Environmental Effects on the Star Formation Activity at z~0.9 in the COSMOS Field
M. Kajisawa, Y. Shioya, Y. Aida, Y. Ideue, Y. Taniguchi, T. Nagao, T. Murayama, K. Matsubayashi, L. Riguccini
aa r X i v : . [ a s t r o - ph . C O ] F e b Draft version December 2, 2017
Preprint typeset using L A TEX style emulateapj v. 12/16/11
ENVIRONMENTAL EFFECTS ON THE STAR FORMATION ACTIVITY AT Z ∼ . * M. Kajisawa , Y. Shioya , Y. Aida , Y. Ideue , Y. Taniguchi , T. Nagao , T. Murayama , K. Matsubayashi ,L. Riguccini Draft version December 2, 2017
ABSTRACTWe investigated the fraction of [O ii ] emitters in galaxies at z ∼ . ii ] emittersare selected by the narrow-band excess technique with the NB711-band imaging data taken withSuprime-Cam on the Subaru telescope. We carefully selected 614 photo-z selected galaxies with M U < − .
31 at z = 0 .
901 – 0.920, which includes 195 [O ii ] emitters, to directly compare resultswith our previous study at z ∼ .
2. We found that the fraction is almost constant at 0 . − < Σ <
10 Mpc − . We also checked the fraction of galaxies with blue rest-frame colors of N U V − R < z ∼ . ii ] emitters decreases from ∼
60% at z ∼ . ∼
30% at z ∼ . ii ] emitter fraction could be explainedmainly by the rapid decrease of the star formation activity in the universe from z ∼ . z ∼ . Subject headings: galaxies: environment — galaxies: evolution — galaxies: star formation INTRODUCTION
It is known that star formation activity in galaxiesstrongly depends on environment in the present uni-verse. The high-density regions such as clusters of galax-ies are dominated by passively evolving early-type galax-ies, while there are many star-forming late-type galaxiesin field (low-density) environments (e.g., Dressler 1980;Goto et al. 2003; Bamford et al. 2009). The fraction ofstar-forming galaxies systematically decreases with in- * Based on observations with the NASA/ESA
Hubble SpaceTelescope , obtained at the Space Telescope Science Institute,which is operated by AURA Inc, under NASA contract NAS5-26555. Also based on observations made with the SpitzerSpace Telescope, which is operated by the Jet Propulsion Labo-ratory, California Institute of Technology, under NASA contract1407. Also based on data collected at; the Subaru Telescope,which is operated by the National Astronomical Observatoryof Japan; the XMM-Newton, an ESA science mission with in-struments and contributions directly funded by ESA MemberStates and NASA; the European Southern Observatory underLarge Program 175.A-0839, Chile; Kitt Peak National Obser-vatory, Cerro Tololo Inter-American Observatory and the Na-tional Optical Astronomy Observatory, which are operated bythe Association of Universities for Research in Astronomy, Inc.(AURA) under cooperative agreement with the National Sci-ence Foundation; and the Canada-France-Hawaii Telescope withMegaPrime/MegaCam operated as a joint project by the CFHTCorporation, CEA/DAPNIA, the NRC and CADC of Canada,the CNRS of France, TERAPIX and the Univ. of Hawaii. Research Center for Space and Cosmic Evolution, EhimeUniversity, Bunkyo-cho, Matsuyama 790-8577, Japan [email protected] Graduate School of Science and Engineering, Ehime Univer-sity, Bunkyo-cho, Matsuyama 790-8577, Japan The Hakubi Project, Kyoto University, Yoshida-Ushinomiya-cho, Sakyo-ku, Kyoto 606-8302, Japan Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan Astronomical Institute, Graduate School of Science, TohokuUniversity, Aramaki, Aoba, Sendai 980-8578, Japan NASA Ames Research Center, Moffett Field, CA 94035 creasing local galaxy density (e.g., G´omez et al. 2003;Balogh et al. 2004; Kauffmann et al. 2004; Tanaka et al.2004). From these findings, it is considered that the starformation history of galaxies in general depends on envi-ronment.In order to understand how the star formation his-tory of galaxies depends on environment, it is impor-tant to investigate the star formation activity of galax-ies as a function of environment in the early universe.Several such environmental studies have been carriedout at z >
1, when the cosmic star formation ratedensity reached its peak. Hayashi et al. (2010) studiedthe fraction of narrow-band selected [O ii ] emitters asa function of the local galaxy density around a clusterat z ∼ .
5, and found that the fraction of such star-forming galaxies is nearly independent of the local den-sity, and does not decrease even in the core of the clus-ter. Tran et al. (2010) also found that the fraction ofactively star-forming galaxies with bright IR luminos-ity slightly increases with the local galaxy density in acluster at z = 1 .
62. Ideue et al. (2009) similarly inves-tigated the fraction of [O ii ] emitters in more generalenvironments at z ∼ .
2, and found that the fractionis almost constant from low-density to medium-densityenvironments. These results suggest that the relationbetween the star formation activity and the galaxy en-vironment changed between z > z ∼
0. Sincethe cosmic star formation rate (SFR) density decreasesfrom z ∼ z ∼
1, when the cosmic SFR density started M. Kajisawa et al.to decrease.At z ∼
1, several studies in general environmentsclaimed that the average star formation rate of galaxiesincreases with the local galaxy density (e.g., Elbaz et al.2007; Cooper et al. 2008), while some studies aroundclusters of galaxies reported that the fraction of star-forming galaxies decreases with the local density (e.g.,Poggianti et al. 2008; Patel et al. 2009; Koyama et al.2010; Patel et al. 2011). Recently, Sobral et al. (2011)carried out a wide-field near-infrared narrow-band sur-vey in the COSMOS and UKIDSS UDS fields. Theyfound that the fraction of narrow-band selected H α emit-ters is nearly constant or slightly increases with the localgalaxy density from low-density to medium-density en-vironments, and then the fraction decreases towards thehighest-density regions such as rich clusters, which is con-sistent with previous studies both in general fields andclusters regions. Such somewhat complicated environ-mental dependence of the star formation activity mightbe considered to be intermediate between those at z > z ∼ z ∼ z = 0.4–0.8 with those at z ∼ z ∼ . z ∼ .
