The Fastest Galaxy Evolution in an Unbiased Compact Group Sample with WISE

1.1 MB·

by  Gwang-Ho Lee

Draftversion January5,2017
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
THE FASTEST GALAXY EVOLUTION IN AN UNBIASED COMPACT GROUP SAMPLE WITH WISE
Gwang-Ho Lee1, Ho Seong Hwang2, Jubee Sohn3, and Myung Gyoon Lee1
1DepartmentofPhysicsandAstronomy,SeoulNational University,1Gwanak-ro,Gwanak-gu, Seoul08826, RepublicofKorea
2SchoolofPhysics,KoreaInstituteforAdvancedStudy, 85Hoegiro,Dongdaemun-gu, Seoul02455, RepublicofKoreaand
3SmithsonianAstrophysicalObservatory,60GardenStreet, Cambridge,MA02138, USA
Draft version January 5, 2017
ABSTRACT
7
1 We study the mid-infrared(MIR) propertiesofgalaxiesincompactgroupsandtheirenvironmental
0 dependence using the Wide-field Infrared Survey Explorer (WISE) data. We use a volume-limited
2 sample of 670 compact groups and their 2175 member galaxies with Mr < −19.77 and 0.01 < z <
0.0741, drawn from Sohn et al. (2016), which were identified using a friends-of-friends algorithm.
n
a Among the 2175 galaxies, 1541 galaxies are detected at WISE 12 µm with a signal-to-noise ratio
J greater than 3. Among the 1541 galaxies,433 AGN-host galaxies are identified by using both optical
and MIR classification scheme. Using the remaining 1108 non-AGN galaxies, we find that the MIR
4
[3.4]−[12] colors of compact group early-type galaxies are on average bluer than those of cluster
] early-type galaxies. When compact groups have both early- and late-type member galaxies,the MIR
A colorsofthelate-typemembersinthosecompactgroupsarebluerthantheMIRcolorsofclusterlate-
G type galaxies. As compact groups are located in denser regions, they tend to have larger early-type
galaxy fractions and bluer MIR color galaxies. These trends are also seen for neighboring galaxies
.
h around compact groups. However, compact group member galaxies always have larger early-type
p galaxy fractions and bluer MIR colors than their neighboring galaxies. Our findings suggest that the
- properties of compact group galaxies depend on both internal and external environments of compact
o
groups, and that galaxy evolution is faster in compact groups than in the central regions of clusters.
r
t Subject headings: galaxies: evolution – galaxies: groups: general – galaxies: interactions
s
a
[
1. INTRODUCTION the compact group should merge into a single elliptical

1 within1Gyr(seealsoRubin et al.1990). Kroupa(2015)
Compact groups of galaxies contain several galaxies
v suggestedthat the abundance of compact groups should
within just a few tens of kilo-parsec scales. This makes
2 decreaseastheredshiftdecreasesfromz ≃0.1toz =0in
compactgroupshaveextremelyhighgalaxynumberden-
0 sities (the median galaxy number density of log(ρ/[h−3 the ΛCDM universe. However, Sohn et al. (2015) found
1 that the abundance of compact groups changes little
1 Mpc3])=4.36, Sohn et al. 2016), higher than the galaxy
with redshifts at 0.01 < z < 0.22, using a spectroscop-
0 number densities of cluster and group environments
. (the median galaxy number density of log(ρ/[h−3 ically complete sample of 332 compact groups. Kroupa
1 (2015) also suggested an alternative scenario that com-
Mpc3])= 1.97 for clusters and groups in the A2199
0 pact groups do not merge, which predicts a constant
supercluster, Lee et al. 2015; Sohn et al. 2015). The
7 abundance of compact groups at z <0.1. However, this
velocity dispersions of compact groups are much smaller
1 alternativescenariois basedonthe assumptionof a uni-
(< 879 km s−1 with a median value of 194 km s−1 with
: verse without dark matter.
v a standard deviation of 156 km s−1, Sohn et al. 2016,
Diaferio et al. (1994) suggested that compact groups
Xi Table 7) than those of clusters (∼400−1300 km s−1 replenish themselves with new members from the sur-
ar wdeitvhiataionmeodfia2n08vaklume so−f1,77R8inkems est−a1l.an20d13a).staTndhaersde rtiomunesditnogtheenvciurorrnemntenetp,octhh.ereTbhye efixntdeinndginogf Stohheniretlifael-.
two characteristics make compact groups an ideal (2015), constant abundance of compact group at 0.01<
environment for frequent interactions and mergers be- z < 0.22, can be explained by this replenishment
tween galaxies (e.g., Rubin et al. 1990; Rodrigue et al. model. This replenishment model is supported by
1995; Amram et al. 2007; Coziol & Plauchu-Frayn several observational findings that showed that many
2007; Plauchu-Frayn& Coziol 2010; Gallagher et al. (> 50%) compact groups are embedded in or as-
2010; Konstantopoulos et al. 2012; Sohn et al. 2013; sociated with larger-scale structures (Rood & Struble
Vogt et al. 2015). 1994; Ramella et al. 1994; de Carvalho et al. 1994;
Numerical simulations suggested that galaxies in a Ribeiro et al.1998;Mendel et al.2011;Pompei & Iovino
compact group should merge into a single ellipti- 2012). However, D´ıaz-Gim´enez & Zandivarez (2015)
cal galaxy within a few Gyrs, causing the compact showed that only 27% of compact groups are embedded
group to disappear (Barnes 1985, 1989; Mamon 1987). in surrounding structures.
Gallagher et al. (2010) also reached a similar conclusion Unfortunately, many existing compact group cata-
through their study of star cluster age-dating and neu- logs suffer from some observational biases, which make
tral hydrogen content in the Hickson compact group 31; them difficult to use for testing compact group forma-
tion models and for studying the relation between com-
pact groups and their surrounding environments. For
[email protected],[email protected]
2 Lee, G.-H., et al.
example, McConnachie et al. (2009) constructed a large from interacting pairs, and from the center of the Coma
sample of compact groups, from the photometric data cluster (Walker et al. 2010, 2012, 2013). Cluver et al.
