15 research outputs found
The Rest-frame Optical Colors of 99,000 SDSS Galaxies
We synthesize the rest-frame Stroemgren colors using SDSS spectra for 99,088
galaxies selected from Data Release 1. This narrow-band ~200 AA photometric
system (uz, vz, bz, yz), first designed for the determination of effective
temperature, metallicity and gravity of stars, measures the continuum spectral
slope of galaxies in the rest-frame 3200-5800 AA wavelength range. Galaxies
form a remarkably narrow locus (~0.03 mag) in the resulting color-color
diagram. The Bruzual & Charlot population synthesis models suggest that the
position of a galaxy along this locus is controlled by a degenerate combination
of metallicity and age of the dominant stellar population. Galaxy distribution
along the locus is bimodal, with the local minimum corresponding to an ~1 Gyr
old single stellar population. The position perpendicular to the locus is
independent of metallicity and age, and reflects the galaxy's dust content, as
implied by both the models and the statistics of IRAS detections. A comparison
of this locus with the galaxy locus in the H_delta-D_n(4000) diagram, utilized
by Kauffmann et al. (2003) to estimate stellar masses, reveals a tight
correlation, although the two analyzed spectral ranges barely overlap. Overall,
the galaxy spectral energy distribution in the entire UV to near-IR range can
be described as a single-parameter family with an accuracy of 0.1 mag, or
better. This nearly one-dimensional distribution of galaxies in the
multi-dimensional space of measured parameters strongly supports the conclusion
of Yip et al. (2004), based on a principal component analysis, that SDSS galaxy
spectra can be described by a small number of eigenspectra. Apparently, the
contributions of stellar populations that dominate the optical emission from
galaxies are combined in a simple and well-defined way.Comment: Accepted for publication in MNRAS; 19 pages, 28 color figure
Source Matching in the SDSS and RASS: Which Galaxies are Really X-ray Sources?
The current view of galaxy formation holds that all massive galaxies harbor a
massive black hole at their center, but that these black holes are not always
in an actively accreting phase. X-ray emission is often used to identify
accreting sources, but for galaxies that are not harboring quasars
(low-luminosity active galaxies), the X-ray flux may be weak, or obscured by
dust. To aid in the understanding of weakly accreting black holes in the local
universe, a large sample of galaxies with X-ray detections is needed. We
cross-match the ROSAT All Sky Survey (RASS) with galaxies from the Sloan
Digital Sky Survey Data Release 4 (SDSS DR4) to create such a sample. Because
of the high SDSS source density and large RASS positional errors, the
cross-matched catalog is highly contaminated by random associations. We
investigate the overlap of these surveys and provide a statistical test of the
validity of RASS-SDSS galaxy cross-matches. SDSS quasars provide a test of our
cross-match validation scheme, as they have a very high fraction of true RASS
matches. We find that the number of true matches between the SDSS main galaxy
sample and the RASS is highly dependent on the optical spectral classification
of the galaxy; essentially no star-forming galaxies are detected, while more
than 0.6% of narrow-line Seyferts are detected in the RASS. Also, galaxies with
ambiguous optical classification have a surprisingly high RASS detection
fraction. This allows us to further constrain the SEDs of low-luminosity active
galaxies. Our technique is quite general, and can be applied to any
cross-matching between surveys with well-understood positional errors.Comment: 10 pages, 10 figures, submitted to The Astronomical Journal on 19
June 200
The UV, Optical, and IR Properties of SDSS Sources Detected by GALEX
We discuss the UV, optical, and IR properties of the SDSS sources detected by
GALEX as part of its All-sky Imaging Survey Early Release Observations.
Virtually all of the GALEX sources in the overlap region are detected by SDSS.
GALEX sources represent ~2.5% of all SDSS sources within these fields and about
half are optically unresolved. Most unresolved GALEX/SDSS sources are bright
blue turn-off thick disk stars and are typically detected only in the GALEX
near-UV band. The remaining unresolved sources include low-redshift quasars,
white dwarfs, and white dwarf/M dwarf pairs, and these dominate the optically
unresolved sources detected in both GALEX bands.
Almost all the resolved SDSS sources detected by GALEX are fainter than the
SDSS 'main' spectroscopic limit. These sources have colors consistent with
those of blue (spiral) galaxies (u-r<2.2), and most are detected in both GALEX
bands. Measurements of their UV colors allow much more accurate and robust
estimates of star-formation history than are possible using only SDSS data.
Indeed, galaxies with the most recent (<20 Myr) star formation can be robustly
selected from the GALEX data by requiring that they be brighter in the far-UV
than in the near-UV band. However, older starburst galaxies have UV colors
similar to AGN, and thus cannot be selected unambiguously on the basis of GALEX
fluxes alone.
