order to obtain reliable measurements of these lines alargeraperturewouldreducethe[NII]/Hαratiointhecase
one needs access to different parts of the spectrum with of AGN-driven nuclear emission, making bona-fide optical
relatively high signal-to-noise. AGNmoredifficulttoidentify.Inpractice,giventhelimited
A more economical way of identifying the sources of spatialresolutionofourground-basedOMEGAimages,the
ionization in a galaxy using only two emission lines, Hα statistical properties of our AGN sample change very little
(cid:13)c 2016RAS,MNRAS000,1–20
8 Rodr´ıguez del Pino et. al.
when the analysis is carried out with the larger aperture 5.0
rather than with the smaller one (see below). 170 # 9020
Bearing these subtleties in mind, four different 1 ×4.5
populations can be identified in the diagram: star-forming 1) −
Å
galaxies, strong AGN (sAGN) or Seyferts, weak AGN 1 −4.0
(wAGN) or LINERs, and the so-called “retired” galaxy 2 s−
population(CidFernandesetal.2011,Yan&Blanton2012, m3.5
c
Stasin´ska et al. 2015). LINER stands for ‘low ionisation gs
er
nuclearemissionregions’(asintroducedbyHeckman1980). x (3.0
u
Therefore, LINERs are galaxies with ‘LINER-like spectra’. Fl
The weak emission lines in these objects can be seen in the 2.5
7620 7650 7680 7710
nucleus, due to the presence of an AGN, but they may also Wavelength (Å)
appearinthegalacticdisk(e.g.,Belfioreetal.2016),where
they can be produced by fast shocks (Dopita & Sutherland
1995), starburst-driven winds (Armus, Heckman & Miley 17108.0 # 19305
×
1F9in9a0l)lyo,rthdei“ffruesteireiodn”izgeadlaxpileassamraeo(bCjoeclltisnsth&ataRraenndol2o0n0g1e)r. 1Å) −7.2
formingstarsandtheirgasisbelievedtobeionizedbyhot, 1 −
low-mass stars in the post-AGB phase. They do not have a 2 s−
m6.4
detectablecontributionfromanAGNandtheirHαemission gs c
is often extended (Singh et al. 2013; Belfiore et al. 2015, er
2016). ux (5.6
Fl
The WHAN diagram for the 360 sources whose central
spectra cover both Hα and [Nii] is shown in Figure 5. Here 7620 7650 7680 7710
Wavelength (Å)
we identify which sources are more likely to be hosting
an AGN or to be dominated by star formation using the 14
probability density distributions of WHα and [Nii]/Hα 170 # 11827
1
obtained from the MCMC fitting of the spectra (cf. Paper ×
I). The sources with probabilities higher than 99.7% ((cid:39)3σ 1) −12
Å
confidence) of being AGN or star-forming are plotted as 1 −
purple circles and green stars, respectively. Based on this 2 s−10
procedure we find 33 objects with high likelihood of being m
c
AGN hosts and 239 likely dominated by star formation. gs
er 8
For the remaining 90 galaxies (plotted as blue squares) the x (
u
classificationismoreuncertain(lessthan99.7%probability) Fl
or intermediate. 6
7620 7650 7680 7710
Using the total flux measurements (within Rtot) Wavelength (Å)
instead of the central ones in the WHAN diagrams yields
marginallyhigherAGNfractions(by∼10%,or3additional
sources),implyingthatonlyasmallfractionofthegalaxies 17106.4 # 15698
×
classified as AGN might be associated with extended 1) −
shocks. Unfortunately, the limited spatial resolution of our Å
ground-baseddatapreventsusfromcarryingoutadetailed 1 −5.6
analysis of the emission outside the region affected by the 2 s−
m
PSF. gs c4.8
er
x (
u4.0
5.1 OMEGA and X-Ray AGN cross-matching Fl
7590 7620 7650 7680
Thepresenceofnuclearactivityingalaxiescanbeobserved
Wavelength (Å)
at different wavelengths. A previous study of AGN activity
in the A901/2 multi-cluster system was performed by
Figure 6. OMEGA spectra of four of the X-ray-detected AGN
Gilmouretal.(2007)usingXMM-NewtonX-raydata.Since in the A901/2 system (shown as crosses in Figure 5). In yellow
this type of data are able to detect luminous X-ray AGN andpurpleweshowthefittotheHαand[Nii]lines,respectively,
but do not detect lower levels of nuclear activity and/or and in green the fit to the whole spectrum. Objects 9020 and
Compton-thick objects, it can be used as a complement 12953 are also identified as optical AGN with OMEGA data.
