arXiv subjects

Jaron Kurk

Publications and source records attributed to Jaron Kurk.

At least 19 recordsLinked to original sources

arXiv2019

A spectroscopic study of a rich cluster at z=1.52 with Subaru $\&$ LBT: the environmental impacts on the mass-metallicity relation

We present the results of our near-infrared (NIR) spectroscopic observations of a rich cluster candidate around a radio galaxy at $z=1.52$ (4C65.22) with Subaru/MOIRCS and LBT/LUCI. We observed 71 galaxies mostly on the star-forming main sequence selected by our previous broad-band (photo-$z$) and narrow-band H$α$ imaging observation in this cluster environment. We successfully confirmed the redshifts of 39 galaxies, and conclude that this is a gravitationally bound, real cluster at $z=1.517$. Our spectroscopic data also suggest a hint of large-scale filaments or sheet-like three-dimensional structures crossing at the highest-density cluster core. By stacking the spectra to derive their average interstellar medium (ISM) gas-phase metallicity based on the [NII]/H$α$ emission line flux ratio, we find that the mass-metallicity relation (MZR) in the 4C65.22 cluster environment is consistent with that of H$α$-selected field galaxies at similar redshifts. Our results suggest that the environmental impacts on the MZR is small at high redshifts, but a larger sample of high-$z$ clusters and their member galaxies is still required to fully address the effect of environment as well as its cluster-cluster variation.

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arXiv2014

Probing Outflows in z= 1~2 Galaxies through FeII/FeII* Multiplets

We report on a study of the 2300-2600ÅFeII/FeII* multiplets in the rest-UV spectra of star-forming galaxies at 1.0 1.5 Åand of the FeII* emission lines to W_r>0.5 Å. Whenever we can measure the systemic redshift of the galaxies from the [OII] emission line, we find that both the FeII and MgII absorption lines are blueshifted, indicative that both species trace gaseous outflows. We also find, however, that the FeII gas has generally lower outflow velocity relative to that of MgII. We investigate the variation of FeII line profiles as a function of the radiative transfer properties of the lines, and find that transitions with higher oscillator strengths are more blueshifted in terms of both line centroids and line wings. We discuss the possibility that FeII lines are suppressed by stellar absorptions. The lower velocities of the FeII lines relative to the MgII doublet, as well as the absence of spatially extended FeII* emission in 2D stacked spectra, suggest that most clouds responsible for the FeII absorption lie close (3~4 kpc) to the disks of galaxies. We show that the FeII/FeII* multiplets offer unique probes of the kinematic structure of galactic outflows.

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arXiv2014

A Consistent Study of Metallicity Evolution at 0.8 < z < 2.6

We present the correlations between stellar mass, star formation rate (SFR) and [NII]/Ha flux ratio as indicator of gas-phase metallicity for a sample of 222 galaxies at 0.8 < z < 2.6 and log(M*/Msun)=9.0-11.5 from the LUCI, SINS/zC-SINF and KMOS3D surveys. This sample provides a unique analysis of the mass-metallicity relation (MZR) over an extended redshift range using consistent data analysis techniques and strong-line metallicity indicator. We find a constant slope at the low-mass end of the relation and can fully describe its redshift evolution through the evolution of the characteristic turnover mass where the relation begins to flatten at the asymptotic metallicity. At fixed mass and redshift, our data do not show a correlation between the [NII]/Ha ratio and SFR, which disagrees with the 0.2-0.3dex offset in [NII]/Ha predicted by the "fundamental relation" between stellar mass, SFR and metallicity discussed in recent literature. However, the overall evolution towards lower [NII]/Ha at earlier times does broadly agree with these predictions.

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arXiv2013

A large-scale galaxy structure at z = 2.02 associated with the radio galaxy MRC0156-252

We present the spectroscopic confirmation of a structure of galaxies surrounding the radio galaxy MRC0156-252 at z = 2.02. The structure was initially discovered as an overdensity of both near-infrared selected z > 1.6 and mid-infrared selected z > 1.2 galaxy candidates. We used the VLT/FORS2 multi-object spectrograph to target ~80 high-redshift galaxy candidates, and obtain robust spectroscopic redshifts for more than half the targets. The majority of the confirmed sources are star-forming galaxies at z > 1.5. In addition to the radio galaxy, two of its close-by companions (< 6'') also show AGN signatures. Ten sources, including the radio galaxy, lie within |z - 2.020 | < 0.015 (i.e., velocity offsets < 1500 km/s) and within projected 2 Mpc comoving of the radio galaxy. Additional evidence suggests not only that the galaxy structure associated with MRC0156-252 is a forming galaxy cluster but also that this structure is most probably embedded in a larger scale structure.

