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Register a new userCO interferometry of gas-rich spiral galaxies in the outskirts of an intermediate redshift cluster
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James E. Geach
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We present IRAM Plateau de Bure Interferometer 3mm observations of CO(1-0) emission in two 24um-selected starburst galaxies in the outskirts (2-3xR_virial) of the rich cluster Cl0024+16 (z=0.395). The galaxies inferred far-infrared luminosities place them in the luminous infrared galaxy class (LIRGs, L_FIR>10^11 L_Sun), with star formation rates of ~60 M_Sun/yr. Strong CO(1-0) emission is detected in both galaxies, and we use the CO line luminosity to estimate the mass of cold molecular gas, M(H_2). Assuming M(H_2)/L_CO = 0.8 M_Sun/(K km^-1 pc^2), we estimate M(H_2) = (5.4-9.1)x10^9 M_Sun for the two galaxies. We estimate the galaxies dynamical masses from their CO line-widths, M_dyn~1-3x10^10 M_Sun, implying large cold gas fractions in the galaxies central regions. At their current rates they will complete the assembly of M_Stars~10^10 M_Sun and double their stellar mass within as little as ~150Myr. If these galaxies are destined to evolve into S0s, then the short time-scale for stellar mass assembly implies that their major episode of bulge growth occurs while they are still in the cluster outskirts, long before they reach the core regions. Subsequent fading of the disc component relative to the stellar bulge after the gas reservoirs have been exhausted could complete the transformation of spiral-to-S0.
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(Abridged) We trace the interaction processes of galaxies at intermediate redshift by measuring the irregularity of their ionized gas kinematics, and investigate these irregularities as a function of the environment (cluster versus field) and of morphological type (spiral versus irregular). Our sample consists of 92 distant galaxies. 16 cluster (z~0.3 and z~0.5) and 29 field galaxies (mean z=0.44) of these have velocity fields with sufficient signal to be analyzed. We find that the fraction of galaxies that have irregular gas kinematics is remarkably similar in galaxy clusters and in the field at intermediate redshifts. The distribution of the field and cluster galaxies in (ir)regularity parameters space is also similar. On the other hand galaxies with small central concentration of light, that we see in the field sample, are absent in the cluster sample. We find that field galaxies at intermediate redshifts have more irregular velocity fields as well as more clumpy and less centrally concentrated light distributions than their local counterparts. Comparison with a SINS sample of 11 z ~ 2 galaxies shows that these distant galaxies have more irregular gas kinematics than our intermediate redshift cluster and field sample. We do not find a dependence of the irregularities in gas kinematics on morphological type. We find that two different indicators of star formation correlate with irregularity in the gas kinematics. More irregular gas kinematics, also more clumpy and less centrally concentrated light distributions of spiral field galaxies at intermediate redshifts in comparison to their local counterparts indicate that these galaxies are probably still in the process of building their disks via mechanisms such as accretion and mergers. On the other hand, they have less irregular gas kinematics compared to galaxies at z ~ 2.
The formation of thick stellar disks in spiral galaxies is studied. Simulations of gas-rich young galaxies show formation of internal clumps by gravitational instabilities, clump coalescence into a bulge, and disk thickening by strong stellar scattering. The bulge and thick disks of modern galaxies may form this way. Simulations of minor mergers make thick disks too, but there is an important difference. Thick disks made by internal processes have a constant scale height with galactocentric radius, but thick disks made by mergers flare. The difference arises because in the first case, perpendicular forcing and disk-gravity resistance are both proportional to the disk column density, so the resulting scale height is independent of this density. In the case of mergers, perpendicular forcing is independent of the column density and the low density regions get thicker; the resulting flaring is inconsistent with observations. Late-stage gas accretion and thin disk growth are shown to preserve the constant scale heights of thick disks formed by internal evolution. These results reinforce the idea that disk galaxies accrete most of their mass smoothly and acquire their structure by internal processes, in particular through turbulent and clumpy phases at high redshift.
We report on the detection of three strong HI absorbers originating in the outskirts (i.e., impact parameter, $rho_{rm cl} approx (1.6-4.7) r_{500}$) of three massive ($M_{500}sim3times10^{14} M_{odot}$) clusters of galaxies at redshift $z_{rm cl} approx 0.46$, in the $Hubble Space Telescope$ Cosmic Origins Spectrograph ($HST$/COS) spectra of 3 background UV-bright quasars. These clusters were discovered by the 2500 deg$^2$ South Pole Telescope Sunyaev$-$Zeldovich (SZ) effect survey. All three COS spectra show partial Lyman limit absorber with $N(HI) > 10^{16.5} rm cm^{-2}$ near the photometric redshifts ($|Delta z/(1+z)| approx 0.03$) of the clusters. The compound probability of random occurrence of all three absorbers is $<0.02$%, indicating that the absorbers are most likely related to the targeted clusters. We find that the outskirts of these SZ-selected clusters are remarkably rich in cool gas compared to existing observations of other clusters in the literature. The effective Doppler parameters of the Lyman series lines, obtained using single cloud curve-of-growth (COG) analysis, suggest a non-thermal/turbulent velocity of a few $times10 rm km s^{-1}$ in the absorbing gas. We emphasize the need for uniform galaxy surveys around these fields and for more UV observations of QSO-cluster pairs in general in order to improve the statistics and gain further insights into the unexplored territory of the largest collapsed cosmic structures.
The purpose of this work is to make available new gas-phase oxygen abundance measurements for a serendipitous sample of 27 galaxies with redshift 0.35<z<0.52. We measured the equivalent widths of the [O II]{lambda}3727, H{beta}, and [O III]{lambda}{lambda}4959, 5007 emission lines observed in the galaxy spectra obtained with the Visible Multi-Object Spectrograph mounted at the Very Large Telescope. For each galaxy, we derived the metallicity-sensitive emission lines ratio R23, ionization-sensitive emission lines ratio O32, and gas-phase oxygen abundance 12+log(O/H). The values of gas-phase oxygen abundance 12+log(O/H) we obtained for the sample galaxies are consistent with previous findings for galaxies at intermediate redshift.
We present the Evolution of molecular Gas in Normal Galaxies (EGNoG) survey, an observational study of molecular gas in 31 star-forming galaxies from z=0.05 to z=0.5, with stellar masses of (4-30)x10^10 M_Sun and star formation rates of 4-100 M_Sun yr^-1. This survey probes a relatively un-observed redshift range in which the molecular gas content of galaxies is expected to have evolved significantly. To trace the molecular gas in the EGNoG galaxies, we observe the CO(1-0) and CO(3-2) rotational lines using the Combined Array for Research in Millimeter-wave Astronomy (CARMA). We detect 24 of 31 galaxies and present resolved maps of 10 galaxies in the lower redshift portion of the survey. We use a bimodal prescription for the CO to molecular gas conversion factor, based on specific star formation rate, and compare the EGNoG galaxies to a large sample of galaxies assembled from the literature. We find an average molecular gas depletion time of 0.76 pm 0.54 Gyr for normal galaxies and 0.06 pm 0.04 Gyr for starburst galaxies. We calculate an average molecular gas fraction of 7-20% at the intermediate redshifts probed by the EGNoG survey. By expressing the molecular gas fraction in terms of the specific star formation rate and molecular gas depletion time (using typical values), we also calculate the expected evolution of the molecular gas fraction with redshift. The predicted behavior agrees well with the significant evolution observed from z~2.5 to today.
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