Extragalactic abstracts - IRSA - California Institute of Technology

Spitzer_Approved_Extragalactic
Mar 25, 10 16:24 Page 223/742
Spitzer Space Telescope − General Observer Proposal #40599
Unveiling the Galaxy Counterparts of Damped Lyman−alpha Absorbers using GRB−DLAs
Principal Investigator: Ranga−Ram Chary
Institution: California Institute of Technology
Technical Contact: Ranga−Ram Chary, California Institute of Technology
Co−Investigators:
Edo Berger, Carnegie Institute of Washington
Andrew Blain, Caltech
George Djorgovski, Caltech
Shrinivas Kulkarni, Caltech
Science Category: high−z galaxies (z>0.5)
Observing Modes: IracMap MipsPhot
Hours Approved: 77.4
Abstract:
Damped Lyman−alpha systems (DLAs) have primarily been detected along the line of
sight to bright quasars (QSOs) through absorption line spectroscopy. They harbor
the bulk of the neutral gas in the Universe between redshifts of 05 redshift solutions which indicate that they are extremely massive galaxies
with stellar masses exceeding 1E11 Msun. If true, these galaxies contribute the
bulk of the stellar mass density at z~6 and the past star−formation in these
galaxies is responsible for reionizing the intergalactic medium at z>>6. The
majority of these galaxies have however found to be faint 24 micron sources
which would instead suggest that they are luminous infrared galaxies (LIRGs)
with L(IR)~3E11 Lsun at z~2. We propose ultradeep Spitzer/IRS LL spectroscopy
which will measure the redshifts of two representative, optically invisible
(i>27 mag) sources in this class and distinguish between these two widely
disparate hypotheses. The detection of polycyclic aromatic hydrocarbons (PAH) in
the spectra of these sources would imply that photometric redshifts of dusty
infrared luminous galaxies are unreliable − a fundamental obstacle in estimating
the comoving luminosity density of the Universe as a function of redshift. It
would allow the shape of the dust extinction curve to be constrained and rule
out the Balmer−"break" color selection as a reliable tracer of redshift. By
virtue of being the deepest IRS/LL observations, it would yield the first
measures of PAH line strengths in high redshift LIRGs. This will help refine the
mid−infrared PAH templates that are used to estimate bolometric luminosities of
galaxies detected in various mid−infrared surveys, including those which will be
undertaken by WISE. The absence of PAH in the proposed spectra would imply the
presence of Compton−thick AGN and/or confirm that we have identified the
galaxies responsible for reionization without the need for changing the stellar
initial mass function at high redshift. Spitzer offers the only opportunity to
resolve this important conundrum until the James Webb Space Telescope.
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