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. =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 ({Delta}T_mb_~3mK) survey for thermal OH emission at 1665 and 1667MHz over a dense, 9x9pix grid covering a 1{deg}x1{deg} patch of sky in the direction of l=105.00{deg}, b=+2.50{deg} toward the Perseus spiral arm of our Galaxy. We compare our Green Bank Telescope 1667MHz OH results with archival ^12^CO(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 A_v_ 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 A_v_ and low volume density n_H_ 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 H_2_. 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{deg},GLAT=+2.50{deg} 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 ^12^CO(1-0) data from the Five College Radio Astronomy Observatory (FCRAO; Heyer+ 1998ApJS..115..241H) CO survey. The ^12^CO(1-0) observations were made between 1995 and 1998 using the 14m telescope of the FCRAO. File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file table2.dat 79 80 OH and CO profile integrals in the current "One-Square-Degree" (OSD) survey -------------------------------------------------------------------------------- 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 -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 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 -------------------------------------------------------------------------------- History: From electronic version of the journal References: Allen et al. Paper I. 2015AJ....149..123A ================================================================================ (End) Prepared by [AAS], Emmanuelle Perret [CDS] 10-Mar-2021