(mr <21)of SloanDigital Sky Survey(SDSS) Data Re- (2013) found that compact group galaxies in the gap
lease6(DR6,Adelman-McCarthy et al.2008),including mostly show enhanced warm H emission from the ob-
2
77,088 compact group candidates and their 313,508ten- servationswiththeSpitzer InfraredSpectrograph,which
tative member galaxies. Their sample is the largest one could be caused by shock heating (Appleton et al. 2006;
currently, but it is highly contaminated with interlop- Cluver et al. 2010). This result implies that shock heat-
ers (> 50%, Mendel et al. 2011; Sohn et al. 2013) be- ing may be responsible for rapid evolution of galaxies in
cause the sample is based only on photometric informa- compact groups.
tionusingHickson’sselectioncriteria(Hickson1982). To However, these results are mainly based on the cat-
reduce this contamination, Sohn et al. (2015) compiled alogs of compact groups selected using Hickson’s crite-
the redshifts from Fred Lawrence Whipple Observatory ria, which can introduce a sample bias. We therefore
(FLWO)/FAST observations and from the SDSS DR12 revisit the issues on the comparison of compact groups
(Alam et al. 2015) for the photometric sample of com- galaxies with cluster and field galaxies using the unbi-
pact group galaxies in McConnachie et al. (2009), and ased sample of compact groups from Sohn et al. (2016).
constructed a catalog of 332 compact groups. This cat- In this paper, we use the Wide-field Infrared Survey Ex-
alog is currently the largest one with complete spec- plorer (WISE, Wright et al. 2010) mid-infrared (MIR)
troscopic redshifts. However, this sample is affected data to study the environmental effects on compact
by an isolation criterion from Hickson’s selection crite- groupgalaxiesand the relationbetween compactgroups
ria. The isolation criterion requires that no galaxies be and their surrounding environments. The MIR data are
present in a radius of up to three times the group ra- usefulindicatorsofmeanstellarages(Piovan et al.2003;
dius. This criterion was adopted to avoid selecting very Ko et al. 2009), especially for compact group galaxies
denseregionslikeclustercores,butthiscouldintroducea that are mainly dominated by old stellar populations.
biasthatmisses nearbycompactgroupswithapparently Red-sequencegalaxieswithsmallamountsofyoung(<1
largesizesandembeddedsystemsindenseenvironments Gyr)andintermediate-age(1-10Gyr)stellarpopulations
(Barton et al. 1996). can be distinguished from those without the popula-
Recently, Sohn et al. (2016) published a new catalog tionsinMIRcolorspace(Ko et al.2013,2016;Lee et al.
of 1588 compact groups and their 5178 member galax- 2015). The sample we use in this paper contains 670
ies at 0.01 < z < 0.19 using SDSS DR12 spectroscopic compact groups with 2175 member galaxies drawn from
data supplemented by additional redshifts from the lit- the catalog of Sohn et al. (2016). When we investigate
erature and from FLWO/FAST observations. They ap- the MIR properties of galaxies in Sections 3 and 4, we
plied a friends-of-friends algorithm to identify the com- use 1541 member galaxies detected at 12 µm with a
pactgroupswithoutusingtheisolationcriterion. Asare- signal-to-noise ratio (S/N) greater than 3. This sam-
sult,theircatalogsuccessfullycontainsnearby(z <0.05) ple is larger than those of previous MIR studies. For
compactgroupsandembeddedsystemsindenseregions. example, the samples of Walker et al. (2012) and sev-
This is important in order to study the relation be- eralpreviousstudies(Johnson et al.2007;Bitsakis et al.
tween compact groups and their surrounding environ- 2010, 2011; Walker et al. 2010) have less than 50 com-
ments without any sample bias. pact groups, the sample of Walker et al. (2013) has 99
Previous observational studies suggested that galaxy compact groups and 348 member galaxies,and the sam-
evolution in compact groups is faster than in ple of Zucker et al. (2016) has 163 compact groups and
the field from the comparisons of Lick indices 567 member galaxies.
(Proctor et al. 2004; Mendes de Oliveira et al. 2005; Section 2 describes the compact group sample and
de la Rosa et al. 2007), the star formation quench- comparisonsamplesofcluster/fieldgalaxies. InSection3
ing timescales (Plauchu-Fraynet al. 2012), specific star wecomparethe propertiesofgalaxiesincompactgroups
formation rates (SFRs) distribution (Tzanavaris et al. with those in other environments, and investigate how
2010; Coenda et al. 2015; Lenki´c et al. 2016), and environmentaffectsthepropertiesofgalaxiesincompact
color distribution (Bitsakis et al. 2010, 2011, 2016; groups. InSection4wediscusstheenvironmentaleffects
Walker et al. 2010, 2012). Proctor et al. (2004) and on galaxy evolution in compact groups and the relation
Mendes de Oliveira et al. (2005) found that the stellar between compact groups and their surrounding environ-
ages and the early-type galaxy fractions of compact ments. InSection5wesummarizeourresultsandpresent
group galaxies are similar to those of cluster galaxies, conclusions. Throughout, we adopt flat ΛCDM cosmo-
which suggests that compact group galaxies evolve as logical parameters: H = 70 km s−1 Mpc−1, Ω = 0.7,
0 Λ
fast as cluster galaxies. and Ωm =0.3.
On the other hand, Johnson et al. (2007) and
2. DATA
Walker et al. (2010, 2012) showed that compact group
galaxies exhibit a bimodal distribution in the Spitzer Sohn et al. (2016) constructed a catalog of compact
IRAC (3.6 − 8.0 µm) color space with a statistically groups using the spectroscopic sample of SDSS DR12
evident gap, so-called “canyon”, between star-forming galaxies with mr < 17.77. The completeness of the
galaxies with MIR red colors and quiescent galaxies spectroscopic data in SDSS is low for bright galaxies at
with MIR blue colors. Recently, Zucker et al. (2016) mr < 14.5 because of saturation and cross-talk in the
newly identified the canyon galaxies in the WISE color- spectrograph,andforthegalaxiesinhigh-densityregions
color diagram. This gap may suggest the accelerated (e.g., galaxy clusters) because of the fiber collision (see
galaxy evolution in compact groups because the gap Figure 1 of Park & Hwang 2009). Therefore,Sohn et al.
is not seen in comparison galaxy samples from field, (2016) supplemented the catalog with redshifts from
Galaxy Evolution in Compact Groups 3
TABLE 1
SDSS-relatedPhysicalParameters ofCompactGroupGalaxies
HostGroupa GalaxyID R.A. Decl Redshift Mb u−rc Morphd
r
V1CG001 1237648705657307198 198.233322 1.007515 0.073839±0.000029 −19.82±2.21 2.28±0.10 2
V1CG001 1237648705657307347 198.229294 1.010990 0.072679±0.000020 −20.11±0.01 2.57±0.06 1
V1CG001 1237648705657307315 198.218872 1.019821 0.070435±0.000017 −21.16±0.01 2.67±0.04 1
V1CG002 1237661126155436166 139.939529 33.745014 0.017118±0.000007 −21.22±0.06 2.77±0.01 1
V1CG002 1237661126155436169 139.922531 33.738174 0.020334±0.000013 −20.72±0.05 2.61±0.01 2
V1CG002 1237661126155436164 139.945221 33.749741 0.023006±0.000003 −22.84±0.01 2.91±0.01 1
V1CG003 1237661136886431890 154.733276 37.285831 0.048809±0.000019 −20.41±0.01 1.91±0.03 2
V1CG003 1237661136886497282 154.743576 37.300205 0.047580±0.000007 −20.84±0.01 1.68±0.01 2
V1CG003 1237661136886497353 154.747711 37.308155 0.047489±0.000008 −20.08±0.01 1.99±0.02 2
Note. —Thefulltableisavailableintheonlinejournal. Aportionisshownhereforguidanceregardingitsformandcontent.