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arXiv2013

The galaxy environment of a QSO at z ~ 5.7

High-redshift quasars are believed to reside in massive halos in the early universe and should therefore be located in fields with overdensities of galaxies, which are thought to evolve into galaxy clusters seen in the local universe. However, despite many efforts, the relationship between galaxy overdensities and z~6 quasars is ambiguous. This can possibly be attributed to the difficulty of finding galaxies with accurate redshifts in the vicinity of z~6 quasars. So far, overdensity searches around z~6 quasars have been based on studies of Lyman break galaxies (LBGs), which probe a redshift range of Delta z ~ 1. This range is large enough to select galaxies that may not be physically related to the quasar. We use deep narrow- and broadband imaging to study the environment of the z=5.72 quasar ULAS J0203+0012. The redshift range probed by our narrow-band selection of Lyman alpha emitters (LAEs) is Delta z ~ 0.1, which is significantly narrower than the LBG searches. This is the first time that LAEs were searched for near a z~6 quasar, in an effort to provide clues about the environments of quasars at the end of the epoch of reionization. We find no enhancement of LAEs in the surroundings of ULAS J0203+0012 in comparison with blank fields. We explore different explanations and interpretations for this non-detection of a galaxy overdensity, including that (1) the strong ionization from the quasar may prevent galaxy formation in its immediate vicinity and (2) high-redshift quasars may not reside in the center of the most massive dark matter halos.

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arXiv2013

On the formation time scale of massive cluster ellipticals based on deep near-IR spectroscopy at z~2

We present improved constraints on the formation time scale of massive cluster galaxies based on rest-frame optical spectra of galaxies in a forming cluster located at z=2.16. The spectra are obtained with MOIRCS on the Subaru telescope with an integration time of ~7 hours. We achieve accurate redshift measurements by fitting SEDs using the spectra and broad-band photometry simultaneously, allowing us to identify probable cluster members. Clusters at low redshifts are dominated by quiescent galaxies, but we find that quiescent galaxies and star forming galaxies co-exist in this z=2 system. Interestingly, the quiescent galaxies form a weak red sequence in the process of forming. By stacking the spectra of star forming galaxies, we observe strong emission lines such as [OII] and [OIII] and we obtain a tentative hint of AGN activities in these galaxies. On the other hand, the stacked spectrum of the quiescent galaxies reveals a clear 4000A break with a possible CaII H+K absorption feature and strong emission lines such as [OII] are absent in the spectrum, confirming the quiescent nature of these galaxies. We then perform detailed spectral analyses of the stacked spectrum, which suggest that these massive quiescent galaxies formed at redshifts between 3 and 4 on a time scale of <~0.5 Gyr. This short formation time scale is not reproduced in recent numerical simulations. We discuss possible mechanisms for how these galaxies form 10^11 Msun stellar mass on a short time scale and become red and quiescent by z=2.

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arXiv2013

On the evolution and environmental dependence of the star formation rate versus stellar mass relation since z~2

This paper discusses the evolution of the correlation between galaxy star formation rates (SFRs) and stellar mass (M*) over the last ~10 Gyrs, particularly focusing on its environmental dependence. We first present the mid-infrared (MIR) properties of the H-alpha-selected galaxies in a rich cluster Cl0939+4713 at z=0.4. With a MIR stacking analysis, we find that the median SFR of H-alpha emitters is higher in higher-density environment at z=0.4. We also find that star-forming galaxies in high-density environment tend to have higher specific SFR (SSFR), although the trend is much less significant compared to that of SFR. This increase of SSFR in high-density environment is not visible when we consider the SFR derived from H-alpha alone, suggesting that the dust attenuation in galaxies depends on environment; galaxies in high-density environment tend to be dustier (by up to ~0.5 mag). We then discuss the environmental dependence of the SFR-M* relation for star-forming galaxies since z~2, by compiling our comparable, narrow-band selected, large H-alpha emitter samples in both distant cluster environments (from MAHALO-Subaru) and field environments (from HiZELS). We find that the SSFR of H-alpha-selected galaxies (at the fixed mass of log(M*/Msun)=10) rapidly evolves as (1+z)^3, but the SFR-M* relation is independent of the environment since z~2, as far as we rely on the H-alpha-based SFRs (with M*-dependent extinction correction). Even if we consider the possible environmental variation in the dust attenuation, we conclude that the difference in the SFR-M* relation between cluster and field star-forming galaxies is always small (~<0.2dex level) at any time in the history of the Universe since z~2. [abridged]

