28 research outputs found
The Evolution of the Far-UV Luminosity Function and Star Formation Rate Density of the Chandra Deep Field South from z=0.2-1.2 with Swift/UVOT
We use deep Swift UV/Optical Telescope (UVOT) near-ultraviolet (1600A to
4000A) imaging of the Chandra Deep Field South to measure the rest-frame far-UV
(FUV; 1500A) luminosity function (LF) in four redshift bins between z=0.2 and
1.2. Our sample includes 730 galaxies with u < 24.1 mag. We use two methods to
construct and fit the LFs: the traditional V_max method with bootstrap errors
and a maximum likelihood estimator. We observe luminosity evolution such that
M* fades by ~2 magnitudes from z~1 to z~0.3 implying that star formation
activity was substantially higher at z~1 than today. We integrate our LFs to
determine the FUV luminosity densities and star formation rate densities from
z=0.2 to 1.2. We find evolution consistent with an increase proportional to
(1+z)^1.9 out to z~1. Our luminosity densities and star formation rates are
consistent with those found in the literature, but are, on average, a factor of
~2 higher than previous FUV measurements. In addition, we combine our UVOT data
with the MUSYC survey to model the galaxies' ultraviolet-to-infrared spectral
energy distributions and estimate the rest-frame FUV attenuation. We find that
accounting for the attenuation increases the star formation rate densities by
~1 dex across all four redshift bins.Comment: 20 pages, 8 figures, 6 tables; accepted for publication in Ap
On the Classification of UGC1382 as a Giant Low Surface Brightness Galaxy
We provide evidence that UGC1382, long believed to be a passive elliptical
galaxy, is actually a giant low surface brightness (GLSB) galaxy which rivals
the archetypical GLSB Malin 1 in size. Like other GLSB galaxies, it has two
components: a high surface brightness disk galaxy surrounded by an extended low
surface brightness (LSB) disk. For UGC1382, the central component is a
lenticular system with an effective radius of 6 kpc. Beyond this, the LSB disk
has an effective radius of ~38 kpc and an extrapolated central surface
brightness of ~26 mag/arcsec^2. Both components have a combined stellar mass of
~8x10^10 M_sun, and are embedded in a massive (10^10 M_sun) low-density (<3
M_sun/pc^2) HI disk with a radius of 110 kpc, making this one of the largest
isolated disk galaxies known. The system resides in a massive dark matter halo
of at least 2x10^12 M_sun. Although possibly part of a small group, its low
density environment likely plays a role in the formation and retention of the
giant LSB and HI disks. We model the spectral energy distributions and find
that the LSB disk is likely older than the lenticular component. UGC1382 has
UV-optical colors typical of galaxies transitioning through the green valley.
Within the LSB disk are spiral arms forming stars at extremely low
efficiencies. The gas depletion time scale of ~10^11 yr suggests that UGC1382
may be a very long term resident of the green valley. We find that the
formation and evolution of the LSB disk is best explained by the accretion of
gas-rich LSB dwarf galaxies.Comment: 17 pages, 16 figures, 4 tables; accepted to the Astrophysical Journa
UVOT Measurements of Dust and Star Formation in the SMC and M33
When measuring star formation rates using ultraviolet light, correcting for
dust extinction is a critical step. However, with the variety of dust
extinction curves to choose from, the extinction correction is quite uncertain.
Here, we use Swift/UVOT to measure the extinction curve for star-forming
regions in the SMC and M33. We find that both the slope of the curve and the
strength of the 2175 Angstrom bump vary across both galaxies. In addition, as
part of our modeling, we derive a detailed recent star formation history for
each galaxy.Comment: 6 pages, 5 figures, conference proceedings from Swift: 10 years of
Discovery, held in Rome (2-5 Dec. 2014
Measuring Dust Attenuation Curves of SINGS/KINGFISH Galaxies Using Swift/UVOT Photometry
We present Swift/Ultraviolet Optical Telescope (UVOT) integrated light
photometry of the Spitzer Infrared Nearby Galaxies Survey (SINGS) and the Key
Insights on Nearby Galaxies: A Far-Infrared Survey with Herschel (KINGFISH)
samples of nearby galaxies. Combining the Swift/UVOT data with archival
photometry, we investigate a variety of dust attenuation curves derived using
MCSED, a flexible spectral energy distribution fitting code. We fit the
panchromatic data using three different star formation history (SFH)
parameterizations: a decaying exponential, a double power law, and a piecewise
function with breaks at physically motivated ages. We find that the average
attenuation law of the sample changes slightly based on the SFH assumed.
Specifically, the exponential SFH leads to the shallowest attenuation curves.
Using simulated data, we also find the exponential SFH fails to outperform the
more complex SFHs. Finally, we find a systematic offset in the derived bump
strength between SED fits with and without UVOT data, where the inclusion of
UVOT data leads to smaller bump strengths, highlighting the importance of the
UVOT data. This discrepancy is not seen in fits to mock photometry.
