arXiv subjects

Emeric Le Floc'h

Publications and source records attributed to Emeric Le Floc'h.

At least 37 records · Page 2Linked to original sources

arXiv2014

Star Formation at $4 < z < 6$ From the Spitzer Large Area Survey with Hyper-Suprime-Cam (SPLASH)

Using the first 50% of data collected for the Spitzer Large Area Survey with Hyper-Suprime-Cam (SPLASH) observations on the 1.8 deg$^2$ Cosmological Evolution Survey (COSMOS) we estimate the masses and star formation rates of 3398 $M_*>10^{10}M_\odot $ star-forming galaxies at $4 < z < 6$ with a substantial population up to $M_* \gtrsim 10^{11.5} M_\odot$. We find that the strong correlation between stellar mass and star formation rate seen at lower redshift (the "main sequence" of star-forming galaxies) extends to $z\sim6$. The observed relation and scatter is consistent with a continued increase in star formation rate at fixed mass in line with extrapolations from lower-redshift observations. It is difficult to explain this continued correlation, especially for the most massive systems, unless the most massive galaxies are forming stars near their Eddington-limited rate from their first collapse. Furthermore, we find no evidence for moderate quenching at higher masses, indicating quenching either has not occurred prior to $z \sim 6$ or else occurs rapidly, so that few galaxies are visible in transition between star-forming and quenched.

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arXiv2014

AGN Emission Line Diagnostics and the Mass-Metallicity Relation up to Redshift z~2: the Impact of Selection Effects and Evolution

Emission line diagnostic diagrams probing the ionization sources in galaxies, such as the Baldwin-Phillips-Terlevich (BPT) diagram, have been used extensively to distinguish AGN from purely star-forming galaxies. Yet, they remain poorly understood at higher redshifts. We shed light on this issue with an empirical approach based on a z~0 reference sample built from ~300,000 SDSS galaxies, from which we mimic selection effects due to typical emission line detection limits at higher redshift. We combine this low-redshift reference sample with a simple prescription for luminosity evolution of the global galaxy population to predict the loci of high-redshift galaxies on the BPT and Mass-Excitation (MEx) diagnostic diagrams. The predicted bivariate distributions agree remarkably well with direct observations of galaxies out to z~1.5, including the observed stellar mass-metallicity (MZ) relation evolution. As a result, we infer that high-redshift star-forming galaxies are consistent with having "normal" ISM properties out to z~1.5, after accounting for selection effects and line luminosity evolution. Namely, their optical line ratios and gas-phase metallicities are comparable to that of low-redshift galaxies with equivalent emission-line luminosities. In contrast, AGN narrow-line regions may show a shift toward lower metallicities at higher redshift. While a physical evolution of the ISM conditions is not ruled out for purely star-forming galaxies, and may be more important starting at z>2, we find that reliably quantifying this evolution is hindered by selections effects. The recipes provided here may serve as a basis for future studies toward this goal. Code to predict the loci of galaxies on the BPT and MEx diagnostic diagrams, and the MZ relation as a function of emission line luminosity limits, is made publicly available.

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arXiv2014

Clustering, host halos and environment of z$\sim$2 galaxies as a function of their physical properties