to the optical AGN detected in OMEGA in order to Note the strong [Nii] lines. The galaxy 11827 has a very low
have a wider coverage of AGN luminosities. It is therefore formalprobabilityofbeinganopticalAGN,withitsweaker[Nii]
lines, although it is located just to the left of the boundary
interestingtocomparethesetwomethodsofdetectingAGN
in the WHAN diagram (see Figure 5). Object 15698 doesn’t
activitytoexploretheirefficiencyandwhethersomesources
meet the selection criteria because [Nii]λ6583 is just outside
are detected in both wavelength ranges. the observed wavelength range. The [Nii]/Hα ratio is therefore
In their X-ray studies, Gilmour et al. (2007) detected notwellconstrainedforthisobject,although[Nii]λ6548appears
12 galaxies as XMM point sources in the A901/902 field reasonablystrong(circledcrossinFigure5).
(cid:13)c 2016RAS,MNRAS000,1–20
OMEGAII.–EnvironmentalinfluenceonintegratedstarformationpropertiesandAGNactivity 9
(seetheirTable12).Theseobjectsarepresentinourparent the OMEGA AGN population). The correlation that exists
sample and we are able to produce OMEGA spectra for betweenX-rayluminosityandopticalemission(albeit,with
all of them. From these 12 objects, the two most luminous largescatter;see,e.g.,Lussoetal.2010;Stern&Laor2012),
sources in the X-ray (12953 and 41435) have photometric suggests that the vast majority of our optical AGN are
redshifts(1.4and0.33,respectively)thatpreventthemfrom probably too faint in X-rays to be detected in the available
being included in our sample following the selection cuts X-ray data.
Section (4.2), although the photometric redshifts of such
strong AGN could be very uncertain.
Of the remaining 10, only 3 have central OMEGA
spectra which fulfil all our selection criteria (including the
detection of Hα and [Nii] at the system redshift). These
5.2 AGN properties
three X-ray AGN have IDs 9020, 11827 and 19305, with
formalprobabilitiesofbeinganopticalAGNof100%,1.1% WenowexplorethepropertiesoftheOMEGAAGNandSF
and 100%, respectively, according to the WHAN diagnostic galaxiesandcomparethemtotheoverallclusterpopulation.
diagram (see black crosses in Figure 5). Two of them (9020 Wecarryoutthisanalysisusingtwosamplesinparallel:one
and 19305) are therefore robust optical AGN. In the case including only the robust detections (probability of being
of 11827, the [Nii]/Hα ratio (which has very small formal an AGN or a SF galaxy (cid:62)99.7% or significance (cid:62)3σ from
errors given the quality of the data) places this galaxy very the WHAN diagram) and another one including also the
closetotheverticalboundarybetweenstar-forminggalaxies objectsthatappearineithertheSeyfert orthestar-forming
and AGN (but just to the left of the line; see Figure 5). regionsoftheWHANdiagram,regardlessofthesignificance
Given the uncertainties in the exact location of this line of their classification. This increases the sample sizes by ∼
(cf. Section 5), it is reasonable to conclude that it is quite 25%inawaythatisstatisticallyrobustwhenanalysingthe
possiblethatthisobjectalsoshowssignsofnuclearactivity propertiesofthesetwopopulations.Inwhatfollowswewill
in the optical. The nuclear spectra of these three galaxies see that the conclusions obtained using both samples are
are shown in Figure 6. The prominent [Nii] lines are clear entirely compatible.
