Search

We study the relationship between stellar mass, star formation rate (SFR), ionization state, and gas-phase metallicity for a sample of 41 normal star-forming galaxies at 3 ≾ z ≾ 3.7. The gas-phase oxygen abundance, ionization parameter, and electron density of ionized gas are derived from rest-frame optical strong emission lines measured on near-infrared spectra obtained with Keck/Multi-Object Spectrograph for Infra-Red Exploration. We remove the effect of these strong emission lines in the broadband fluxes to compute stellar masses via spectral energy distribution fitting, while the SFR is derived from the dust-corrected ultraviolet luminosity. The ionization parameter is weakly correlated with the specific SFR, but otherwise the ionization parameter and electron density do not correlate with other global galaxy properties such as stellar mass, SFR, and metallicity. The mass–metallicity relation (MZR) at z ≃ 3.3 shows lower metallicity by ≃ 0.7 dex than that at z = 0 at the same stellar mass. Our sample shows an offset by ≃ 0.3 dex from the locally defined mass–metallicity–SFR relation, indicating that simply extrapolating such a relation to higher redshift may predict an incorrect evolution of MZR. Furthermore, within the uncertainties we find no SFR–metallicity correlation, suggesting a less important role of SFR in controlling the metallicity at high redshift. We finally investigate the redshift evolution of the MZR by using the model by Lilly et al., finding that the observed evolution from z = 0 to z ≃ 3.3 can be accounted for by the model assuming a weak redshift evolution of the star formation efficiency.

Even 10 billion years ago, the cores of the first galaxy clusters are often found to host a characteristic population of massive galaxies with already suppressed star formation. Here we search for distant cluster candidates at z similar to 2 using massive passive galaxies as tracers. With a sample of similar to 40 spectroscopically confirmed passive galaxies at 1.3 < z < 2.1, we tuned photometric redshifts of several thousand passive sources in the 2 sq. deg COSMOS field. This allowed us to map their density in redshift slices, probing the largescale structure in the COSMOS field as traced by passive sources. We report here on the three strongest passive galaxy overdensities that we identify in the range 1.5 < z < 2.5. While the actual nature of these concentrations still needs to be confirmed, we discuss their identification procedure and the arguments supporting them as candidate galaxy clusters (probably in the mid-10(13) M-circle dot range). Although this search approach is probably biased toward more evolved structures, it has the potential of selecting still rare, clusterlike environments close to their epoch of first appearance, enabling new investigations of the evolution of galaxies in the context of structure growth.

We investigate the stellar population properties of a sample of 24 massive quenched galaxies at 1.25 < z(spec) < 2.09 identified in the COSMOS field with our Subaru/Multi-object Infrared Camera and Spectrograph near-IR spectroscopic observations. Tracing the stellar population properties as close to their major formation epoch as possible, we try to put constraints on the star formation history, post-quenching evolution, and possible progenitor star-forming populations for such massive quenched galaxies. By using a set of Lick absorption line indices on a rest-frame optical composite spectrum, the average age, metallicity [Z/H], and alpha-to-iron element abundance ratio [alpha/Fe] are derived as log(age Gyr) = 0.04(-0.08)(+0.10-), [Z/H] = 0.24(-0.14)(+0.20), and [alpha/Fe] = 0.3(-0.12)(+0.12), respectively. If our sample of quenched galaxies at < z > = 1.6 is evolved passively to z = 0, their stellar population properties will align in excellent agreement with local counterparts at similar stellar velocity dispersions, which qualifies them as progenitors of local massive early-type galaxies. Redshift evolution of stellar population ages in quenched galaxies combined with low redshift measurements from the literature suggests a formation redshift of z(r) similar to 2.3, around which the bulk of stars in these galaxies have been formed. The measured [alpha/Fe] value indicates a star formation timescale of less than or similar to 1 Gyr, which can be translated into a specific star formation rate of similar or equal to 1 Gyr(-1) prior to quenching. Based on these findings, we discuss identifying possible progenitor star-forming galaxies at z similar or equal to 2.3. We identify normal star-forming galaxies, i.e., those on the star-forming main sequence, followed by a rapid quenching event, as likely precursors of the quenched galaxies at < z > = 1.6 presented here.