Using a sample of 25683 star-forming and 2821 passive galaxies at $z\sim2$, selected in the COSMOS field following the BzK color criterion, we study the hosting halo mass and environment of galaxies as a function of their physical properties. Spitzer and Herschel provide accurate SFR estimates for starburst galaxies. We measure the auto- and cross-correlation functions of various galaxy sub-samples and infer the properties of their hosting halos using both an HOD model and the linear bias at large scale. We find that passive and star-forming galaxies obey a similarly rising relation between the halo and stellar mass. The mean host halo mass of star forming galaxies increases with the star formation rate between 30 and 200 M$_\odot$.yr$^{-1}$, but flattens for higher values, except if we select only main-sequence galaxies. This reflects the expected transition from a regime of secular co-evolution of the halos and the galaxies to a regime of episodic starburst. We find similar large scale biases for main-sequence, passive, and starburst galaxies at equal stellar mass, suggesting that these populations live in halos of the same mass. We detect an excess of clustering on small scales for passive galaxies and showed, by measuring the large-scale bias of close pairs, that this excess is caused by a small fraction ($\sim16%$) of passive galaxies being hosted by massive halos ($\sim 3 \times 10^{13}$ M$_\odot$) as satellites. Finally, extrapolating the growth of halos hosting the z$\sim$2 population, we show that M$_\star \sim 10^{10}$ M$_\odot$ galaxies at z$\sim$2 will evolve, on average, into massive (M$_\star \sim 10^{11}$ M$_\odot$), field galaxies in the local Universe and M$_\star \sim 10^{11}$ M$_\odot$ galaxies at z=2 into local, massive, group galaxies. The most massive main-sequence galaxies and close pairs of massive, passive galaxies end up in today's clusters.

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arXiv2013

Multi-Wavelength SEDs of Herschel Selected Galaxies in the COSMOS Field

We combine Herschel PACS and SPIRE maps of the full 2 deg^2 COSMOS field with existing multi-wavelength data to obtain template and model-independent optical-to-far-infrared spectral energy distributions (SEDs) for 4,218 Herschel-selected sources with log(L_IR/L_sun) = 9.4-13.6 and z = 0.02-3.54. Median SEDs are created by binning the optical to far-infrared (FIR) bands available in COSMOS as a function of infrared luminosity. Herschel probes rest-frame wavelengths where the bulk of the infrared radiation is emitted, allowing us to more accurately determine fundamental dust properties of our sample of infrared luminous galaxies. We find that the SED peak wavelength (lambda_peak) decreases and the dust mass (M_dust) increases with increasing total infrared luminosity (L_IR). In the lowest infrared luminosity galaxies (log(L_IR/L_sun) = 10.0-11.5), we see evidence of Polycyclic Aromatic Hydrocarbons (PAH) features (lambda=7-9 um), while in the highest infrared luminosity galaxies (L_IR > 10^12 L_sun) we see an increasing contribution of hot dust and/or power-law emission, consistent with the presence of heating from an active galactic nucleus (AGN). We study the relationship between stellar mass and star formation rate of our sample of infrared luminous galaxies and find no evidence that Herschel-selected galaxies follow the SFR/M_* "main sequence" as previously determined from studies of optically selected, star-forming galaxies. Finally, we compare the mid-infrared (MIR) to FIR properties of our infrared luminous galaxies using the previously defined diagnostic, IR8 = L_IR / L_8, and find that galaxies with L_IR > 10^11.3 L_sun tend to systematically lie above (~3-5) the IR8 "infrared main sequence", suggesting either suppressed PAH emission or an increasing contribution from AGN heating.

astro-ph.CO↗

arXiv2013

The role of galaxy interaction in the SFR-M relation: characterizing morphological properties of Herschel-selected galaxies at 0.2<z<1.5

Galaxy interactions/mergers have been shown to dominate the population of IR luminous galaxies (log(LIR)>11.6Lsun) in the local Universe (z<0.25). Recent studies based on the relation between galaxies' star formation rates and stellar mass (the SFR-M relation or the galaxy main sequence (MS)) have suggested that galaxy interaction/mergers may only become significant when galaxies fall well above the galaxy MS. Since the typical SFR at given M increases with redshift, the existence of galaxy MS implies that massive, IR-luminous galaxies at high-z may not necessarily be driven by galaxy interactions. We examine the role of galaxy interactions in the SFR-M relation by carrying out a morphological analysis of 2084 Herschel-selected galaxies at 0.2 < z < 1.5 in the COSMOS field. Herschel-PACS and -SPIRE observations covering the full 2-deg^2 COSMOS field provide one of the largest far-IR selected samples of high-redshift galaxies with well-determined redshifts to date, with sufficient sensitivity at z ~ 1, to sample objects lying on and above the galaxy MS. Using a detailed visual classification scheme, we show that the fraction of "disk galaxies" decreases and the fraction of "irregular" galaxies increases systematically with increasing LIR out to z ~ 1.5 and z ~ 1.0, respectively. At log(LIR) > 11.5 Lsun, >50% of the objects show evident features of strongly interacting/merger systems, where this percentage is similar to the studies of local IR-luminous galaxies. The fraction of interacting/merger systems also systematically increases with the deviation from the SFR-M relation, supporting the view that galaxies fall above the MS are more dominated by mergers than the MS galaxies. Meanwhile, we find that ~18% of massive IR-luminous MS galaxies are classified as interacting systems, where this population may not evolve through the evolutionary track predicted by a simple gas exhaustion model.