a GroupIDfromTable5ofSohnetal.(2016).
b Thekz=0.1-,andevolution-corrected r-bandPetrosianabsolutemagnitudes.
c TheSDSSextinction- andk-correctedmodelmagnitudes.
d Galaxymorphology. 1indicates anearly-typegalaxy,while2indicatesalate-typegalaxies.
the literature (Hill & Oegerle 1993, 1998; Wegner et al. only galaxies detected at 12 µm with S/N>3. Because
1996, 1999; Slinglend et al. 1998; Falco et al. 1999; see the ALLWISE magnitudes with S/N < 2 are just up-
also Hwang et al. 2010) and from FAST observations at per limits, we choose only the galaxies with S/N > 3
FLWO (Sohn et al. 2015). They applied the friends- to safely use the magnitudes. Among the 2175 galaxies,
of-friends algorithm with a projected linking length of 1541 galaxies (71%) are detected at 12 µm. If we adopt
∆D = 50 h−1 kpc and a radial linking length of the S/N cut of 5, the sample size is reduced from 1541
|∆V| = 1000 km s−1, and constructed a magnitude- to 863 galaxies by 44%. However, we confirm that our
limited (mr < 17.77) sample of 1588 compact groups conclusionsdonotchangemuchevenifweusethissmall
with each consisting of three or more member galaxies sample.
at 0.01 < z < 0.19. This new catalog contains 18 times To compare the physical properties of compact group
as many systems and reaches three times the depth of galaxies with those in other environments, we use
the catalog of Barton et al. (1996), which is also based two comparison samples of cluster and field galaxies.
on the friends-of-friends algorithm. Hwang et al. (2012a) constructed a sample of 129 re-
Sohn et al.(2016)alsoconstructedtwovolume-limited laxed Abell clusters at 0.02 < z < 0.14 using the
subsamples: theV1sampleofgalaxieswithMr <−19.77 spectroscopicsampleoftheSDSSDR7(Abazajian et al.
and 0.01 < z < 0.0741 and the V2 sample of galax- 2009). The cluster sample also includes the galaxies at
ies with Mr < −20.77 and 0.01 < z < 0.1154 (V2)1. R<10R200 (R200 is the virial radius of the cluster) and
These volume-limitedsamples contain670and297com- ∆V = |V −V | < 1000 km s−1 (V and V are ra-
gal cl gal cl
pact groups that are independently identified through dial velocity of a galaxy and systematic velocity of the
the friends-of-friends algorithm. Unlike systems in the cluster). From this sample, we selected 2433 galaxies at
magnitude-limited sample, the volume-limited samples R < R as the cluster galaxy sample, and 6312 galax-
200
have systems with a median stellar mass independent of ies at 5R < R < 10R as the field galaxy sample.
200 200
redshift. Therefore,the volume-limitedsamples allowus Park & Hwang (2009) showedthat the fraction of early-
to study the properties of compact groups without any type galaxiesdecreaseswith increasingclustercentricra-
sample bias that could be introduced in the magnitude- dius at R<3R . However,the early-type galaxy frac-
200
limited sample. Details for the catalogs and the com- tion is nearly constant at R > 4R (see their Figure
200
pact groupselection are described in Sections 2 and 3 of 4), suggesting that the galaxies at 5R < R < 10R
200 200
Sohn et al. (2016). can be considered to be field galaxies. The galaxies in
In this study, we use the V1 sample of 670 com- the two comparison samples also satisfy the criterion of
pact groups and their 2175 member galaxies with Mr < Mr < −19.77 and 0.01 < z < 0.0741 as do those in the
−19.77 and 0.01<z <0.0741. To study the MIR prop- V1 sample.
erties of these galaxies, we cross-correlate the galaxies The optical parameters of galaxies that we consider
withtheobjectsintheALLWISE sourcecatalog2usinga in this study are r-band Petrosian absolute magni-
′′
matching tolerance of 3 , correspondingto about half of tude (Mr), u − r color, and morphology. The Mr
theFWHMofthePSFat3.4µm. Amongthe2175galax- was computed using the Galactic reddening correction
ies,2067(95%)arematchedwithALLWISE sources. We (Schlegel et al. 1998), K-corrections (Blanton & Roweis
use the profile-fitting magnitudes of the sources at four 2007), shifted to z = 0.1, and evolution correction,
MIR bands (3.4 µm, 4.6 µm, 12 µm, and 22 µm). When E(z) = 1.6(z −0.1) (Tegmark et al. 2004). The u−r
we investigate the MIR properties of galaxies, we use color was computed using extinction- and K-corrected
(to z = 0.1) u- and r-band model magnitudes. Galaxy
morphologydataaremainlyfromtheKoreaInstitutefor
1 Sohnetal.(2016)usedMr <−19+5logh andMr <−20+ Advanced Study (KIAS) DR7 value-added galaxy cata-
5logh with h = 1 (i.e., H0 = 100 kms−1 Mpc−1). However, we
adopt h = 0.7 (i.e., H0 = 70 kms−1 Mpc−1), which results in log (Choi et al. 2010). Galaxies in this catalog are mor-
−19+5logh≃−19.77and−20+5logh≃−20.77. phologically classified into early (E/S0) and late types
2 http://wise2.ipac.caltech.edu/docs/release/allwise/ (S/Irr) based on an automated scheme with u−r color,
4 Lee, G.-H., et al.
11..00 (c) Compact group (a) CG galaxies Total (1541)
max galaxies max 1.0 Non-AGN (1108)
N Cluster galaxies N AGN fraction
/ 00..55 /
N Field galaxies N 0.5
00..00 0.0
(b) Non-AGN Cluster galaxies (R<R , 1137)
3.0 max 1.0 Cluster galaxies (R<0.250R0200, 617)
N CG galaxies (1108)
/
2.5 N 0.5
B)
A
r ( 2.0 0.0
u- -2 -1 0 1 2
[3.4]-[12] (AB)
1.5
Fig. 2.—(a)[3.4]−[12]colordistributionfortheall1541(filled)
andthe1108non-AGN(hatched)compactgroupgalaxies. Squares
1.0 (a) (b) represent the fraction of AGN as a function of [3.4]−[12]. The
error bars indicate the 1σ deviation from 1000-times bootstrap
-2 -1 0 1 2 0.0 0.5 1.0 resamplings. (b) Comparison of the color distributions between
[3.4]-[12] (AB) N/N the 1108 non-AGN compact group galaxies (hatched), the 1137
max
non-AGN cluster galaxies at R < R200 (filled), and the 617 non-
Fig. 1.—(a)u−rversus[3.4]−[12]diagramforthe1541compact AGNclustergalaxiesatR<0.5R200 (open).
groupgalaxiesdetectedat12µm. Contoursrepresentthenumber
density distribution. In panels (b) and (c), we display the u−r
Walker et al. (2013) and Zucker et al. (2016).
and [3.4]−[12] distribution for the compact group galaxies with
Optical red-sequence galaxies, located in a narrow
thefilledhistograms. Forcomparison, wealsoplottheMIR color
distributionofthe1461clustergalaxies(openhistogramwiththick u−r range of 2.5−3.2, have MIR colors ranging from
lines) and for the 5084 field galaxies (open histogram with thin [3.4]−[12]≃−2.4to[3.4]−[12]≃0. Lee et al.(2015)di-
lines).
videdopticalred-sequencegalaxiesintoMIRblue galax-
ies (i.e., [3.4]−[12] < −1.55) and MIR green galaxies
the g −i color gradient, and the i-band concentration
(i.e., −1.55 ≤ [3.4] − [12] < −0.3)4. The MIR blue
index (Park & Choi 2005). For galaxies not included in
galaxies are composed of stellar populations older than
the KIAS DR7 value-added galaxy catalog, Sohn et al.