9, when the cosmic SFR density began todecrease. Using the optical narrow-band imaging datain the COSMOS survey (Scoville et al. 2007) obtainedwith Suprime-Cam on the Subaru Telescope, we investi-gated the fraction of narrow-band selected [O ii ] emittersin galaxies at z ∼ . ii ] emitter selection allows usto construct a large sample of star-forming galaxies witha secure redshift identification. By carefully choosingthe selection criteria for our sample and adopting thesame method for the estimate of the local galaxy den-sity as our previous study at z ∼ . z ∼ . z ∼ .
2. Section 2 describes the data andthe methods for the sample selection and the environ-ment estimate. In Section 3, we show the fraction of [O ii ] emitters in galaxies at z ∼ . z ∼ .
2, andthen check the robustness of the results. In Section 4,we compare our results with previous studies, and thendiscuss the evolution of the fraction in the redshift inter-val and its relation to the decrease of the star formationactivity in the universe.Throughout this paper, magnitudes are given in theAB system. We adopt a flat universe with Ω matter = 0 . Λ = 0 .
7, and H = 70 km s − Mpc − . SAMPLE AND ANALYSIS
Samples
In this study, we use a sample of galaxies with photo-metric redshifts of z = 0 .
901 – 0.920 from the COSMOSphotometric redshift catalog (Ilbert et al. 2009). We can
20 25−20−15 i’ M U Fig. 1.— M U vs. i ′ for galaxies at 0 . ≤ z phot ≤ .
920 fromthe COSMOS photometric redshift catalog. Horizontal dashed lineand vertical dotted line show M U = − .
31 and i ′ = 24 . M U < − .
31 satisfy i ′ < . select [O ii ] emitters using the NB711 narrow-band datafor the redshift interval. In order to directly comparewith our previous results at z ∼ . z ∼ .
2. In I09, we used thesample of galaxies with i ′ <
24 at z ∼ .
2, which cor-responds to the rest-frame 3500 ˚A absolute magnitudeof M U < − .
71. Therefore we constructed a sam-ple of galaxies with M U < − .
71 at z = 0 .
901 –0.920 (hereafter Sample A). The rest-frame 3500 ˚A ab-solute magnitude was calculated from the best-fit SEDtemplate derived in the photo- z calculation (Ilbert et al.2009) for each galaxy.In addition to the Sample A, we also construct an-other sample of galaxies at z = 0 .
901 – 0.920 usingthe different magnitude limit, for which the luminosityevolution at the rest-frame 3500 ˚A between z ∼ . z ∼ . z ∼ . z ∼ . M ⋆ = − .
55 and log φ ⋆ = − .
01 for the z ∼ . M ⋆ = − .
95 and log φ ⋆ = − . z ∼ . α = − . z ∼ . z ∼ .
9, and used − .
71 + 0 . − .
31 as another magnitude limit. Thesample of galaxies with M U < − .
31 at z = 0 .
901 –0.920 is referred to as Sample B.For the redshift range, we can almost completely sam-ple galaxies even with M U < − .
31 (Figure 1). Allgalaxies in the samples have i ′ <
24, and the photometricredshift accuracy is the same as in our previous study at z ∼ .
2. The numbers of galaxies in the samples are 373for the Sample A and 733 for the Sample B. The effectiveO ii ] Emitters at z ≃ . and the redshift interval of z = 0 .
901 – 0.920 corresponds to the co-moving depth of50 Mpc. Then our effective survey volume is 2 . × Mpc given the assumed cosmology. [O ii ] Emitter Selection We select [O ii ] emitters from the samples mentionedabove as star-forming galaxies using the narrow-band ex-cess technique. In order to select the [O ii ] emitter,we use the r ′ , i ′ , and NB711 bands photometry fromthe COSMOS photometric catalog (Capak et al. 2007).NB711 is a narrow band with a central wavelength of7119.6 ˚A and a full width at half maximum of 72.5 ˚A,which covers the redshifted [O ii ] λλ z = 0 .
901 – 0.920. We used the 3 ′′ diam-eter aperture magnitudes in three bands. We adoptedthe correction for the photometric zero point presentedby Ilbert et al. (2009), which is calculated by comparingthe observed multi broad-band photometry for galaxieswith spectroscopic identification with the best-fit modeltemplates. The zero-point corrections are 0.003, 0.019,and 0.014 mag for r ′ , i ′ , and NB711 bands, respectively.The limiting magnitudes are r ′ lim = 26 . i ′ lim = 26 . lim = 24 .
6, for a 3 σ detection on a 3 ′′ φ diam-eter aperture. It is noted that we use the CFHT i ∗ -bandmagnitude instead of the Subaru/Suprime-Cam i ′ -bandone for galaxies brighter than i ′ = 21 because such brightgalaxies appear to be affected by the saturation effect inthe Suprime-Cam data. Details of the imaging data andthe photometry are given in Taniguchi et al. (2007) andCapak et al. (2007).In order to select NB711-band excess objects, we cal-culated a continuum magnitude at the wavelength ofNB711 from r and i -bands magnitudes as f ri = 0 . f r ′ +0 . f i ′ , where f r ′ and f i ′ are the r ′ and i ′ flux densities,respectively. Its 3 σ limiting magnitude is ri ≃ . ′′ φ aperture. For the bright galaxies with i ′ <
21, the ri continuum is calculated as f ri = 0 . f r ′ + 0 . f i ∗ , where f i ∗ is the CFHT i ∗ flux density.Then, we select NB711-band excess objects using thefollowing criteria: ri − N B ≥ .
285 (1)and ri − N B > σ ri − NB , (2)where3 σ ri − NB = − . (cid:16) − p ( f σ NB ) + ( f σ ri ) /f NB (cid:17) . (3)The former criterion corresponds to the rest-frame equiv-alent width EW ([O ii ]) ≥
12 ˚A, which is the same asthat in our previous study at z ∼ . z = 0 .
901 – 0.920. Red symbols represent galaxies with M U < − .