indicators of AGN activity in 9020 and 19305, while they ThetoptwopanelsofFigure7presentthestar-forming
are weaker in 11827. and AGN galaxy fractions of the total cluster Hα-emitting
A fourth source (15698) is detected in Hα and meets galaxy population as a function of stellar mass and
the redshift criteria, but it doesn’t make the selection morphology. Only objects where both Hα and [Nii] have
because [Nii]λ6583 is just outside the observed wavelength been observed in the central aperture (R ) spectra are
PSF
range and therefore the [Nii]/Hα ratio cannot be well used when computing these fractions since both lines are
constrained,although[Nii]λ6548appearsreasonablystrong. needed to classify the galaxies. Optical AGN hosts span a
Its spectrum is also shown in Figure 6, and the location of wide range in masses although they are more frequent at
this galaxy on the WHAN diagram (Figure 5) is indicated highermasses.Moreover,theseAGNappearingalaxieswith
with a circled black cross. The available data suggest that allmorphologicaltypes,withasmalltendencytofavourdisk
thisgalaxycouldalsobeanopticalAGN,albeitwithhigher galaxies with prominent bulges (S0–Sb).
uncertainty. The fraction of Hα-detected elliptical galaxies that are
In summary, at least two of the X-ray AGN (9020 and classified as star-forming rather than AGN seems to be
19305) are classified as bona-fide optical AGN, with the relatively high, but the uncertainty on this fraction is large
possibilityoftwomore,11827and15698,alsobeingoptical sinceonly∼10%ofalltheellipticalgalaxiesaredetectedin
AGN. This result implies that, out of the 12 X-ray AGN Hα (cf. Figure 4). Taken at face value, it would imply that
detectedbyGilmouretal.(2007),atleasttwo,andperhaps when Hα emission is present in elliptical galaxies it is more
four galaxies (17–33%) also show optical signatures of the often associated with residual star formation than with an
presence of AGN. If we only consider the 10 X-ray AGN optical AGN. However, it is also possible that because the
whose spectroscopic or photometric redshifts place them in Hα emission detected in these galaxies tends to be quite
the cluster structure, and thus within the detection limits weak, some of them may be misclassified LINERs.
of OMEGA, the fraction of X-ray AGN with optical AGN Finally, we explore the SED types of the AGN
signaturescouldbeashighas20–40%.Martinietal.(2006) population by plotting the WHAN diagram colour-coded
found that only 4 of at least 35 X-ray objects (∼ 11%) bySEDtypeinFigure8.Whereas,asexpected,mostofthe
would be classified as optical AGN from their emission-line star-forming population belongs to the blue cloud (86%),
signatures,whichisasmallerproportionthanwhatwefind. there is a significant fraction of AGN that are classified as
The lack of optical signatures in X-ray AGN might be dusty reds (42%). This class of galaxies have reduced SFR
aconsequenceoftheseobjectsbeingheavilyobscuredAGN andthereforetendtohavelowerW .WiththeSF-related
Hα
and therefore they would not be expected to show optical Hα emission suppressed, any other source of ionization,
signatures of nuclear activity. On the other hand, the fact includinganAGN,wouldbeeasiertodetectthaningalaxies
that in the optical we detect about three times more AGN with more dominant star formation. This could explain
galaxiesthaninX-raysclearlyshowsthatOMEGAisableto theirrelativelyhighpresenceamongtheobjectsclassifiedas
detect a large number of optical AGN with relatively weak AGN in the WHAN diagram. There are also a few galaxies
X-rayemission.ThefourX-rayAGNweidentifyassecureor classifiedasold red amongthelow-W population.Thisis
Hα
possible optical AGN are among the brightest Hα emitters notsurprisingbecauseourdeepHαimagingisabletodetect
in the OMEGA-identified optical AGN sample (their Hα very low levels of SF or AGN activity which are too weak
luminosity is, on average, 2.5 times brighter than that of to affect significantly the colours or SEDs of the galaxies.
