269 research outputs found
The Star-Formation Histories of z~2 DOGs and SMGs
The Spitzer Space Telescope has identified a population of ultra-luminous
infrared galaxies (ULIRGs) at z ~ 2 that may play an important role in the
evolution of massive galaxies. We measure the stellar masses of two populations
of Spitzer-selected ULIRGs, both of which have extremely red R-[24] colors
(dust-obscured galaxies, or DOGs) and compare our results with sub-millimeter
selected galaxies (SMGs). One set of 39 DOGs has a local maximum in their
mid-IR spectral energy distribution (SED) at rest-frame 1.6um associated with
stellar emission ("bump DOGs"), while the other set of 51 DOGs has a power-law
dominated mid-IR SED with spectral features typical of obscured AGN ("power-law
DOGs"). We use stellar population synthesis models applied self-consistently to
broad-band photometry in the rest-frame ultra-violet, optical, and
near-infrared of each of these populations and test a variety of stellar
population synthesis codes, star-formation histories (SFHs), and initial mass
functions (IMFs). Assuming a simple stellar population SFH and a Chabrier IMF,
we find that the median and inner quartile stellar masses of SMGs, bump DOGs
and power-law DOGs are given by log(M_*/M_sun) = 10.42_-0.36^+0.42,
10.62_-0.32^+0.36, and 10.71_-0.34^+0.40, respectively. Implementing more
complicated SFHs with multiple age components increases these mass estimates by
up to 0.5 dex. Our stellar mass estimates are consistent with physical
mechanisms for the origin of z~2 ULIRGs that result in high star-formation
rates for a given stellar mass. Such mechanisms are usually driven by a major
merger of two gas-rich systems, rather than smooth accretion of gas and small
satellites.Comment: 17 pages, 7 figures, 3 tables. Plus figures showing SEDs and best-fit
synthesized stellar population model. Accepted to the Ap
Spectroscopic Confirmation of an Ultramassive and Compact Galaxy at Z = 3.35: A Detailed Look at an Early Progenitor of Local Giant Ellipticals
We present the first spectroscopic confirmation of an ultra-massive galaxy at redshift z\u3e3 using data from Keck-NIRSPEC, VLT-Xshooter, and GTC-Osiris. We detect strong [OIII] and Lyα emission, and weak [OII], CIV, and HeII, placing C1-23152 at a spectroscopic redshift of zspec=3.351. The modeling of the emission-line corrected spectral energy distribution results in a best-fit stellar mass of Mâ=3.1+0.6â0.7Ă1011Mâ, a star-formation rate of \u3c7 Mâyrâ1, and negligible dust extinction. The stars appear to have formed in a short intense burst ~300-500 Myr prior to the observation epoch, setting the formation redshift of this galaxy at z~4.1. From the analysis of the line ratios and widths, and the observed flux at 24ÎŒm, we confirm the presence of a luminous hidden active galactic nucleus (AGN), with bolometric luminosity of ~1046ergsâ1. Potential contamination to the observed SED from the AGN continuum is constrained, placing a lower limit on the stellar mass of 2Ă1011Mâ. HST/WFC3 H160 and ACS I814 images are modeled, resulting in an effective radius of re~1 kpc in the H160 band and a Sersic index n~4.4. This object may be a prototype of the progenitors of local most massive elliptical galaxies in the first 2 Gyr of cosmic history, having formed most of its stars at z\u3e4 in a highly dissipative, intense, and short burst of star formation. C1-23152 is completing its transition to a post-starburst phase while hosting a powerful AGN, potentially responsible for the quenching of the star formation activity
A Star Catalog for the Open Cluster NGC188
We present new BVRI broad-band photometry for the old open cluster NGC188
based upon analysis of 299 CCD images either obtained by us, donated by
colleagues, or retrieved from public archives. We compare our results on a
star-by-star basis with data from eleven previous photometric investigations of
the cluster. We homogenize and merge the data from all the photometric studies,
and also merge membership probabilities from four previous proper-motion
studies of the cluster field. Fiducial cluster sequences in the BV (Johnson) RI
(Cousins) photometric system of Landolt (1992, AJ, 104, 340) represent the
principal result of this paper. These have been compared to reference samples
defined by (a) Landolt's standard stars, (b) the old open clusters M67 and
NGC6791, and (c) stars within 25 pc having modern photometry and precise
Hipparcos parallaxes. In a companion paper we show that our derived cluster
results agree well with the predictions of modern stellar-interior and
-evolution theory, given reasonable estimates of the cluster chemical
abundances and foreground reddening. The individual and combined datasets for
NGC188 have been made available through our web site.Comment: Accepted for PAS
LBT and Spitzer Spectroscopy of Star-Forming Galaxies at 1 < z < 3: Extinction and Star Formation Rate Indicators
We present spectroscopic observations in the rest-frame optical and near- to
mid-infrared wavelengths of four gravitationally lensed infrared (IR) luminous
star-forming galaxies at redshift 1 < z < 3 from the LUCIFER instrument on the
Large Binocular Telescope and the Infrared Spectrograph on Spitzer. The sample
was selected to represent pure, actively star-forming systems, absent of active
galactic nuclei. The large lensing magnifications result in high
signal-to-noise spectra that can probe faint IR recombination lines, including