71 (Sample A), and blue ones show galax-ies with − . < M U < − .
31. The narrow andbroad-bands data are deep enough to almost completelysample galaxies with EW ([O ii ]) ≥
12 ˚A. Here, weexclude X-ray sources as active galactic nuclei (AGNs)based on the X-ray information given in the COSMOSphoto- z catalog (Ilbert et al. 2009). Finally, we select118 [O ii ] emitters out of 373 galaxies in the Sample A
20 22 24−10123 r i − N B NB711
Fig. 2.— ri − NB 711 vs. NB 711 for galaxies with M U < − .
31 at 0 . ≤ z phot ≤ . ii ] emitters. Red symbols show galaxies with M U ≤ − .
71 (the Sample A), while blue symbols representthose with − . < M U < − .
31 (i.e., red + blue = theSample B). The dotted line corresponds to the minimum excessof ri − NB711 = 0 . σ error of ri − NB
711 color. The dashed line represents the 3 σ sensitivitylimit for ri magnitude. and 233 [O ii ] emitters out of 733 galaxies in the SampleB.We also examine how many AGNs are included inour sample using Spitzer IRAC mid-infrared colors.Lacy et al. (2004) and Stern et al. (2005) pointed outthat AGNs can be distinguished from star-forming galax-ies using Spitzer IRAC colors, e.g., [3 . − [4 . λ < µ m) continuumof star-forming galaxies is the composite stellar contin-uum that peaks at ∼ . µ m, an AGN continuum is wellfit by a power law. The infrared colors of AGNs tendto be systematically redder than star-forming galaxies.As in I09, we select objects with a mid-infrared color of[3 . − [4 . > ii ] emitters detected in both 3.6 and 4.5 µ m. Wefind that 2 out of the 207 [O ii ] emitters in our samplesatisfy this criterion; the fraction of the possible AGNsis 2/207 = 1.0 % at most. Therefore, we consider thatthe AGN contamination does not affect our discussionbelow. Local Surface Density
As in I09, we use the 10th nearest neighbor method toestimate the local surface density of galaxies as a mea-surement of the galaxy environment. The projected sur-face density is calculated asΣ = 11 πr , (4)where r is the distance to 10th nearest neighbor. Wecalculate this distance for galaxies within the redshift z ± σ z taking account of the error of the photometric M. Kajisawa et al. TABLE 1Summary of the samples photometric limiting total number ii ] emittersredshift magnitude (Σ available) (Σ available)Sample A 0.901–0.920 M U < − .
71 373 (291) 118 (95)Sample B 0.901–0.920 M U < − .
31 733 (614) 233 (195) redshift ( σ z = 0 .
023 at z ∼ . z ± σ z corresponds to the co-moving depth of 118 Mpc,while that in our previous study at z ∼ . r is larger thanthe distance to the edge of the field. This proceduredecreases the numbers of galaxies in our samples to 291(95 [O ii ] emitters) for the Sample A and 614 (195 [O ii ]emitters) for the Sample B. We summarize our samplesin Table 1. RESULTS
Fraction of [O ii ] emitters at z ∼ . as a functionof local density Figure 3 shows the fraction of [O ii ] emitters in galaxiesat z ∼ . z ∼ . ii ] emitters in the bothSamples A and B. The fraction of [O ii ] emitters is nearlyconstant ( ∼ .
3) for between Σ ∼ . − and ∼
10 Mpc − . Even if we take the effect of luminosityevolution of galaxies into account, the flat distributionholds. The fraction of [O ii ] emitters at z ∼ . ∼ . z ∼ .
9. Thefraction of [O ii ] emitters decreases from ∼ . ∼ . z ∼ . z ∼ . Fraction of galaxies with blue
N U V − R color In the analysis of the previous section, we mainly usedthe sample of galaxies with 0 . ≤ z phot ≤ .
920 (i.e.,∆ z = 0 . ii ] emitters as the star-formingpopulation, while the local galaxy density are calcu-lated with galaxies within a redshift slice of z ± σ z (i.e.,∆ z = 0 . ii ]emitters selected by the NB711-band excess tend to havehigher photo-z accuracy than the other galaxies withoutthe narrow-band excess.In order to check whether they affect the environmentaldependence of the fraction of star-forming galaxies, weselected star-forming galaxies by the rest-frame N U V − R −1 0 100.51 fr ac ti on o f [ O II] e m tt e r s log Σ (Mpc −2 ) Fig. 3.—
Fraction of [O ii ] emitters as a function of the galaxylocal density. Filled circles show the result for the Sample A andopen squares show that for the Sample B. Small open circles rep-resent the result at z ∼ . color estimated from the COSMOS multi-band photo-metric data instead of EW ([O ii ]). The EW ([O ii ]) isthe ratio of the line luminosity L ([O ii ]) and the contin-uum luminosity at the rest-frame 3727˚A. While L ([O ii ])mainly depend on SFR, the continuum luminosity at3727˚A depends on both SFR and stellar mass. There-fore we consider that the rest-frame N U V − R color ismore suitable for a substitute for EW ([O ii ]) than thesimple rest-frame UV luminosity. If we ignore the effectof the dust reddening, star-forming galaxies with large EW ([O ii ]) are expected to have blue N U V − R colors.Since the rest-frame N U V − R selection is not limited tothe narrow redshift range of z phot = 0.901–0.920, we canestimate the fraction of blue star-forming galaxies andthe local galaxy density with the same sample within aredshift slice of ∆ z = 0 . N U V − R vs. R − J for galaxies at 0 . ≤ z phot ≤ . ii ] emitters tend to show blue N U V − R colors as expected. Since more than 90% of [O ii ]emitters have N U V − R <
2, we use
N U V − R <
N U V − R < ii ] emitters. Figure 5 shows the fraction ofgalaxies with N U V − R < . ≤ z phot ≤ .
