221 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
Ionisation Feedback in Star and Cluster Formation Simulations
Feedback from photoionisation may dominate on parsec scales in massive
star-forming regions. Such feedback may inhibit or enhance the star formation
efficiency and sustain or even drive turbulence in the parent molecular cloud.
Photoionisation feedback may also provide a mechanism for the rapid expulsion
of gas from young clusters' potentials, often invoked as the main cause of
'infant mortality'. There is currently no agreement, however, with regards to
the efficiency of this process and how environment may affect the direction
(positive or negative) in which it proceeds. The study of the photoionisation
process as part of hydrodynamical simulations is key to understanding these
issues, however, due to the computational demand of the problem, crude
approximations for the radiation transfer are often employed. We will briefly
review some of the most commonly used approximations and discuss their major
drawbacks. We will then present the results of detailed tests carried out using
the detailed photoionisation code MOCASSIN and the SPH+ionisation code iVINE
code, aimed at understanding the error introduced by the simplified
photoionisation algorithms. This is particularly relevant as a number of new
codes have recently been developed along those lines. We will finally propose a
new approach that should allow to efficiently and self-consistently treat the
photoionisation problem for complex radiation and density fields.Comment: Invited review presented at the IAU Symposium 270: Computational Star
Formation held in Barcelona (May 31st- June 4th 2010) - Refereed paper
version; 8 Pages, 4 Figure
GMC Collisions as Triggers of Star Formation. V. Observational Signatures
We present calculations of molecular, atomic and ionic line emission from
simulations of giant molecular cloud (GMC) collisions. We post-process
snapshots of the magneto-hydrodynamical simulations presented in an earlier
paper in this series by Wu et al. (2017) of colliding and non-colliding GMCs.
Using photodissociation region (PDR) chemistry and radiative transfer we
calculate the level populations and emission properties of CO ,
[CI] at m, [CII] m and [OI]
transition at m. From integrated
intensity emission maps and position-velocity diagrams, we find that
fine-structure lines, particularly the [CII] m, can be used as a
diagnostic tracer for cloud-cloud collision activity. These results hold even
in more evolved systems in which the collision signature in molecular lines has
been diminished.Comment: 10 pages, 7 figures, accepted for publication in ApJ, comments
welcom
An Alternative Accurate Tracer of Molecular Clouds: The "-Factor"
We explore the utility of CI as an alternative high-fidelity gas mass tracer
for Galactic molecular clouds. We evaluate the X-factor for the 609
m carbon line, the analog of the CO X-factor, which is the ratio of the
H column density to the integrated CO(1-0) line intensity. We use
3D-PDR to post-process hydrodynamic simulations of turbulent, star-forming
clouds. We compare the emission of CI and CO for model clouds irradiated by 1
and 10 times the average background and demonstrate that CI is a comparable or
superior tracer of the molecular gas distribution for column densities up to cm. Our results hold for both reduced and full chemical
networks. For our fiducial Galactic cloud we derive an average of
cmKkms and of cmKkms.Comment: 5 pages, 4 figures, 1 table, accepted to MNRAS Letter
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
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
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