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Register a new userField E+A Galaxies at Intermediate Redshifts (0.3<z<1)
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We select E+A candidates from a spectroscopic dataset of ~800 field galaxies and measure the E+A fraction at 0.3<z<1 to be 2.7+/-1.1%, a value lower than that in galaxy clusters at comparable redshifts (11+/-3%). HST/WFPC2 imaging for five of our six E+As shows they have a heterogeneous parent population: these E+As span a range in half-light radius (0.8-8 kpc) and estimated internal velocity dispersion (50-220 km/s), and they include luminous systems (-21.6<M_Bz-5logh<-19.2). Despite their diversity in some aspects, the E+As share several common characteristics that indicate the E+A phase is an important link in the evolution of star-forming galaxies into passive systems: the E+As are uniformly redder than the blue, star-forming galaxies that make up the majority of the field, they are more likely to be bulge-dominated than the average field galaxy, and they tend to be morphologically irregular. We find E+As make up ~9% of the absorption line systems in this redshift range, and estimate that ~25% of passive galaxies in the local field had an E+A phase at z<1.
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We present the first study of large-scale bars in clusters at intermediate redshifts (z=0.4-0.8). We compare the properties of the bars and their host galaxies in the clusters with those of a field sample in the same redshift range. We use a sample of 945 moderately inclined disk galaxies drawn from the EDisCS project. The morphological classification of the galaxies and the detection of bars are based on deep HST/ACS F814W images. The total optical bar fraction in the redshift range z=0.4-0.8, averaged over the entire sample, is 25%. This is lower than found locally, but in good agreement with studies of bars in field environments at intermediate redshifts. For the cluster and field subsamples, we measure bar fractions of 24% and 29%, respectively. In agreement with local studies, we find that disk-dominated galaxies have a higher bar fraction than bulge-dominated galaxies. We also find, based on a small subsample, that bars in clusters are on average longer than in the field and preferentially found close to the cluster center, where the bar fraction is somewhat higher than at larger distances.
The nature of absorption-selected galaxies and their connection to the general galaxy population have been open issues for more than three decades, with little information available on their gas properties. Here we show, using detections of carbon monoxide (CO) emission with the Atacama Large Millimeter/submillimeter Array (ALMA), that five of seven high-metallicity, absorption-selected galaxies at intermediate redshifts, $z approx 0.5-0.8$, have large molecular gas masses, $M_{rm Mol} approx (0.6 - 8.2) times 10^{10} : {rm M}_odot$ and high molecular gas fractions ($f_{rm Mol} equiv : M_{rm Mol}/(M_ast + M_{rm Mol}) approx 0.29-0.87)$. Their modest star formation rates (SFRs), $approx (0.3-9.5) : {rm M}_odot$ yr$^{-1}$, then imply long gas depletion timescales, $approx (3 - 120)$ Gyr. The high-metallicity absorption-selected galaxies at $z approx 0.5-0.8$ appear distinct from populations of star-forming galaxies at both $z approx 1.3-2.5$, during the peak of star formation activity in the Universe, and lower redshifts, $z lesssim 0.05$. Their relatively low SFRs, despite the large molecular gas reservoirs, may indicate a transition in the nature of star formation at intermediate redshifts, $z approx 0.7$.
Despite the increasing number of studies of barred galaxies at intermediate and high redshifts, double-barred (S2B) systems have only been identified in the nearby (z<0.04) universe thus far. In this feasibility study we demonstrate that the detection and analysis of S2Bs is possible at intermediate redshifts (0.1 < z < 0.5) with the exquisite resolution of the Hubble Space Telescope Advanced Camera for Surveys (HST/ACS). We identify barred galaxies in the HST/ACS data of the Great Observatories Origins Deep Survey (GOODS) using a novel method. The radial profile of the Gini coefficient -- a model-independent structure parameter -- is able to detect bars in early-type galaxies that are large enough that they might host an inner bar of sufficient angular size. Using this method and subsequent examination with unsharp masks and ellipse fits we identified the two most distant S2Bs currently known (at redshifts z=0.103 and z=0.148). We investigate the underlying stellar populations of these two galaxies through a detailed colour analysis, in order to demonstrate the analysis that could be performed on a future sample of intermediate-redshift S2Bs. We also identify two S2Bs and five S2B candidates in the HST/ACS data of the Cosmic Evolution Survey (COSMOS). Our detections of distant S2Bs show that deep surveys like GOODS and COSMOS have the potential to push the limit for S2B detection and analysis out by a factor of ten in redshift and lookback time (z=0.5, t=5Gyr) compared to the previously known S2Bs. This in turn would provide new insight into the formation of these objects.
Spatially resolved velocity profiles are presented for nine faint field galaxies in the redshift range 0.1 < z < 1, based on moderate-resolution spectroscopy obtained with the Keck 10 m telescope. These data were augmented with high-resolution HST images from WFPC2, which provided V and I photometry, galaxy type, orientation, and inclination. The effects of seeing, slit width, and slit misalignment with respect to galaxy major axis were modeled along with inclination for each source, in order to derive a maximum circular velocity from the observed rotation curve. The lowest redshift galaxy, though highly elongated, shows a distorted low-amplitude rotation curve that suggests a merger in progress seen perpendicular to the collision path. The remaining rotation curves appear similar to those of local galaxies in both form and amplitude, implying that some massive disks were in place at z ~ 1. The key result is that the kinematics of these distant galaxies show evidence for only a modest increase in luminosity of delta M_B < 0.6 compared to velocity-luminosity (Tully-Fisher) relations for local galaxies.
(abridged) We measure spectral indices for 1823 galaxies in the CNOC1 sample of fifteen X-ray luminous clusters at 0.18<z<0.55, to investigate the mechanisms responsible for differential evolution between the galaxy cluster and field environments. The radial trends of D4000, Hdelta and [OII] are all consistent with an age sequence, in the sense that the last episode of star formation occurred more recently in galaxies farthest from the cluster center. Throughout the cluster environment, galaxies show evidence for older mean stellar populations than field galaxies. From the subsample of galaxies more luminous than M_r=-18.8 + 5log h, we identify a sample of K+A galaxies, which may result from recently terminated star formation. Corrected for a systematic effect which results from the large uncertainties on individual spectral index measurements, we estimate that K+A galaxies make up only 1.5 +/- 0.8 % of the cluster sample, and 1.2 +/- 0.8 % of the field. We compare our data with spectrophotometric models and conclude that up to 1.9 +/- 0.8 % of the cluster population may have had its star formation recently truncated without a starburst. However, this is still not significantly greater than the fraction of such galaxies in the field, 3.1 +/- 1.0 %. Furthermore, we do not detect an excess of cluster galaxies that have unambiguously undergone a starburst within the last 1 Gyr. Our results imply that these cluster environments are not responsible for inducing starbursts; thus, the increase in cluster blue galaxy fraction with redshift may not be a strictly cluster--specific phenomenon. We suggest that the truncation of star formation in clusters may largely be a gradual process, perhaps due to the exhaustion of gas in the galactic disk over fairly long timescales.
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