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arXiv2013

The SINS/zC-SINF Survey of z~2 Galaxy Kinematics: The Nature of Dispersion Dominated Galaxies

We analyze the spectra, spatial distributions and kinematics of Ha, [NII] and [SII] emission in a sample of 42, z~2.2 UV/optically selected star forming galaxies (SFGs) from the SINS & zC-SINF surveys, 35 of which were observed in the adaptive optics mode of SINFONI. This is supplemented by kinematic data from 48 z~1-2.5 galaxies from the literature. We find that the kinematic classification of the high-z SFGs as `dispersion dominated' or `rotation dominated' correlates most strongly with their intrinsic sizes. Smaller galaxies are more likely `dispersion-dominated' for two main reasons: 1) The rotation velocity scales linearly with galaxy size but intrinsic velocity dispersion does not depend on size, and as such, their ratio is systematically lower for smaller galaxies, and 2) Beam smearing strongly decreases large-scale velocity gradients and increases observed dispersion much more for galaxies with sizes at or below the resolution. Dispersion dominated SFGs may thus have intrinsic properties similar to `rotation dominated' SFGs, but are primarily more compact, lower mass, less metal enriched and may have higher gas fractions, plausibly because they represent an earlier evolutionary state.

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arXiv2012

The SINS/zC-SINF survey of z~2 galaxy kinematics: Outflow properties

Based on SINFONI Ha, [NII] and [SII] AO data of 30 z \sim 2 star-forming galaxies (SFGs) from the SINS and zcSINF surveys, we find a strong correlation of the Ha broad flux fraction with the star formation surface density of the galaxy, with an apparent threshold for strong outflows occurring at 1 Msun yr^-1 kpc^-2. Above this threshold, we find that SFGs with logm_\ast>10 have similar or perhaps greater wind mass loading factors (eta = Mdotout/SFR) and faster outflow velocities than lower mass SFGs. This trend suggests that the majority of outflowing gas at z \sim 2 may derive from high-mass SFGs, and that the z \sim 2 mass-metallicity relation is driven more by dilution of enriched gas in the galaxy gas reservoir than by the efficiency of outflows. The mass loading factor is also correlated with the SFR and inclination, such that more star-forming and face-on galaxies launch more powerful outflows. For galaxies that have evidence for strong outflows, we find that the broad emission is spatially extended to at least the half-light radius (\sim a few kpc). We propose that the observed threshold for strong outflows and the observed mass loading of these winds can be explained by a simple model wherein break-out of winds is governed by pressure balance in the disk. Using the ratio of the [SII] doublet in a broad and narrow component, we find that outflowing gas has a density of \sim10-100 cm^-3, significantly less than that of the star forming gas (600 cm^-3).

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arXiv2012

Massive starburst galaxies in a z=2.16 proto-cluster unveiled by panoramic H-alpha mapping

We present a panoramic narrow-band study of H-alpha emitters in the field of the z=2.16 proto-cluster around PKS1138-262 using MOIRCS on the Subaru Telescope. We find 83 H-alpha emitters down to a SFR(Ha)~10Msun/yr across a ~7'x7' region centered on the radio galaxy, and identify ~10-Mpc scale filaments of emitters running across this region. By examining the properties of H-alpha emitters within the large-scale structure, we find that galaxies in the higher-density environments at z=2.16 tend to have redder colours and higher stellar masses compared to galaxies in more underdense regions. We also find a population of H-alpha emitters with red colours ((J-Ks)>1), which are much more frequent in the denser environments and which have apparently very high stellar masses with M*>~10^11Msun, implying that these cluster galaxies have already formed a large part of their stellar mass before z~2. Spitzer Space Telescope 24-micron data suggests that many of these red H-alpha emitters are bright, dusty starbursts (rather than quiescent sources). We also find that the proto-cluster galaxies follow the same correlation between SFR and M* (the "main sequence") of z~2 field star-forming galaxies, but with an excess of massive galaxies. These very massive star-forming galaxies are not seen in our similar, previous study of z~1 clusters, suggesting that their star-formation activity has been shut off at 1<~z<~2. We infer that the massive red (but active) galaxies in this rich proto-cluster are likely to be the products of environmental effects, and they represent the accelerated galaxy formation and evolution in a biased high density region in the early Universe.

