29 research outputs found
Effective destruction of CO by cosmic rays: implications for tracing H gas in the Universe
We report on the effects of cosmic rays (CRs) on the abundance of CO in clouds under conditions typical for star-forming galaxies in the Universe.
We discover that this most important molecule for tracing H gas is very
effectively destroyed in ISM environments with CR energy densities , a range expected in numerous
star-forming systems throughout the Universe. This density-dependent effect
operates volumetrically rather than only on molecular cloud surfaces (i.e.
unlike FUV radiation that also destroys CO), and is facilitated by: a) the
direct destruction of CO by CRs, and b) a reaction channel activated by
CR-produced He. The effect we uncover is strong enough to render
Milky-Way type Giant Molecular Clouds (GMCs) very CO-poor (and thus
CO-untraceable), even in ISM environments with rather modestly enhanced average
CR energy densities of . We conclude
that the CR-induced destruction of CO in molecular clouds, unhindered by dust
absorption, is perhaps the single most important factor controlling the
CO-visibility of molecular gas in vigorously star-forming galaxies. We
anticipate that a second order effect of this CO destruction mechanism will be
to make the H distribution in the gas-rich disks of such galaxies appear
much clumpier in CO =1--0, 2--1 line emission than it actually is. Finally
we give an analytical approximation of the CO/H abundance ratio as a
function of gas density and CR energy density for use in galaxy-size or
cosmological hydrodynamical simulations, and propose some key observational
tests.Comment: Accepted for publication in ApJ, 29 page
New places and phases of CO-poor/CI-rich molecular gas in the Universe
In this work we extend the work on the recently discovered role of Cosmic
Rays (CRs) in regulating the average CO/ abundance ratio in molecular
clouds (and thus their CO line visibility) in starburst galaxies, and find that
it can lead to a CO-poor/CI-rich gas phase even in environments with
Galactic or in only modestly enhanced CR backgrounds expected in ordinary
star-forming galaxies. Furthermore, the same CR-driven astro-chemistry raises
the possibility of a widespread phase transition of molecular gas towards a
CO-poor/CI-rich phase in: a) molecular gas outflows found in star-forming
galaxies, b) active galactic nuclei (AGNs), and c) near synchrotron-emitting
radio jets and the radio-loud cores of powerful radio galaxies. For main
sequence galaxies we find that CRs can render some of their molecular gas mass
CO-invisible, compounding the effects of low metallicities. Imaging the two
fine structure lines of atomic carbon with resolution high enough to search
beyond the CI/CO-bright line regions associated with central starbursts can
reveal such a CO-poor/CI-rich molecular gas phase, provided that relative
brightness sensitivity levels of (CI )/(CO )0.15
are reached. The capability to search for such gas in the Galaxy is now at hand
with the new high-frequency survey telescope HEAT deployed in Antarctica and
future ones to be deployed in Dome A. ALMA can search for such gas in
star-forming spiral disks, galactic molecular gas outflows and the CR-intense
galactic and circumgalactic gas-rich environments of radio-loud objects.Comment: 11 pages, 5 figures, MNRAS accepte
Cosmic-ray induced destruction of CO in star-forming galaxies
We explore the effects of the expected higher cosmic ray (CR) ionization
rates on the abundances of carbon monoxide (CO), atomic carbon
(C), and ionized carbon (C) in the H clouds of star-forming galaxies.
The study of Bisbas et al. (2015) is expanded by: a) using realistic
inhomogeneous Giant Molecular Cloud (GMC) structures, b) a detailed chemical
analysis behind the CR-induced destruction of CO, and c) exploring the thermal
state of CR-irradiated molecular gas. CRs permeating the interstellar medium
with (Galactic) are found to significantly
reduce the [CO]/[H] abundance ratios throughout the mass of a GMC. CO
rotational line imaging will then show much clumpier structures than the actual
ones. For (Galactic) this bias becomes
severe, limiting the utility of CO lines for recovering structural and
dynamical characteristics of H-rich galaxies throughout the Universe,
including many of the so-called Main Sequence (MS) galaxies where the bulk of
cosmic star formation occurs. Both C and C abundances increase with rising
, with C remaining the most abundant of the two throughout
H clouds, when (Galactic). C starts
to dominate for (Galactic). The thermal
state of the gas in the inner and denser regions of GMCs is invariant with
for (Galactic).
