Observations of various molecular lines toward a disk region of a nearby
galaxy are now feasible, and they are being employed as diagnostic tools to
study star-formation activities there. However, the spatial resolution
attainable for a nearby galaxy with currently available radio telescopes is
$10-1000$ pc, which is much larger than the scales of individual star-forming
regions and molecular cloud cores. Hence, it is of fundamental importance to
elucidate which part of an interstellar medium such spatially-unresolved
observations are tracing. Here we present sensitive measurements of the H$_2$CO
($1_01-0_00$) line at 72 GHz toward giant molecular clouds (GMCs) in the
spiral arm of M51 using the NRO 45 m and IRAM 30 m telescopes. In conjunction
with the previously observed H$_2$CO ($2_02-1_01$) and CS ($2-1$ and $3-2$)
lines, we derive the H$_2$ density of the emitting regions to be
$(0.6-2.6)\times10^4$ cm$^-3$ and $(2.9-12)\times10^4$ cm$^-3$ for H$_2$CO
and CS, respectively, by the non-LTE analyses, where we assume the source size
of $0.8-1$ kpc and the gas kinetic temperature of $10-20$ K. The derived H$_2$
density indicates that the emission of H$_2$CO and CS is not localized to
star-forming cores, but is likely distributed over an entire region of GMCs.
Such widespread distributions of H$_2$CO and CS are also supported by models
assuming lognormal density distributions over the 1 kpc region. Thus,
contributions from the widespread less-dense components should be taken into
account for interpretation of these molecular emission observed with a
GMC-scale resolution. The different H$_2$ densities derived for H$_2$CO and CS
imply their different distributions. We discuss this differences in terms of
the formation processes of H$_2$CO and CS.
Beschreibung
Molecular Gas Density Measured with H$_2$CO and CS toward a Spiral Arm of M51
%0 Generic
%1 nishimura2019molecular
%A Nishimura, Yuri
%A Watanabe, Yoshimasa
%A Harada, Nanase
%A Kohno, Kotaro
%A Yamamoto, Satoshi
%D 2019
%K CS H2CO molecular-gas
%T Molecular Gas Density Measured with H$_2$CO and CS toward a Spiral Arm
of M51
%U http://arxiv.org/abs/1905.11087
%X Observations of various molecular lines toward a disk region of a nearby
galaxy are now feasible, and they are being employed as diagnostic tools to
study star-formation activities there. However, the spatial resolution
attainable for a nearby galaxy with currently available radio telescopes is
$10-1000$ pc, which is much larger than the scales of individual star-forming
regions and molecular cloud cores. Hence, it is of fundamental importance to
elucidate which part of an interstellar medium such spatially-unresolved
observations are tracing. Here we present sensitive measurements of the H$_2$CO
($1_01-0_00$) line at 72 GHz toward giant molecular clouds (GMCs) in the
spiral arm of M51 using the NRO 45 m and IRAM 30 m telescopes. In conjunction
with the previously observed H$_2$CO ($2_02-1_01$) and CS ($2-1$ and $3-2$)
lines, we derive the H$_2$ density of the emitting regions to be
$(0.6-2.6)\times10^4$ cm$^-3$ and $(2.9-12)\times10^4$ cm$^-3$ for H$_2$CO
and CS, respectively, by the non-LTE analyses, where we assume the source size
of $0.8-1$ kpc and the gas kinetic temperature of $10-20$ K. The derived H$_2$
density indicates that the emission of H$_2$CO and CS is not localized to
star-forming cores, but is likely distributed over an entire region of GMCs.
Such widespread distributions of H$_2$CO and CS are also supported by models
assuming lognormal density distributions over the 1 kpc region. Thus,
contributions from the widespread less-dense components should be taken into
account for interpretation of these molecular emission observed with a
GMC-scale resolution. The different H$_2$ densities derived for H$_2$CO and CS
imply their different distributions. We discuss this differences in terms of
the formation processes of H$_2$CO and CS.
@misc{nishimura2019molecular,
abstract = {Observations of various molecular lines toward a disk region of a nearby
galaxy are now feasible, and they are being employed as diagnostic tools to
study star-formation activities there. However, the spatial resolution
attainable for a nearby galaxy with currently available radio telescopes is
$10-1000$ pc, which is much larger than the scales of individual star-forming
regions and molecular cloud cores. Hence, it is of fundamental importance to
elucidate which part of an interstellar medium such spatially-unresolved
observations are tracing. Here we present sensitive measurements of the H$_2$CO
($1_{01}-0_{00}$) line at 72 GHz toward giant molecular clouds (GMCs) in the
spiral arm of M51 using the NRO 45 m and IRAM 30 m telescopes. In conjunction
with the previously observed H$_2$CO ($2_{02}-1_{01}$) and CS ($2-1$ and $3-2$)
lines, we derive the H$_2$ density of the emitting regions to be
$(0.6-2.6)\times10^4$ cm$^{-3}$ and $(2.9-12)\times10^4$ cm$^{-3}$ for H$_2$CO
and CS, respectively, by the non-LTE analyses, where we assume the source size
of $0.8-1$ kpc and the gas kinetic temperature of $10-20$ K. The derived H$_2$
density indicates that the emission of H$_2$CO and CS is not localized to
star-forming cores, but is likely distributed over an entire region of GMCs.
Such widespread distributions of H$_2$CO and CS are also supported by models
assuming lognormal density distributions over the 1 kpc region. Thus,
contributions from the widespread less-dense components should be taken into
account for interpretation of these molecular emission observed with a
GMC-scale resolution. The different H$_2$ densities derived for H$_2$CO and CS
imply their different distributions. We discuss this differences in terms of
the formation processes of H$_2$CO and CS.},
added-at = {2019-05-28T12:56:58.000+0200},
author = {Nishimura, Yuri and Watanabe, Yoshimasa and Harada, Nanase and Kohno, Kotaro and Yamamoto, Satoshi},
biburl = {https://www.bibsonomy.org/bibtex/25da7209b399609fab9ac38184f91a852/heh15},
description = {Molecular Gas Density Measured with H$_2$CO and CS toward a Spiral Arm of M51},
interhash = {341090d01c84f78d61a5bd372f2076f7},
intrahash = {5da7209b399609fab9ac38184f91a852},
keywords = {CS H2CO molecular-gas},
note = {cite arxiv:1905.11087Comment: 12 pages, 6 figures, accepted for publication in ApJ},
timestamp = {2019-05-28T12:56:58.000+0200},
title = {Molecular Gas Density Measured with H$_2$CO and CS toward a Spiral Arm
of M51},
url = {http://arxiv.org/abs/1905.11087},
year = 2019
}