44 research outputs found
On the maximum age resolution achievable through stellar population synthesis models
As the reconstruction of the star-formation histories (SFH) of galaxies from
spectroscopic data becomes increasingly popular, we explore the best age
resolution achievable with stellar population synthesis (SPS) models, relying
on different constraints: broad-band colours, absorption indices, a combination
of the two, and the full spectrum. We perform idealized experiments on SPS
models and show that the minimum resolvable relative duration of a
star-formation episode (time difference between 10% and 90% of the stellar mass
formed divided by the median age) is never better than 0.4, even when using
spectra with signal-to-noise ratio (SNR) larger than 100 per AA. Typically, the
best relative age resolution ranges between 0.4 and 0.7 over most of the
age-metallicity plane, corresponding to minimum bin sizes for SFH sampling
between 0.15 and 0.25 dex. This resolution makes the spectroscopic exploration
of distant galaxies mandatory in order to reconstruct the early phases of
galaxies' SFHs. We show that spectroscopy with SNR 2/AA is essential
for good age resolution. Remarkably, using the full spectrum does not prove
significantly more effective than relying on absorption indices, especially at
SNR 20/AA. We discuss the physical origins of the age resolution
trends as a function of age and metallicity, and identify the presence of
maxima in age resolution (i.e. minima in measurable relative time duration) at
the characteristic ages that correspond to quick time variations in spectral
absorption features. We connect these maxima to bumps commonly observed in
reconstructed SFHs.Comment: Accepted for publication on MNRAS - 16 page
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 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 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 0.07 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
Resolving the age bimodality of galaxy stellar populations on kpc scales
Galaxies in the local Universe are known to follow bimodal distributions in
the global stellar populations properties. We analyze the distribution of the
local average stellar-population ages of 654,053 sub-galactic regions resolved
on ~1-kpc scales in a volume-corrected sample of 394 galaxies, drawn from the
CALIFA-DR3 integral-field-spectroscopy survey and complemented by SDSS imaging.
We find a bimodal local-age distribution, with an old and a young peak
primarily due to regions in early-type galaxies and star-forming regions of
spirals, respectively. Within spiral galaxies, the older ages of bulges and
inter-arm regions relative to spiral arms support an internal age bimodality.
Although regions of higher stellar-mass surface-density, mu*, are typically
older, mu* alone does not determine the stellar population age and a bimodal
distribution is found at any fixed mu*. We identify an "old ridge" of regions
of age ~9 Gyr, independent of mu*, and a "young sequence" of regions with age
increasing with mu* from 1-1.5 Gyr to 4-5 Gyr. We interpret the former as
regions containing only old stars, and the latter as regions where the relative
contamination of old stellar populations by young stars decreases as mu*
increases. The reason why this bimodal age distribution is not inconsistent
with the unimodal shape of the cosmic-averaged star-formation history is that
i) the dominating contribution by young stars biases the age low with respect
to the average epoch of star formation, and ii) the use of a single average age
per region is unable to represent the full time-extent of the star-formation
history of "young-sequence" regions.Comment: 17 pages, 11 figures, MNRAS accepte
An exquisitely deep view of quenching galaxies through the gravitational lens: Stellar population, morphology, and ionized gas
This work presents an in-depth analysis of four gravitationally lensed red
galaxies at z = 1.6-3.2. The sources are magnified by factors of 2.7-30 by
foreground clusters, enabling spectral and morphological measurements that are
otherwise challenging. Our sample extends below the characteristic mass of the
stellar mass function and is thus more representative of the quiescent galaxy
population at z > 1 than previous spectroscopic studies. We analyze deep
VLT/X-SHOOTER spectra and multi-band Hubble Space Telescope photometry that
cover the rest-frame UV-to-optical regime. The entire sample resembles stellar
disks as inferred from lensing-reconstructed images. Through stellar population
synthesis analysis we infer that the targets are young (median age = 0.1-1.2
Gyr) and formed 80% of their stellar masses within 0.07-0.47 Gyr. Mg II
absorption is detected across the sample.
