Sean M. Moran

Evolution since z = 1 of the Morphology-Density Relation for Galaxies

We measure the morphology-density relation of galaxies at z = 1 across the full 3 orders of magnitude in projected galaxy density available in low-redshift studies. Our study adopts techniques that are comparable with those applied at lower redshifts, allowing a direct investigation of how the morphological segregation of galaxies has evolved over the last 8 Gyr. Although the morphology-density relation, as described by the fraction of early-type (E+S0) galaxies, was in place at z = 1, its form differs from that observed at both z = 0 and z = 0.5. In the highest density regions the early-type fraction has increased steadily with time from fE+S0 = 0.7 ± 0.1 at z = 1 to fE+S0 = 0.9 ± 0.1 at the present epoch. However, in intermediate-density regions corresponding to groups and the accretion regions of rich clusters, significant evolution appears to begin only after z = 0.5. Finally, at the lowest densities, no evolution is observed for the early-type fraction of field galaxies, which remains constant at fE+S0 = 0.4 ± 0.1 at all epochs. We examine a simple picture consistent with these observations where the early-type population at z = 1 is comprised largely of elliptical galaxies. Subsequent evolution in both intermediate and dense regions is attributed to the transformation of spirals into lenticulars. Further progress in verifying our hypothesis may be achieved through distinguishing ellipticals and lenticulars at these redshifts through resolved dynamical studies of representative systems.

A Wide-Field Survey of Two z ~ 0.5 Galaxy Clusters: Identifying the Physical Processes Responsible for the Observed Transformation of Spirals into S0s

We present new results from our comparative survey of two massive, intermediate-redshift galaxy clusters, Cl 0024+17 (z = 0.39) and MS 0451–03 (z = 0.54). Combining optical and UV imaging with spectroscopy of member galaxies, we identify and study several key classes of transition objects whose stellar populations or dynamical states indicate a recent change in morphology and star formation rate. For the first time, we have been able to conclusively identify spiral galaxies in the process of transforming into S0 galaxies. This has been accomplished by locating both spirals whose star formation is being quenched and their eventual successors, the recently created S0s. Differences between the two clusters in both the timescales and spatial location of this conversion process allow us to evaluate the relative importance of several proposed physical mechanisms that could be responsible for the transformation. Combined with other diagnostics that are sensitive to either ICM-driven galaxy evolution or galaxy-galaxy interactions, we describe a self-consistent picture of galaxy evolution in clusters. We find that spiral galaxies within infalling groups have already begun a slow process of conversion into S0s, likely via gentle galaxy-galaxy interactions. The fates of spirals upon reaching the core of the cluster depend heavily on the cluster ICM, with rapid conversion of all remaining spirals into S0s via ram pressure stripping in clusters where the ICM is dense. In the presence of a less dense ICM, the conversion continues at a slower pace, with other mechanisms continuing to play a role. We conclude that the buildup of the local S0 population through the transformation of spiral galaxies is a heterogeneous process that nevertheless proceeds robustly across a variety of different environments.

A Panoramic Mid-Infrared Survey of Two Distant Clusters

We present panoramic Spitzer MIPS 24 μm observations, covering ~9 × 9 Mpc2 (25 × 25) fields around two massive clusters, Cl 0024+16 and MS 0451-03, at z = 0.39 and z = 0.55, respectively, reaching a 5 σ flux limit of ~200 μJy. Our observations cover a very wide range of environments within these clusters, from high-density regions around the cores out to the turnaround radius. Cross-correlating the mid-infrared catalogs with deep optical and near-infrared imaging of these fields, we investigate the optical/near-infrared colors of the mid-infrared sources. We find excesses of mid-infrared sources with the optical/near-infrared colors expected of cluster members in the two clusters and test this selection using spectroscopically confirmed 24 μm members. The much more significant excess is associated with Cl 0024+16, whereas MS 0451-03 has comparatively few mid-infrared sources. The mid-infrared galaxy population in Cl 0024+16 appears to be associated with dusty star-forming galaxies (typically redder than the general cluster population by up to AV ~ 1-2 mag) rather than emission from dusty tori around active galactic nuclei in early-type hosts. We compare the star formation rates derived from the total infrared (8-1000 μm) luminosities for the mid-infrared sources in Cl 0024+16 with those estimated from a published Hα survey, finding rates 5 times those found from Hα, indicating significant obscured activity in the cluster population. Compared to previous mid-infrared surveys of clusters from z ~ 0-0.5, we find evidence for strong evolution of the level of dust-obscured star formation in dense environments to z = 0.5, analogous to the rise in the fraction of optically selected star-forming galaxies seen in clusters and the field out to similar redshifts. However, there are clearly significant cluster-to-cluster variations in the populations of mid-infrared sources, probably reflecting differences in the intracluster media and recent dynamical evolution of these systems.

