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Register a new userZFIRE: Measuring Electron Density with [OII] as a function of environment at z = 1.62
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The global star formation rates (SFR) of galaxies at fixed stellar masses increase with redshift and are known to vary with environment unto z~2. We explore here whether the changes in the star formation rates can also apply to the electron densities of the inter-stellar medium (ISM) by measuring the [OII] (3727A/3729A) ratio for cluster and field galaxies at z~2. We measure a median electron density of ne = 366+/-84 cm-3 for six galaxies (with 1-sigma scatter = 163 cm-3) in the UDS proton-cluster at z=1.62. We find that the median electron density of galaxies in the UDS photo-cluster environment is three times higher compared to the median electron density of field galaxies (ne = 113+/- 63 cm-3 and 1-sigma scatter = 79 cm-3) at comparable redshifts, stellar mass and SFR. However, we note that a sample of six photo-cluster galaxies is insufficient to reliably measure the electron density in the average porto-cluster environment at z~2. We conclude that the electron density increases with redshift in both cluster and field environments up to z~2 (ne = 30 +/- 1 cm-3 for z ~ 0 to ne =254+/- 76 cm-3 for z~1.5). We find tentative evidence (~2.6 sigma ) for a possible dependence of electron density on environment, but the results require confirmation with larger sample sizes.
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We perform a kinematic analysis of galaxies at $zsim2$ in the COSMOS legacy field using near-infrared (NIR) spectroscopy from Keck/MOSFIRE as part of the ZFIRE survey. Our sample consists of 75 Ks-band selected star-forming galaxies from the ZFOURGE survey with stellar masses ranging from log(M$_{star}$/M$_{odot}$)$=9.0-11.0$, 28 of which are members of a known overdensity at $z=2.095$. We measure H$alpha$ emission-line integrated velocity dispersions ($sigma_{rm int}$) from 50$-$230 km s$^{-1}$, consistent with other emission-line studies of $zsim2$ field galaxies. From these data we estimate virial, stellar, and gas masses and derive correlations between these properties for cluster and field galaxies at $zsim2$. We find evidence that baryons dominate within the central effective radius. However, we find no statistically significant differences between the cluster and the field, and conclude that the kinematics of star-forming galaxies at $zsim2$ are not significantly different between the cluster and field environments.
We use ZFIRE and ZFOURGE observations with the Spectral Energy Distribution (SED) fitting tool Prospector to reconstruct the star formation histories (SFHs) of proto-cluster and field galaxies at $zsim 2 $ and compare our results to the TNG100 run of the IllustrisTNG cosmological simulation suite. In the observations, we find that massive proto-cluster galaxies ($log[{rm M}_{ast}/{rm M}_{odot}]>$10.5) form $45 pm 8 %$ of their total stellar mass in the first $2$ Gyr of the Universe compared to $31 pm 2 %$ formed in the field galaxies. In both observations and simulations, massive proto-cluster galaxies have a flat/declining SFH with decreasing redshift compared to rising SFH in their field counterparts. Using IllustrisTNG, we find that massive galaxies ($log[{rm M}_{ast}/{rm M}_{odot}] geq 10.5$) in both environments are on average $approx190$ Myr older than low mass galaxies ($log[{rm M}_{ast}/{rm M}_{odot}]= 9-9.5$). However, the difference in mean stellar ages of cluster and field galaxies is minimal when considering the full range in stellar mass ($log[{rm M}_{ast}/{rm M}_{odot}] geq 9$). We explore the role of mergers in driving the SFH in IllustrisTNG and find that massive cluster galaxies consistently experience mergers with low gas fraction compared to other galaxies after 1 Gyr from the Big Bang. We hypothesize that the low gas fraction in the progenitors of massive cluster galaxies is responsible for the reduced star formation.