Understanding dust attenuation in the local universe is key to understanding
high redshift objects where rest-frame far-infrared data is unavailable.Comment: 30 pages, 13 figures, accepted for publication in Ap
The Quest for the Missing Dust: II -- Two Orders of Magnitude of Evolution in the Dust-to-Gas Ratio Resolved Within Local Group Galaxies
We explore evolution in the dust-to-gas ratio with density within four
well-resolved Local Group galaxies - the LMC, SMC, M31, and M33. We do this
using new maps, which restore extended emission that was
missed by previous reductions. This improved data allows us to
probe the dust-to-gas ratio across 2.5 orders of magnitude in ISM surface
density. We find significant evolution in the dust-to-gas ratio, with
dust-to-gas varying with density within each galaxy by up to a factor 22.4. We
explore several possible reasons for this, and our favored explanation is dust
grain growth in denser regions of ISM. We find that the evolution of the
dust-to-gas ratio with ISM surface density is very similar between M31 and M33,
despite their large differences in mass, metallicity, and star formation rate;
conversely, we find M33 and the LMC to have very different dust-to-gas
evolution profiles, despite their close similarity in those properties. Our
dust-to-gas ratios address previous disagreement between UV- and FIR-based
dust-to-gas estimates for the Magellanic Clouds, removing the disagreement for
the LMC, and considerably reducing it for the SMC - with our new dust-to-gas
measurements being factors of 2.4 and 2.0 greater than the previous
far-infrared estimates, respectively. We also observe that the dust-to-gas
ratio appears to fall at the highest densities for the LMC, M31, and M33; this
is unlikely to be an actual physical phenomenon, and we posit that it may be
due to a combined effect of dark gas, and changing dust mass opacity.Comment: Accepted for publication in the Astrophysical Journa
Towards Space-like Photometric Precision from the Ground with Beam-Shaping Diffusers
We demonstrate a path to hitherto unachievable differential photometric
precisions from the ground, both in the optical and near-infrared (NIR), using
custom-fabricated beam-shaping diffusers produced using specialized
nanofabrication techniques. Such diffusers mold the focal plane image of a star
into a broad and stable top-hat shape, minimizing photometric errors due to
non-uniform pixel response, atmospheric seeing effects, imperfect guiding, and
telescope-induced variable aberrations seen in defocusing. This PSF reshaping
significantly increases the achievable dynamic range of our observations,
increasing our observing efficiency and thus better averages over
scintillation. Diffusers work in both collimated and converging beams. We
present diffuser-assisted optical observations demonstrating
ppm precision in 30 minute bins on a nearby bright star
16-Cygni A (V=5.95) using the ARC 3.5m telescope---within a factor of 2
of Kepler's photometric precision on the same star. We also show a transit of
WASP-85-Ab (V=11.2) and TRES-3b (V=12.4), where the residuals bin down to
ppm in 30 minute bins for WASP-85-Ab---a factor of 4 of
the precision achieved by the K2 mission on this target---and to 101ppm for
TRES-3b. In the NIR, where diffusers may provide even more significant
improvements over the current state of the art, our preliminary tests have
demonstrated ppm precision for a star on the 200"
Hale Telescope. These photometric precisions match or surpass the expected
photometric precisions of TESS for the same magnitude range. This technology is
inexpensive, scalable, easily adaptable, and can have an important and
immediate impact on the observations of transits and secondary eclipses of
exoplanets.Comment: Accepted for publication in ApJ. 30 pages, 20 figure
Toward Space-like Photometric Precision from the Ground with Beam-shaping Diffusers
We demonstrate a path to hitherto unachievable differential photometric precisions from the ground, both in the optical and near-infrared (NIR), using custom-fabricated beam-shaping diffusers produced using specialized nanofabrication techniques. Such diffusers mold the focal plane image of a star into a broad and stable top-hat shape, minimizing photometric errors due to non-uniform pixel response, atmospheric seeing effects, imperfect guiding, and telescope-induced variable aberrations seen in defocusing. This PSF reshaping significantly increases the achievable dynamic range of our observations, increasing our observing efficiency and thus better averages over scintillation. Diffusers work in both collimated and converging beams. We present diffuser-assisted optical observations demonstrating 62_(-16)^(+26) ppm precision in 30 minute bins on a nearby bright star 16 Cygni A (V = 5.95) using the ARC 3.5 m telescope—within a factor of ~2 of Kepler's photometric precision on the same star. We also show a transit of WASP-85-Ab (V = 11.2) and TRES-3b (V = 12.4), where the residuals bin down to 180_(-41)^(+66) ppm in 30 minute bins for WASP-85-Ab—a factor of ~4 of the precision achieved by the K2 mission on this target—and to 101 ppm for TRES-3b. In the NIR, where diffusers may provide even more significant improvements over the current state of the art, our preliminary tests demonstrated 137_(-36)^(+64) ppm precision for a K_S = 10.8 star on the 200 inch Hale Telescope. These photometric precisions match or surpass the expected photometric precisions of TESS for the same magnitude range. This technology is inexpensive, scalable, easily adaptable, and can have an important and immediate impact on the observations of transits and secondary eclipses of exoplanets