920 as the main sample and measuredO ii ] Emitters at z ≃ . −1 0 1 20246 N U V − R R−J [OII] emittersMIPS 24 m sources µ Fig. 4.—
The rest-frame
NUV − R vs. R − J diagram forthe Sample B ( M U < − . ii ]emitters selected by the NB711-band excess. Red circles rep-resent bright Spitzer/MIPS 24 µ m sources with f µ m & µ Jy at z phot =0.901–0.920, which include those objects with M U > − . −1 0 100.51 fr ac ti on o f b l u e g a l a x i e s log Σ (Mpc −2 ) Fig. 5.—
Fraction of blue galaxies with the rest
NUV − R < M U < − .
71 at 0 . ≤ z phot ≤ . z ± .
023 centered on its redshift, as in Figure 3. Onthe other hand, open circles show the case that both the fraction ofblue galaxies and the local galaxy density are measured from thesame galaxy sample at 0 . − . ≤ z phot ≤ .
91 + 0 . the local density for each sample galaxy by using galaxieswithin a redshift slice of z ± σ z centered on its redshift,as in the case of [O ii ] emitters (solid circles in Figure5). Then we estimated both the fraction of blue galaxies and the local galaxy density from the same galaxies at0 . − . ≤ z phot ≤ .
91 + 0 .
023 (open circles in thefigure). The fractions of galaxies with
N U V − R <
Comparison with the semi-analytic model
Although more than 30 photometric bands of the COS-MOS data set provide the very accurate photometric red-shift, the error of σ z = 0 .
023 at z ∼ . ∼
30 Mpc. The effects ofthe projection over the redshift slice of z ± σ z ( ∼ M U < − . M U < − .
31 from a snapshot at z ∼ .
9, and thencalculated the local galaxy densities with three ways; 1)3-dimensional density based on the 10th nearest neighbor(i.e., true density), 2) 2-dimensional projected densitybased on the 10th nearest neighbor which is calculatedfrom samples within a slice of 60 Mpc in physical scale,3) the same as 2) but calculated from samples within aslice of 60 Mpc after galaxies are randomly shifted alongthe depth direction of the slice with a offset of the Gaus-sian distribution with σ = 30 Mpc in order to take thephotometric redshift error of the observed sample into ac-count. We can check the projection effect by comparing1) and 2), and see the effect of the photometric redshiftuncertainty from 3). Figure 6 shows the comparison be-tween 1) and 2) (left panel) and that between 1) and 3)(right panel) for the sample with M U < − .
71. It isseen from the left panel that there is a clear correlationwith a scatter of σ ∼ . M U < − .
31 are the same except that theboth true and projected densities become slightly largersimply because of the increase of the number of galaxiesin the sample. We confirmed that these results do notdepend on a choice of the direction of the slice in thesimulation box.We also checked the environmental dependence of thefraction of [O ii ] emitters in the simulation by calculat-ing EW ([O ii ]) of model galaxies from their L ([OII])and M U in the mock catalogue. Figure 7 shows thefraction of galaxies with EW ([O ii ]) >
12 ˚A as a func-tion of the 3-dimensional density (left panel) and the 2- M. Kajisawa et al.
10 10 10 10 10 10 10 10 10 Σ ρ t h (Mpc ) ( M p c ) -3 - -1 0 1 2 -3 -2 -1 0 1
10 10 10 10 10 10 10 10 10 Σ ρ t h (Mpc ) ( M p c ) -3 - -1 0 1 2 -3 -2 -1 0 1 with photo-z error Fig. 6.— left)
Comparison between the 3-dimensional (true) local galaxy density and the projected 2-dimensional density for ∼ M U < − .
71 at z ∼ . z = 0 .
046 at z ∼ .
9. Solid line shows the median value of the projected density as a function of the 3-dimensionaldensity, while dashed lines represent the 16 and 84 percentiles. right)
The same as the left panel but the projected density is calculatedfrom model galaxies within a slice of 60 Mpc after galaxies are randomly shifted with a offset of the Gaussian distribution with σ = 30Mpc in order to take account of the photometric redshift error (see text). F r a c t i on o f [ O II] e m i tt e r s EW([OII])>12AEW([OII])>36A Σ (Mpc ) -2 -1 0 1 2 F r a c t i on o f [ O II] e m i tt e r s EW([OII])>12AEW([OII])>36A 10 10 10 10 10 ρ (Mpc ) -3-3 -2 -1 0 1 Fig. 7.— left)
Fraction of [O ii ] emitters in mock galaxies with M U < − .
71 at z ∼ . EW ([O ii ]) >
12 ˚A, and dashed lineshows that of galaxies with EW ([O ii ]) >
36 ˚A. right)
The same as the left panel but as a function of the 2-dimensional projected densitywith the photometric redshift error of σ z = 0 . dimensional projected density with the photometric red-shift error (right panel). Since the overall fraction isslightly larger than the observed fraction of [O ii ] emit-ters, we also show the fraction of model galaxies with EW ([O ii ]) >
36 ˚A for comparison. It is seen from theleft panel that the fraction of star-forming galaxies se-lected by the EW ([O ii ]) criteria clearly decreases withthe local galaxy density in the semi-analytic model asprevious studies have already reported (e.g., Elbaz et al. 2007). Even if we use the projected density with thephotometric redshift error, the fraction of [O ii ] emittersclearly depends on the density, although the environmen-tal dependence of the fraction becomes slightly weaker.This suggests that the effects of the projection and thephotometric redshift error in our density measurementdo not smear out the relation between the fraction of [O ii ] emitters and local density. The observed fraction inFigure 3 does not seem to decrease with the local density,O ii ] Emitters at z ≃ . ii ] emitters with the semi-analytic model does notconsider the fact that [O ii ] emitters are selected by thenarrow-band excess within a narrower redshift width of∆ z = 0 . N U V − R colors as the star-forming popula-tion within the same redshift width of ∆ z = 0 .
046 as inthe local density measurement. Therefore, we considerthat it does not affect the comparison between our resultand the model prediction.
Incompleteness and contamination due to thephotometric redshift error
We here check the incompleteness and contaminationdue to the photometric redshift error in our sample byusing spectroscopic redshifts from the zCOSMOS red-shift survey (Lilly et al. 2007; Lilly et al. 2009). Sincethe NB711 data are used in the photometric redshiftmeasurement (Ilbert et al. 2009), the photometric red-shifts of [O ii ] emitters with the narrow-band excess areexpected to be more accurate than the other galaxieswithout the narrow-band excess (hereafter, non-[O ii ]emitters) in our sample. Therefore we investigated theincompleteness and contamination for [O ii ] emitters andnon-[O ii ] emitters separately.We first examined the spectroscopic redshift distribu-tion for the photo-z selected galaxies with M U < − .