Pa-alpha and Br-alpha at high redshifts. The sample was augmented by three
lensed galaxies with similar suites of unpublished data and observations from
the literature, resulting in the final sample of seven galaxies. We use the IR
recombination lines in conjunction with H-alpha observations to probe the
extinction, Av, of these systems, as well as testing star formation rate (SFR)
indicators against the SFR measured by fitting spectral energy distributions to
far-IR photometry. Our galaxies occupy a range of Av from ~0 to 5.9 mag, larger
than previously known for a similar range of IR luminosities at these
redshifts. Thus, estimates of SFR even at z ~ 2 must take careful count of
extinction in the most IR luminous galaxies. We also measure extinction by
comparing SFR estimates from optical emission lines with those from far-IR
measurements. The comparison of results from these two independent methods
indicates a large variety of dust distribution scenarios at 1 < z < 3. Without
correcting for dust extinction, the H-alpha SFR indicator underestimates the
SFR; the size of the necessary correction depends on the IR luminosity and dust
distribution scenario. Individual SFR estimates based on the 6.2 micron PAH
emission line luminosity do not show a systematic discrepancy with extinction,
although a considerable, ~0.2 dex scatter is observed.Comment: Accepted for publication in The Astrophysical Journal; 14 pages, 8
figure
A Potential Galaxy Threshing System in the Cosmos Field
We report on the discovery of a new potential galaxy threshing system in the
COSMOS 2 square degree field using the prime-focus camera, Suprime-Cam, on the
8.2 m Subaru Telescope. This system consists of a giant elliptical galaxy with
and a tidally disrupted satellite galaxy with at a photometric redshift of . This redshift is
consistent with the spectroscopic redshift of 0.079 for the giant elliptical
galaxy obtained from the Sloan Digital Sky Survey (SDSS) archive. The
luminosity masses of the two galaxies are
and , respectively. The distance between the
two galaxies is greater than 100 kpc. The two tidal tails emanating from the
satellite galaxy extend over 150 kpc. This system would be the second
well-defined galaxy threshing system found so far.Comment: 17 pages, 7 figures, accepted for the COSMOS special issue of ApJ
General Approach for Combining Diverse Rare Variant Association Tests Provides Improved Robustness Across a Wider Range of Genetic Architectures
The widespread availability of genome sequencing data made possible by way of next-generation technologies has yielded a flood of different gene-based rare variant association tests. Most of these tests have been published because they have superior power for particular genetic architectures. However, for applied researchers it is challenging to know which test to choose in practice when little is known a priori about genetic architecture. Recently, tests have been proposed which combine two particular individual tests (one burden and one variance components) to minimize power loss while improving robustness to a wider range of genetic architectures. In our analysis we propose an expansion of these approaches, yielding a general method that works for combining any number of individual tests. We demonstrate that running multiple different tests on the same dataset and using a Bonferroni correction for multiple testing is never better than combining tests using our general method. We also find that using a test statistic that is highly robust to the inclusion of non-causal variants (Joint-infinity) together with a previously published combined test (SKAT-O) provides improved robustness to a wide range of genetic architectures and should be considered for use in practice. Software for this approach is supplied. We support the increased use of combined tests in practice-- as well as further exploration of novel combined testing approaches using the general framework provided here--to maximize robustness of rare-variant testing strategies against a wide range of genetic architectures
HST Morphologies of z ~ 2 Dust-Obscured Galaxies II: Bump Sources
We present Hubble Space Telescope (HST) imaging of 22 ultra-luminous infrared
galaxies (ULIRGs) at z~2 with extremely red R-[24] colors (called dust-obscured
galaxies, or DOGs) which have a local maximum in their spectral energy
distribution (SED) at rest-frame 1.6um associated with stellar emission. These
sources, which we call "bump DOGs", have star-formation rates of 400-4000
Msun/yr and have redshifts derived from mid-IR spectra which show strong
polycyclic aromatic hydrocarbon emission --- a sign of vigorous on-going
star-formation. Using a uniform morphological analysis, we look for
quantifiable differences between bump DOGs, power-law DOGs (Spitzer-selected
ULIRGs with mid-IR SEDs dominated by a power-law and spectral features that are
more typical of obscured active galactic nuclei than starbursts),
sub-millimeter selected galaxies (SMGs), and other less-reddened ULIRGs from
the Spitzer extragalactic First Look Survey (XFLS). Bump DOGs are larger than
power-law DOGs (median Petrosian radius of 8.4 +/- 2.7 kpc vs. 5.5 +/- 2.3 kpc)
and exhibit more diffuse and irregular morphologies (median M_20 of -1.08 +/-
0.05 vs. -1.48 +/- 0.05). These trends are qualitatively consistent with
expectations from simulations of major mergers in which merging systems during
the peak star-formation rate period evolve from M_20 = -1.0 to M_20 = -1.7.
