Environmental Dependence Of Star Formation Research Articles

Critical Stellar Central Densities Drive Galaxy Quenching in the Nearby Universe

Abstract We study the structural and environmental dependence of star formation on the plane of stellar mass versus central core density (Σ1 kpc) in the nearby universe. We study the central galaxies in the sparse environment and find a characteristic population-averaged Σ1 kpc ∼ 109–109.2 M ⊙ kpc−2, above which quenching is operating. This Σ 1 kpc crit only weakly depends on the stellar mass, suggesting that the mass quenching of the central galaxies is closely related to the processes that operate in the central region rather than over the entire galaxies. For satellites, at a given stellar mass, environment quenching appears to operate in a similar fashion as mass quenching in centrals, also starting from galaxies with high Σ1 kpc to low Σ1 kpc, and Σ 1 kpc crit becomes strongly mass-dependent, in particular in dense regions. This is because (1) more low-mass satellites are quenched by the environmental effects in denser regions and (2) at fixed stellar mass and environment, the environment-quenched satellites have, on average, larger Σ1 kpc, M 1 kpc/M ⋆, and Sérsic index n, and as well as smaller size. These results imply that either some dynamical processes change the structure of the satellites during quenching or the satellites with higher Σ1 kpc are more susceptible to environmental effects.

Molecular Gas in a Gravitationally Lensed Galaxy Group at z = 2.9

Abstract Most molecular gas studies of z > 2.5 galaxies are of intrinsically bright objects, despite the galaxy population being primarily normal galaxies with less extreme star formation rates. Observations of normal galaxies at high redshift provide a more representative view of galaxy evolution and star formation, but such observations are challenging to obtain. In this work, we present Atacama Large Millimeter/submillimeter Array 12CO(J = 3 → 2) observations of a submillimeter selected galaxy group at z = 2.9, resulting in spectroscopic confirmation of seven images from four member galaxies. These galaxies are strongly lensed by the MS 0451.6-0305 foreground cluster at z = 0.55, allowing us to probe the molecular gas content on levels of 109–1010 M ⊙. Four detected galaxies have molecular gas masses of (0.2–13.1) × 1010 M ⊙, and the nondetected galaxies have inferred molecular gas masses of <8.0 × 1010 M ⊙. We compare these new data to a compilation of 546 galaxies up to z = 5.3, and find that depletion times decrease with increasing redshift. We then compare the depletion times of galaxies in overdense environments to the field-scaling relation from the literature, and find that the depletion time evolution is steeper for galaxies in overdense environments than for those in the field. More molecular gas measurements of normal galaxies in overdense environments at higher redshifts (z > 2.5) are needed to verify the environmental dependence of star formation and gas depletion.

Star formation in a high-pressure environment: An SMA view of the dust ridge

AbstractThe star formation rate in the Central Molecular Zone is an order of magnitude lower than in the disk of the Galaxy, given the amount of dense gas there. Understanding why star formation is different in this region is crucial if we are to understand the environmental dependence of star formation. Here, we present the detection of high-mass cores in the CMZ’s ‘dust ridge’ that have been discovered with the Submillimeter Array. These cores range in mass from ~ 50 – 1800 M⊙ within radii of 0.1 – 0.25 pc. All are young (pre-UCHII), meaning that they are prime candidates for representing the initial conditions of high-mass stars and sub-clusters. We compare these with high-mass cores and clouds in the Galactic disk and find that they are very similar in terms of their masses and sizes, despite being subjected to external pressures that are several magnitudes greater (~ 108 K cm−3). The fact that > 80% of these cores do not show any signs of star-forming activity in such a high-pressure environment leads us to conclude that this is further evidence of the critical density for star formation being heightened in the CMZ due to turbulence.

Environmental dependence of star formation induced by cloud collisions in a barred galaxy

Abstract Cloud collision has been proposed as a way to link the small-scale star formation process with the observed global relation between the surface star formation rate and gas surface density. We suggest that this model can be improved further by allowing the productivity of such collisions to depend on the relative velocity of the two clouds. Our adjustment implements a simple step function that results in the most successful collisions being at the observed velocities for triggered star formation. By applying this to a high-resolution simulation of a barred galaxy, we successfully reproduce the observational result that the star formation efficiency (SFE) in the bar is lower than that in the spiral arms. This is not possible when we use an efficiency dependent on the internal turbulence properties of the clouds. Our results suggest that high-velocity collisions driven by the gravitational pull of the clouds are responsible for the low bar SFE.