With the aid of 2MASS data, we construct and discuss median 10 band
UV-optical-IR spectral energy distributions for turn-off stars, hot white
dwarfs, low-redshift quasars, and spiral and elliptical galaxies. We point out
the high degree of correlation between the UV color and the contribution of the
UV flux to the UV-optical-IR flux of galaxies detected by GALEX.Comment: 35 pages, 11 figures, 3 tables; to appear in the AJ. PS with better
figures available from http://www.astro.washington.edu/agueros/pub
A new method to separate star forming from AGN galaxies at intermediate redshift: The submillijansky radio population in the VLA-COSMOS survey
We explore the properties of the submillijansky radio population at 20 cm by
applying a newly developed optical color-based method to separate star forming
(SF) from AGN galaxies at intermediate redshifts (z<1.3). Although optical
rest-frame colors are used, our separation method is shown to be efficient, and
not biased against dusty starburst galaxies. This classification method has
been calibrated and tested on a local radio selected optical sample. Given
accurate multi-band photometry and redshifts, it carries the potential to be
generally applicable to any galaxy sample where SF and AGN galaxies are the two
dominant populations.
In order to quantify the properties of the submillijansky radio population,
we have analyzed ~2,400 radio sources, detected at 20 cm in the VLA-COSMOS
survey. 90% of these have submillijansky flux densities. We classify the
objects into 1) star candidates, 2) quasi stellar objects, 3) AGN, 4) SF, and
5) high redshift (z>1.3) galaxies. We find, for the composition of the
submillijansky radio population, that SF galaxies are not the dominant
population at submillijansky flux levels, as previously often assumed, but that
they make up an approximately constant fraction of 30-40% in the flux density
range of ~50 microJy to 0.7 mJy. In summary, based on the entire VLA-COSMOS
radio population at 20 cm, we find that the radio population at these flux
densities is a mixture of roughly 30-40% of SF and 50-60% of AGN galaxies, with
a minor contribution (~10%) of QSOs.Comment: 26 pages, 26 figures; accepted for publication in ApJ
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The Ultraviolet, Optical, and Infrared Properties of Sloan Digital Sky Survey Sources Detected by GALEX
We discuss the ultraviolet, optical, and infrared properties of the Sloan Digital Sky Survey (SDSS) sources detected by the Galaxy Evolution Explorer (GALEX) as part of its All-sky Imaging Survey Early Release Observations. Virtually all (>99%) the GALEX sources in the overlap region are detected by SDSS; those without an SDSS counterpart within our 6'' search radius are mostly unflagged GALEX artifacts. GALEX sources represent ~2.5% of all SDSS sources within these fields, and about half are optically unresolved. Most unresolved GALEX-SDSS sources are bright (r < 18 mag), blue, turnoff, thick-disk stars and are typically detected only in the GALEX near-ultraviolet (NUV) band. The remaining unresolved sources include low-redshift quasars (z < 2.2), white dwarfs, and white dwarfāM dwarf pairs, and these dominate the optically unresolved sources detected in both GALEX bands. Almost all the resolved SDSS sources detected by GALEX are fainter than the SDSS main spectroscopic limit. (Conversely, of the SDSS galaxies in the main spectroscopic sample, about 40% are detected in at least one GALEX band.) These sources have colors consistent with those of blue (spiral) galaxies (u - r < 2.2), and most are detected in both GALEX bands. Measurements of their UV colors allow much more accurate and robust estimates of star formation history than are possible using only SDSS data. Indeed, galaxies with the most recent (lesssim20 Myr) star formation can be robustly selected from the GALEX data by requiring that they be brighter in the far-ultraviolet (FUV) than in the NUV band. However, older starburst galaxies have UV colors similar to those of active galactic nuclei and thus cannot be selected unambiguously on the basis of GALEX fluxes alone. Additional information, such as spatially resolved FUV emission, optical morphology, or X-ray and radio data, is needed before blue GALEX colors can be unambiguously interpreted as a sign of recent star formation. With the aid of Two Micron All Sky Survey data, we construct and discuss median 10-band UV through infrared spectral energy distributions for turnoff stars, hot white dwarfs, low-redshift quasars, and spiral and elliptical galaxies. We point out the high degree of correlation between the UV color and the contribution of the UV flux to the UV through infrared flux of galaxies detected by GALEX; for example, this correlation can be used to predict the SDSS z-band measurement, using only two GALEX fluxes, with a scatter of only 0.7 mag
The census of nuclear activity of late-type galaxies in the Virgo cluster
The first spectroscopic census of AGNs associated to late-type galaxies in
the Virgo cluster is carried on by observing 213 out of a complete set of 237
galaxies more massive than M_dyn>10^{8.5} solar masses. Among them, 77 are
classified as AGNs (including 21 transition objects, 47 LINERs and 9 Seyferts),
and comprize 32% of the late-type galaxies in Virgo. Due to spectroscopic
