82 research outputs found
Predicting dust extinction from the stellar mass of a galaxy
We investigate how the typical dust extinction of H-alpha luminosity from a
star-forming galaxy depends upon star formation rate (SFR), metallicity and
stellar mass independently, using a sample of ~90,000 galaxies from Data
Release 7 of the Sloan Digital Sky Survey (SDSS). We measure extinctions
directly from the Balmer decrement of each source, and while higher values of
extinction are associated with an increase in any of the three parameters, we
demonstrate that the fundamental property that governs extinction is stellar
mass. After this mass-dependent relationship is removed, there is very little
systematic dependence of the residual extinctions with either SFR or
metallicity, and no significant improvement is obtained from a more general
parameterisation. In contrast to this, if either a SFR-dependent or
metallicity-dependent extinction relationship is applied, the residual
extinctions show significant trends that correlate with the other parameters.
Using the SDSS data, we present a relationship to predict the median dust
extinction of a sample of galaxies from its stellar mass, which has a scatter
of ~0.3 mag. The relationship was calibrated for H-alpha emission, but can be
more generally applied to radiation emitted at other wavelengths. These results
have important applications for studies of high-redshift galaxies, where
individual extinction measurements are hard to obtain but stellar mass
estimates can be relatively easily estimated from long-wavelength data.Comment: Accepted for publication in MNRAS. 12 pages. Dedicated to the memory
of Timothy Gar
Clustering around radio galaxies at z~1.5
The importance of studying old elliptical galaxies at redshift z ~ 1.5 is
reviewed, considering both what can be learned by extending studies of the
evolution of cluster galaxy scaling relations to earlier cosmic epochs, and the
age-dating of old elliptical galaxies at high redshifts. Following this, the
first results are provided of an on-going project to find such distant
elliptical galaxies, through an investigation of the cluster environments of
powerful radio sources with redshifts 1.44 < z < 1.7. These studies show a
considerable excess of red galaxies in the radio sources fields, with the
magnitudes (K >~ 17.5) and colours (R-K > 4) expected of old passively evolving
galaxies at the radio source redshift. The red galaxy overdensities are found
on two different scales around the radio sources; a pronounced small-scale peak
at radial distances of <~ 150 kpc, and a weaker large-scale excess extending
out to 1 - 1.5 Mpc. The presence and richness of these red galaxy excesses
varies considerably from source to source. An interpretation of these results
is provided.Comment: LaTeX, 6 pages, Elsevier Science format. To appear in "Radio
galaxies: past, present & future". eds. M. Jarvis et al., Leiden, Nov 200
Radio Jets in Galaxies with Actively Accreting Black Holes: new insights from the SDSS
The majority of nearby radio-loud AGN are found in massive, old elliptical
galaxies with weak emission lines. At high redshifts,however, most known radio
AGN have strong emission lines. In this paper, we examine a subset of radio AGN
with emission lines selected from the Sloan Digital Sky Survey. The probability
for a nearby radio AGN to have emission lines is a strongly decreasing function
of galaxy mass and an increasing function of radio luminosity above 10^25 W/Hz.
Emission line and radio luminosities are correlated, but with large dispersion.
At a given radio power, AGN with small black holes have higher [OIII]
luminosities (which we interpret as higher accretion rates) than AGN with big
black holes. However, if we scale both radio and emission line luminosities by
the black hole mass, we find a correlation between normalized radio power and
accretion rate in Eddington units that is independent of black hole mass. There
is also a clear correlation between normalized radio power and the age of the
stellar population in the galaxy. Present-day AGN with the highest normalized
radio powers are confined to galaxies with small black holes. High-redshift,
high radio-luminosity AGN could be explained if big black holes were similarly
active at earlier cosmic epochs. To investigate why only a small fraction of
emission line AGN become radio loud, we create matched samples of radio-loud
and radio-quiet AGN and compare their host galaxy properties and environments.
The main difference lies in their environments; our local density estimates are
a factor 2 larger around the radio-loud AGN. We propose a scenario in which
radio-loud AGN with emission lines are located in galaxies where accretion of
both cold and hot gas can occur simultaneously. (Abridged)Comment: 18 figures, submitted to MNRA
How special are Brightest Group and Cluster Galaxies?
We use the Sloan Digital Sky Survey to construct a sample of 625 brightest
group and cluster galaxies (BCGs) together with control samples of non-BCGs
matched in stellar mass, redshift, and color. We investigate how the systematic
properties of BCGs depend on stellar mass and on their privileged location near
the cluster center. The groups and clusters that we study are drawn from the C4
catalogue of Miller et al. (2005) but we have developed improved algorithms for
identifying the BCG and for measuring the cluster velocity dispersion. Since
the SDSS photometric pipeline tends to underestimate the luminosities of large
galaxies in dense environments, we have developed a correction for this effect
which can be readily applied to the published catalog data. We find that BCGs
are larger and have higher velocity dispersions than non-BCGs of the same
stellar mass, which implies that BCGs contain a larger fraction of dark matter.
