Misc,
Molecular and atomic line surveys of galaxies I: the dense, star-forming phase as a beacon
(2012)cite arxiv:1206.4693Comment: 16 pages, 9 figures, accepted for publication in ApJ.
Abstract
We predict the space density of molecular gas reservoirs in the Universe, and
place a lower limit on the number counts of carbon monoxide (CO), hydrogen
cyanide (HCN) molecular and CII atomic emission lines in blind redshift
surveys in the submillimeter-centimeter spectral regime. Our model uses: (a)
recently available HCN Spectral Line Energy Distributions (SLEDs) of local
Luminous Infrared Galaxies (LIRGs, L_IR>10^11 L_sun), (b) a value for
epsilon=SFR/M_dense(H_2) provided by new developments in the study of star
formation feedback on the interstellar medium and (c) a model for the evolution
of the infrared luminosity density. Minimal 'emergent' CO SLEDs from the dense
gas reservoirs expected in all star-forming systems in the Universe are then
computed from the HCN SLEDs since warm, HCN-bright gas will necessarily be
CO-bright, with the dense star-forming gas phase setting an obvious minimum to
the total molecular gas mass of any star-forming galaxy. We include CII as
the most important of the far-infrared cooling lines. Optimal blind surveys
with the Atacama Large Millimeter Array (ALMA) could potentially detect very
distant (z~10-12) CII emitters in the >ULIRG galaxy class at a rate of ~0.1-1
per hour (although this prediction is strongly dependent on the star formation
and enrichment history at this early epoch), whereas the (high-frequency)
Square Kilometer Array (SKA) will be capable of blindly detecting z>3 low-J CO
emitters at a rate of ~40-70 per hour. The CII line holds special promise for
the detection of metal-poor systems with extensive reservoirs of CO-dark
molecular gas where detection rates with ALMA can reach up to 2-7 per hour in
Bands 4-6.