71 (Sample A) in order to estimate the fraction ofthe contamination from outside the redshift range intothe photo-z selected sample. There are 24 [O ii ] emittersand 65 non-[O ii ] emitters with spectroscopic identifica-tion in the Sample A, and we can use these galaxies forthe purpose. Out of 24 [O ii ] emitters, 19 have spectro-scopic redshifts of 0 . ≤ z spec ≤ . z spec < .
901 or z spec > . /
24 = 21%. Actually, fourof these 5 objects have slightly lower redshifts of z spec =0.893–0.899, for which the [O ii ] emission enters into theshort-wavelength wing of the NB711 filter, while one ob-ject is a [O III ] emitter at z spec = 0 . z spec = 0.893–0.899 into the [O ii ] emit-ter sample, the contamination rate becomes 1 /
24 = 4%.On the other hand, 39 of 65 non-[O ii ] emitters in theSample A lie within 0 . ≤ z spec ≤ . z spec < . z spec > . ii ] emitters is 26 /
65 = 40%.Next, we checked the photometric redshift distribu-tion of galaxies with z spec = 0.901–0.920 and M U < − .
71 to estimate the incompleteness due to the photo-z error for the Sample A. From the spectroscopic cata-logue, we could use 97 objects with z spec = 0.901–0.920and M U < − .
71, and found that 58 out of 97 havephotometric redshifts of 0 . ≤ z phot ≤ . z spec = 0.901–0.920,20 objects shows a significant NB711-band excess andsatisfy the criteria for [O ii ] emitters, and the other 77objects are non-[O ii ] emitters. Similarly, out of 58 ob-jects with z spec = 0.901–0.920 and z phot = 0.901–0.920,19 objects satisfy the criteria for [O ii ] emitters, and theother 39 objects are non-[O ii ] emitters. Therefore, for[O ii ] emitters, 19 out of 20 galaxies with z spec = 0.901–0.920 are selected to the Sample A, and the completenessis 19 /
20 = 95%. On the other hand, 39 out of 77 non-[O ii ] emitters with z spec = 0.901–0.920 are included intothe sample. Therefore the completeness for non-[O ii ]emitters is 39 /
77 = 51%.By adopting these contamination and completenessrates, which is estimated from the spectroscopic sam-ple, for all the photo-z selected sample, we examinedthe effects of the contamination and incompleteness dueto the photo-z error on the fraction of [O ii ] emittersin the photo-z selected sample. Since the contamina-tion and completeness rates for [O ii ] emitters are 21%and 95%, respectively, we select 95% of the real [O ii ]emitters and also pick up the contaminants which ac-count for 21% of the observed number. Therefore, thenumber of [O ii ] emitters are expected to be overesti-mated by ∼ . / (1 − . ∼ .
20. Similarly,from the contamination rate of 40% and the complete-ness of 51%, we can calculate that the observed num-ber of non-[O ii ] emitters is underestimated by ∼ . / (1 − . ∼ .
85. Since the observed num-bers of [O ii ] emitters and non-[O ii ] emitters are 95and 196 respectively, the number of all photo-z selectedgalaxies is expected to be underestimated by ∼ / (95 / .
21 + 196 / . ∼ .
94. As a result,the number of [O ii ] emitters is overestimated by ∼ ∼ ii ] emitterscould be overestimated by ∼ . / .
94 = 1 . ii ] emitters with z spec =0.893–0.899 as the contaminants, the contamination ratefor [O ii ] emitters decreases from 21% to 4% as men-tioned above, and the overestimation of the fraction of[O ii ] emitters becomes ∼ ii ] emitters at z ∼ . ∼
17% due to the photo-z er-ror, the fractions of [O ii ] emitters at z ∼ . z ∼ . DISCUSSION
Comparison with other studies
We here compare our results in the COSMOS fieldwith previous studies of the environmental dependenceof the star formation activity in galaxies at similar red-shifts. Elbaz et al. (2007) investigated the average SFRof galaxies with M B < −
20 at 0 . ≤ z ≤ . ∼ − .Our results seem to be consistent with their results inthat the fraction of actively star-forming galaxies doesnot decreases with the local density on average, whichis different from the SFR-density relation seen in thepresent universe and from the predictions by the semi-analytic models of the galaxy formation. However, the M. Kajisawa et al. −1 0 14243 log Σ l og L ([ O II])
Fig. 8.— [O ii ] luminosity vs. the local galaxy density for [O ii ]emitters with M U < − .
71. The [O ii ] luminosity is correctedfor the dust extinction under the assumption of A H α = 1 mag forall [O ii ] emitters (see Appendix). fraction of [O ii ] emitters at z ∼ . − . In orderto examine the environmental dependence of the averageSFR in our sample, we plot L ([O ii ]) of [O ii ] emitters asa function of the local density in Figure 8. L ([O ii ]) isestimated from the ri and N B
711 magnitudes (see Ap-pendix for detail). It is seen that the L ([O ii ]) of [O ii ]emitters is also independent of the density. Therefore weexpect that the average L ([O ii ]) also does not signifi-cantly depend on the local density.A possible origin of the different behaviors in theSFR-density relation between Elbaz et al. (2007) andthis study is the effect of the dust extinction. We se-lected star-forming galaxies with the [O ii ] emitter selec-tion, while Elbaz et al. (2007) estimated SFRs of galax-ies from the Spitzer/MIPS 24 µ m fluxes. Although wecorrect L ([O ii ]) for the dust extinction assuming A H α = 1 mag, the [O ii ] emitter selection itself could missdusty star-forming galaxies. In order to check this, wecross-matched the 24 µ m source catalogue from the S-COSMOS survey (Sanders et al. 2007) to galaxies at0 . ≤ z phot ≤ .