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arXiv2013

Characterisation of SCUBA-2 450um and 850um-selected Galaxies in the COSMOS Field

We present deep 450um and 850um observations of a large, uniformly covered 394arcmin^2 area in the COSMOS field obtained with the SCUBA-2 instrument on the James Clerk Maxwell Telescope (JCMT). We achieve root-mean-square noise values of 4.13mJy at 450um and 0.80mJy at 850um. The differential and cumulative number counts are presented and compared to similar previous works. Individual point sources are identified at >3.6sigma significance, a threshold corresponding to a 3-5% sample contamination rate. We identify 78 sources at 450um and 99 at 850um, with flux densities S450=13-37mJy and S850=2-16mJy. Only 62-76% of 450um sources are 850um detected and 61-81% of 850um sources are 450um detected. The positional uncertainties at 450um are small (1-2.5") and therefore allow a precise identification of multiwavelength counterparts without reliance on detection at 24um or radio wavelengths; we find that only 44% of 450um-selected galaxies and 60% of 850um-sources have 24um or radio counterparts. 450um-selected galaxies peak at =1.95+-0.19 and 850um=selected galaxies peak at =2.16+-0.11. The two samples occupy similar parameter space in redshift and luminosity, while their median SED peak wavelengths differ by ~10-50um (translating to deltaTdust =8-12K, where 450um-selected galaxies are warmer). The similarities of the 450um and 850um populations, yet lack of direct overlap between them, suggests that submillimeter surveys conducted at any single far-infrared wavelength will be significantly incomplete (~>30%) at censusing infrared-luminous star formation at high-z.

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arXiv2013

Encoding the infrared excess (IRX) in the NUVrK color diagram for star-forming galaxies

We present an empirical method of assessing the star formation rate (SFR) of star-forming galaxies based on their locations in the rest-frame color-color diagram (NUV-r) vs (r-K). By using the Spitzer 24 micron sample in the COSMOS field (~16400 galaxies with 0.2 < z < 1.3) and a local GALEX-SDSS-SWIRE sample (~700 galaxies with z < 0.2), we show that the mean infrared excess = < L_IR / L_UV > can be described by a single vector, NRK, that combines the two colors. The calibration between and NRK allows us to recover the IR luminosity, L_IR, with an accuracy of ~0.21 dex for the COSMOS sample and ~0.27 dex for the local one. The SFRs derived with this method agree with the ones based on the observed (UV+IR) luminosities and on the spectral energy distribution fitting for the vast majority (~85 %) of the star-forming population. Thanks to a library of model galaxy SEDs with realistic prescriptions for the star formation history, we show that we need to include a two-component dust model (i.e., birth clouds and diffuse ISM) and a full distribution of galaxy inclinations in order to reproduce the behavior of the stripes in the NUVrK diagram. In conclusion, the NRK method, based only on rest-frame UV and optical colors available in most of the extragalactic fields, offers a simple alternative of assessing the SFR of star-forming galaxies in the absence of far-IR or spectral diagnostic observations.

astro-ph.CO↗

arXiv2013

A correlation between star formation rate and average black hole accretion in star forming galaxies