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arXiv2012

A large Hα survey at z=2.23, 1.47, 0.84 & 0.40: the 11 Gyr evolution of star-forming galaxies from HiZELS

This paper presents new deep and wide narrow-band surveys undertaken with UKIRT, Subaru and the VLT; a combined effort to select large, robust samples of H-alpha (Ha) emitters at z=0.40, 0.84, 1.47 and 2.23 (corresponding to look-back times of 4.2, 7.0, 9.2 and 10.6 Gyrs) in a uniform manner over ~2 deg^2 in the COSMOS and UDS fields. The deep Ha surveys reach ~3M_sun/yr out to z=2.2 for the first time, while the wide area and the coverage over two independent fields allow to greatly overcome cosmic variance. A total of 1742, 637, 515 and 807 Ha emitters are homogeneously selected at z=0.40, 0.84, 1.47 and 2.23, respectively, and used to determine the Ha luminosity function and its evolution. The faint-end slope is found to be -1.60+-0.08 over z=0-2.23, showing no evolution. The characteristic luminosity of SF galaxies, L*, evolves significantly as log[L*(z)]=0.45z+log[L*(z=0)]. This is the first time Ha has been used to trace SF activity with a single homogeneous survey at z=0.4-2.23. Overall, the evolution seen in Ha is in good agreement with the evolution seen using inhomogeneous compilations of other tracers of star formation, such as FIR and UV, jointly pointing towards the bulk of the evolution in the last 11 Gyrs being driven by a strong luminosity increase from z~0 to z~2.2. Our uniform analysis reveals an Ha star formation history of the Universe which can be simply parameterised by log(SFRD)=-2.1/(1+z) for z<2.23. Both the shape and normalisation of the Ha star formation history are consistent with the measurements of the stellar mass density growth, confirming that our Ha analysis traces the bulk of the formation of stars in the Universe up to z~2.2. Star formation activity since z~2.2 is responsible for ~95% of the total stellar mass density observed locally today (with half being assembled between z=1.2-2.2, and the rest since z<1.2).

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arXiv2012

GMASS ultradeep spectroscopy of galaxies at z~2 - VII. Sample selection and spectroscopy

The populations of both quiescent and actively star-forming galaxies at 1 1.4, called Galaxy Mass Assembly ultra-deep Spectroscopic Survey (GMASS). To determine redshifts and physical properties, such as metallicity, dust content, dynamical masses, and star formation history, we performed ultra-deep spectroscopy with the red-sensitive optical spectrograph FORS2 at the VLT. Our sample consists of objects, within the CDFS/GOODS area, detected at 4.5 micron, to be sensitive to stellar mass rather than star formation intensity. The spectroscopic targets were selected with a photometric redshift constraint (z>1.4) and magnitude constraints (B(AB)<26, I(AB)<26.5), which should ensure that these are faint, distant, and fairly massive galaxies. We present the sample selection, survey design, observations, data reduction, and spectroscopic redshifts. Up to 30 hours of spectroscopy of 174 spectroscopic targets and 70 additional objects enabled us to determine 210 redshifts, of which 145 are at z>1.4. From the redshifts and photometry, we deduce that the BzK selection criteria are efficient (82%) and suffer low contamination (11%). Several papers based on the GMASS survey show its value for studies of galaxy formation and evolution. We publicly release the redshifts and reduced spectra. In combination with existing and on-going additional observations in CDFS/GOODS, this data set provides a legacy for future studies of distant galaxies.