920 in our sample. The flux limitof the 24 µ m catalogue is ∼ µ Jy. We plot the24 µ m sources as red circles in Figure 4. These bright24 µ m sources tend to show red rest-frame N U V − R and R − J colors, and the overlap between these 24 µ msources and [O ii ] emitters is relatively small. Since star-forming galaxies are expected to distribute over a di-agonal region from ( R − J ∼ N U V − R ∼
1) to( R − J ∼ . N U V − R ∼ .
5) (e.g., Bundy et al.2010), the [O ii ] selection seems to miss star-forminggalaxies with relatively red N U V − R colors such asthe bright 24 µ m sources. We also compare the stellarmass of [O ii ] emitters with that of these bright 24 µ msources in Figure 9. The stellar mass of each galaxyis estimated by fitting the multi-band photometric data N u m be r Stellar mass (Msun)Sample B[OII] emitters24 µ m sources Fig. 9.—
Distribution of the stellar mass of galaxies for the Sam-ple B ( M U < − . ii ] emit-ters with M U < − .
31. The dashed histogram represents thebright MIPS 24 µ m sources with f µ m & µ Jy. from UV to MIR wavelength with the population syn-thesis model by Bruzual & Charlot (2003) (Ilbert et al.2010). Chabrier (2003)’s IMF is assumed. Figure 9 showsthat most of [O ii ] emitters have relatively low stellarmass of M star . M ⊙ , while the all photo-z selectedgalaxies distribute over 10 M ⊙ . M star . M ⊙ . Onthe other hand, the bright 24 µ m sources show systemat-ically larger stellar mass of ∼ –10 M ⊙ . The 24 µ m-selected star-forming galaxies in Elbaz et al. (2007) alsodistribute over 10 M ⊙ . M star . M ⊙ . Thereforethe [O ii ] emitter selection seems to miss massive (dusty)star-forming population. Since star-forming galaxieswith 10 –10 M ⊙ have a large contribution to the cos-mic SFR density at z ∼ ∼ − seen in Elbaz et al. (2007). However, we notethat the fraction of star-forming galaxies in our sampledoes not significantly depend on the density, even if weinclude the bright 24 µ m sources into the star-formingpopulation.Another possible origin of the different results isthe different scales of the local density measurement.Elbaz et al. (2007) measured the density with a box of1.5 Mpc × ×
40 Mpc in comoving scale, whilewe used the 10th nearest neighbor method with galaxieswithin a comoving depth of 118 Mpc. In the 10th nearestneighbor method, a radius used in the density measure-ment depends on the local density itself. For example,the local density of Σ = 1 Mpc − corresponds to aradius of r ∼ . ∼ . − ,a radius becomes 1.1 Mpc in comoving scale. Since wetypically investigate galaxies with Σ ∼ − O ii ] Emitters at z ≃ . - -
10 10 10 10 10 10 10 10 10 10 ρ ( z =1.2, Mpc ) -3-3 -2 -1 0 1 ρ t h ( z = . , M < - . , M p c ) U -3 -2 -1 0 1
10 10 10 10 10 10 10 10 10 10 ρ ( z =1.2, Mpc ) -3-3 -2 -1 0 1 ρ t h ( z = . , M < - . , M p c ) U -3 -2 -1 0 1
10 10 10 10 Σ ( z =1.2, Mpc ) -2 -1 0 1 2
10 10 10 10 Σ t h ( z = . , M < - . , M p c ) - -1 0 1 2 U
10 10 10 10 Σ ( z =1.2, Mpc ) -2 -1 0 1 2
10 10 10 10 Σ t h ( z = . , M < - . , M p c ) - -1 0 1 2 U Fig. 10.—
Evolution of the local galaxy density between z ∼ . z ∼ . z ∼ . M U < − .
71. Left panels show the case that the local density at z ∼ . M U < − .
71, while right panels show the results for the density at z ∼ . M U < − .
31. Solid line shows themedian value of the density at z ∼ . z ∼ .
2, while dashed lines represent the 16 and 84 percentiles. (Figure 3), the local density in our analysis is measuredwith a radius of 1.1–3.6 Mpc, which corresponds to a di-ameter of 2.2–7.2 Mpc. This scale is larger than that inElbaz et al. (2007) (1.5 Mpc), especially for low-densityenvironments. The depth of 118 Mpc used in our den-sity measurement is also much larger than 40 Mpc usedin Elbaz et al. (2007). For local galaxies, Blanton et al.(2006) pointed out that the environment dependence ofthe star formation activity in galaxies could vary withthe scale used for the density measurement. Elbaz et al.(2007) also discussed that the peak of the average SFRaround a density of ∼ − seen in their result couldbe caused by active star formation in galaxies duringgroup formation at the scale of ∼ z =0.4–0.8. They found that the fraction of [O ii ] emitters ingroups or outskirts of clusters is similar with that in fieldenvironments at the same redshift, while the fraction de-creases toward the highest-density region at the coresof clusters. The weak or no environmental dependenceof the fraction of [O ii ] emitters at the relatively low- density environment (Σ c < − ) might be con-sistent with our results, although the fraction of ∼ EW ([O ii ]) > ii ] emitters by using the spectroscopicdata. They also pointed out that the average SFR of[O ii ] emitters has a peak at Σ c ∼ − , whilewe found no significant environmental dependence of the[O ii ] luminosity at the lower density. Patel et al. (2011)similarly studied SFRs of galaxies at 0 . < z < . M star > . M ⊙ ,while [O ii ] emitters in this study tend to have smallermass of M star < M ⊙ as seen in Figure 9.Recently, Sobral et al. (2011) investigated the fractionof H α emitters in galaxies with K <
23 at z ∼ .
84 in theCOSMOS and UKIDSS UDS fields (total ∼ . ), us-ing a narrow-band observations at 1.211 µ m. They alsoused the 10th nearest neighbor method to measure thelocal density and studied the environmental dependenceof the fraction of star-forming galaxies. They found that0 M. Kajisawa et al. z=0.9z=1.2 0 0.2 0.4 0.6 0.8 1 10 10 10 10 F r a c t i on o f [ O II] e m i tt e r s Σ (Mpc ) -2 -1 0 1 2
10 10 10 10 Σ (Mpc ) -2 -1 0 1 2 EW([OII]) > 12A EW([OII]) > 36A
Fig. 11.—
Evolution of the fraction of [O ii ] emitters between z ∼ . z ∼ . M U < − .