In contrast to non-BCGs, the dynamical mass-to-light ratio of BCGs does not
vary as a function of galaxy luminosity. Hence BCGs lie on a different
fundamental plane than ordinary elliptical galaxies. BCGs also follow a steeper
Faber-Jackson relation than non-BCGs, as suggested by models in which BCGs
assemble via dissipationless mergers along preferentially radial orbits. We
find tentative evidence that this steepening is stronger in more massive
clusters. BCGs have similar mean stellar ages and metallicities to non-BCGs of
the same mass, but they have somewhat higher alpha/Fe ratios, indicating that
star formation may have occurred over a shorter timescale in the BCGs. Finally,
we find that BCGs are more likely to host radio-loud active galactic nuclei
than other galaxies of the same mass, but are less likely to host an optical
AGN. The differences we find are more pronounced for the less massive BCGs.Comment: Replaced with slightly modified version accepted by MNRAS. 28 pages,
25 figures. Version with full resolution figures available at
http://www.mpa-garching.mpg.de/~anja/bcgs_avdl.pd
Recommended from our members
Dense Cores in Perseus: The Influence of Stellar Content and Cluster Environment
We present the chemistry, temperature, and dynamical state of a sample of 193 dense cores or core candidates in the Perseus Molecular cloud and compare the properties of cores associated with young stars and clusters with those which are not. The combination of our NH3 and CCS observations with previous millimeter, submillimeter, and Spitzer data available for this cloud enables us both to determine core properties precisely and to accurately classify cores as starless or protostellar. The properties of cores in different cluster environments and before-and-after star formation provide important constraints on simulations of star formation, particularly under the paradigm that the essence of star formation is set by the turbulent formation of prestellar cores. We separate the influence of stellar content from that of the cluster environment and find that cores within clusters have (1) higher kinetic temperatures (12.9 K versus 10.8 K) and, (2) lower fractional abundances of CCS versus and versus . Cores associated with protostars have (1) slightly higher kinetic temperatures (11.9 K versus 10.6 K), (2) higher NH3 excitation temperatures (7.4 K versus 6.1 K), (3) are at higher column density versus , have (4) slightly more nonthermal/turbulent line widths versus , have (5) higher masses versus , and have (6) lower fractional abundance of CCS versus . All values are medians. We find that neither cluster environment nor protostellar content makes a significant difference to the dynamical state of cores as estimated by the virial parameterâmost cores in each category are gravitationally bound. Only the high precision of our measurements and the size of our sample make such distinctions possible. Overall, cluster environment and protostellar content have a smaller influence on the properties of the cores than is typically assumed, and the variation within categories is larger than the differences between categories.AstronomyEngineering and Applied SciencesOther Research Uni
Identifying the Low Luminosity Population of Embedded Protostars in the c2d Observations of Clouds and Cores
We present a search for all embedded protostars with internal luminosities <
1 solar luminosity in the sample of nearby, low-mass star-forming regions
surveyed by the Spitzer Space Telescope c2d Legacy Project. The internal
luminosity (Lint) of a source is the luminosity of the central source and
excludes luminosity arising from external heating. On average, the c2d data are
sensitive to embedded protostars with Lint > 4E-3 (d/140 pc)^2 solar
luminosities, a factor of 25 better than the sensitivity of IRAS to such
objects. We present selection criteria used to identify candidates from the
Spitzer data and examine complementary data to decide whether each candidate is
truly an embedded protostar. We find a tight correlation between the 70 micron
flux and internal luminosity of a protostar, an empirical result based on
observations and two-dimensional radiative transfer models of protostars. We
identify 50 embedded protostars with Lint < 1 solar luminosities; 15 have Lint
< 0.1 solar luminosities. The intrinsic distribution of source luminosities
increases to lower luminosities. While we find sources down to the above
sensitivity limit, indicating that the distribution may extend to luminosities
lower than probed by these observations, we are able to rule out a continued
rise in the distribution below 0.1 solar luminosities. Between 75-85% of cores
classified as starless prior to being observed by Spitzer remain starless to
our luminosity sensitivity; the remaining 15-25% harbor low-luminosity,
embedded protostars. We compile complete Spectral Energy Distributions for all
50 objects and calculate standard evolutionary signatures, and argue that these
objects are inconsistent with the simplest picture of star formation wherein
mass accretes from the core onto the protostar at a constant rate.Comment: 97 pages, 11 Tables, 22 Figures. Accepted by ApJS. Version with
high-resolution figures available at http://peggysue.as.utexas.edu/SIRT
Metallicity Gradients at Large Galactocentric Radii Using the Near-infrared Calcium Triplet
We describe a new spectroscopic technique for measuring radial metallicity
gradients out to large galactocentric radii. We use the DEIMOS multi-object
spectrograph on the Keck telescope and the galaxy spectrum extraction technique
of Proctor et al. (2009). We also make use of the metallicity sensitive
near-infrared (NIR) Calcium triplet (CaT) features together with single stellar
population models to obtain metallicities. Our technique is applied as a pilot
study to a sample of three relatively nearby (<30 Mpc) intermediate-mass to
massive early-type galaxies. Results are compared with previous literature
inner region values and generally show good agreement. We also include a
comparison with profiles from dissipational disk-disk major merger simulations.