incompleteness at most 21 AGNs are missed in the survey, so that the fraction
would increase up to 41%. Using corollary Near-IR observations, that enable us
to estimate galaxies dynamical masses, it is found that AGNs are hosted
exclusively in massive galaxies, i.e. M_dyn\gsim 10^{10} solar masses. Their
frequency increases steeply with the dynamical mass from zero at
M_dyn\approx10^{9.5} solar masses to virtually 1 at M_dyn>10^{11.5} solar
masses. These frequencies are consistent with the ones of low luminosity AGNs
found in the general field by the SDSS. Massive galaxies that harbor AGNs
commonly show conspicuous r-band star-like nuclear enhancements. Conversely
they often, but not necessarily contain massive bulges. Few well known AGNs
(e.g. M61, M100, NGC4535) are found in massive Sc galaxies with little or no
bulge. The AGN fraction seems to be only marginally sensitive to galaxy
environment. We infer the black hole masses using the known scaling relations
of quiescent black holes. No black holes lighter than \msol are
found active in our sample.Comment: The paper contains 13 figures and 5 tables; accepted for publication
in MNRA
Panchromatic Properties of 99,000 Galaxies Detected by SDSS, and (some by) ROSAT, GALEX, 2MASS, IRAS, GB6, FIRST, NVSS and WENSS Surveys
We discuss the panchromatic properties of 99,088 galaxies selected from the
SDSS Data Release 1 spectroscopic sample (a flux-limited sample for 1360
deg^2). These galaxies are positionally matched to sources detected by ROSAT,
GALEX, 2MASS, IRAS, GB6, FIRST, NVSS and WENSS. We find strong correlations
between the detection fraction at other wavelengths and optical properties such
as flux, colors, and emission-line strengths. Using GALEX, SDSS, and 2MASS
data, we construct the UV-IR broad-band spectral energy distributions for
various types of galaxies, and find that they form a nearly one-parameter
family. For example, based on SDSS u- and r-band data, supplemented with
redshift, the K-band 2MASS magnitudes can be "predicted" with an rms scatter of
only 0.2 mag. When a dust content estimate determined from SDSS data by
Kauffmann et al. (2003) is also utilized, this scatter decreases to 0.1 mag. We
demonstrate that this dust content is indeed higher for galaxies detected by
IRAS and that it can be used to "predict" measured IRAS 60 micron flux density
within a factor of two using only SDSS data. We also show that the position of
a galaxy in the emission-line-based Baldwin-Phillips-Terlevich diagram is
correlated with the optical light concentration index and u-r color determined
from the SDSS broad-band imaging data, and discuss changes in the morphology of
this diagram induced by requiring detections at other wavelengths. We study the
IR-radio correlation and find evidence that its slope may be different for AGN
and star-forming galaxies and related to the H_alpha/H_beta line strength
ratio.Comment: Accepted for publication in MNRAS; 25 pages, 32 color figure
The star-formation history of K-selected galaxies
We have studied the uJy radio properties of K-selected galaxies detected in
the Ultra-Deep Survey portion of UKIDSS using 610- and 1,400-MHz images from
the VLA and GMRT. These deep radio mosaics, combined with the largest and
deepest K image currently available, allow high-S/N detections of many
K-selected sub-populations, including sBzK and pBzK star-forming and passive
galaxies. We find a strong correlation between the radio and K fluxes and a
linear relationship between SFR and K luminosity. We find no evidence, from
either radio spectral indices or a comparison with submm-derived SFRs, that the
full sample is strongly contaminated by AGN. The sBzK and pBzK galaxies have
similar levels of radio flux, SFR and specific SFR (SSFR) at z < 1.4,
suggesting there is strong contamination of the pBzK sample by star-forming
galaxies. At z > 1.4, pBzK galaxies become difficult to detect in the radio
stack, though the implied SFRs are still much higher than expected for
passively evolving galaxies. Their radio emission may come from low-luminosity
AGN. EROs straddle the passive and star-forming regions of the BzK diagram and
also straddle the two groups in terms of their radio properties. K-bright ERO
samples are dominated by passive galaxies and faint ERO samples contain more
star-forming galaxies. The star-formation history (SFH) from stacking all K
sources in the UDS agrees well with that derived for other wavebands and other
radio surveys, at least out to z ~ 2. The radio-derived SFH then appears to
fall more steeply than that measured at other wavelengths. The SSFR for
K-selected sources rises strongly with redshift at all stellar masses, and
shows a weak dependence on stellar mass. High- and low-mass galaxies show a
similar decline in SSFR since z ~ 2 (abridged).Comment: Published in MNRAS, 20 pages, colou
The ASTRODEEP Frontier Fields catalogues. I. Multiwavelength photometry of Abell-2744 and MACS-J0416
Context. The Frontier Fields survey is a pioneering observational program aimed at collecting photometric data, both from space (Hubble Space Telescope and Spitzer Space Telescope) and from ground-based facilities (VLT Hawk-I), for six deep fields pointing at clusters of galaxies and six nearby deep parallel fields, in a wide range of passbands. The analysis of these data is a natural outcome of the Astrodee