We present a measurement of the average supermassive black hole accretion rate (BHAR) as a function of star formation rate (SFR) for galaxies in the redshift range 0.25<z<0.8. We study a sample of 1,767 far-IR selected star-forming galaxies in the 9 deg^2 Boötes multiwavelength survey field. The SFR is estimated using 250 micron observations from the Herschel Space Observatory, for which the contribution from the AGN is minimal. In this sample, 121 AGNs are directly identified using X-ray or mid-IR selection criteria. We combined these detected AGNs and an X-ray stacking analysis for undetected sources to study the average BHAR for all of the star-forming galaxies in our sample. We find an almost linear relation between the average BHAR (in M_sun/year) and the SFR (in M_sun/year) for galaxies across a wide SFR range 0.85<log SFR<2.56 : log BHAR=(-3.72\pm0.52)+(1.05\pm0.33) log SFR. This global correlation between SFR and average BHAR is consistent with a simple picture in which SFR and AGN activity are tightly linked over galaxy evolution timescales.

astro-ph.CO↗

arXiv2012

The Clustering of Extremely Red Objects

We measure the clustering of Extremely Red Objects (EROs) in ~8 deg^2 of the NOAO Deep Wide Field Survey Boötes field in order to establish robust links between ERO z~1.2 and local galaxy z<0.1 populations. Three different color selection criteria from the literature are analyzed to assess the consequences of using different criteria for selecting EROs. Specifically, our samples are (R-K_s)>5.0 (28,724 galaxies), (I-K_s)>4.0 (22,451 galaxies) and (I-[3.6])>5.0 (64,370 galaxies). Magnitude-limited samples show the correlation length (r_0) to increase for more luminous EROs, implying a correlation with stellar mass. We can separate star-forming and passive ERO populations using the (K_s-[24]) and ([3.6]-[24]) colors to K_s=18.4 and [3.6]=17.5, respectively. Star-forming and passive EROs in magnitude limited samples have different clustering properties and host dark halo masses, and cannot be simply understood as a single population. Based on the clustering, we find that bright passive EROs are the likely progenitors of >4L^* elliptical galaxies. Bright EROs with ongoing star formation were found to occupy denser environments than star-forming galaxies in the local Universe, making these the likely progenitors of >L^* local ellipticals. This suggests that the progenitors of massive >4L^* local ellipticals had stopped forming stars by z>1.2, but that the progenitors of less massive ellipticals (down to L^*) can still show significant star formation at this epoch.

astro-ph.CO↗

arXiv2012

PAH Emission in Powerful High-Redshift Radio Galaxies

We present the mid-infrared spectra of seven of the most powerful radio-galaxies known to exist at 1.5 < z < 2.6. The radio emission of these sources is dominated by the AGN with 500 MHz luminosities in the range 10^27.8 - 10^29.1 W/Hz. The AGN signature is clearly evident in the mid-infrared spectra, however, we also detect polycyclic aromatic hydrocarbons emission, indicative of prodigious star formation at a rate of up to ~1000 Msun/yr. Interestingly, we observe no significant correlation between AGN power and star formation in the host galaxy. We also find most of these radio galaxies to have weak 9.7 um silicate absorption features (tau_{9.7 um} < 0.8) which implies that their mid-IR obscuration is predominantly due to the dusty torus that surrounds the central engine, rather than the host galaxy. The tori are likely to have an inhomogeneous distribution with the obscuring structure consisting of individual clouds. We estimate that these radio galaxies have already formed the bulk of their stellar mass and appear to lie at a stage in their evolution where the obscured AGN dominates the energy output of the system but star formation is also prevalent.