Based on our new extended metallicity gradients combined with other
observational evidence and theoretical predictions, we discuss possible
formation scenarios for the galaxies in our sample. The limitations of our new
technique are also discussed.Comment: 13 Pages, 9 Figures, 7 Tables, Accepted for publication in MNRA
Stellar population gradients in the cores of nearby field E+A galaxies
We have selected a sample of local E+A galaxies from the Sloan Digital Sky
Survey (SDSS) Data Release 7 for follow up integral field spectroscopy with the
Wide Field Spectrograph (WiFeS) on the ANU 2.3-m telescope. The sample was
selected using the Halpha line in place of the [OII]3727 line as the indicator
of on-going star formation (or lack thereof). This allowed us to select a lower
redshift sample of galaxies than available in the literature since the
[OII]3727 falls off the blue end of the wavelength coverage in the SDSS for the
very lowest redshift objects. This low redshift selection means that the
galaxies have a large angular to physical scale which allows us to resolve the
central ~1kpc region of the galaxies; the region where stellar population
gradients are expected. Such observations have been difficult to make using
other higher redshift samples because even at redshifts z~0.1 the angular to
physical scale is similar to the resolution provided by ground based seeing.
Our integral field spectroscopy has enabled us to make the first robust
detections of Balmer line gradients in the centres of E+A galaxies. Six out of
our sample of seven, and all the galaxies with regular morphologies, are
observed to have compact and centrally-concentrated Balmer line absorption.
This is evidence for compact young cores and stellar population gradients which
are predicted from models of mergers and tidal interactions which funnel gas
into the galaxy core. Given the generally isolated nature of our sample this
argues for the galaxies being seen in the late stage of a merger where the
progenitors have already coalesced.Comment: accepted to MNRA
Dynamical Delays Between Starburst and AGN Activity in Galaxy Nuclei
Observations of AGN have suggested a possible delay between the peak of star
formation (on some scale) and AGN activity. Inefficient fueling (and/or
feedback) from fast stellar winds has been invoked to explain this, but we
argue this is unlikely in bright systems accreting primarily cold dense gas. We
show that such a delay can arise even in bright quasars for purely dynamical
reasons. If some large-scale process produces rapid inflow, smaller scales will
quickly become gas-dominated. As the gas density peaks, so does the SFR.
However, gravitational torques which govern further inflow are relatively
inefficient in gas-dominated systems; as more gas is turned into stars, the
stars provide an efficient angular momentum sink allowing more rapid inflow.
Moreover, the gas provided to the central regions in mergers or strong disk
instabilities will typically be ~100 times larger than that needed to fuel the
BH; the system is effectively in the 'infinite gas supply' limit. BH growth can
therefore continue for some time while the gas supply exhausts, until it has
significantly depleted to the point where the BH is finally 'starved.' Both of
these effects act together with comparable magnitude, and mean that the peak of
BH growth can lag the peak in the SFR measured at a given scale by a timescale
corresponding to the gas exhaustion time on that scale (~ 10-100 local
dynamical times). This predicts that the inferred delay will vary in a specific
manner with the radius over which the star formation rate is measured. We
discuss possible implications for the role of AGN feedback in suppressing star
formation activity.Comment: 5 pages, 2 figures, accepted to MNRAS Letters (matches accepted
version
The evolution of the star formation activity in galaxies and its dependence on environment
We study how the proportion of star-forming galaxies evolves between z=0.8
and z=0 as a function of galaxy environment, using the [OII] line in emission
as a signature of ongoing star formation. Our high-z dataset comprises 16
clusters, 10 groups and another 250 galaxies in poorer groups and the field at
z=0.4-0.8 from the ESO Distant Cluster Survey, plus another 9 massive clusters
at similar redshifts. As a local comparison, we use samples of galaxy systems
selected from the Sloan Digital Sky Survey at 0.04< z < 0.08. At high-z most
systems follow a broad anticorrelation between the fraction of star-forming
galaxies and the system velocity dispersion. At face value, this suggests that
at z=0.4-0.8 the mass of the system largely determines the proportion of
galaxies with ongoing star formation. At these redshifts the strength of star
formation (as measured by the [OII] equivalent width) in star-forming galaxies
is also found to vary systematically with environment. Sloan clusters have much
lower fractions of star-forming galaxies than clusters at z=0.4-0.8 and, in
contrast with the distant clusters, show a plateau for velocity dispersions , where the fraction of galaxies with [OII] emission does not
vary systematically with velocity dispersion. We quantify the evolution of the
proportion of star-forming galaxies as a function of the system velocity
dispersion and find it is strongest in intermediate-mass systems (sigma ~
500-600 km s^-1 at z=0). To understand the origin of the observed trends, we
use the Press-Schechter formalism and the Millennium Simulation and show that
galaxy star formation histories may be closely related to the growth history of
clusters and groups. We propose a scheme that is able to account for the
observed relations between the star-forming fraction and \sigma [abridged].Comment: 28 pages, 19 figures, ApJ in pres
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