astro-ph.CO↗

arXiv2012

HST narrow-band search for extended Ly-alpha emission around two z>6 quasars

We search for extended Ly-alpha emission around two z>6 quasars, SDSS J1030+0524 (z=6.309) and SDSS J1148+5251 (z=6.419) using WFC3 narrow-band filters on board the Hubble Space Telescope. For each quasar, we collected two deep, narrow-band images, one sampling the Ly-alpha line+continuum at the quasar redshifts and one of the continuum emission redwards of the line. After carefully modeling the Point Spread Function, we find no evidence for extended Ly-alpha emission. These observations set 2-sigma limits of L(Ly-alpha, extended) < 3.2 x 10^{44} erg/s for J1030+0524 and L(Ly-alpha, extended) < 2.5 x 10^{44} erg/s for J1148+5251. Given the star formation rates typically inferred from (rest-frame) far-infrared measurements of z~6 quasars, these limits are well below the intrinsic bright Ly-alpha emission expected from the recombination of gas photoionized by the quasars or by the star formation in the host galaxies, and point towards significant Ly-alpha suppression or dust attenuation. However, small extinction values have been observed along the line of sight to the nuclei, thus reddening has to be coupled with other mechanisms for Ly-alpha suppression (e.g., resonance scattering). No Ly-alpha emitting companions are found, down to a 5-sigma sensitivity of ~ 1 x 10^{-17} erg/s/cm^2/arcsec^2 (surface brightness) and ~ 5 x 10^{-17} erg/s/cm^2 (assuming point sources).

astro-ph.CO↗

arXiv2012

Shocked Superwinds from the z~2 Clumpy Star-forming Galaxy, ZC406690

We have obtained high-resolution data of the z 2 ring-like, clumpy star-forming galaxy (SFG) ZC406690 using the VLT/SINFONI with AO (in K-band) and in seeing-limited mode (in H- and J-band). Our data includes all of the main strong optical emission lines: [OII], [OIII], Ha, Hb, [NII] and [SII]. We find broad, blueshifted Ha and [OIII] emission line wings in the spectra of the galaxy's massive, star-forming clumps (sigma \sim 85 km s^-1) and even broader wings (up to 70% of the total Ha flux, with sigma \sim 290 km s^-1) in regions spatially offset from the clumps by \sim 2 kpc. The broad emission likely originates from large-scale outflows with mass outflow rates from individual clumps that are 1-8x the SFR of the clumps. Based on emission line ratio diagnostics ([NII]/Ha and [SII]/Ha) and photoionization and shock models, we find that the emission from the clumps is due to a combination of photoionization from the star-forming regions and shocks generated in the outflowing component, with 5-30% of the emission deriving from shocks. In terms of the ionization parameter (6x10^7-10^8 cm/s, based on both the SFR and the O32 ratio), density (local electron densities of 300-1800 cm^-3 in and around the clumps, and ionized gas column densities of 1200-8000 Msol/pc^2), and SFR (10-40 Msol/yr), these clumps more closely resemble nuclear starburst regions of local ULIRGs and dwarf irregulars than HII regions in local galaxies. However, the star-forming clumps are not located in the nucleus as in local starburst galaxies but instead are situated in a ring several kpc from the center of their high-redshift host galaxy, and have an overall disk-like morphology. The two brightest clumps are quite different in terms of their internal properties, energetics and relative ages, and thus we are given a glimpse at two different stages in the formation and evolution of rapidly star-forming giant clumps at high-z.

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arXiv2011

Discovery Of Cold, Pristine Gas Possibly Accreting Onto An Overdensity Of Star-Forming Galaxies At Redshift z ~ 1.6

We report the discovery of large amounts of cold (T ~ 10^4 K), chemically young gas in an overdensity of galaxies at redshift z ~ 1.6 in the Great Observatories Origins Deep Survey southern field (GOODS-S). The gas is identified thanks to the ultra-strong Mg II absorption features it imprints in the rest-frame UV spectra of galaxies in the background of the overdensity. There is no evidence that the optically-thick gas is part of any massive galaxy (i.e. M_star > 4x10^9 M_sun), but rather is associated with the overdensity; less massive and fainter galaxies (25.5 < z_850 < 27.5 mag) have too large an impact parameter to be causing ultra-strong absorption systems, based on our knowledge of such systems. The lack of corresponding Fe II absorption features, not detected even in co-added spectra, suggests that the gas is chemically more pristine than the ISM and outflows of star-forming galaxies at similar redshift, including those in the overdensity itself, and comparable to the most metal-poor stars in the Milky Way halo. A crude estimate of the projected covering factor of the high-column density gas (N_H >~ 10^20 cm-2) based on the observed fraction of galaxies with ultra-strong absorbers is C_F ~ 0.04. A broad, continuum absorption profile extending to the red of the interstellar Mg II absorption line by <~ 2000 km/s is possibly detected in two independent co-added spectra of galaxies of the overdensity, consistent with a large-scale infall motion of the gas onto the overdensity and its galaxies. Overall, these findings provides the first tentative evidence of accretion of cold, chemically young gas onto galaxies at high redshift, possibly feeding their star formation activity. The fact that the galaxies are members of a large structure, as opposed to field galaxies, might play a significant role in our ability to detect the accreting gas.