astro-ph.CO↗

arXiv2012

Star formation in LINER host galaxies at z~0.3

We present the results of a Herschel-PACS study of a sample of 97 LINERs at redshift z\sim 0.3 selected from the zCOSMOS survey. Of these sources, 34 are detected in a least one PACS band, enabling reliable estimates of the far-infrared L(FIR) luminosities, and a comparison to the FIR luminosities of local LINERs. Many of our PACS-detected LINERs are also UV sources detected by Galex. Assuming that the FIR is produced in young dusty star-forming regions, the typical star-formation rates (SFRs) for the host galaxies in our sample is \sim 10 M_Sun yr-1, 1-2 orders of magnitude larger than in many local LINERs. Given stellar masses inferred from optical/NIR photometry of the (unobscured) evolved stellar populations, we find that the entire sample lies close to the star-forming "main sequence" for galaxies at redshift 0.3. For young star-forming regions, the Hα- and UV-based estimates of the SFRs are much smaller than the FIR-based estimates, by factors \sim 30, even assuming that all of the Hα emission is produced by O-star ionization rather than by the AGNs. These discrepancies may be due to large (and uncertain) extinctions towards the young stellar systems. Alternatively, the Hα and UV emissions could be tracing residual star-formation in an older less obscured population with decaying star formation. We also compare L(SF) and L(AGN) in local LINERs and in our sample and comment on the problematic use of several line diagnostic diagrams in cases similar to the sample under study.

astro-ph.CO↗

arXiv2012

An Observed Link between Active Galactic Nuclei and Violent Disk Instabilities in High-Redshift Galaxies

We provide evidence for a correlation between the presence of giant clumps and the occurrence of active galactic nuclei (AGN) in disk galaxies. Giant clumps of 10^8-9 Msun arise from violent gravitational instability in gas-rich galaxies, and it has been proposed that this instability could feed supermassive black holes (BH). We use emission line diagnostics to compare a sample of 14 clumpy (unstable) disks and a sample of 13 smoother (stable) disks at redshift z~0.7. The majority of clumpy disks in our sample have a high probability of containing AGN. Their [OIII] emission line is strongly excited, inconsistent with low-metallicity star formation alone. [NeIII] excitation is also higher. Stable disks rarely have such properties. Stacking ultra sensitive Chandra observations (4 Ms) reveals an X-ray excess in clumpy galaxies, which cannot be solely due to star formation and confirms the presence of AGN. The clumpy galaxies in our intermediate-redshift sample have properties typical of gas-rich disk galaxies rather than mergers, being in particular on the Main Sequence of star formation. This suggests that our findings apply to the physically-similar and numerous gas-rich unstable disks at z>1. Using the observed [OIII] and X-ray luminosities, we conservatively estimate that AGN hosted by clumpy disks have typical bolometric luminosities of the order of a few 10^43 erg/s, BH growth rates ~10^-2 Msun/yr, and that these AGN are substantially obscured in X-rays. This moderate-luminosity mode could be sufficient to provide a large fraction of today's BH mass over a couple of Gyr given that our observations suggest a high duty cycle (>10%), accretion bursts with higher luminosities being possible over shorter phases. The observed evolution of disk instabilities with mass and redshift could explain the simultaneous downsizing of star formation and of BH growth.

astro-ph.CO↗

arXiv2012

CO J=2-1 line emission in cluster galaxies at z~1: fueling star formation in dense environments

We present observations of CO J=2-1 line emission in infrared-luminous cluster galaxies at z~1 using the IRAM Plateau de Bure Interferometer. Our two primary targets are optically faint, dust-obscured galaxies (DOGs) found to lie within 2 Mpc of the centers of two massive (>10^14 Msun) galaxy clusters. CO line emission is not detected in either DOG. We calculate 3-sigma upper limits to the CO J=2-1 line luminosities, L'_CO < 6.08x10^9 and < 6.63x10^9 K km/s pc^2. Assuming a CO-to-H_2 conversion factor derived for ultraluminous infrared galaxies in the local Universe, this translates to limits on the cold molecular gas mass of M_H_2 < 4.86x10^9 Msun and M_H_2 < 5.30x10^9 Msun. Both DOGs exhibit mid-infrared continuum emission that follows a power-law, suggesting that an AGN contributes to the dust heating. As such, estimates of the star formation efficiencies in these DOGs are uncertain. A third cluster member with an infrared luminosity, L_IR < 7.4x10^11 Lsun, is serendipitously detected in CO J=2-1 line emission in the field of one of the DOGs located roughly two virial radii away from the cluster center. The optical spectrum of this object suggests that it is likely an obscured AGN, and the measured CO line luminosity is L'_CO = (1.94 +/- 0.35)x10^10 K km/s pc^2, which leads to an estimated cold molecular gas mass M_H_2 = (1.55+/-0.28)x10^10 Msun. A significant reservoir of molecular gas in a z~1 galaxy located away from the cluster center demonstrates that the fuel can exist to drive an increase in star-formation and AGN activity at the outskirts of high-redshift clusters.