10 Gyr, while the MIR green galaxies have small (∼5%)
(2016) classified them into early and late types through
mass fractions of relatively young (0.5−1 Gyr) or inter-
visualinspection. InTable1,welistthegroupID,galaxy
mediate (1−10 Gyr) stellar populations (Piovan et al.
ID, R.A., declination, redshift, Mr, u−r, and morphol-
2003; Ko et al. 2013; Lee et al. 2015; Ko et al. 2016).
ogy of the 2175 compact group galaxies.
Optical colors trace star formation on timescales up to
1 Gyr (Schawinski et al. 2014). Therefore, the MIR
3. RESULTS
greengalaxiesarenotdistinguishablefromtheMIRblue
3.1. Galaxy Color Distributions galaxies in optical colors because both lie in the optical
Figure 1 shows the u−r and [3.4]−[12] distributions red-sequence (Walker et al. 2012, 2013; Ko et al. 2013;
of the 1541 compact group galaxies detected at 12 µm. Lee et al. 2015; Zucker et al. 2016). Thus, we need to
The mean u−r error is 0.07,while the mean [3.4]−[12] use MIR colors instead of optical colors to investigate
erroris0.12. Inthisdiagram,compactgroupgalaxiesare whether galaxies in compact groups experienced differ-
distributed from the top-left corner to the bottom-right ent star formation histories from those in clusters, or
corner because galaxieswith bluer optical colorstend to not.
have redder MIR colors. In panel (c) of Figure 1, we compare the MIR color
Inpanels (b)and(c), weplotthe u−r and[3.4]−[12] distributions between the three different galaxy sam-
distributions of compact group galaxies. For compari- ples. The cluster galaxy sample shows a blue peak at
son, we also plot the distributions of cluster and field [3.4]− [12] ≃ −1.7 and a red peak [3.4] − [12] ≃ 1.3.
galaxies. As shown in panel (b), u−r histograms for The blue peak is higher than the red peak. The com-
the three galaxy samples peak at u−r ≃2.6. However, pact group galaxy sample shows a strong blue peak and
the fraction of galaxies with u−r >2.5 in the compact a red bump, but the field galaxy sample shows a dif-
group galaxy sample is 73.8% ± 1.1%, which is larger ferent color distribution with a blue bump and a strong
than that of the cluster galaxy sample (64.1%±1.2%) red peak. The mean [3.4]−[12] is −0.54±0.03 for the
andofthefieldgalaxysample(38.4%±0.7%). Themean compactgroupgalaxy sample, −0.29±0.04for the clus-
u−r is 2.65±0.02in the compactgroupgalaxysample, ter galaxy sample, and 0.46±0.02 for the field galaxy
which is also larger than the 2.50±0.01 of the cluster sample. These results suggest that compared to cluster
galaxy sample and the 2.22 ± 0.01 of the field galaxy galaxies,compactgroupgalaxiesaremoredominatedby
sample3. These suggest that compact group galaxies
aredominatedbyopticalred-sequencegalaxiesmuchlike 4 This criterion is determined from the decomposition of the
[3.4]−[12] color distribution with three Gaussian functions using
cluster galaxies, which is consistent with the findings of
theGaussianmixturemodeling(Muratov&Gnedin2010,seeFig-
ure 2 in Leeetal. 2015). We stress that this MIR green color
3 The errors in the fractions and in the mean values are the selectioncriterionisnotidenticaltothe“MIRgreenvalley”onein
standarddeviations derivedfrom1000-timesresamplings. Section3.2.
Galaxy Evolution in Compact Groups 5
MIR blue galaxies: the fraction of MIR blue galaxies is galaxysample,butsmallerthanthe 43.9%±1.5%inthe
32.6%±1.2% in the compact group galaxy sample and R < R cluster galaxy sample. The mean [3.4]−[12]
200
23.7%±1.1%intheclustergalaxysample. Thissuggests for the compact group galaxy sample (−0.57±0.04) is
thatthe meanstellar agesofcompactgroupgalaxiesare similar to the −0.52± 0.06 for the R < 0.5R clus-
200
on average older than those of cluster galaxies. ter galaxy sample, but smaller than the −0.30 ± 0.04
On the other hand, because compact groups favor for the R < R cluster galaxy sample. The early-
200
stronggalaxyinteractions,thepresenceofactivegalactic type galaxy fraction in the R < 0.5R cluster galaxy
200
nuclei (AGN) should be expected (Coziol et al. 1998a,b; sample (66.0%±1.9%) is similar to that in the compact
Mart´ınez et al. 2010; Tzanavaris et al. 2014). Because group galaxy sample (62.0%±1.5%), while the fraction
the contribution of AGN to the MIR fluxes of the host is 57.5%±1.5% in the R < R cluster galaxy sample.
200
galaxies is not negligible, we identify the AGN in our Theseresultssuggestthatcompactgroupsarecomposed
sample and remove them. First, we identify optically of galaxy populations similar to those in cluster central
selected AGN using the scheme given by Kewley et al. regions.
(2006) who classified star-forming galaxies from AGN However, we find that the fraction of MIR green (i.e.,
(Seyferts and LINERs) and composite galaxies in the −1.55 ≤ [3.4]−[12] < −0.3) galaxies is smaller in the
[NII]/Hα versus [OII]/Hβ diagram (see their Figure 1). compact group galaxy sample (28.5%± 1.3%) than in
We remove 281 AGN and 149 composite galaxies from the R < 0.5R cluster galaxy sample (34.5%±1.9%).