We have analyzed Hubble Space Telescope WFPC2 F555W and F814W (i.e., V and I) images for 15 elliptical galaxies with kinematically distinct cores. For each of them? we have derived surface brightness and isophotal parameter profiles in the two bands, color maps, and radial profiles in V-I. Most galaxies show patchy dust absorption close to their nuclei, However, there are generally no indications of homogeneous, diffuse dust components close to the nuclei. The nuclear colors in the unobscured regions are most likely representative of the central stellar populations. We have detected photometric evidence for faint stellar disks, on scales of a few tens to a few arcseconds, in seven galaxies, namely NGC 1427, 1439, 1700, 4365, 4406, 4494, and 5322, In NGC 1700, the isophotes are slightly boxy at the scale of the counterrotating component and disky at larger radii. We find no difference in V-I color greater than 0.02 mag between these disks and the surrounding galactic regions. Hence, the stellar populations in the kinematically distinct cores are not strongly deviant from the population of the main body. Specifically, there is no evidence for a dominating population of blue, very metal weak stars as predicted by some of the formation scenarios. This argues against models in which small galaxies fall in and survive in the nuclei, unless supermassive black holes are present. These would likely disrupt the accreted small systems. For one galaxy, NGC 4365, the innermost region is bluer than the surrounding regions. This area extends to similar to 15 pc and contains a luminosity of similar to 2.5 x 10(6) L.. If interpreted as a stellar population effect, an age difference of similar to 3-4 Gyr, or an [Fe/H] variation of about 0.2 dex? is derived. The nuclear intensity profiles show a large variety: some galaxies have steep cusp profiles, while others have shallow cusps and a ''break radius.'' The nuclear cusps of galaxies with kinematically distinct cores follow the same trends as the nuclei of normal galaxies. We have not been able to identify a unique, qualifying feature in the WFPC2 images that distinguishes the galaxies with kinematically distinct cores from the kinematically normal cares. It is possible that statistical differences exist: possibly, the kinematically distinct cores have a higher fraction of nuclear disks. The similarity of both types of cores puts strong constraints on the formation scenarios. Simulations of galaxy mergers, with the inclusion of star formation and nuclear black holes, are needed to resolve the question of how these structures may have formed. Spectra with high spatial resolution are needed to study the nuclear structure of the distinct component in detail.

FOS spectra and FOC photometry of two centrally located, UV-bright spikes in the elliptical galaxy NGC 4552 and the bulge-dominated early spiral NGC 2681, are presented. These spectra reveal that such point-like UV sources detected by means of HST within a relatively large fraction ~15% of bulges can be related to radically different phenomena. While the UV unresolved emission in NGC 4552 represents a transient event likely induced by an accretion event onto a supermassive black hole, the spike seen at the center of NGC 2681 is not variable and it is stellar in nature., 4 pages, 4 figures (LaTeX, cupconf.sty). To appear in the proceedings of the STScI workshop "When and How do Bulges Form and Evolve", eds. C. M. Carollo, H. C. Ferguson, and R. F. G. Wyse, Cambridge: CUP

We use HST/WFC3 imaging to study the red population in the IR-selected, X-ray detected, low-mass cluster Cl J1449+0856 at z=2, one of the few bona-fide established clusters discovered at this redshift, and likely a typical progenitor of an average massive cluster today. This study explores the presence and significance of an early red sequence in the core of this structure, investigating the nature of red sequence galaxies, highlighting environmental effects on cluster galaxy populations at high redshift, and at the same time underlining similarities and differences with other distant dense environments. Our results suggest that the red population in the core of Cl J1449+0856 is made of a mixture of quiescent and dusty star-forming galaxies, with a seedling of the future red sequence already growing in the very central cluster region, and already characterising the inner cluster core with respect to lower density environments. On the other hand, the color-magnitude diagram of this cluster is definitely different from that of lower-redshift (z, 5 pages, 5 figures. ApJ Letters, in press