The Different Environmental Dependencies of Star Formation for Giant and Dwarf Galaxies

We examine the origins of the bimodality observed in the global properties of galaxies around a stellar mass of 3x10^10 M_sun by comparing the environmental dependencies of star-formation for the giant and dwarf galaxy populations. The Sloan Digital Sky Survey DR4 spectroscopic dataset is used to produce a sample of galaxies in the vicinity of the supercluster centered on the cluster A2199 at z=0.03 that is ~90% complete to a magnitude limit of M*+3.3. From these we measure global trends with environment for both giant (M_r 7 Gyr) or passive (EW[H_alpha]<4 A) falls gradually from ~80% in the cluster cores to ~40% in field regions beyond 3-4 R_virial, as found in previous studies. In contrast, we find that the dwarf galaxy population shows a sharp transition at ~1 R_virial, from being predominantly old/passive within the cluster, to outside where virtually all galaxies are forming stars and old/passive galaxies are only found as satellites to more massive galaxies. These results imply fundamental differences in the evolution of giant and dwarf galaxies: whereas the star-formation histories of giant galaxies are determined primarily by their merger history, star-formation in dwarf galaxies is much more resilient to the effects of major mergers. Instead dwarf galaxies become passive only once they become satellites within a more massive halo, by losing their halo gas reservoir to the host halo, or through other environment-related processes such as galaxy harassment and/or ram-pressure stripping.

CAIRNS: The Cluster and Infall Region Nearby Survey. III. Environmental Dependence of Hα Properties of Galaxies

We investigate the environmental dependence of star formation in cluster virial regions and infall regions as part of the Cluster and Infall Region Nearby Survey (CAIRNS), a large spectroscopic survey of the infall regions surrounding nine nearby rich clusters of galaxies. We use complete, homogeneous spectroscopic surveys of Ks-limited samples in eight of the CAIRNS clusters. Our long-slit spectroscopy yields estimates of star formation rates in environments from cluster cores to the general large-scale structure. Galaxies in infall regions probe whether processes affecting star formation are effective over scales larger than cluster virial regions. The fraction of galaxies with current star formation in their inner disks as traced by Hα emission increases with distance from the cluster and converges to the "field" value only at 2–3 virial radii, in agreement with other investigations. However, among galaxies with significant current star formation (EW[Hα] ≥ 2 Å), there is no difference in the distribution of EW[Hα] inside and outside the virial radius. This surprising result, first seen by Carter and coworkers, suggests that (1) star formation is truncated either on very short timescales or only at moderate and high redshifts or (2) that projection effects contaminate the measurement. We quantify the possible impact of mechanisms that only affect the outer parts of galaxies and thus might not be detected in this survey or any fiber-based survey. The number density profiles of star-forming and non-star-forming galaxies indicate that, among galaxies projected inside the virial radius, at least half the former and 20% of the latter are "infall interlopers," galaxies in the infall region but outside the virial region. We show that the kinematics of star-forming galaxies in the infall region closely match those of absorption-dominated galaxies. This result shows that the star-forming galaxies in the infall regions are not interlopers from the field and excludes one model of the backsplash scenario of galaxy transformation. Finally, we quantify systematic uncertainties in estimating the global star formation in galaxies from their inner disks.

Galaxy groups at 0.3 ≤z≤ 0.55 - II. Evolution toz∼ 0

We compare deep Magellan spectroscopy of 26 groups at 0.3 ≤ z ≤ 0.55, selected from the Canadian Network for Observational Cosmology 2 field survey, with a large sample of nearby groups from the 2PIGG catalogue. We find that the fraction of group galaxies with significant [OII]λ3727 emission (≥5 A) increases strongly with redshift, from ∼29 per cent in 2dFGRS to ∼58 per cent in CNOC2, for all galaxies brighter than ∼M* + 1.75. This trend is parallel to the evolution of field galaxies, where the equivalent fraction of emission-line galaxies increases from ∼53 to ∼75 per cent. The fraction of emission-line galaxies in groups is lower than in the field, across the full redshift range, indicating that the history of star formation in groups is influenced by their environment. We show that the evolution required to explain the data is inconsistent with a quiescent model of galaxy evolution; instead, discrete events in which galaxies cease forming stars (truncation events) are required. We constrain the probability of truncation (P trunc ) and find that a high value is required in a simple evolutionary scenario neglecting galaxy mergers (P trunc ≥ 0.3 Gyr -1 ). However, without assuming significant density evolution, P trunc is not required to be larger in groups than in the field, suggesting that the environmental dependence of star formation was embedded at redshifts z ≥ 0.45.