200
the sample of 1541 compact group galaxies. Second, This may suggest that galaxy transition occurs faster in
we identify MIR selected AGN using the classification compactgroupsthaninclusterenvironments. Hereafter,
method introduced by Mateos et al. (2012) who defined weuseonlytheR<0.5R clustergalaxysampleinthe
200
theAGNwedgeintheWISE [3.4]−[4.6]versus[4.6]−[12] comparisonwith the compact group galaxy sample.
color-color diagram (see their Figure 2). We find 8 Figure3(a)showsagaintheMIRcolordistributionsof
MIRselectedAGNinthe compactgroupgalaxysample. the 1108 non-AGN compact group galaxies and of the
Among the 8 MIR selected AGN, 5 are alsooptically se- 617 non-AGN cluster galaxies. In Figure 3 we divide
lected AGN. In total, 433 galaxies (28%) are classified these two galaxy samples into early-type and late-type
as AGN-host galaxies among the 1541 compact group galaxy samples. In Figure 3(b) we compare the MIR
galaxies. color distribution of the 687 compact group early-type
We compare the MIR color distribution of all 1541 galaxies with that of the 407 cluster early-type galaxies.
compactgroupgalaxieswiththatof1108non-AGNcom- The P and P values of < 0.001 reject the null hy-
KS AD
pact group galaxies in Figure 2(a). The difference be- pothesis for the two colordistributions ata >3σ signifi-
tween the two distributions is remarkable at −1.5 . cance. WefindthatthefractionofMIRgreengalaxiesis
[3.4]−[12] . 0.5 where the AGN fraction is larger than smaller in the compact group early-type galaxy sample
30%. This color range is similar to the WISE infrared (37.6%±1.8%)thanintheclusterearly-typegalaxysam-
transition zone, −1.71 . [3.4]−[12] . 0.02, defined by ple (47.4%±2.5%), and that the mean [3.4]−[12] value
Alatalo et al. (2014)5. They showed that Seyferts and is smaller in the compact group early-type galaxy sam-
LINERs are representatives of the zone, which is consis- ple (−1.40±0.03) than in the cluster early-type galaxy
tent with our result. sample (−1.28±0.04).
In Figure 2(b), we compare the MIR color distribu- InFigure3(c)wecomparethecolordistributionofthe
tion of non-AGN compact group galaxies with that of 421 compact grouplate-type galaxy sample with that of
non-AGN cluster galaxies. In this comparison, we use the 210 cluster late-type galaxy sample. The difference
1137 non-AGN cluster galaxies (R <R ) after remov- inthecolordistributionforthetwolate-typegalaxysam-
200
ing 324 AGN-host galaxies (22%) from the 1461 cluster ples is not highly significant (< 1.9σ), compared to the
galaxysample. To statisticallyexamine the difference in case for early-type galaxy samples. However, the mean
the color distributions of the two samples, we compute [3.4]−[12]valueissmallerinthecompactgrouplate-type
p-values from the Kolmogorov-Smirnov (KS) test (P ) galaxysample (0.80±0.06)than in the cluster late-type
KS
and the Anderson-Darling (AD) k-sample test (P ). galaxysample(0.95±0.07). TheMIRgreengalaxyfrac-
AD
The P and P indicate the probability of rejecting tionislargerinthecompactgrouplate-typegalaxysam-
KS AD
the nullhypothesisthatthe twosamplesaredrawnfrom ple (13.8%±1.7%) than in the cluster late-type galaxy
the same parent sample. The two tests on the two color sample (9.5%±1.9%).
distributions give the P and P of ≪0.01, rejecting Because galaxy properties are dependent on their
KS AD
the null hypothesis at a significance of >3σ level. stellar masses (or luminosities), we compare the MIR
In Figure 2(b), we use another comparison sample of colordistributionsofcompactgroupgalaxiesandcluster
617 cluster galaxies located at R < 0.5R200. In com- galaxiesby using a fixed Mr. We use Mr instead ofstel-
parison with the R < R cluster galaxy sample, the larmassesbecausethe formerisavailableforallgalaxies
200
R < 0.5R cluster galaxy sample displays the MIR in both compact group and cluster galaxy samples. We
200
color distribution more colsely to that of the compact findthatthenullhypothesisisrejectedinthecomparison
group galaxy sample (PKS = 0.076 and PAD = 0.035; for early-type galaxies with −20.77 ≥ Mr > −21.77 at
the null hypothesis is rejected at the <1.8σ level). The the&2.2σ level(panelh; PKS =0.03andPAD =0.001).
fractionsofMIR red(i.e., [3.4]−[12]≥−0.3)galaxiesin Forthetotalgalaxysamplewith−20.77≥Mr >−21.77
the compactgroup galaxysample is 35.4%±1.4%. This (panelg),PAD rejectsthenullhypothesisatthe2σlevel,
issimilartothe 34.7%±1.8%intheR<0.5R200 cluster but PKS does not. The other cases show lower signifi-
cance of rejection. This indicates that the difference in
5 Thiscolorrangeisconvertedfromthe0.8<[4.6]−[12]<2.4 the MIR color distribution between the compact group
inVegamagnitudeinAlataloetal.(2014). galaxies and the cluster galaxies is mainly attributed to
6 Lee, G.-H., et al.
AAllll Early Types Late Types
(a) Total (b) Total (c) Total
Nmax 11..05 (1108/617) PPKASD==00..007325 (687/407) PPKASD<<00..000011 (421/210) PPKASD==00..002676
/
N
0.5
0.0
(d) -19.77 ≥ M > -20.77 (e) -19.77 ≥ M > -20.77 (f) -19.77 ≥ M > -20.77
Nmax 11..05 (427/309) r PPKASD==00..112769 (196/165) r PPKASD==00..748758 (231/144) r PPKASD==00..135249
/
N
0.5
0.0
(g) -20.77 ≥ M > -21.77 (h) -20.77 ≥ M > -21.77 (i) -20.77 ≥ M > -21.77
Nmax 11..05 (407/226) r PPKASD==00..303435 (261/170) r PPKASD==00..003001 (146/ 56) r PPKASD==00..528290
/
N
0.5
0.0
(j) M ≤ -21.77 (k) M ≤ -21.77 (l) M ≤ -21.77
Nmax 11..05 (274/r 82) PPKASD==00..101895 (230/r 72) PPKASD==00..113606 ( 44/r 10) ppKASD==00..844424
/
N
0.5
0.0
-2 -1 0 1 2 -2 -1 0 1 2 -2 -1 0 1 2
[3.4]-[12] (AB) [3.4]-[12] (AB) [3.4]-[12] (AB)
Fig. 3.—Comparisonof[3.4]−[12]colordistributionsforcompactgroupgalaxies(filledhistograms)withthoseofclustergalaxies(open
histograms). The middle column is for early-type galaxies, and the right-hand column is for late-type galaxies. The left-hand column is
thesum. Thesecondrowisforfaintgalaxies with−19.77≥Mr >−20.77,thethirdrowisforgalaxieswith−20.77≥Mr >−21.77,and
the bottom row is for bright galaxies with Mr ≤−21.77. We list p-values from the KS and AD k-sample tests for the two distributions
(forcompactgroupgalaxiesandforclustergalaxies)ineachpanel.