Less obscured ULIRGs (i.e., non-DOGs) tend to have more regular, centrally
peaked, single-object morphologies rather than diffuse and irregular
morphologies. This distinction in morphologies may imply that less obscured
ULIRGs sample the merger near the end of the peak star-formation rate period.
Alternatively, it may indicate that the intense star-formation in these
less-obscured ULIRGs is not the result of a recent major merger.Comment: Accepted to ApJ; 22 pages, 8 Figures, 7 Table
Dust extinction from Balmer decrements of star-forming galaxies at 0.75<z<1.5 with HST/WFC3 spectroscopy from the WISP survey
Spectroscopic observations of Halpha and Hbeta emission lines of 128
star-forming galaxies in the redshift range 0.75<z<1.5 are presented. These
data were taken with slitless spectroscopy using the G102 and G141 grisms of
the Wide-Field-Camera 3 (WFC3) on board the Hubble Space Telescope as part of
the WFC3 Infrared Spectroscopic Parallel (WISP) survey. Interstellar dust
extinction is measured from stacked spectra that cover the Balmer decrement
(Halpha/Hbeta). We present dust extinction as a function of Halpha luminosity
(down to 3 x 10^{41} erg/s), galaxy stellar mass (reaching 4 x 10^{8} Msun),
and rest-frame Halpha equivalent width. The faintest galaxies are two times
fainter in Halpha luminosity than galaxies previously studied at z~1.5. An
evolution is observed where galaxies of the same Halpha luminosity have lower
extinction at higher redshifts, whereas no evolution is found within our error
bars with stellar mass. The lower Halpha luminosity galaxies in our sample are
found to be consistent with no dust extinction. We find an anti-correlation of
the [OIII]5007/Halpha flux ratio as a function of luminosity where galaxies
with L_{Halpha}<5 x 10^{41} erg/s are brighter in [OIII]5007 than Halpha. This
trend is evident even after extinction correction, suggesting that the
increased [OIII]5007/Halpha ratio in low luminosity galaxies is likely due to
lower metallicity and/or higher ionization parameters.Comment: 11 pages, 9 figures, 2 tables; version addressing the referee
comment
The WFC3 Infrared Spectroscopic Parallel (WISP) Survey
We present the WFC3 Infrared Spectroscopic Parallel (WISP) Survey. WISP is
obtaining slitless, near-infrared grism spectroscopy of ~ 90 independent,
high-latitude fields by observing in the pure parallel mode with Wide Field
Camera-3 on the Hubble Space Telescope for a total of ~ 250 orbits. Spectra are
obtained with the G102 (lambda=0.8-1.17 microns, R ~ 210) and G141 grisms
(lambda=1.11-1.67 microns, R ~ 130), together with direct imaging in the J- and
H-bands (F110W and F140W, respectively). In the present paper, we present the
first results from 19 WISP fields, covering approximately 63 square arc
minutes. For typical exposure times (~ 6400 sec in G102 and ~ 2700 sec in
G141), we reach 5-sigma detection limits for emission lines of 5 x 10^(-17)
ergs s^(-1) cm^(-2) for compact objects. Typical direct imaging 5sigma-limits
are 26.8 and 25.0 magnitudes (AB) in F110W and F140W, respectively. Restricting
ourselves to the lines measured with highest confidence, we present a list of
328 emission lines, in 229 objects, in a redshift range 0.3 < z < 3. The
single-line emitters are likely to be a mix of Halpha and [OIII]5007,4959 A,
with Halpha predominating. The overall surface density of high-confidence
emission-line objects in our sample is approximately 4 per arcmin^(2).These
first fields show high equivalent width sources, AGN, and post starburst
galaxies. The median observed star formation rate of our Halpha selected sample
is 4 Msol/year. At intermediate redshifts, we detect emission lines in galaxies
as faint as H_140 ~ 25, or M_R < -19, and are sensitive to star formation rates
down to less than 1 Msol/year. The slitless grisms on WFC3 provide a unique
opportunity to study the spectral properties of galaxies much fainter than L*
at the peak of the galaxy assembly epoch.Comment: 15 pages, 12 figures, submitted to Ap
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