25 research outputs found
The similar stellar populations of quiescent spiral and elliptical galaxies
We compare the stellar population properties in the central regions of visually classified non-star-forming spiral and elliptical galaxies from Galaxy Zoo and Sloan Digital Sky Survey (SDSS) Data Release 7. The galaxies lie in the redshift range 0.04 < z < 0.1 and have stellar masses larger than log M-* = 10.4. We select only face-on spiral galaxies in order to avoid contamination by light from the disc in the SDSS fibre and enabling the robust visual identification of spiral structure. Overall, we find that galaxies with larger central stellar velocity dispersions, regardless of morphological type, have older ages, higher metallicities and an increased overabundance of alpha-elements. Age and alpha-enhancement, at fixed velocity dispersion, do not depend on morphological type. The only parameter that, at a given velocity dispersion, correlates with morphological type is metallicity, where the metallicity of the bulges of spiral galaxies is 0.07 dex higher than that of the ellipticals. However, for galaxies with a given total stellar mass, this dependence on morphology disappears. Under the assumption that, for our sample, the velocity dispersion traces the mass of the bulge alone, as opposed to the total mass (bulge+disc) of the galaxy, our results imply that the formation epoch of galaxy and the duration of its star-forming period are linked to the mass of the bulge. The extent to which metals are retained within the galaxy, and not removed as a result of outflows, is determined by the total mass of the galaxy
The similar stellar populations of quiescent spiral and elliptical galaxies
We compare the stellar population properties in the central regions of visually classified non-star-forming spiral and elliptical galaxies from Galaxy Zoo and Sloan Digital Sky Survey (SDSS) Data Release 7. The galaxies lie in the redshift range 0.04 < z < 0.1 and have stellar masses larger than logâM* = 10.4. We select only face-on spiral galaxies in order to avoid contamination by light from the disc in the SDSS fibre and enabling the robust visual identification of spiral structure. Overall, we find that galaxies with larger central stellar velocity dispersions, regardless of morphological type, have older ages, higher metallicities and an increased overabundance of α-elements. Age and α-enhancement, at fixed velocity dispersion, do not depend on morphological type. The only parameter that, at a given velocity dispersion, correlates with morphological type is metallicity, where the metallicity of the bulges of spiral galaxies is 0.07âdex higher than that of the ellipticals. However, for galaxies with a given total stellar mass, this dependence on morphology disappears. Under the assumption that, for our sample, the velocity dispersion traces the mass of the bulge alone, as opposed to the total mass (bulge+disc) of the galaxy, our results imply that the formation epoch of galaxy and the duration of its star-forming period are linked to the mass of the bulge. The extent to which metals are retained within the galaxy, and not removed as a result of outflows, is determined by the total mass of the galaxy
Systematic errors in weighted twoâpoint correlation functions: an application to interactionâinduced star formation
Weighted correlation functions are an increasingly important tool for understanding how galaxy properties depend on their separation from each other. We use a mock galaxy sample drawn from the Millennium Simulation, assigning weights using a simple prescription to illustrate and explore how well a weighted correlation function recovers the true separation dependence of the input weights. We find that the use of a weighted correlation function results in a dilution of the magnitude of any separation dependence of properties and a smearing out of that dependence in radius, compared to the input behaviour. We present a quantitative discussion of the dilution in the magnitude of separation dependence in properties in the special case of a constant enhancement at r < r c . In this particular case where there was a star formation rate (SFR) enhancement at small radius r < r c = 35âkpc, the matching of one member of an enhanced pair with an unenhanced galaxy in the same group gives an artificial enhancement out to large radius âŒ200âkpc. We compare this with the observations of SFR enhancement from the Sloan Digital Sky Survey, finding very similar behaviour â a significant enhancement at radii <40âkpc and a weak enhancement out to more than 150âkpc. While we explore a particular case in this paper, it is easy to see that the phenomenon is general, and precision analysis of weighted correlation functions will need to account carefully for this effect using simulated mock catalogues.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94502/1/mnr20325.pd
The Majority of Compact Massive Galaxies at z~2 are Disk Dominated
We investigate the stellar structure of massive, quiescent galaxies at z~2,
based on Hubble Space Telescope/WFC3 imaging from the Early Release Science
program. Our sample of 14 galaxies has stellar masses of M* > 10^{10.8} Msol
and photometric redshifts of 1.5 < z < 2.5. In agreement with previous work,
their half-light radii are <2 kpc, much smaller than equally massive galaxies
in the present-day universe. A significant subset of the sample appears highly
flattened in projection, which implies, considering viewing angle statistics,
that a significant fraction of the galaxies in our sample have pronounced
disks. This is corroborated by two-dimensional surface brightness profile fits.