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arXiv2011

Evidence for non-evolving FeII/MgII ratios in rapidly accreting z~6 QSOs

[abridged] Quasars (QSOs) at the highest known redshift (z~6) are unique probes of the early growth of supermassive black holes (BHs). Until now, only the most luminous QSOs have been studied, often one object at a time. Here we present the most extensive consistent analysis to date of z>4 QSOs with observed NIR spectra, combining three new z~6 objects from our ongoing VLT-ISAAC program with nineteen 4 =-0.37 (Lbol/LEdd~0.43) with a scatter of 0.20 dex for the z>4 sample and the =-0.80 (Lbol/LEdd~0.16) with a scatter of 0.24 dex for the 0.35 4 sources are accreting significantly faster than the lower-redshift ones. We show that the derived FeII/MgII ratios depend sensitively on the performed analysis: our self-consistent, homogeneous analysis significantly reduces the FeII/MgII scatter found in previous studies. The measured FeII/MgII line ratios show no sign of evolution with cosmic time in the redshift range 4<z<6.5 [...]

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arXiv2010

Galaxy protocluster candidates at 1.6<z<2

We present a study of protoclusters associated with high redshift radio galaxies. We imaged MRC1017-220 (z=1.77) and MRC0156-252 (z=2.02) using the near-infrared wide-field (7.5'x7.5') imager VLT/HAWK-I in the Y, H and Ks bands. We present the first deep Y-band galaxy number counts within a large area (200 arcmin2). We then develop a purely near-infrared colour selection technique to isolate galaxies at 1.6<z<3 that may be associated with the two targets, dividing them into (i) red passively evolving or dusty star-forming galaxies or (ii) blue/star-formation dominated galaxies with little or no dust. Both targeted fields show an excess of star-forming galaxies with respect to control fields. No clear overdensity of red galaxies is detected in the surroundings of MRC1017-220 although the spatial distribution of the red galaxies resembles a filament-like structure within which the radio galaxy is embedded. In contrast, a significant overdensity of red galaxies is detected in the field of MRC0156-252, ranging from a factor of 2-3 times the field density at large scales (2.5Mpc, angular distance) up to a factor of 3-4 times the field density within a 1Mpc radius of the radio galaxy. Half of these red galaxies have colours consistent with red sequence models at z~2, with a large fraction being bright (Ks<21.5, i.e. massive). In addition, we also find a small group of galaxies within 5" of MRC0156-252 suggesting that the radio galaxy has multiple companions within ~50 kpc. We conclude that the field of MRC0156-252 shows many remarkable similarities with the well-studied protocluster surrounding PKS1138-262 (z=2.16) suggesting that MRC0156-252 is associated with a galaxy protocluster at z~2.

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arXiv2010

The Environmental Dependence of Galaxy Properties at z=2

We report on the environmental dependence of galaxy properties at z=2.15. We construct multi-band photometric data sets in the (proto-)cluster PKS1138-26 field and in the GOODS field. We then fit spectral energy distributions of the galaxies with model templates generated with the latest stellar population synthesis code and derive physical properties of galaxies from the fits. To quantify the environmental dependence of galaxy properties, a special care is taken of systematic errors -- we use data sets that have almost the same wavelength samplings, use the same code to fit SEDs with the same set of templates, and compare relative differences between the two samples. We find that the PKS1138 galaxies have similar ages, shorter star formation time scales, lower star formation rates, and weaker dust extinction compared to the GOODS galaxies at z~2. This trend is similar to that observed locally, suggesting that the environmental dependence of galaxy properties is already partly in place as early as z=2.15. We show that the PKS1138 galaxies assemble the bulk of their masses ~1 Gyr earlier than field galaxies, i.e., the galaxy formation depends on environment. Galaxy mergers should frequently occur during the first collapse of clusters and they might play an important role in driving the observed environmental dependence of galaxy properties at z=2.15.

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