TABLE 2
WISE-related PhysicalParameters ofCompactGroupGalaxies
HostGroup GalaxyID [3.4]−[12]a (S/N)b12 log(L12/L⊙)c AGNd M3e.4 log(Mstar/Msun)f
V1CG001 1237648705657307198 −9.99±0.00 -9.9 −9.99±0.00 0 −9.99±0.00 0.00+0.00
−0.00
V1CG001 1237648705657307347 0.00±0.00 1.4 8.60±0.00 0 −23.04±0.03 9.92+0.07
−0.05
V1CG001 1237648705657307315 −0.92±0.00 0.2 8.50±0.00 0 −23.71±0.04 10.61+0.12
−0.17
V1CG002 1237661126155436166 −0.87±0.05 26.5 8.40±0.02 1 −23.42±0.02 10.70+0.12
−0.15
V1CG002 1237661126155436169 0.69±0.03 44.9 8.95±0.01 1 −23.22±0.02 10.63+0.08
−0.10
V1CG002 1237661126155436164 −2.03±0.05 23.2 8.61±0.02 0 −25.10±0.02 11.59+0.07
−0.13
V1CG003 1237661136886431890 0.81±0.11 9.3 8.71±0.05 0 −22.51±0.03 10.13+0.09
−0.12
V1CG003 1237661136886497282 2.04±0.05 31.7 9.67±0.01 1 −23.67±0.04 10.20+0.09
−0.13
V1CG003 1237661136886497353 0.41±0.10 10.6 8.74±0.04 0 −23.00±0.03 10.14+0.08
−0.08
Note. —Thefulltableisavailableintheonlinejournal. Aportionisshownhereforguidanceregardingitsformandcontent.
a TheALLWISE profile-fittingABmagnitudes. −9.99indicatesnoALLWISE photometry.
bThesignal-to-ratioat12µm. −9.9indicatesnoALLWISE photometry. When(S/N)12 ≤3,weassignzeroerrorsto[3.4]−[12]
andlog(L12/L⊙).
c 12µmluminosities. −9.99indicates noALLWISE photometry.
d AGNclassification(0: non-AGN,1: opticalAGN,2: MIRAGN,and3: optical+MIRAGN)
e 3.4µmabsolutemagnitudes. −9.99±0.000indicatesnoALLWISE photometry.
f StellarmassescalculatedusingH0=70kms−1 Mpc−1. 0.00meansnomeasurement.
Galaxy Evolution in Compact Groups 7
Compact Groups Clusters (R < 0.5R ) Field
200
2 (a) Total (b) Total (c) Total
B)
A 1
2] ( 0 33.2%±1.4% 32.1%±1.9% 71.7%±0.7%
1
4]-[ -1 30.3%±1.4% 37.1%±2.0% 17.4%±0.6%
3.
[
-2 36.5%±1.4% 30.8%±1.9% 10.9%±0.5%
2 (d) Early Types (e) Early Types (f) Early Types
B)
A 1
2] ( 0 6.1%±0.9% 6.4%±1.2% 13.0%±1.1%
1
4]-[ -1 38.4%±1.9% 49.4%±2.5% 48.2%±1.6%
3.
[
-2 55.5%±1.9% 44.2%±2.5% 38.8%±1.6%
2 (g) Late Types (h) Late Types (i) Late Types
B)
A 1
2] ( 0 77.4%±2.1% 81.9%±2.6% 93.3%±0.5%
1
4]-[ -1 17.1%±1.8% 13.3%±2.3% 6.1%±0.5%
3.
[
-2 5.5%±0.9% 4.8%±1.1% 0.6%±0.2%
107 108 109 1010 107 108 109 1010 107 108 109 1010
L12µm/Lsun L12µm/Lsun L12µm/Lsun
Fig. 4.—MIRcolorvs. luminositydiagramsforcompactgroupgalaxies(left),cluster(R<0.5R200)galaxies(middle),andfieldgalaxies
(right). Orange and blue dots represent early- and late-type galaxies, respectively. We divide the galaxies into three classes following
the classification scheme of Leeetal. (2015): MIR star-formingsequence galaxies (above the long-dashed lines), MIR bluecloud galaxies
(below short-dashed lines), and MIR green valley galaxies (between the two lines). Middle and bottom rows are for early- and late-type
galaxiesindifferentenvironments. WelistthefractionsofdifferentMIRclassesineachpanel.
8 Lee, G.-H., et al.
the galaxies with −20.77 ≥ Mr > −21.77. We exam- fraction of MIR star-forming sequence galaxies in com-
ine galaxy properties using the luminosity-limited sub- pact groups is consistent with the case that the star for-
samples in the following analysis, but find no significant mation quenching for late-type galaxies occurs more ef-
difference from the results based on the total sample. fectively in compact groups than in clusters and in the
We thereforepresenttheresultsinthefollowinganalysis field. We note that the results in this section do not
based on the total sample for better statistics. changesignificantlyifweuse −1.55≤[3.4]−[12]<−0.3
asthe criteriontoselectMIRgreengalaxies. InTable2,
3.2. MIR Color-Luminosity Diagram welisttheWISE-relatedparametersandAGNclassesof
InFigure4weinvestigatethe [3.4]−[12]versus12µm compact group galaxies.
luminosity distribution of galaxies in compact groups,
clusters, and field. In the [3.4]−[12] versus 12 µm lu- 3.3. Environments of Compact Group Galaxies
minosity diagram,galaxies are divided into three classes To investigate the environmental effects on galaxy
following the classification scheme of Lee et al. (2015): properties in compact groups, we use three kinds of
MIR star-forming sequence galaxies (above the inclined environmental parameters. The first parameter is
dashed lines6), MIR blue cloud galaxies ([3.4]−[12] < the number of neighboring galaxies (N ) of compact
nei
−1.55), and MIR green valley galaxies located between groups. Sohn et al. (2016) calculated N around com-
nei
the two classes. In this section we use this classifica- pact groups within a comoving cylinder defined by pro-
tion scheme to calculate the fractions of different MIR jected group centric radius R = 1 Mpc and ∆V =
classes. In particular, MIR green valley galaxies are not |V −V |=1000kms−1,whereV andV arearadial
gal gr gal gr
necessarily “MIR green” in the sense of Section 3.1. velocityofagalaxyandameanvelocityofmembergalax-
In the three panels in the top row of Figure 4, we ies in a compact group. In the N count, Sohn et al.
nei
plot 1108 compact group galaxies, 617 cluster galaxies, (2016) used galaxies in the V2 sample (Mr < −20.77
and 3854 field galaxies, separately. All these galaxies and0.01<z <0.115). Inthisstudy, werecalculateN
nei
are not AGN-host galaxies. We find that the fraction of for 670 compact groups using galaxies in the V1 sample
MIR blue cloud galaxies is larger in the compact group (Mr < −19.77 and 0.01 < z < 0.0741). We exclude the
galaxysample (36.5%±1.4%)thanin the cluster galaxy compact group members in the N count. We do not
nei
sample (30.8%± 1.9%) and in the field galaxy sample assign N to 59 compact groups close to the lower and
nei
(10.9%±0.5%). Intheclustergalaxysampleandthefield upper redshift limits because their N can be underes-
nei
galaxy sample, the fractions of MIR blue cloud galaxies timated, resultinginthatwe assignN to 611compact
nei
aresmallerthanthefractionsofMIRgreenvalleygalax- groups.
ies. In contrast, in the compact group galaxy sample, The second environmental parameter is the fraction
thefractionofMIRbluecloudgalaxiesislargerthanthe of early-type member galaxies in compact groups (f ).
