J/ApJ/688/290 GASS HI survey of the lower halo (Ford+, 2008)
H I clouds in the lower halo.
I. The Galactic All-Sky Survey pilot region.
Ford H.A., McClure-Griffiths N.M., Lockman F.J., Bailin J.,
Calabretta M.R., Kalberla P.M.W., Murphy T., Pisano D.J.
<Astrophys. J., 688, 290-305 (2008)>
=2008ApJ...688..290F 2008ApJ...688..290F
ADC_Keywords: Molecular clouds ; H I data ; Radio lines ; Surveys ;
Interstellar medium
Keywords: galaxies: structure - Galaxy: halo - ISM: clouds - ISM: structure -
radio lines: ISM
Abstract:
We have detected over 400 HI clouds in the lower halo of the Galaxy
within the pilot region of the Galactic All-Sky Survey (GASS), a
region of the fourth quadrant that spans 18° in longitude, 40°
in latitude, and is centered on the Galactic equator. These clouds
have a median peak brightness temperature of 0.6K, a median velocity
width of 12.8km/s, and angular sizes ≲1°. The motion of these
clouds is dominated by Galactic rotation with a random cloud-to-cloud
velocity dispersion of 18km/s. A sample of clouds likely to be near
tangent points was analyzed in detail. These clouds have radii on the
order of 30pc and a median HI mass of 630M☉. The population has
a vertical scale height of 400pc and is concentrated in Galactocentric
radius, peaking at R=3.8kpc. This confined structure suggests that the
clouds are linked to spiral features, while morphological evidence
that many clouds are aligned with loops and filaments is suggestive of
a relationship with star formation. The clouds might result from
supernovae and stellar winds in the form of fragmenting shells and gas
that has been pushed into the halo rather than from a galactic
fountain.
Description:
The data presented in this paper are from the Galactic All-Sky Survey
(GASS), a fully sampled Galactic HI survey that covers the entire sky
south of declination δ=0°. GASS observations, with the 21cm
multibeam receiver at the Parkes Radio Telescope, began in 2005
January and were completed by the end of 2006. The spectral resolution
of the data is 0.8km/s.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 66 403 HI clouds - Observed properties
table2.dat 80 81 Tangent point HI clouds - Derived properties
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See also:
VIII/75 : HI survey of the sky DE<+25deg (Bajaja+, 2005)
VIII/73 : HI Parkes All Sky Survey Catalogue (HIPASS) (Meyer+, 2004)
VIII/54 : Atlas of Galactic Neutral Hydrogen (Hartmann+, 1997)
J/ApJ/626/887 : HI clouds in Southern Galactic Plane Survey (Kavars+, 2005)
J/ApJS/146/125 : O VI in the galactic halo (Savage+, 2003)
J/AJ/123/873 : HIPASS high-velocity clouds (Putman+, 2002)
J/ApJS/140/331 : High-velocity HI observations at 21cm (Lockman+, 2002)
J/A+AS/142/25 : High velocity HI Southern Survey (Morras+, 2000)
J/MNRAS/201/495 : HI Survey of Southern Galactic Plane (Strong+ 1982)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 6 F6.2 deg GLON Galactic longitude (1)
8- 13 F6.2 deg GLAT Galactic latitude (1)
15- 20 F6.1 km/s VLSR Local Standard of Rest velocity (2)
22- 25 F4.1 km/s e_VLSR Uncertainty in VLSR
27- 30 F4.2 K Tpk Peak brightness temperature (3)
32- 35 F4.1 km/s DelV Velocity profile FWHM (4)
37- 40 F4.1 km/s e_DelV Uncertainty in DelV
42- 46 F5.2 10+19/cm2 NHI HI column density (5)
48- 51 F4.2 10+19/cm2 e_NHI Uncertainty in NHI
53- 54 I2 arcmin amin Minor axis (6)
55 A1 --- --- [x]
56- 58 I3 arcmin amaj Major axis (6)
60- 64 F5.1 Msun/kpc2 MHI/D2 HI surface density
(mass divided by squared distance) (7)
66 A1 --- Note Details of prior detections in literature (8)
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Note (1): Of the cloud at the position of peak brightness temperature.