Blue-shifted absorption and/or redshifted emission of Mg II is found in the two
youngest sources, indicative of a galactic-scale outflow of warm (
K) gas. The [O III] luminosity is higher for the two young
sources (median age less than 0.4 Gyr) than the two older ones, perhaps
suggesting a decline in nuclear activity as quenching proceeds. Despite
high-velocity ( km s) galactic-scale outflows seen in the
most recently quenched galaxies, warm gas is still present to some extent long
after quenching. Altogether our results indicate that star formation quenching
at high redshift must have been a rapid process (< 1 Gyr) that does not
synchronize with bulge formation or complete gas removal. Substantial bulge
growth is required if they are to evolve into the metal-rich cores of
present-day slow-rotators.Comment: Accepted for publication in the Astrophysical Journal. 37 pages, 20
figures, 10 table
Stars, gas, and star formation of distant post-starburst galaxies
We present a comprehensive multi-wavelength study of 5 poststarburst galaxies
with at , examining their stars, gas, and
current and past star-formation activities. Using optical images from the
Subaru telescope and Hubble Space Telescope, we observe a high incidence of
companion galaxies and low surface brightness tidal features, indicating that
quenching is closely related to interactions between galaxies. From optical
spectra provided by the LEGA-C survey, we model the stellar continuum to derive
the star-formation histories and show that the stellar masses of progenitors
ranging from to , undergoing a burst of
star formation several hundred million years prior to observation, with a decay
time scale of million years. Our ALMA observations detect CO(2-1)
emission in four galaxies, with the molecular gas spreading over up to ,
or kpc, with a mass of up to . However,
star-forming regions are unresolved by either the slit spectra or 3~GHz
continuum observed by the Very Large Array. Comparisons between the
star-formation rates and gas masses, and the sizes of CO emission and
star-forming regions suggest a low star-forming efficiency. We show that the
star-formation rates derived from IR and radio luminosities with commonly-used
calibrations tend to overestimate the true values because of the prodigious
amount of radiation from old stars and the contribution from AGN, as the
optical spectra reveal weak AGN-driven outflows.Comment: Accepted by Ap
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
Optically-passive spirals: The missing link in gradual star formation suppression upon cluster infall
Galaxies migrate from the blue cloud to the red sequence when their star
formation is quenched. Here, we report on galaxies quenched by environmental
effects and not by mergers or strong AGN as often invoked: They form stars at a
reduced rate which is optically even less conspicuous, and manifest a
transition population of blue spirals evolving into S0 galaxies. These
'optically passive' or 'red spirals' are found in large numbers in the STAGES
project (and by Galaxy Zoo) in the infall region of clusters and groups.Comment: Proceedings of "The Starburst-AGN connection" conference held in
Shanghai, Oct 27-31, 200
The STAGES view of red spirals and dusty red galaxies: Mass-dependent quenching of star-formation in cluster infall
We investigate the properties of optically passive spirals and dusty red
galaxies in the A901/2 cluster complex at redshift ~0.17 using restframe
near-UV-optical SEDs, 24 micron IR data and HST morphologies from the STAGES
dataset. The cluster sample is based on COMBO-17 redshifts with an rms
precision of sigma_cz~2000 km/sec. We find that 'dusty red galaxies' and
'optically passive spirals' in A901/2 are largely the same phenomenon, and that
they form stars at a substantial rate, which is only 4x lower than that in blue
spirals at fixed mass. This star formation is more obscured than in blue
galaxies and its optical signatures are weak. They appear predominantly in the
stellar mass range of log M*/Msol=[10,11] where they constitute over half of
the star-forming galaxies in the cluster; they are thus a vital ingredient for
understanding the overall picture of star formation quenching in clusters. We
find that the mean specific SFR of star-forming galaxies in the cluster is
clearly lower than in the field, in contrast to the specific SFR properties of
blue galaxies alone, which appear similar in cluster and field. Such a rich red
spiral population is best explained if quenching is a slow process and
morphological transformation is delayed even more. At log M*/Msol<10, such
galaxies are rare, suggesting that their quenching is fast and accompanied by
morphological change. We note, that edge-on spirals play a minor role; despite
being dust-reddened they form only a small fraction of spirals independent of
environment.Comment: Accepted for publication in MNRA
Less than 10 percent of star formation in z=0.6 massive galaxies is triggered by major interactions
Both observations and simulations show that major tidal interactions or
mergers between gas-rich galaxies can lead to intense bursts of starformation.
Yet, the average enhancement in star formation rate (SFR) in major mergers and
the contribution of such events to the cosmic SFR are not well estimated. Here
we use photometric redshifts, stellar masses and UV SFRs from COMBO-17, 24
micron SFRs from Spitzer and morphologies from two deep HST cosmological survey
fields (ECDFS/GEMS and A901/STAGES) to study the enhancement in SFR as a
function of projected galaxy separation. We apply two-point projected
correlation function techniques, which we augment with morphologically-selected
very close pairs (separation <2 arcsec) and merger remnants from the HST
imaging. Our analysis confirms that the most intensely star-forming systems are
indeed interacting or merging. Yet, for massive (M* > 10^10 Msun) star-forming
galaxies at 0.4<z<0.8, we find that the SFRs of galaxies undergoing a major
interaction (mass ratios <1:4 and separations < 40 kpc) are only 1.80 +/- 0.30
times higher than the SFRs of non-interacting galaxies when averaged over all
interactions and all stages of the interaction, in good agreement with other
observational works.
We demonstrate that these results imply that <10% of star formation at 0.4 <
z < 0.8 is triggered directly by major mergers and interactions; these events
are not important factors in the build-up of stellar mass since z=1.Comment: Submitted to ApJ. 41 pages, 11 figure