11 research outputs found
Stellar SEDs from 0.3-2.5 Microns: Tracing the Stellar Locus and Searching for Color Outliers in SDSS and 2MASS
The Sloan Digital Sky Survey (SDSS) and Two Micron All Sky Survey (2MASS) are
rich resources for studying stellar astrophysics and the structure and
formation history of the Galaxy. As new surveys and instruments adopt similar
filter sets, it is increasingly important to understand the properties of the
ugrizJHKs stellar locus, both to inform studies of `normal' main sequence stars
as well as for robust searches for point sources with unusual colors. Using a
sample of ~600,000 point sources detected by SDSS and 2MASS, we tabulate the
position and width of the ugrizJHKs stellar locus as a function of g-i color,
and provide accurate polynomial fits. We map the Morgan-Keenan spectral type
sequence to the median stellar locus by using synthetic photometry of spectral
standards and by analyzing 3000 SDSS stellar spectra with a custom spectral
typing pipeline. We develop an algorithm to calculate a point source's minimum
separation from the stellar locus in a seven-dimensional color space, and use
it to robustly identify objects with unusual colors, as well as spurious
SDSS/2MASS matches. Analysis of a final catalog of 2117 color outliers
identifies 370 white-dwarf/M dwarf (WDMD) pairs, 93 QSOs, and 90 M giant/carbon
star candidates, and demonstrates that WDMD pairs and QSOs can be distinguished
on the basis of their J-Ks and r-z colors. We also identify a group of objects
with correlated offsets in the u-g vs. g-r and g-r vs. r-i color-color spaces,
but subsequent follow-up is required to reveal the nature of these objects.
Future applications of this algorithm to a matched SDSS-UKIDSS catalog may well
identify additional classes of objects with unusual colors by probing new areas
of color-magnitude space.Comment: 23 pages in emulateapj format, 17 figures, 7 tables. Accepted for
publication in the Astronomical Journal. To access a high-resolution version
of this paper, as well as machine readable tables and an archive of 'The
Hammer' spectral typing suite, see http://www.cfa.harvard.edu/~kcovey v2 --
fixed typos in Table 7 (mainly affecting lines for M8-M10 III stars
Clues to Nuclear Star Cluster Formation from Edge-on Spirals
We find 9 nuclear cluster candidates in a sample of 14 edge-on, late-type
galaxies observed with HST/ACS. These clusters have magnitudes (M_I ~ -11) and
sizes (r_eff ~ 3pc) similar to those found in previous studies of face-on,
late-type spirals and dE galaxies. However, three of the nuclear clusters are
significantly flattened and show evidence for multiple, coincident structural
components. The elongations of these three clusters are aligned to within 10
degrees of the galaxies' major axes. Structurally, the flattened clusters are
well fit by a combination of a spheroid and a disk or ring. The nuclear cluster
disks/rings have F606W-F814W (~V-I) colors 0.3-0.6 magnitudes bluer than the
spheroid components, suggesting that the stars in these components have ages <
1 Gyr. In NGC 4244, the nearest of the nuclear clusters, we further constrain
the stellar populations and provide a lower limit on the dynamical mass via
spectroscopy. We also present tentative evidence that another of the nuclear
clusters (in NGC 4206) may also host a supermassive black hole. Based on our
observational results we propose an in situ formation mechanism for nuclear
clusters in which stars form episodically in compact nuclear disks, and then
lose angular momentum or heat vertically to form an older spheroidal structure.
We estimate the period between star formation episodes to be 0.5 Gyr and
discuss possible mechanisms for tranforming the disk-like components into
spheroids. We also note the connection between our objects and massive globular
clusters (e.g. Cen), UCDs, and SMBHs. (Abridged)Comment: Accepted for publication in the A
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
The Second Byurakan Survey Galaxies. I. The Optical Database
A database for the entire catalog of the Second Byurakan Survey (SBS)
galaxies is presented. It contains new measurements of their optical parameters
and additional information taken from the literature and other databases. The
measurements were made using Ipg(near-infrared), Fpg(red) and Jpg(blue) band
images from photographic sky survey plates obtained by the Palomar Schmidt
telescope and extracted from the STScI Digital Sky Survey (DSS). The database
provides accurate coordinates, morphological type, spectral and activity
classes, apparent magnitudes and diameters, axial ratios, and position angles,
as well as number counts of neighboring objects in a circle of radius 50 kpc.
The total number of individual SBS objects in the database is now 1676. The 188
Markarian galaxies which were re-discovered by SBS are not included in this
database. We also include redshifts that are now available for 1576 SBS
objects, as well as 2MASS infrared magnitudes for 1117 SBS galaxies.Comment: 13 pages, 1 figure, 1 tabl
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 600 search radius are mostly unflagged GALEX artifacts. GALEX sources represent similar to 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 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. ( 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 (less than or similar to 20 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