(cid:13)c 2016RAS,MNRAS000,1–20
10 Rodr´ıguez del Pino et. al.
1.0
AGN
0.8
SF
s
n0.6
o
cti
a0.4
Fr
0.2
0.0
7 8 9 10 11 12
log (M [M ])
fl
1.0
0.8
s
n0.6
o
cti
a0.4
Fr
0.2
Figure 8. WHAN diagram colour-coded by the SED type of
0.0 the galaxies. In the bottom corner we show the 25, 50 and 75
E S0 Sa Sb Sc Sd Irr
percentiles of the errors for reference. Filled stars and circles
Morphology
correspond to galaxies robustly classified as SF and AGN,
1.0
respectively,whileemptysquaresrepresentgalaxiesthatcontain
0.8 mixed sources of ionization and/or the classification is more
uncertain.
s
n0.6
o
cti
a0.4
Fr
diagram,thislineratiocanbeusedtoseparatestar-forming
0.2
galaxies and AGN for W > 6˚A. Therefore, rather than
Hα
0.0 useabinarySF/AGNdivisionwecanusethislineratioasa
11.0 11.5 12.0 12.5
continuousmeasureofAGNactivity.Thishastheadvantage
log(Σ/Mpc2)
of being independent on the exact location of the SF/AGN
boundary in the diagram.
Figure 7. Fractions of the cluster Hα-emitting galaxies that The distributions of [Nii]/Hα in the infall and core
are star-forming (green) or AGN (purple) as a function of regions for galaxies with W > 6˚A are shown in Figure
the galaxies’ stellar mass (top), morphology (middle), and Hα
9. The [Nii]/Hα distributions of the infall and core regions
environmentalstellarmassdensity(bottom).Onlyobjectswhere
both Hα and [Nii] have been observed in the central aperture are quite similar. A K–S test gives a probability ∼ 67%
of them being drawn from the same population, which is
(RPSF) spectra are used when computing these fractions. The
filled symbols and solid lines correspond to galaxies with robust consistentwiththelackofenvironmentaldependenceofthe
classifications ((cid:62) 3σ; see text for details). The open symbols AGN fraction we found before.
and dashed lines represent the AGN and star-forming fractions OurresultsareincontrastwiththefindingsofGilmour
including all objects in the corresponding region of the WHAN etal.(2007).TheyfoundevidencesuggestingthattheX-ray
diagram. We only show error bars for the fractions of robust AGNinA901/2tendtoavoidthehighestandlowestdensity
classifications for clarity. The stellar mass and environmental regions. Given that the optical and X-ray AGN samples
densitybinsaremadetocontainthesamenumberofobjects.
are very different, it is not surprising that they occupy
regionsofdifferentenvironmentaldensities.Thisabsenceof
environmentaldependenceagreeswithwhathasbeenfound
in previous studies (Martini, Kelson, Mulchaey, & Trager
5.3 Environmental influence on AGN activity
2002, Miller, Nichol, Go´mez, Hopkins, & Bernardi 2003,
Apart from studying the properties of the objects hosting Martini, Kelson, Kim, Mulchaey, & Athey 2006), although
an AGN, our observations allow us to explore whether the lack of true field galaxies in our sample prevents a
the presence of AGN is related to the local environment directcomparisonwithotherworkssuchasKauffmannetal.
within the A901/2 system. The bottom panel of Figure 7 (2004) that found the fraction of high-luminosity optical
shows the fraction of Hα-emitting galaxies that are AGN AGNinthefieldtobearoundtwicelargerthaninthecluster
or star-forming as a function of environmental stellar mass environment.
surface density. Within the A901/2 cluster environment we This diversity in the results found on the dependence
findthatthefractionofAGNremainsroughlyconstantatall of AGN activity on environment might be to some extent
densities,withvaluesalwaysbelow15%oftheHα-emitting due to the use of different samples and indicators to
population. probe AGN activity, together with inconsistent definitions
The dependence of AGN activity on environment can ofenvironment.InthecaseofA901/2itwouldbeextremely
be further evaluated by comparing the [Nii]/Hα line ratio useful to have a comparable sample of Hα-selected galaxies
distributionsofthegalaxypopulationsresidingintheinfall in the field in order to test whether there is any difference
and core regions, as defined in Section 4.4. In the WHAN in the AGN content with respect to the cluster system. It
(cid:13)c 2016RAS,MNRAS000,1–20