Note (2): Measured as the velocity of the cloud's peak brightness temperature
after background subtraction.
Note (3): After background subtraction. Uncertainties are 0.07K.
Note (4): Determined by inspection after background subtraction.
Note (5): NHI, at the cloud center is 1.94x1018TpkΔνcm-2 in the
optically thin limit, an assumption that is reasonable because the
emission is faint. An integrated intensity map was not used to
subtract a background when determining NHI, and was only used when
determining Θmaj, Θmin, and MHId-2. The background
subtraction from the line profile in the determination of Tpk was
inherently included in the calculation of NHI.
Note (6): Maximum and minimum extents of the cloud were determined by
inspection from the integrated intensity map of the cloud and are
in units of arcminutes. Uncertainties are dominated by background
levels surrounding the cloud and are assumed to be 25% of the
estimated values.
Note (7): HI mass of cloud where "D" is distance to the cloud. Uncertainties
are dominated by the interactive process used in mass determination
and are assumed to be 40% of the estimated values.
Note (8): Clouds that have been cataloged elsewhere in the literature are noted
by the following labels:
1 = detected by Putman et al. (2002, Cat. J/AJ/123/873) and
2 = detected by Wakker & van Woerden (1991A&A...250..509W 1991A&A...250..509W).
Although the clouds detected elsewhere do not necessarily have the
exact Galactic coordinates and VLSR as listed here, it is likely that
they are the same cloud and that the differences are a result of
observational constraints. Also, we note that Morras et al. (2000,
Cat. J/A+AS/142/25) detected HI in some areas of these clouds but
such detections were not identifed as individual objects.
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 6 F6.2 deg GLON Galactic longitude (1)
8- 13 F6.2 deg GLAT Galactic latitude (1)
15- 20 F6.1 km/s VLSR Local Standard of Rest velocity (2)
22- 25 F4.1 km/s e_VLSR Uncertainty in VLSR
27- 31 F5.1 km/s Vdev Deviation velocity (3)
33- 36 F4.1 km/s e_Vdev Uncertainty in Vdev
38- 40 F3.1 kpc Dist Heliocentric distance (4)
42- 44 F3.1 kpc e_Dist Uncertainty in Dist
46- 48 F3.1 kpc R Galactocentric distance (5)
50- 52 F3.1 kpc e_R Uncertainty in R
54- 58 F5.2 kpc z Height from Galactic plane (6)
60- 63 F4.2 kpc e_z Uncertainty in z
65- 66 I2 pc Rad Radius of HI cloud (7)
68- 69 I2 pc e_Rad Uncertainty in Rad
71- 75 I5 solMass MHI Physical HI mass (8)
77- 80 I4 solMass e_MHI Uncertainty in MHI
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Note (1): Of the cloud at the position of peak brightness temperature.
Note (2): Measured as the velocity of the cloud's peak brightness temperature
after background subtraction.
Note (3): Where Vdev=VLSR-Vt (Wakker, B.P., 1991A&A...250..499W 1991A&A...250..499W), where Vt
is the most negative velocity expected from Galactic rotation in the
fourth quadrant and was determined by McClure-Griffiths & Dickey
(2007ApJ...671..427M 2007ApJ...671..427M) from H I observations for GLON≤339.695 and by
Luna et al. (2006ApJ...641..938L 2006ApJ...641..938L) from CO observations for all
remaining longitudes.
Note (4): Along the line-of-sight from the Sun to the cloud determined by
assuming the cloud is at the tangent point:
Dist=R0*cos(GLON)/cos(GLAT)
Note (5): Where R=R0|sin(GLON)|, of the tangent point at the cloud's
location. Along a given line of sight, the smallest Galactocentric
radius possible is at the tangent point. If the cloud is not
located at the tangent point it must be further away from the
center and the error on R must be positive.
Note (6): Determined geometrically to be z=Dist*sin(GLAT).
Note (7): Determined by sqrt{rmaj*rmin}.
Note (8): Determined as in MHI/D2 in Table 1 but with the tangent point
distance assumed.
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Nomenclature notes:
HI clouds are <[FML2008] LLL.ll+BB.bb> in Simbad.
History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Emmanuelle Perret [CDS] 14-Jan-2011