E
fraction of MIR green valley galaxies (30.3%±1.4%). Bitsakis et al. (2010, 2011, 2015) used this f to divide
E
When we consider only early-type galaxies, as shown compactgroupsintodynamicallyoldandyoungsystems
in the middle row of Figure 4, we obtain similar results. based on the assumption that dynamically old systems
The fraction of early-type galaxies is 62.0% (687/1108) are dominated by early-type galaxies that could form
inthecompactgroupgalaxysample,66.0%(407/617)in throughrepeatedinteractionsbetweenmember galaxies.
the clustergalaxysample,and26.5%(1023/3854)inthe Thus,f istheparameterreflectingtheinternalenviron-
E
field galaxy sample. Among the three early-type galaxy ment of compact groups.
samples, the compact group early-type galaxy sample The third environmental parameter is the rest-frame
showsthehighestMIRbluecloudgalaxyfraction. Inad- velocity dispersions of compact group member galaxies
dition,intheclusterearly-typegalaxysampleandinthe (σ ). We adopt the σ from Table 5 in Sohn et al.
CG CG
fieldearly-typegalaxysample,the fractionsofMIRblue (2016). Sohn et al. (2016) found that compact groups
cloud galaxies is smaller than those of MIR green val- with low σ (. 100 km s−1) show features of ongoing
CG
ley galaxies. However, in the compact group early-type interactions among member galaxies, but that compact
galaxy sample, the fraction of MIR blue cloud galaxies groups with σ & 300 km s−1 do not. Thus, σ is
CG CG
(55.5%±1.9%) is significantly larger than the fraction anotherparameterthatreflectstheinternalenvironment
of MIR green valley galaxies (38.4%±1.9%). These re- of compact groups.
sultssuggestnotonlythatearly-typegalaxiesincompact Wenotethatweuseallthemembergalaxiesregardless
groups are on averageolder than those in other environ- of 12 µm detection when we calculate f and σ . In
E CG
ments,butalsothatthetimescalesforearly-typegalaxies Table3,welisttheID,R.A.,declination,redshift,aswell
to stay in the MIR green valley are shorter in compact as three environmental parameters of the 670 compact
groups than in other environments. groups. Figure 5 shows the relations between the three
In the bottom row of Figure 4, we plot only late-type environmental parameters. We plot only 611 systems
galaxies. The fraction of MIR star-forming sequence withcompleteN . TheN histogrampeaksatN ≃
nei nei nei
galaxies decreases from the field (93.3%±0.5%) to clus- 3−6, andstretches outto morethan N =40. The f
nei E
ters (81.9%±2.6%) to compact groups (77.4%±2.1%). mainly has discontinuous values (e.g., 0, 1/3, 2/3, and
Thus, the fractions of MIR blue cloud galaxies and 1) because the majority (495/611, 81%) of our systems
of MIR green valley galaxies are smallest in the field have three member galaxies. The σ histogram peaks
CG
(0.6%±0.2% and 6.1%±0.5%) and largest in compact at ∼80 km s−1, and stretches out to more than 600 km
groups (5.5%±0.9% and 17.1%±1.8%). The smallest s−1. The median σ is ∼200 km s−1.
CG
Le6ee[t3.a4l].−(2[01125])=folrogd(eLta1i2l/s.Lsun)×0.62−5.51. SeeSection 5.1of 3.3.1. Environmental Dependence on Nnei
Galaxy Evolution in Compact Groups 9
TABLE 3
EnvironmentalParametersof CompactGroups
GroupIDa R.A.a Decl.a Redshifta Nmbem Nncei fEd σCG (kms−1)e
V1CG001 198.227173 1.012775 0.0723±0.0008 3 -99 0.67 482±122
V1CG002 139.935760 33.744308 0.0202±0.0012 3 11 0.67 866±228
V1CG003 154.741531 37.298065 0.0480±0.0003 3 5 0.00 210±37
V1CG004 158.222275 12.086633 0.0330±0.0004 3 11 0.67 242±69
V1CG005 127.709404 28.573534 0.0657±0.0001 3 1 0.33 26±4
V1CG006 149.408493 37.324886 0.0611±0.0002 3 4 0.33 100±26
V1CG007 240.816635 45.356598 0.0420±0.0002 3 3 0.33 89±15
V1CG008 239.193481 46.345490 0.0420±0.0009 3 3 1.00 483±83
V1CG009 204.321213 45.249256 0.0620±0.0003 3 7 0.67 189±47
V1CG010 161.923065 38.933136 0.0360±0.0004 4 14 0.50 272±75
Note. — Thefulltable isavailable inthe onlinejournal. A portionis shownherefor guidance
regardingitsformandcontent.
a GroupIDfromTable5ofSohnetal.(2016).
b Thenumberofmembergalaxiesineachcompactgroup.
c The number of neighboring galaxies (Mr < −19.77) in the comoving cylinder. For 59 compact
groupsnearthelowerandupperredshiftlimits,Nnei arenotassigned(Nnei=−99).
d Thefractionofearly-typegalaxiesamongmembergalaxiesofacompact group.
eTherest-frameline-of-sightvelocitydispersionofgalaxiesineachcompactgroupandthe1σerror
derivedfrom1000-timesbootstrapresamplings,fromTable5ofSohnetal.(2016).
To examine the effects of N on galaxy properties columns in Figure 6. The KS and AD k-sample tests for
nei
in compact groups, we investigate how the MIR color early-type galaxy samples in the four different N bins
nei
distribution of compact group galaxies varies with N . (e−h)rejectthenullhypothesisatthe<1.3σlevel. They
nei
Figure 6 shows the MIR colorhistogramsof the galaxies also reject the null hypothesis for late-type galaxy sam-
in different N bins. The left-hand column shows that ples in the four different N bins (i−l) at the < 1.8σ
nei nei
thefractionofMIRred(i.e.,[3.4]−[12]>−0.3)galaxies level. These imply that N does not affect the MIR
nei
dramatically decreases as N increases. The MIR red colorsofearly-andlate-type galaxiesin compactgroups
nei
galaxies are mainly late-type galaxies. The fraction of directly. Thus,theN -dependenceofMIRcolorsshown
nei
late-typegalaxiesgraduallydecreasesfrom56.2%±3.0% in the left column (a−d) results from the fact that com-
(panel a) to 15.9%±2.6% (panel d) as N increases. pact groups located in denser regions (larger N ) tend
nei nei
For comparison, we also plot the MIR color distribution tohavemoreearly-typemember galaxieswith MIRblue
of cluster galaxies as open histograms. We find that the colors.
fraction of late-type galaxies in the N ≥ 8 compact The middle column in Figure 6 shows that compact
nei
groups is smaller than the fraction of late-type galaxies group early-type galaxies have MIR colors bluer than
(34.0%±1.9%) in the cluster galaxy sample. cluster early-type galaxies regardless of N . The color
nei
We investigate the MIR color distributions for early- difference is significant (& 2.1σ) in the N < 20 com-
nei
andlate-typegalaxiesseparatelyinthe middle andright pactgroups,butitismarginal(∼1.5σ)intheN ≥20
nei
groups. We find no color difference between cluster late-
type galaxies and compact group late-type galaxies at
100
the N < 8 groups. However, late-type galaxies at the
N 50 (d) N ≥ne8i groupshaveMIRcolorsbluerthanclusterlate-
0 nei
type galaxies. The significance of the color difference is
60 (c) (e)
∼2−3σ.