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arXiv2012

Resolving the Galaxies within a Giant Lya Nebula: Witnessing the Formation of a Galaxy Group?

Detailed analysis of the substructure of Lya nebulae can put important constraints on the physical mechanisms at work and the properties of galaxies forming within them. Using high resolution HST imaging of a Lya nebula at z~2.656, we have taken a census of the compact galaxies in the vicinity, used optical/near-infrared colors to select system members, and put constraints on the morphology of the spatially-extended emission. The system is characterized by (a) a population of compact, low luminosity (~0.1 L*) sources --- 17 primarily young, small (Re~1-2 kpc), disky galaxies including an obscured AGN --- that are all substantially offset (>20 kpc) from the line-emitting nebula; (b) the lack of a central galaxy at or near the peak of the Lya emission; and (c) several nearly coincident, spatially extended emission components --- Lya, HeII, and UV continuum --- that are extremely smooth. These morphological findings are difficult to reconcile with theoretical models that invoke outflows, cold flows, or resonant scattering, suggesting that while all of these physical phenomena may be occurring, they are not sufficient to explain the powering and large extent of Lya nebulae. In addition, although the compact galaxies within the system are irrelevant as power sources, the region is significantly overdense relative to the field galaxy population (by at least a factor of 4). These observations provide the first estimate of the luminosity function of galaxies within an individual Lya nebula system, and suggest that large Lya nebulae may be the seeds of galaxy groups or low-mass clusters.

astro-ph.CO↗

arXiv2011

Galaxy Structure and Mode of Star Formation in the SFR-Mass Plane from z ~ 2.5 to z ~ 0.1

We analyze the dependence of galaxy structure (size and Sersic index) and mode of star formation (Σ_SFR and SFR_IR/SFR_UV) on the position of galaxies in the SFR versus Mass diagram. Our sample comprises roughly 640000 galaxies at z~0.1, 130000 galaxies at z~1, and 36000 galaxies at z~2. Structural measurements for all but the z~0.1 galaxies were based on HST imaging, and SFRs are derived using a Herschel-calibrated ladder of SFR indicators. We find that a correlation between the structure and stellar population of galaxies (i.e., a 'Hubble sequence') is already in place since at least z~2.5. At all epochs, typical star-forming galaxies on the main sequence are well approximated by exponential disks, while the profiles of quiescent galaxies are better described by de Vaucouleurs profiles. In the upper envelope of the main sequence, the relation between the SFR and Sersic index reverses, suggesting a rapid build-up of the central mass concentration in these starbursting outliers. We observe quiescent, moderately and highly star-forming systems to co-exist over an order of magnitude or more in stellar mass. At each mass and redshift, galaxies on the main sequence have the largest size. The rate of size growth correlates with specific SFR, and so does Σ_SFR at each redshift. A simple model using an empirically determined SF law and metallicity scaling, in combination with an assumed geometry for dust and stars is able to relate the observed Σ_SFR and SFR_IR/SFR_UV, provided a more patchy dust geometry is assumed for high-redshift galaxies.

