J/ApJ/883/158 Dark molecular gas in the Galaxy II. Perseus arm (Busch+, 2019)
The structure of dark molecular gas in the Galaxy.
II. Physical state of "CO-dark" gas in the Perseus Arm.
Busch M.P., Allen R.J., Engelke P.D., Hogg D.E., Neufeld D.A., Wolfire M.G.
<Astrophys. J., 883, 158 (2019)>
=2019ApJ...883..158B 2019ApJ...883..158B
ADC_Keywords: Radio lines; Interstellar medium; Molecular data; Surveys
Keywords: Galaxy: disk ; ISM: molecules ; ISM: structure ;
local interstellar matter ; radio lines: ISM ; surveys
Abstract:
We report the results from a new, highly sensitive (ΔTmb∼3mK)
survey for thermal OH emission at 1665 and 1667MHz over a dense,
9x9pix grid covering a 1°x1° patch of sky in the direction of
l=105.00°, b=+2.50° toward the Perseus spiral arm of our
Galaxy. We compare our Green Bank Telescope 1667MHz OH results with
archival 12CO(1-0) observations from the Five College Radio
Astronomy Observatory Outer Galaxy Survey within the velocity range of
the Perseus Arm at these galactic coordinates. Out of the
81 statistically independent pointings in our survey area, 86% show
detectable OH emission at 1667MHz, and 19% of them show detectable CO
emission. We explore the possible physical conditions of the observed
features using a set of diffuse molecular cloud models. In the context
of these models, both OH and CO disappear at current sensitivity
limits below an Av of 0.2, but the CO emission does not appear until
the volume density exceeds 100-200cm-3. These results demonstrate
that a combination of low column density Av and low volume density
nH can explain the lack of CO emission along sight lines exhibiting
OH emission. The 18cm OH main lines, with their low critical density
of n*∼1cm-3, are collisionally excited over a large fraction of the
quiescent galactic environment and, for observations of sufficient
sensitivity, provide an optically thin radio tracer for diffuse H2.
Description:
We carried out highly sensitive observations of main-line OH emission
at 18cm with the Robert C. Byrd Green Bank Telescope (GBT) in project
AGBT14B_031 at 81 pointings (9x9 pixels grid) in the direction of
GLON=105.00°,GLAT=+2.50° toward the Perseus spiral arm of our
Galaxy.
The observations discussed in this paper were made between 2015
September 11 and 2016 January 31 in the frequency range 1.15-1.73GHz
(L-Band).
For comparison with the OH data, we used the 12CO(1-0) data from the
Five College Radio Astronomy Observatory (FCRAO;
Heyer+ 1998ApJS..115..241H 1998ApJS..115..241H) CO survey. The 12CO(1-0) observations
were made between 1995 and 1998 using the 14m telescope of the FCRAO.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 79 80 OH and CO profile integrals in the current
"One-Square-Degree" (OSD) survey
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See also:
VI/128 : Log of CGPS exposures (Taylor+, 2003-)
VIII/40 : GB6 catalog of radio sources (Gregory+ 1996)
J/A+AS/126/413 : 21cm radio continuum survey. II. (Reich+ 1997)
J/ApJ/783/130 : Parallaxes of high mass star forming regions (Reid+, 2014)
J/ApJ/810/14 : Third catalog of LAT-detected AGNs (3LAC) (Ackermann+, 2015)
Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 7 F7.3 deg GLON [104.5/105.5] Galactic Longitude
9- 13 F5.3 deg GLAT [2/3] Galactic Latitude
15- 22 F8.5 K.km/s OHSum [-0.03/0.104] OH profile integral
24- 30 F7.5 K.km/s e_OHSum [0.001/0.004] OH profile integral error
32- 39 F8.4 km/s OHVel [-71.5/-50.6]?=0 OH median velocity
41- 46 F6.4 km/s e_OHVel [0.003/8.5]?=0 OH median velocity error
48- 56 F9.6 K.km/s COSum [-0.3/2.6]?=0 CO profile integral
58- 63 F6.4 K.km/s e_COSum [0.008/0.24]?=0 CO profile integral error
65- 72 F8.4 km/s COVel [-73.2/-28.8]?=0 CO median velocity
74- 79 F6.3 km/s e_COVel [0.06/72]?=0 CO median velocity error
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History:
From electronic version of the journal
References:
Allen et al. Paper I. 2015AJ....149..123A 2015AJ....149..123A
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 10-Mar-2021