71 in thesemi-analytic model as a function of the 2-dimensional projected density with the photometric redshift error at each redshift. Solid lineshows the fraction at z ∼ .
9, while dashed line represents that at z ∼ .
2. Left panel shows the fraction of model galaxies with EW ([O ii ]) >
12 ˚A, while right panel shows the fraction of those with EW ([O ii ]) >
36 ˚A. the fraction of H α emitters with EW obs (H α ) >
50 ˚A is ∼
30% almost independent of the local density in rela-tively low-density environments of their Σ c <
10 Mpc − .On the other hand, the fraction rapidly decreases with in-creasing the local density in higher-density environments.If the environments we investigated are mainly field en-vironments, our result at z ∼ . ii ] emitters between z ∼ . z ∼ . z ∼ . z ∼ . Evolution of the fraction of [O ii ] emitters between z ∼ . and z ∼ . ii ] emitters from ∼
60% at z ∼ . ∼
30% at z ∼ .
9. At first, we examinedhow the local density for each galaxy evolves between z ∼ . z ∼ . z ∼ .
2, we similarly calculated the 3-dimensional den-sity and the 2-dimensional projected density includingthe random offsets due to the photo-z error at z ∼ .
2. Note that the average number densities of galaxies at z ∼ . ∼
2. It is seen from the upper panels of the figurethat the evolution of the true 3-dimensional density isrelatively small (a factor of .
2) for most model galax-ies except for the high density region. The projecteddensity also mildly evolves at 0.3 Mpc − ≤ Σ ≤ − especially for the Sample B. By combining thiswith the lack of the environmental dependence at theboth redshifts, we infer that the change in the local den-sity probably does not strongly affect the evolution of thefraction of [O ii ] emitters in the range of the density weinvestigated. Therefore we can fairly compare the frac-tions of [O ii ] emitters at the same range of the localdensity between z ∼ . z ∼ . ii ] emitters between z ∼ . z ∼ . EW ([O ii ]), and it could be slightly large at Σ ∼ − . The observed strength of the evolution seenin Figure 3 is larger than that in the model.We next consider the evolution of the EW ([O ii ]) dis-tribution. Figure 12 shows a comparison of the observed EW ([O ii ]) distribution between the z ∼ . z ∼ . EW ([O ii ]) for each [O ii ] emitter is cal-culated from the narrow-band excess (NB711 − ri ′ ). InFigure 12, the fraction of galaxies with EW ([O ii ]) > z ∼ . z ∼ . EW ([O ii ]) of allstar-forming galaxies decreases by the same factor from z ∼ . z ∼ .
9, the observed EW ([O ii ]) distri-butions suggest that the EW ([O ii ]) needs to decreaseby a factor of 1 . +0 . − . in this redshift interval in orderto reproduce the fraction of [O ii ] emitters with EW ([O ii ]) ≥
12 ˚A at z ∼ . ∼ . ± . EW ([O ii ]) evolution asthe evolution of star formation activity in galaxies. Fig-ure 13 compares [O ii ] LFs between z ∼ . z ∼ . ii ] Emitters at z ≃ . F r ac ti on EW ([OII]) (A) o z = 0.91= 1.19 z F r ac ti on EW ([OII]) (A) o z = 0.91= 1.19 z ( /1.6) EW ( /1.3) EW Fig. 12.— left)
Comparison of the normalized differential distributions of EW ([O ii ]) between z ∼ . z ∼ .
2. black and greyhistograms show that for galaxies at z ∼ . z ∼ .
2, respectively. Vertical dashed line shows the equivalent width limit of EW ([O ii ]) = 12 ˚A for our [O ii ] emitter selection. Note that only galaxies classified as [O ii ] emitters are plotted, while the calculation of thenormalization constant takes account of all galaxies including objects which do not satisfy the selection criteria for the [O ii ] emitter. right) The same as the left panel, but the observed EW ([O ii ]) for [O ii ] emitters at z ∼ . EW ([O ii ]) from z ∼ . z ∼ .
41 42 43 4410 −6 −4 −2 φ log ([OII]) (erg s −1 ) L ( l og ) ( l og − M p c − ) LL (α = −1.0)(α = −1.2)(α = −1.4) z ~ 0.9, SDF (Ly et al. 07)~ 1.2, COSMOS~ 1.2, SDF (Takahashi et al. 07)~ 0.9, COSMOS zzz Fig. 13.—
The [O ii ] luminosity function for [O ii ] emitters at z ∼ . z ∼ .
2. The solid circles show the result for our [O ii ]emitter sample at z ∼ . α = − . − .
2, and − .
4, respectively. Thin dashed-dotted lineshows [O ii ] LF for [O ii ] emitters at z ∼ . z ∼ . ii ] LF at z ∼ . in the COSMOS field and the Subaru Deep Field (SDF).The [O ii ] LFs in the COSMOS field are derived from oursamples at z ∼ . z ∼ . ∼ L ∗ ∼ . z ∼ . L ∗ ∼ . z ∼ . z ∼ . z ∼ .
9, this luminosity evolu-tion reflects the decrease of the star formation activity ingalaxies. Many previous studies of the evolution of thecosmic SFR density also suggest that the star formationactivity in the universe decreases by a factor of ∼ z ∼ . z ∼ . EW ([O ii ]) for the case that the star for-mation rate decreases by a factor of 2 in the redshiftinterval. The [O ii ] line luminosity is simply expected todecreases by the same factor as the SFR, if we ignore themetallicity/dust extinction evolution. If we assume ex-ponentially decaying star formation histories (i.e., SFR ∝ exp( − age /τ )) between z ∼ . z ∼ . ∼ ii ] luminosity function at z ∼ . z ∼ . τ =1.2–1.8 Gyr. In this case, the continuum luminos-ity at the rest-frame 3727 ˚A is expected to decrease bya factor of ∼ EW ([O ii ]) decreases bya factor of 1.3–1.6 between z ∼ . z ∼ .
