53 research outputs found
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
Search with EGRET for a Gamma Ray Line from the Galactic Center
We search data from the Energetic Gamma Ray Experiment Telescope (EGRET) for
a gamma-ray line in the energy range 0.1-10 GeV from the 10 X 10 degree region
around the Galactic center. Our null results lead to upper limits to the line
flux from the Galactic center. Such lines may have appeared if the dark matter
in the Galactic halo is composed of weakly-interacting massive particles
(WIMPs) in the mass range 0.1-10 GeV. For a given dark-matter-halo model, our
null search translates to upper limits to the WIMP two-photon annihilation
cross section as a function of WIMP mass. We show that for a toy model in which
Majorana WIMPs in this mass range annihilate only to electron-positron pairs,
these upper limits supersede those derived from measurements of the 511-keV
line and continuum photons from internal bremsstrahlung at the Galactic center.Comment: Accepted for publication in PRD; 8 pages; 9 figures; 3 tables; text
changed to clarify several points; conclusions unchange
Spitzer Observations of the z=2.73 Lensed Lyman Break Galaxy, MS1512-cB58
We present Spitzer infrared (IR) photometry and spectroscopy of the lensed
Lyman break galaxy (LBG), MS1512-cB58 at z=2.73. The large (factor ~30)
magnification allows for the most detailed infrared study of an L*_UV(z=3) LBG
to date. Broadband photometry with IRAC (3-10 micron), IRS (16 micron), and
MIPS (24, 70 & 160 micron) was obtained as well as IRS spectroscopy spanning
5.5-35 microns. A fit of stellar population models to the optical/near-IR/IRAC
photometry gives a young age (~9 Myr), forming stars at ~98 M_sun/yr, with a
total stellar mass of ~10^9 M_sun formed thus far. The existence of an old
stellar population with twice the stellar mass can not be ruled out. IR
spectral energy distribution fits to the 24 and 70 micron photometry, as well
as previously obtained submm/mm, data give an intrinsic IR luminosity L_IR =
1-2 x10^11 L_sun and a star formation rate, SFR ~20-40 M_sun/yr. The UV derived
star formation rate (SFR) is ~3-5 times higher than the SFR determined using
L_IR or L_Halpha because the red UV spectral slope is significantly over
predicting the level of dust extinction. This suggests that the assumed
Calzetti starburst obscuration law may not be valid for young LBGs. We detect
strong line emission from Polycyclic Aromatic Hydrocarbons (PAHs) at 6.2, 7.7,
and 8.6 microns. The line ratios are consistent with ratios observed in both
local and high redshift starbursts. Both the PAH and rest-frame 8 micron
luminosities predict the total L_IR based on previously measured relations in
starbursts. Finally, we do not detect the 3.3 micron PAH feature. This is
marginally inconsistent with some PAH emission models, but still consistent
with PAH ratios measured in many local star-forming galaxies.Comment: Accepted for publication in ApJ. aastex format, 18 pages, 7 figure
Spitzer Observations of the Prototypical Extremely Red Objects HR10 and LBDS53W091: Separating Dusty Starbursts from Old Elliptical Galaxies
We present Spitzer Space Telescope observations of the well-studied extremely
red objects (EROs) HR10 and LBDS53W091 from 3.6 to 160 microns. These galaxies
are the prototypes of the two primary classes of EROs: dusty starbursts and
old, evolved galaxies, respectively. Both galaxies, as well as LBDS53W069,
another example of an old, quiescent galaxy, are well-detected out to 8
microns. However, only the dusty starburst HR10 is detected in the
far-infrared. All three EROs have stellar masses of a few times 10^11 M(sun).
Using evolutionary model fits to their multiband photometry, we predict the
infrared colors of similar EROs at 1<z<2. We find that blueward of observed 10
microns, the two ERO classes are virtually indistinguishable photometrically.
Deep spectroscopy and 24 micron data allow the classes to be separated.Comment: 23 pages, 4 figures; accepted to the Astronomical Journa
The Spitzer Warm Mission Science Prospects
After exhaustion of its cryogen, the Spitzer Space telescope will still have a fully functioning two-channel mid-IR camera that will have sensitivities better than any other ground or space-based telescopes until the launch of JWST. This document provides a description of the expected capabilities of Spitzer during its warm mission phase, and provides brief descriptions of several possible very large science programs that could be conducted. This information is intended to serve as input to a wide ranging discussion of the warm mission science, leading up to the Warm Mission Workshop in June 2007
Origins of the extragalactic background at 1mm from a combined analysis of the AzTEC and MAMBO data in GOODS-N
We present a study of the cosmic infrared background, which is a measure of
the dust obscured activity in all galaxies in the Universe. We venture to
isolate the galaxies responsible for the background at 1mm; with spectroscopic
and photometric redshifts we constrain the redshift distribution of these
galaxies. We create a deep 1.16mm map (sigma ~ 0.5mJy) by combining the AzTEC
1.1mm and MAMBO 1.2mm datasets in GOODS-N. This combined map contains 41 secure
detections, 13 of which are new. By averaging the 1.16mm flux densities of
individually undetected galaxies with 24um flux densities > 25uJy, we resolve
31--45 per cent of the 1.16mm background. Repeating our analysis on the SCUBA
850um map, we resolve a higher percentage (40--64 per cent) of the 850um
background. A majority of the background resolved (attributed to individual
galaxies) at both wavelengths comes from galaxies at z > 1.3. If the ratio of
the resolved submillimeter to millimeter background is applied to a reasonable
scenario for the origins of the unresolved submillimeter background, 60--88 per
cent of the total 1.16mm background comes from galaxies at z > 1.3.Comment: 12 pages, 10 figures. Accepted by MNRAS. The combined map is publicly
available at http://www.astro.umass.edu/~pope/goodsn_mm
IR Background Anisotropies in Spitzer GOODS images and constraints on first galaxies
We describe the angular power spectrum of unresolved 3.6 micron IR light in
Spitzer GOODS fields. The amplitude of the anisotropy spectrum decreases with
decreasing flux threshold to which resolved sources are removed from images.
When all pixels brighter than a Vega magnitude of 24.6 are removed, the
amplitude of the power spectrum at arcminute angular scales can be described
with an extra component of z>8 sources with a IRB contribution around 0.4 nW
m^-2 sr-1. The shape of the power spectrum, however, is more consistent with
that expected for unresolved, faint galaxies at lower redshifts with Vega
magnitudes fainter than 23 with a total 3.6 micron intensity between 0.1 to 0.8
nW m^-2 sr^-1. We confirm this assumption by showing that large-scale power
decreases rapidly when the unresolved clustering spectrum is measured from a
processed HDF-N IRAC image where locations of faint ACS sources with no IR
counterparts were also masked. Based on resolved counts and unresolved
fluctuations, we find that, at most, about 7.0 nW m^-2 sr^-1 can be ascribed to
galaxies.Comment: 4 pages, 3 figures; for additional information see astro-ph/0609451 ;
data products are available at http://www.cooray.org/spitzer.htm
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