We estimate that 65% +/- 15% of the population of massive, quiescent z~2
galaxies are disk-dominated. The median disk scale length is 1.5 kpc,
substantially smaller than the disks of equally massive galaxies in the
present-day universe. Our results provide strong observational evidence that
the much-discussed ultra-dense high-redshift galaxies should generally be
thought of as disk-like stellar systems with the majority of stars formed from
gas that had time to settle into a disk.Comment: published versio
The merger-driven evolution of massive galaxies
We explore the rate and impact of galaxy mergers on the massive galaxy
population using the amplitude of the two-point correlation function on small
scales for M > 5e10 M_sun galaxies from the COSMOS and COMBO-17 surveys. Using
a pair fraction derived from the Sloan Digital Sky Survey as a low-redshift
benchmark, the large survey area at intermediate redshifts allows us to
determine the evolution of the close pair fraction with unprecedented accuracy
for a mass-selected sample: we find that the fraction of galaxies more massive
than 5e10M_sun in pairs separated by less than 30 kpc in 3D space evolves as
F(z) = (0.0130+/-0.0019)x(1+z)^1.21+/-0.25 between z = 0 and z = 1.2. Assuming
a merger time scale of 0.5 Gyrs, the inferred merger rate is such that galaxies
with mass in excess of 1e11 M_sun have undergone, on average, 0.5 (0.7) mergers
involving progenitor galaxies both more massive than 5e10 M_sun since z = 0.6
(1.2). We also study the number density evolution of massive red sequence
galaxies using published luminosity functions and constraints on the M/L
evolution from the fundamental plane. Moreover, we demonstrate that the
measured merger rate of massive galaxies is sufficient to explain this observed
number density evolution in massive red sequence galaxies since z = 1.Comment: Accepted in Ap
The bulk of the black hole growth since z ~ 1 occurs in a secular universe: no major merger-AGN connection
What is the relevance of major mergers and interactions as triggering mechanisms for active galactic nuclei (AGNs)
activity? To answer this long-standing question, we analyze 140 XMM-Newton-selected AGN host galaxies and
a matched control sample of 1264 inactive galaxies over z ~ 0.3â1.0 and M_â < 10^(11.7) M_â with high-resolution
Hubble Space Telescope/Advanced Camera for Surveys imaging from the COSMOS field. The visual analysis of
their morphologies by 10 independent human classifiers yields a measure of the fraction of distorted morphologies
in the AGN and control samples, i.e., quantifying the signature of recent mergers which might potentially be
responsible for fueling/triggering the AGN. We find that (1) the vast majority (>85%) of the AGN host galaxies
do not show strong distortions and (2) there is no significant difference in the distortion fractions between active
and inactive galaxies. Our findings provide the best direct evidence that, since z ~ 1, the bulk of black hole (BH)
accretion has not been triggered by major galaxy mergers, therefore arguing that the alternative mechanisms, i.e.,
internal secular processes and minor interactions, are the leading triggers for the episodes of major BH growth.We
also exclude an alternative interpretation of our results: a substantial time lag between merging and the observability
of the AGN phase could wash out the most significant merging signatures, explaining the lack of enhancement
of strong distortions on the AGN hosts. We show that this alternative scenario is unlikely due to (1) recent major
mergers being ruled out for the majority of sources due to the high fraction of disk-hosted AGNs, (2) the lack of
a significant X-ray signal in merging inactive galaxies as a signature of a potential buried AGN, and (3) the low
levels of soft X-ray obscuration for AGNs hosted by interacting galaxies, in contrast to model predictions
Obscured star formation in intermediate-density environments:A Spitzer study of the Abell 901/902 supercluster