A Wide-Field Hubble Space Telescope Survey of the Cluster Cl 0024+16 at z = 0.4. III. Spectroscopic Signatures of Environmental Evolution in Early-Type Galaxies

We report results from a panoramic spectroscopic survey of 955 objects in the field of the rich cluster Cl 0024+1654 (z 0.4), complementing the HST imaging presented in the first paper in this series. Combining with previous work, we compile a catalog of 1394 unique redshifts in the field of this cluster, including 486 cluster members spread across an area 10 Mpc in diameter. Our new spectroscopic sample includes over 200 high-quality spectra of cluster members. We examine the properties of a large sample of 104 cluster early-type galaxies as a function of cluster radius and local density, using them as sensitive tracers of the various physical processes that may be responsible for galaxy evolution. By constructing the fundamental plane of Cl 0024, we infer an evolution in the mean mass-to-light ratio of early-type galaxies with respect to z = 0 of Δlog(M/LV) = -0.14 ± 0.02. In the cluster center, we detect a significantly increased scatter in the relationship compared to that seen in local clusters. Moreover, we observe a clear radial trend in the mass-to-light ratios of individual early-type galaxies, with the oldest galaxies located in the cluster core. Galaxies are apparently younger at larger radius, with E+S0 galaxies in the periphery having M/LV ratios that nearly match values seen in the field at a similar redshift. The strong radial trend is seen even when the sample is restricted to a narrow range in galaxy mass. Independent spectral indicators used in combination reveal an abrupt interaction with the cluster environment that occurs near the virial radius of Cl 0024, revealed by small bursts of star formation in a population of dim early-type galaxies, as well as by enhanced Balmer absorption for a set of larger E+S0 galaxies closer to the cluster core. We construct a simple infall model used to compare the timescales and strengths of the observed interactions in this cluster. We examine the possibility that bursts of star formation are triggered when galaxies suffer shocks as they encounter the intracluster medium, or by the onset of galaxy harassment.

Dynamical Evidence for Environmental Evolution of Intermediate-Redshift Spiral Galaxies

Combining resolved optical spectroscopy with panoramic HST imaging, we study the dynamical properties of spiral galaxies as a function of position across two intermediate-redshift clusters, and we compare the cluster population to field galaxies in the same redshift range. By modeling the observed rotation curves, we derive maximal rotation velocities for 40 cluster spirals and 37 field spirals, yielding one of the largest matched samples of cluster and field spirals at intermediate redshift. We construct the Tully-Fisher (TF) relation in both V and K_s bands, and find that the cluster Tully-Fisher relation exhibits significantly higher scatter than the field relation, in both V and K_s bands. Under the assumption that this increased scatter is due to an interaction with the cluster environment, we examine several dynamical quantities (dynamical mass, mass-to-light ratio, and central mass density) as a function of cluster environment. We find that the central mass densities of star-forming spirals exhibit a sharp break near the cluster virial radius, with spirals in the cluster outskirts exhibiting significantly lower densities. We argue that the lower density spirals in the cluster outskirts, combined with the high scatter in both K_s- and V-band TF relations, demonstrate that cluster spirals are kinematically disturbed by their environment, even as far as 2R_(vir) from the cluster center. We propose that such disturbances may be due to a combination of galaxy merging and harassment.

The Dynamical Influence of a Planet at Semimajor Axis 3.4 AU on the Dust around ε Eridani

Precise Doppler experiments suggest that a massive (m sin i = 0.86MJ) planet orbits at semimajor axis a = 3.4 AU around Eri, a nearby star with a massive debris disk. The dynamical perturbations from such a planet would mold the distribution of dust around this star. We numerically integrated the orbits of dust grains in this system to predict the central dust-cloud structure. For a supply of grains that begin in low-inclination, low-eccentricity orbits at 15 AU, the primary feature of the dust distribution is a pair of dense clumps containing dust particles trapped in mean-motion resonances of the form n : 1. These clumps appear to revolve around the star once every two planet revolutions. Future observations with the IRAM Plateau de Bure Interferometer, the Submillimeter Array (SMA), or the Atacama Large Millimeter Array (ALMA) could detect these clumps, confirming the existence of the planet and revealing its location.