We identify a strong lensing galaxy in the cluster IRC 0218 (also known as XMM-LSS J02182$-$05102) that is spectroscopically confirmed to be at $z=1.62$, making it the highest-redshift strong lens galaxy known. The lens is one of the two brightest cluster galaxies and lenses a background source galaxy into an arc and a counterimage. With Hubble Space Telescope (HST) grism and Keck/LRIS spectroscopy, we measure the source redshift to be $z_{rm S}=2.26$. Using HST imaging in ACS/F475W, ACS/F814W, WFC3/F125W, and WFC3/F160W, we model the lens mass distribution with an elliptical power-law profile and account for the effects of the cluster halo and nearby galaxies. The Einstein radius is $theta_{rm E}=0.38^{+0.02}_{-0.01}$ ($3.2_{-0.1}^{+0.2}$ kpc) and the total enclosed mass is M$_{rm tot} (< theta_{rm E})=1.8^{+0.2}_{-0.1}times10^{11}~{rm M}_{odot}$. We estimate that the cluster environment contributes $sim10$% of this total mass. Assuming a Chabrier IMF, the dark matter fraction within $theta_{{rm E}}$ is $f_{rm DM}^{{rm Chab}} = 0.3_{-0.3}^{+0.1}$, while a Salpeter IMF is marginally inconsistent with the enclosed mass ($f_{rm DM}^{{rm Salp}} = -0.3_{-0.5}^{+0.2}$). The total magnification of the source is $mu_{rm tot}=2.1_{-0.3}^{+0.4}$. The source has at least one bright compact region offset from the source center. Emission from Ly$alpha$ and [O III] are likely to probe different regions in the source.
We report the discovery of a galaxy cluster at z=1.62 located in the Spitzer Wide-Area Infrared Extragalactic survey XMM-LSS field. This structure was selected solely as an overdensity of galaxies with red Spitzer/IRAC colors, satisfying [3.6]-[4.5] > -0.1 AB mag. Photometric redshifts derived from Subaru XMM Deep Survey (BViz-bands), UKIRT Infrared Deep Survey-Ultra-Deep Survey (UKIDSS-UDS, JK-bands), and from the Spitzer Public UDS survey (3.6-8.0 micron) show that this cluster corresponds to a surface density of galaxies at z ~ 1.6 that is more than 20 sigma above the mean at this redshift. We obtained optical spectroscopic observations of galaxies in the cluster region using IMACS on the Magellan telescope. We measured redshifts for seven galaxies in the range z=1.62-1.63 within 2.8 arcmin (<1.4 Mpc) of the astrometric center of the cluster. A posteriori analysis of the XMM data in this field reveal a weak (4 sigma) detection in the [0.5-2 keV] band compatible with the expected thermal emission from such a cluster. The color-magnitude diagram of the galaxies in this cluster shows a prominent red-sequence, dominated by a population of red galaxies with (z-J) > 1.7 mag. The photometric redshift probability distributions for the red galaxies are strongly peaked at z=1.62, coincident with the spectroscopically confirmed galaxies. The rest-frame (U-B) color and scatter of galaxies on the red-sequence are consistent with a mean luminosity-weighted age of 1.2 +/- 0.1 Gyr, yielding a formation redshift z_f = 2.35 +/- 0.10, and corresponding to the last significant star-formation period in these galaxies.
The galaxy cluster CLG0218.3-0510 at z=1.62 is one of the most distant galaxy clusters known, with a rich muti-wavelength data set that confirms a mature galaxy population already in place. Using very deep, wide area (20x20 Mpc) imaging by Spitzer/MIPS at 24um, in conjunction with Herschel 5-band imaging from 100-500um, we investigate the dust-obscured, star-formation properties in the cluster and its associated large scale environment. Our galaxy sample of 693 galaxies at z=1.62 detected at 24um (10 spectroscopic and 683 photo-z) includes both cluster galaxies (i.e. within r <1 Mpc projected clustercentric radius) and field galaxies, defined as the region beyond a radius of 3 Mpc. The star-formation rates (SFRs) derived from the measured infrared luminosity range from 18 to 2500 Ms/yr, with a median of 55 Ms/yr, over the entire radial range (10 Mpc). The cluster brightest FIR galaxy, taken as the centre of the galaxy system, is vigorously forming stars at a rate of 256$pm$70 Ms/yr, and the total cluster SFR enclosed in a circle of 1 Mpc is 1161$pm$96 Ms/yr. We estimate a dust extinction of about 3 magnitudes by comparing the SFRs derived from [OII] luminosity with the ones computed from the 24um fluxes. We find that the in-falling region (1-3 Mpc) is special: there is a significant decrement (3.5x) of passive relative to star-forming galaxies in this region, and the total SFR of the galaxies located in this region is lower (130 Ms/yr/Mpc2) than anywhere in the cluster or field, regardless of their stellar mass. In a complementary approach we compute the local galaxy density, Sigma5, and find no trend between SFR and Sigma5. However, we measure an excess of star-forming galaxies in the cluster relative to the field by a factor 1.7, that lends support to a reversal of the SF-density relation in CLG0218.
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