We explore the amount of obscured star formation as a function of environment in the Abell 901/902 (A901/902) supercluster at z = 0.165 in conjunction with a field sample drawn from the A901 and CDFS fields, imaged with the Hubble Space Telescope as part of the Space Telescope A901/902 Galaxy Evolution Survey and Galaxy Evolution from Morphology and Spectral Energy Distributions (SEDs) Survey. We combine the combo-17 near-UV/optical SED with Spitzer 24 mu m photometry to estimate both the unobscured and obscured star formation in galaxies with M-* > 10(10) M-circle dot. We find that the star formation activity in massive galaxies is suppressed in dense environments, in agreement with previous studies. Yet, nearly 40% of the star-forming (SF) galaxies have red optical colors at intermediate and high densities. These red systems are not starbursting; they have star formation rates (SFRs) per unit stellar mass similar to or lower than blue SF galaxies. More than half of the red SF galaxies have low infrared-to-ultraviolet (IR-to-UV) luminosity ratios, relatively high Sersicindices, and they are equally abundant at all densities. They might be gradually quenching their star formation, possibly but not necessarily under the influence of gas-removing environmental processes. The other greater than or similar to 40% of the red SF galaxies have high IR-to-UV luminosity ratios, indicative of high dust obscuration. They have relatively high specific SFRs and are more abundant at intermediate densities. Our results indicate that while there is an overall suppression in the SF galaxy fraction with density, the small amount of star formation surviving the cluster environment is to a large extent obscured, suggesting that environmental interactions trigger a phase of obscured star formation, before complete quenching
The Majority of Compact Massive Galaxies at z~2 are Disk Dominated
We investigate the stellar structure of massive, quiescent galaxies at z~2,
based on Hubble Space Telescope/WFC3 imaging from the Early Release Science
program. Our sample of 14 galaxies has stellar masses of M* > 10^{10.8} Msol
and photometric redshifts of 1.5 < z < 2.5. In agreement with previous work,
their half-light radii are <2 kpc, much smaller than equally massive galaxies
in the present-day universe. A significant subset of the sample appears highly
flattened in projection, which implies, considering viewing angle statistics,
that a significant fraction of the galaxies in our sample have pronounced
disks. This is corroborated by two-dimensional surface brightness profile fits.
We estimate that 65% +/- 15% of the population of massive, quiescent z~2
galaxies are disk-dominated. The median disk scale length is 1.5 kpc,
substantially smaller than the disks of equally massive galaxies in the
present-day universe. Our results provide strong observational evidence that
the much-discussed ultra-dense high-redshift galaxies should generally be
thought of as disk-like stellar systems with the majority of stars formed from
gas that had time to settle into a disk.Comment: published versio
Major Merging: The Way to Make a Massive, Passive Galaxy
We analyze the projected axial ratio distribution, p(b/a), of galaxies that
were spectroscopically selected from the Sloan Digital Sky Survey (DR6) to have
low star-formation rates. For these quiescent galaxies we find a rather abrupt
change in p(b/a) at a stellar mass of ~10^{11} M_sol: at higher masses there
are hardly any galaxies with b/a<0.6, implying that essentially none of them
have disk-like intrinsic shapes and must be spheroidal. This transition mass is
~3-4 times higher than the threshold mass above which quiescent galaxies
dominate in number over star-forming galaxies, which suggests these mass scales
are unrelated. At masses lower than ~10^{11} M_sol, quiescent galaxies show a
large range in axial ratios, implying a mix of bulge- and disk-dominated
galaxies. Our result strongly suggests that major merging is the most
important, and perhaps only relevant, evolutionary channel to produce massive
(>10^{11} M_sol), quiescent galaxies, as it inevitably results in spheroids.Comment: Minor changes to match published version in ApJ Letter