40 3.3.2. Environmental Dependence on fE
nei
N Figure7showstheMIRcolordistributionsofcompact
20 group early-type galaxies in different f bins: f < 0.5,
E E
0.5 ≤ f < 1, and f = 1. We find no significant dif-
E E
0 ferences in the MIR colors between the three f bins.
E
1.0 The mean [3.4]−[12] value is −1.49±0.08 at fE < 0.5,
−1.44± 0.04 at 0.5 ≤ f < 0.1, and −1.43± 0.03 at
0.8 E
f = 1, respectively. These mean values agree well
E
0.6 within 1σ errors. The KS and the AD k-sample tests
E
f 0.4 do not reject the null hypothesis, either.
Bitsakis et al. (2011) showed that early-type galax-
0.2
ies in dynamically young (f < 0.25) compact groups
0.0 (a) (b) (f) have bluer NUV−r colors thaEn those in dynamically old
0 20 40 60 0 400 800 0 200 (fE > 0.25) groups. However, the number of early-type
N σ (km s-1) N galaxiesintheirdynamicallyyounggroupsis onlythree.
nei CG
Inoursample,therearenoearly-typegalaxiesbelonging
Fig.5.— Relation between the three environmental parameters
forcompactgroups: (a)fE vs. Nnei,(b)fE vs. σCG,and(c)Nnei to the fE < 0.25 groups, and only six early-type galax-
vs. σCG diagrams. (d−f)Histogramsforthethreeparameters. ies belonging to the fE < 0.3 groups. However, we find
10 Lee, G.-H., et al.
TToottaall Early Types Late Types
11..55 (a) Nnei < 3 (Ngal=265) (e) Nnei < 3 (Ngal=116) (i) Nnei < 3 (Ngal=149)
maxmax fL=56.2%±3.0% PKS<0.001 PKS=0.026 PKS=0.375
NN 11..00 P <0.001 P =0.004 P =0.198
AD AD AD
//
NN CG
00..55
Cluster
00..00
1.5 (b) 3 ≤ Nnei < 8 (Ngal=306) (f) 3 ≤ Nnei < 8 (Ngal=173) (j) 3 ≤ Nnei < 8 (Ngal=133)
max fL=43.5%±2.8% PKS=0.100 PKS=0.038 PKS=0.215
N 1.0 P =0.146 P =0.016 P =0.265
AD AD AD
/
N
0.5
0.0
1.5 (c) 8 ≤ Nnei < 20 (Ngal=261) (g) 8 ≤ Nnei < 20 (Ngal=190) (k) 8 ≤ Nnei < 20 (Ngal= 71)
max fL=27.2%±2.7% PKS<0.001 PKS<0.001 PKS=0.002
N 1.0 P <0.001 P <0.001 P =0.005
AD AD AD
/
N
0.5
0.0
1.5 (d) Nnei ≥ 20 (Ngal=195) (h) Nnei ≥ 20 (Ngal=164) (l) Nnei ≥ 20 (Ngal=31)
max fL=15.9%±2.6% PKS<0.001 PKS=0.144 PKS=0.054
N 1.0 P <0.001 P =0.044 P =0.026
AD AD AD
/
N
0.5
0.0
-2 -1 0 1 2 -2 -1 0 1 2 -2 -1 0 1 2
[3.4]-[12] (AB) [3.4]-[12] (AB) [3.4]-[12] (AB)
Fig. 6.—Dependence ofthe[3.4]−[12]colorofcompact groupgalaxies onthenumberof neighboringgalaxies(Nnei). Intheleft-hand
column, we list the fraction of late-type galaxies (fL). In the middle and right-hand columns, we plot early-type galaxies and late-type
galaxies,separately. Forcomparison,wealsoplotthecolordistributionsforclustergalaxies(openhistograms).
(a) f < 0.5 (N = 66) Early Types (a) f = 0 (N =110) Late Types
NNmaxmax 11..00 E gal NNmaxmax 11..00 ECG gCalluster
N/N/ 00..55 PPKASD==00..001045 N/N/ 00..55 PPKASD<<00..000011
00..00 00..00
(b) 0.5 ≤ f < 1 (N =233) CG (b) 0 < f < 0.5 (N =175)
max 1.0 E gal Cluster max 1.0 E gal
N/N 0.5 PPKS==00..001169 N/N 0.5 PPKASD==00..000011
AD
0.0 0.0
(c) f = 1 (N =388) (c) 0.5 ≤ f < 1 (N =136)
max 1.0 E gal max 1.0 E gal
N/N 0.5 PPKS=<00..000031 N/N 0.5 PPKASD==00..000013
AD
0.0 0.0
-2 -1 0 1 2 -2 -1 0 1 2
[3.4]-[12] (AB) [3.4]-[12] (AB)
Fig.7.— MIR colordistributions forearly-type galaxies inthe Fig.8.— MIR color distributions for late-type galaxies in the
(a)fE<0.5,(b)0.5≤fE<1,and(c)fE=1compactgroups. For (a) fE = 0, (b) 0 < fE < 0.5, and (c) 0.5 ≤ fE < 1 compact
comparison,wealsoplotthe (filled)histogramsfortheearly-type groups. For comparison, we also plot the (filled) histograms for
galaxiesinourclustersample. thelate-typegalaxiesinourclustersample.
thattheMIRcolordistributionforthesixgalaxiesisnot Figure8showstheMIRcolordistributionsofcompact
differentsignificantlyfromthatforearly-typegalaxiesin grouplate-typegalaxiesindifferentf bins: f =0,0<
E E
the f <0.5 groups. f <0.5,and0.5≤f <1. Forcomparison,wealsoplot
E E E
On the other hand, we find that in all three f bins, the color distributions for the cluster late-type galaxy
E
compactgroupearly-typegalaxieshaveMIRcolorsbluer sample. Late-typegalaxiesinthef =0compactgroups
E
than cluster early-type galaxies. The P and P are have MIR colors redder than cluster late-type galaxies.
KS AD
< 0.019, suggesting that the color difference between However, late-type galaxies in the f > 0 groups have
E
compact group and cluster early-type galaxy samples is MIR colors bluer than cluster late-type galaxies. In all
significant(>2.4σ). This is consistentwith the resultin three bins, P and P are ≤0.03, indicating that the
KS AD
Figure 6. color differences are significant (> 3σ). On the other