9, tak-ing account of the possible effect of the Balmer break onthe measurement of the narrow-band excess (NB711 − ri ′ ). This is roughly consistent with the evolution of the2 M. Kajisawa et al. EW ([O ii ]) expected from the evolution in the fractionof [O ii ] emitters (1 . +0 . − . ). In the right panel of Figure12, we compare the observed EW ([O ii ]) distribution at z ∼ . EW ([O ii ]) at z ∼ . EW ([O ii ]) distri-bution is relatively good especially in the case that the EW ([O ii ]) decreases by a factor of 1.6, although theobserved EW ([O ii ]) at z ∼ . ii ] emitters from z ∼ . z ∼ . SUMMARY
We investigated the fraction of [O ii ] emitters in thephoto-z selected galaxies at z ∼ . ii ] emittersare selected by the narrow-band excess technique withthe NB711-band data taken with Subaru/Suprime-Cam.We used the magnitude limits and selection criteria for [O ii ] emitters which are consistent with our previous studyat z ∼ . z ∼ . z ∼ .
2. Our final sample consists of 614(291) photo-z selected galaxies with M U < − . M U < − .
71) at z = 0 .
901 – 0.920, which includes195 (95) [O ii ] emitters. Our main results are as follows. • The fraction of [O ii ] emitters at z ∼ . ∼ .
3) at 0 . − < Σ <
10 Mpc − . The flat distribution holds, even if weuse the magnitude limit for which the luminosityevolution of galaxies is taken into account. No sig-nificant environmental dependence is similar withthe result in our previous study at z ∼ . • The fraction of [O ii ] emitters decreases from ∼ . z ∼ . ∼ . z ∼ . • Instead of [O ii ] emitters, we used galaxies withblue rest-frame colors of N U V − R < . − . ≤ z phot ≤ .
91 + 0 .
023 as the star-forming population in order to measure both the fraction ofstar-forming galaxies and local galaxy density fromthe same sample. The fraction of blue galaxies with
N U V − R < • We checked the effects of the projection over theredshift slice and the photometric redshift error onour density measurement, using the semi-analyticmodel by Font et al. (2008). Although these ef-fects seem to smear out very high and low densityregions in some degree, we confirmed that the frac-tion of [O ii ] emitters clearly depends on the pro-jected density in the simulation, which is differentfrom the observed results. • Most of [O ii ] emitters have relatively small stel-lar mass of M star < M ⊙ , and the overlap be-tween [O ii ] emitters and bright 24 µ m sources with f µ m & µ Jy is relatively small. The [O ii ]emitter selection seems to miss massive dusty star-forming galaxies. Such a selection bias might causethe different behaviors in the SFR-density relationamong studies with the different SFR indicators. • If we simply assume SFRs of star-forming galaxiesdecrease by a factor of 2 from z ∼ . z ∼ . ii ] luminosity function and cosmic SFR density inthe redshift interval, the expected evolution of the EW ([O ii ]) is roughly consistent with the observed EW ([O ii ]) distributions at z ∼ . z ∼ . ii ] emit-ters from z ∼ . z ∼ . APPENDIX
LUMINOSITY FUNCTION OF [O II] EMITTERS AT Z ∼ In this Appendix, we describe the derivation of the [O ii ] luminosity function at z ∼ . ii ] flux, we have used the total flux density of r ′ , i ′ (or i ∗ ), and NB711. The flux of [O ii ] emissionline is given by f [O ii ] = ∆NB f NB − f ri − . / ∆ i ′ ) . (A1)where f NB is the total flux density of NB711, f ri is the ri continuum flux density, ∆NB and ∆ i ′ are the effectivebandwidth of the NB711 and i ′ filters, respectively: ∆NB711 = 72.5 ˚A and ∆ i ′ =1489.4 ˚A. Since the flux of the [O ii ] emission line is affected by the dust obscuration, it is necessary to correct the extinction effect. Here, we apply aconstant extinction of A [O ii ] = 1.87 mag, which corresponds to A H α = 1 mag, following previous studies (Hopkins2004; Takahashi et al. 2007; Sobral et al. 2012). We also apply the filter transmission effect since the actual NB711filter transmission is not rectangular. We adopt a factor of 1.24 following Shioya et al. (2009).O ii ] Emitters at z ≃ . TABLE 2Best-fit Schechter parameters for the [O ii] luminosity function at z ∼ . in the COSMOS field. α log L ∗ log φ ∗ -1.00 42 . ± . − . ± . . ± . − . ± . . ± . − . +0 . − . Then the [O ii ] flux is given by f cor ([O ii ]) = f [O ii ] × . A [O ii ] × . , (A2)and the [O ii ] luminosity is estimated by L ([O ii ]) = 4 πd f cor ([O ii ]) (A3)where d L is the luminosity distance: d L = 5883 Mpc.The [O ii ] luminosity function (LF) is constructed by the following formula,Φ (log L i ) = 1∆ log L X j V j , (A4)with | log L j − log L i | < ∆ log L , where ∆ log L is the logarithmic bin size and V j is the volume covered by the filter.Here we use ∆log L = 0 .
2, and V j = 2 . × Mpc . We show the [O ii ] LF in Figure 13.We fit the [O ii ] LF with the Schechter function (Schechter 1976),Φ( L )d L = φ ⋆ L ⋆ (cid:18) LL ⋆ (cid:19) α exp (cid:18) − LL ⋆ (cid:19) d L , (A5)by the STY method (Sandage et al. 1979). Before fitting the [O ii ] LF, we estimate the lower and upper limiting lumi-nosities ( L low and L up ) that evaluate whether the sample is complete or not. Using the observed limiting magnitudesof ri and NB711, we obtain log L low = 41 .
83 erg s − . On the other hand, the saturation magnitude of r ′ gives theupper limiting luminosity of log L up = 43 .
65 erg s − . Since it is difficult to estimate the power index α accuratelybecause of incompleteness at the faint end, we show our results for the following three cases, α = − . , − .
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