The Environmental Dependence of Galaxy Properties in the Local Universe: Dependences on Luminosity, Local Density, and System Richness

We investigate the environmental dependence of star formation and the morphology of galaxies in the local universe based on a volume-limited sample constructed from the data of the Sloan Digital Sky Survey. The sample galaxies (19,714 in total) are restricted to the redshift range of 0.030 200 km s-1 show no dependence on system richness, and most of the galaxies in those systems are non?star-forming early-type galaxies. Star formation activities of galaxies are different from those of field galaxies even in systems as poor as ? ~ 100 km s-1. This result suggests that environmental mechanisms that are effective only in rich systems, such as ram pressure stripping of cold gas and harassment, have not played a major role in transforming galaxies into red early-type galaxies. Strangulation and interactions between galaxies, however, remain candidates of the driver of the environmental dependence. In the dense environment in the local universe, the slow transformation of faint galaxies occurs to some extent, but the transformation of bright galaxies is not clearly visible. We suggest that the evolution of bright galaxies is not strongly related to galaxy system, such as groups and clusters, while the evolution of faint galaxies is likely to be closely connected to galaxy system.

The Influence of Environment on the Star Formation Rates of Galaxies

We have used a sample of 15,749 galaxies taken from the Las Campanas Redshift Survey to investigate the effects of environment on the rate of star formation in galaxies. For each galaxy, we derive a measure of the star formation rate (SFR) based on the strength of the [O II] emission line and a measure of galactic structure based on the central concentration of the galaxy's light, which is used to decouple the effect of morphology-environment relation from the SFR. Galactic environment is characterized both by the three-space local density of galaxies and by membership in groups and clusters. The size and homogeneity of this data set allows us to sample, for the first time, the entire range of galactic environments, from the lowest density voids to the richest clusters, in a uniform manner. Thus, we could expand our research from the conventional cluster versus field comparison to a new environmental investigation by decoupling the local galaxy density from the membership in associations. This decoupling is crucial for constraining the physical processes responsible for the observed environmental dependencies of star formation. On the other hand, the use of an automatic measure of galactic structure (concentration index) rather than Hubble type, which is a subjective and star formation-contaminated estimate of galactic morphology, allows us to separate cleanly the morphological component from the SFR-environment relationship. We find that, when a cluster/field comparison is made, cluster galaxies exhibit reduced star formation for the same concentration index. This result supports several previous studies based on Hubble type reporting similar suppression of star formation among cluster galaxies for the same Hubble type. We did not find any qualitatively different responses to environments between early- and late-type spirals, which were also previously reported. On the other hand, a further division of clusters by richness reveals a new possible excitation of starbursts in groups and poor clusters. Meanwhile, a more general environmental investigation shows that the star formation rate of galaxies with a given concentration index is sensitive to local galaxy density and shows a continuous correlation with the local density, in such a way that galaxies show higher levels of star formation in lower density environments. Interestingly, this trend is also observed both inside and outside of clusters, implying that physical processes responsible for this correlation might not operate intrinsically in the cluster environment. Furthermore, a more complex facet of the dependence of SFR on local density is also revealed; galaxies with differing levels of star formation appear to respond differently to the local density. Low levels of star formation, corresponding to those expected in normal members of the Hubble sequence, are more sensitive to environment inside than outside of clusters. In contrast, high levels of star formation, identified as starbursts, are at least as sensitive to local density in the field as in clusters. We conclude that at least two separate processes are responsible for the environmental sensitivity of the SFR and tentatively identify gas removal processes as responsible for the variation with density of the SFR of normal galaxies and galaxy-galaxy interactions as responsible for the prevalence of starbursts in intermediate density environments.