1,384 research outputs found
Characterizing Dust Attenuation in Local Star-Forming Galaxies: Near-Infrared Reddening and Normalization
We characterize the near-infrared (NIR) dust attenuation for a sample of
~5500 local (z<0.1) star-forming galaxies and obtain an estimate of their
average total-to-selective attenuation . We utilize data from the
United Kingdom Infrared Telescope (UKIRT) and the Two Micron All-Sky Survey
(2MASS), which is combined with previously measured UV-optical data for these
galaxies. The average attenuation curve is slightly lower in the far-UV than
local starburst galaxies, by roughly 15%, but appears similar at longer
wavelengths with a total-to-selective normalization at V-band of
. Under the assumption of energy balance,
the total attenuated energy inferred from this curve is found to be broadly
consistent with the observed infrared dust emission () in a small
sample of local galaxies for which far-IR measurements are available. However,
the significant scatter in this quantity among the sample may reflect large
variations in the attenuation properties of individual galaxies. We also derive
the attenuation curve for sub-populations of the main sample, separated
according to mean stellar population age (via ), specific star
formation rate, stellar mass, and metallicity, and find that they show only
tentative trends with low significance, at least over the range which is probed
by our sample. These results indicate that a single curve is reasonable for
applications seeking to broadly characterize large samples of galaxies in the
local Universe, while applications to individual galaxies would yield large
uncertainties and is not recommended.Comment: 14 pages, 10 figures, 1 table. Accepted for publication in Ap
Spectral Variations of the Sky: Constraints on Alternate Universes
The fine tuning of parameters required to reproduce our present day universe suggests that our universe may simply be a region within an eternally inflating super-region. Many other regions beyond our observable universe would exist with each such region governed by a different set of physical parameters. Collision between these regions, if they occur, should leave signatures of anisotropy in the cosmic microwave background (CMB) but have not been seen. We analyze the spectral properties of masked, foreground-cleaned maps between 100 and 545 GHz constructed from the Planck data set. Four distinct ~2°–4° regions associated with CMB cold spots show anomalously strong 143 GHz emission but no correspondingly strong emission at either 100 or 217 GHz. The signal to noise of this 143 GHz residual emission is at the ≳6σ level which reduces to 3.2–5.4σ after subtraction of remaining synchrotron/free–free foregrounds. We assess different mechanisms for this residual emission and conclude that although there is a 30% probability that noise fluctuations may cause foregrounds to fall within 3σ of the excess, there is less than a 0.5% probability that foregrounds can explain all the excess. A plausible explanation is that the collision of our universe with an alternate universe whose baryon to photon ratio is a factor of ~4500 larger than ours, could produce enhanced hydrogen Paschen-series emission at the epoch of recombination. Future spectral mapping and deeper observations at 100 and 217 GHz are needed to mitigate systematics arising from unknown Galactic foregrounds and to confirm this unusual hypothesis
Mass-Richness relations for X-ray and SZE-selected clusters at as seen by at 4.5m
We study the mass-richness relation of 116 spectroscopically-confirmed
massive clusters at by mining the archive. We
homogeneously measure the richness at 4.5m for our cluster sample within a
fixed aperture of radius and above a fixed brightness threshold,
making appropriate corrections for both background galaxies and foreground
stars. We have two subsamples, those which have a) literature X-ray
luminosities and b) literature Sunyaev-Zeldovich effect masses. For the X-ray
subsample we re-derive masses adopting the most recent calibrations. We then
calibrate an empirical mass-richness relation for the combined sample spanning
more than one decade in cluster mass and find the associated uncertainties in
mass at fixed richness to be dex. We study the dependance of the
scatter of this relation with galaxy concentration, defined as the ratio
between richness measured within an aperture radius of 1 and 2 arcminutes. We
find that at fixed aperture radius the scatter increases for clusters with
higher concentrations. We study the dependance of our richness estimates with
depth of the [4.5]m imaging data and find that reaching a depth of at
least [4.5]= 21 AB mag is sufficient to derive reasonable mass estimates. We
discuss the possible extension of our method to the mid-infrared all-sky
survey data, and the application of our results to the mission. This
technique makes richness-based cluster mass estimates available for large
samples of clusters at very low observational cost.Comment: Submitted to ApJ on Aug 31 2016, Revised version resubmitted on Apr
11th 201
The Stellar Initial Mass Function at the Epoch of Reionization
I provide estimates of the ultraviolet and visible light luminosity density
at z~6 after accounting for the contribution from faint galaxies below the
detection limit of deep Hubble and Spitzer surveys. I find the rest-frame
V-band luminosity density is a factor of ~2-3 below the ultraviolet luminosity
density at z~6. This implies that the maximal age of the stellar population at
z~6, for a Salpeter initial mass function, and a single, passively evolving
burst, must be <100 Myr. If the stars in z~6 galaxies are remnants of the
star-formation that was responsible for ionizing the intergalactic medium,
reionization must have been a brief process that was completed at z<7. This
assumes the most current estimates of the clumping factor and escape fraction
and a Salpeter slope extending up to 200 M_{\sun} for the stellar initial mass
function (IMF; dN/dM \propto M^{\alpha}, \alpha=-2.3). Unless the ratio of the
clumping factor to escape fraction is less than 60, a Salpeter slope for the
stellar IMF and reionization redshift higher than 7 is ruled out. In order to
maintain an ionized intergalactic medium from redshift 9 onwards, the stellar
IMF must have a slope of \alpha=-1.65 even if stars as massive as ~200 M_{\sun}
are formed. Correspondingly, if the intergalactic medium was ionized from
redshift 11 onwards, the IMF must have \alpha~-1.5. The range of stellar mass
densities at z~6 straddled by IMFs which result in reionization at z>7 is
1.3+/-0.4\times10^{7} Msun/Mpc^3.Comment: 25 pages, 2 tables, 6 figures, ApJ, in press, v680 n
Spitzer Observations of Gamma-Ray Burst Host Galaxies: A Unique Window into High Redshift Chemical Evolution and Star-formation
We present deep Spitzer 3.6 micron observations of three z~5 GRB host
galaxies. Our observations reveal that z~5 GRB hosts are a factor of 3 less
luminous than the median rest-frame V-band luminosity of spectroscopically
confirmed z~5 galaxies in the GOODS fields and the UDF. The strong connection
between GRBs and massive star formation implies that not all star-forming
galaxies at these redshifts are currently being accounted for in deep surveys
and GRBs provide a unique way to measure the contribution to the star-formation
rate density from galaxies at the faint end of the galaxy luminosity function.
By correlating the co-moving star-formation rate density with co-moving GRB
rates at lower redshifts, we estimate a lower limit to the star-formation rate
density of 0.12+/-0.09 and 0.09+/-0.05 M_sun/yr/Mpc^3 at z~4.5 and z~6,
respectively. Finally, we provide evidence that the average metallicity of
star-forming galaxies evolves as (stellar mass density)^(0.69+/-0.17) between
and , probably indicative of the loss of a significant
fraction of metals to the intergalactic medium, particularly in low-mass
galaxies.Comment: ApJ, in pres
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