GALEX Observations of “Passive Spirals” in the Cluster Cl 0024+17: Clues to the Formation of S0 Galaxies

We present new results from deep GALEX UV imaging of the cluster Cl 0024+17 at z ~ 0.4. Rest-frame far-UV emission is detected from a large fraction of so-called passive spiral galaxies—a significant population that exhibits spiral morphology with little or no spectroscopic evidence of ongoing star formation. This population is thought to represent infalling galaxies whose star formation has been somehow truncated by environmental processes, possibly in morphological transition to S0 galaxies. Compared to normal cluster spirals, we find that passive spirals are redder in FUV-optical color, while exhibiting much stronger UV emission than cluster E/S0 galaxies—as expected for recently truncated star formation. By modeling the different temporal sensitivities of UV and spectroscopic data to recent activity, we show that star formation in passive spirals decayed on timescales of less than 1 Gyr, consistent with gas starvation—a process where the cluster environment prevents cold gas from accreting onto the spiral disk. Intriguingly, the fraction of spirals currently observed in the passive phase is consistent with the longer period expected for the morphological transformation and the subsequent buildup of cluster S0 galaxies observed since z 0.4.

The Dynamical Distinction between Elliptical and Lenticular Galaxies in Distant Clusters: Further Evidence for the Recent Origin of S0 Galaxies

We examine resolved spectroscopic data obtained with the Keck II telescope for 44 spheroidal galaxies in the fields of two rich clusters, Cl 0024+16 (z = 0.40) and MS 0451-03 (z = 0.54) and contrast this with similar data for 23 galaxies within the redshift interval 0.3 < z < 0.65 in the GOODS northern field. For each galaxy we examine the case for systemic rotation, derive central stellar velocity dispersions σ and photometric ellipticities e. Using morphological classifications obtained via Hubble Space Telescope imaging as the basis, we explore the utility of our kinematic quantities in distinguishing between pressure-supported ellipticals and rotationally supported lenticulars (S0s). We demonstrate the reliability of using the v/(1 - e ) versus σ and v/σ versus distributions as discriminators, finding that the two criteria correctly identify 63% ± 3% and 80% ± 2% of S0s at z ~ 0.5, respectively, along with 76^[+8][-3]% and 79% ± 2% of ellipticals. We test these diagnostics using equivalent local data in the Coma Cluster, and find that the diagnostics are similarly accurate at z = 0. Our measured accuracies are comparable to the accuracy of visual classification of morphologies, but avoid the band-shifting and surface brightness effects that hinder visual classification at high redshifts. As an example application of our kinematic discriminators, we then examine the morphology-density relation for elliptical and S0 galaxies separately at z ~ 0.5. We confirm, from kinematic data alone, the recent growth of rotationally supported spheroidals. We discuss the feasibility of extending the method to a more comprehensive study of cluster and field galaxies to z ≃ 1, in order to verify in detail the recent density-dependent growth of S0 galaxies.

Evolution of the Stellar Mass–Metallicity Relation. I. Galaxies in the z ∼ 0.4 Cluster Cl0024

We present the stellar mass–stellar metallicity relationship (MZR) in the galaxy cluster Cl0024+1654 at z ~ 0.4 using full-spectrum stellar population synthesis modeling of individual quiescent galaxies. The lower limit of our stellar mass range is M* = 10^(9.7) M ⊙, the lowest galaxy mass at which individual stellar metallicity has been measured beyond the local universe. We report a detection of an evolution of the stellar MZR with observed redshift at 0.037 ± 0.007 dex per Gyr, consistent with the predictions from hydrodynamical simulations. Additionally, we find that the evolution of the stellar MZR with observed redshift can be explained by an evolution of the stellar MZR with the formation time of galaxies, i.e., when the single stellar population (SSP)-equivalent ages of galaxies are taken into account. This behavior is consistent with stars forming out of gas that also has an MZR with a normalization that decreases with redshift. Lastly, we find that over the observed mass range, the MZR can be described by a linear function with a shallow slope ([Fe/H] ∝ (0.16 ± 0.03) log M*. The slope suggests that galaxy feedback, in terms of mass-loading factor, might be mass-independent over the observed mass and redshift range.

Spectroscopic signatures of galaxy evolution in C10024 + 16 at z~0.4

We report the first results from a panoramic spectroscopic survey of galaxies in the rich cluster Cl0024+1654 (z ≃ 0.4). Using HST imaging we examine the properties of nearly-types as a function of cluster radius. At all cluster radii, our sample lies on a selfconsistent Fundamental Plane whose zero point implies evolution since z = 0 corresponding to Δ[log(M/L_V )] = 0.14 ± 0.06, an overall trend consistent with previous work. Using diagnostic n[O II] emission and Balmer absorption lines, we locate a population of intrinsically faint galaxies at 1–2.4 Mpc radius undergoing a period of star formation. The luminosity-dependent radial trends are suggestive of the gradual quenching of star formation for infalling galaxies. We discuss physical mechanisms that may be responsible for this environmental evolution.