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arXiv2011

Constraining Dust and Molecular Gas Properties in Lyman Alpha Blobs at z~3

In order to constrain the bolometric luminosities, dust properties and molecular gas content of giant Lyman alpha nebulae, the so-called Lyman alpha blobs, we have carried out a study of dust continuum and CO line emission in two well-studied representatives of this population at z ~ 3: a Lya blob discovered by its strong Spitzer MIPS 24um detection (LABd05; Dey et al. 2005) and the Steidel blob 1 (SSA22-LAB01; Steidel et al. 2000). We find that the spectral energy distribution of LABd05 is well described by an AGN-starburst composite template with L(FIR) = (4.0 +/- 0.5) x 10^12 Lsun, comparable to high-z sub-millimeter galaxies and ultraluminous infrared galaxies. New APEX/LABOCA 870um measurements rule out the reported SCUBA detection of the SSA22-LAB01 (S[850um] = 16.8 mJy) at the > 4sigma level. Consistent with this, ultra-deep Plateau de Bure Interferometer (PdBI) observations with ~2arcsec spatial resolution also fail to detect any 1.2mm continuum source down to ~0.45mJy per beam (3sigma). Combined with the existing (sub)mm observations in the literature, we conclude that the FIR luminosity of SSA22-LAB01 remains uncertain. No CO line is detected in either case down to integrated flux limits of (Snu dV) < 0.25--1.0 Jy km/s, indicating a modest molecular gas reservoir, M(H_2) < 1--3 x 10^10 Msun. The non-detections exclude, with high significance (12 sigma), the previous tentative detection of a CO(4-3) line in the SSA22-LAB01. The increased sensitivity afforded by ALMA will be critical in studying molecular gas and dust in these interesting systems.

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arXiv2011

Galaxies in X-ray Groups I: Robust Membership Assignment and the Impact of Group Environments on Quenching

Understanding the mechanisms that lead dense environments to host galaxies with redder colors, more spheroidal morphologies, and lower star formation rates than field populations remains an important problem. As most candidate processes ultimately depend on host halo mass, accurate characterizations of the local environment, ideally tied to halo mass estimates and spanning a range in halo mass and redshift are needed. In this work, we present and test a rigorous, probabalistic method for assigning galaxies to groups based on precise photometric redshifts and X-ray selected groups drawn from the COSMOS field. The groups have masses in the range 10^13 < M_200c/M_sun < 10^14 and span redshifts 0<z<1. We characterize our selection algorithm via tests on spectroscopic subsamples, including new data obtained at the VLT, and by applying our method to detailed mock catalogs. We find that our group member galaxy sample has a purity of 84% and completeness of 92% within 0.5 R200c. We measure the impact of uncertainties in redshifts and group centering on the quality of the member selection with simulations based on current data as well as future imaging and spectroscopic surveys. As a first application of our new group member catalog which will be made publicly available, we show that member galaxies exhibit a higher quenched fraction compared to the field at fixed stellar mass out to z~1, indicating a significant relationship between star formation and environment at group scales. We also address the suggestion that dusty star forming galaxies in such groups may impact the high-l power spectrum of the cosmic microwave background and find that such a population cannot explain the low power seen in recent SZ measurements.

astro-ph.CO↗

arXiv2011

The Dirt on Dry Mergers

Using data from the Spitzer Space Telescope, we analyze the mid-infrared (3-70 micron) spectral energy distributions of dry merger candidates in the Bootes field of the NOAO Deep Wide-Field Survey. These candidates were selected by previous authors to be luminous, red, early-type galaxies with morphological evidence of recent tidal interactions. We find that a significant fraction of these candidates exhibit 8 and 24 micron excesses compared to expectations for old stellar populations. We estimate that a quarter of dry merger candidates have mid-infrared-derived star formation rates greater than ~1 MSun/yr. This represents a "frosting" on top of a large old stellar population, and has been seen in previous studies of elliptical galaxies. Further, the dry merger candidates include a higher fraction of starforming galaxies relative to a control sample without tidal features. We therefore conclude that the star formation in these massive ellipticals is likely triggered by merger activity. Our data suggest that the mergers responsible for the observed tidal features were not completely dry, and may be minor mergers involving a gas-rich dwarf galaxy.

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