We present a spectroscopic survey of galaxies in the COSMOS field using the Fiber Multi-Object Spectrograph (FMOS), a near-infrared instrument on the Subaru Telescope. Our survey is specifically designed to detect the Halpha emission line that falls within the H-band (1.6-1.8 um) spectroscopic window from star-forming galaxies with 1.4 < z < 1.7 and M_stellar>~10^10 Msolar. With the high multiplex capability of FMOS, it is now feasible to construct samples of over one thousand galaxies having spectroscopic redshifts at epochs that were previously challenging. The high-resolution mode (R~2600) effectively separates Halpha and [NII]6585 thus enabling studies of the gas-phase metallicity and photoionization state of the interstellar medium. The primary aim of our program is to establish how star formation depends on stellar mass and environment, both recognized as drivers of galaxy evolution at lower redshifts. In addition to the main galaxy sample, our target selection places priority on those detected in the far-infrared by Herschel/PACS to assess the level of obscured star formation and investigate, in detail, outliers from the star formation rate - stellar mass relation. Galaxies with Halpha detections are followed up with FMOS observations at shorter wavelengths using the J-long (1.11-1.35 um) grating to detect Hbeta and [OIII]5008 that provides an assessment of extinction required to measure star formation rates not hampered by dust, and an indication of embedded Active Galactic Nuclei. With 460 redshifts measured from 1153 spectra, we assess the performance of the instrument with respect to achieving our goals, discuss inherent biases in the sample, and detail the emission-line properties. Our higher-level data products, including catalogs and spectra, are available to the community., 26 pages, Updated version resubmitted to ApJSS; Data products and catalogs are now available at http://member.ipmu.jp/fmos-cosmos/

International audience; We present the first results from a near-IR spectroscopic survey of the COSMOS field, using the Fiber Multi-Object Spectrograph on the Subaru telescope, designed to characterize the star-forming galaxy population at 1.4 \textless z \textless 1.7. The high-resolution mode is implemented to detect H alpha in emission between 1.6-1.8 mu m with f(H alpha) greater than or similar to 4 x 10(-17) erg cm(-2) s(-1). Here, we specifically focus on 271 sBzK-selected galaxies that yield a H alpha detection thus providing a redshift and emission line luminosity to establish the relation between star formation rate and stellar mass. With further J-band spectroscopy for 89 of these, the level of dust extinction is assessed by measuring the Balmer decrement using co-added spectra. We find that the extinction (0.6 less than or similar to A(H alpha) less than or similar to 2.5) rises with stellar mass and is elevated at high masses compared to low-redshift galaxies. Using this subset of the spectroscopic sample, we further find that the differential extinction between stellar and nebular emission E-star(B - V)/E-neb(B - V) is 0.7-0.8, dissimilar to that typically seen at low redshift. After correcting for extinction, we derive an H alpha-based main sequence with a slope (0.81 +/- 0.04) and normalization similar to previous studies at these redshifts.

We investigate the relationships between stellar mass, gas-phase oxygen abundance (metallicity), star formation rate, and dust content of star-forming galaxies at z$\sim$1.6 using Subaru/FMOS spectroscopy in the COSMOS field. The mass-metallicity relation at $z\sim1.6$ is steeper than the relation observed in the local Universe. The steeper MZ relation at $z\sim1.6$ is mainly due to evolution in the stellar mass where the MZ relation begins to turnover and flatten. This turnover mass is 1.2 dex larger at $z\sim1.6$. The most massive galaxies at $z\sim1.6$ ($\sim 10^{11}M_\odot$) are enriched to the level observed in massive galaxies in the local Universe. The mass-metallicity relation we measure at $z\sim1.6$ supports the suggestion of an empirical upper metallicity limit that does not significantly evolve with redshift. We find an anti-correlation between metallicity and star formation rate for galaxies at a fixed stellar mass at $z\sim1.6$ which is similar to trends observed in the local Universe. We do not find a relation between stellar mass, metallicity and star formation rate that is independent of redshift; our data suggest that there is redshift evolution in this relation. We examine the relation between stellar mass, metallicity and dust extinction. We find that at a fixed stellar mass dustier galaxies tend to be more metal rich. From examination of the stellar masses, metallicities, SFRs and dust extinctions we conclude that stellar mass is most closely related to dust extinction., Comment: 21 pages, 14 figures. Updated to accepted version