For (Galactic) this is no longer the case and
are reached. Finally we identify OH as the key
species whose sensitive abundance could mitigate the destruction
of CO at high temperatures.Comment: 17 pages, 12 figures, accepted by Ap
Molecular and atomic line surveys of galaxies I: the dense, star-forming phase as a beacon
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.Comment: 16 pages, 9 figures, accepted for publication in Ap
Deviations from the Schmidt-Kennicutt relations during early galaxy evolution
We utilize detailed time-varying models of the coupled evolution of stars and
the HI, H_2, and CO-bright H_2 gas phases in galaxy-sized numerical simulations
to explore the evolution of gas-rich and/or metal-poor systems, expected to be
numerous in the Early Universe. The inclusion of the CO-bright H_2 gas phase,
and the realistic rendering of star formation as an H_2-regulated process (and
the new feedback processes that this entails) allows the most realistic
tracking of strongly evolving galaxies, and much better comparison with
observations. We find that while galaxies eventually settle into states
conforming to Schmidt-Kennicutt (S-K) relations, significant and systematic
deviations of their star formation rates (SFRs) from the latter occur,
especially pronounced and prolonged for ...
...This indicates potentially serious limitations of (S-K)-type relations as
reliable sub-grid elements of star formation physics in simulations of
structure formation in the Early Universe. We anticipate that galaxies with
marked deviations from the S-K relations will be found at high redshifts as
unbiased inventories of total gas mass become possible with ALMA and the EVLA.Comment: 13 pages, 3 figures, accepted for publication in the Astrophysical
Journa
Physical conditions of molecular gas in the Circinus galaxy Multi-J CO and Ci ^3P_1 →^3P_0 observations
We report mapping observations of the ^(12)CO J = 3 → 2, 4 → 3, 6 → 5, and 7 → 6 transitions and the Ci ^3P→^3P_0 (Ci) 492GHz transition toward the central 40" × 40" region of the Circinus galaxy, using the Atacama Pathfinder EXperiment (APEX) telescope. We also detected ^(13)COJ = 3 → 2 at the central position of Circinus. These observations are to date the highest CO transitions reported in Circinus. With large velocity gradient (LVG) modeling and likelihood analysis we try to obtain density, temperature, and column density of the molecular gas in three regions: the nuclear region (D < 18" ~ 360 pc), the entire central 45" (D < 45" ~ 900 pc) region, and the star-forming (S-F) ring (18" < D < 45"). In the nuclear region, we can fit the CO excitation with a single excitation component, yielding an average condition of n_H_2~10^(3.2) cm^(-3), T_(kin)~ 200 K, and dν/dr~3 km s^(-1) pc^(-1). In the entire 45" region, which covers both the nucleus and the S-F ring, two excitation components are needed with n_H_2~ 10^(4.2) cm^(-3) and 10^(3.0) cm^(-3), T_(kin)~ 60 K and 30 K, and M_H_2~2.3 × 10^7 M_⊙ and 6.6 × 10^7 M_⊙, respectively. The gas excitation in the S-F ring can also be fitted with two LVG components, after subtracting the CO fluxes in the 18" nuclear region. The S-F ring region contributes 80% of the molecular mass in the 45" region. For the entire 45" region, we find a standard conversion factor of N(H_2) /I_(CO 1 → 0) = 0.37 × 10^(20)cm^(-2)(K km s^(-1))^(-1), about 1/5 of the Galactic disk value. The luminosity ratios of Ci and ^(12)COJ = 3 → 2 (R_(CI/CO 3 → 2)) in Circinus basically follow a linear trend, similar to that obtained in high-redshift galaxies. The average R_(CI/CO J = 3 → 2) in Circinus is found to be ~0.2, lying at an intermediate value between non-AGN nuclear regions and high-redshift galaxies