J/ApJS/209/12 High-velocity clouds from GASS. I. (Moss+, 2013)
High-velocity clouds in the Galactic All Sky Survey.
I. Catalog.
Moss V.A., McClure-Griffiths N.M., Murphy T., Pisano D.J., Kummerfeld J.K.,
Curran J.R.
<Astrophys. J. Suppl. Ser., 209, 12 (2013)>
=2013ApJS..209...12M 2013ApJS..209...12M
ADC_Keywords: H I data ; Diffuse clouds ; Surveys ; Cross identifications
Keywords: catalogs - Galaxy: halo - ISM: clouds - radio lines: general - surveys
Abstract:
We present a catalog of high-velocity clouds (HVCs) from the Galactic
All Sky Survey (GASS) of southern sky neutral hydrogen, which has 57mK
sensitivity and 1 km/s velocity resolution and was obtained with the
Parkes Telescope. Our catalog has been derived from the
stray-radiation-corrected second release of GASS. We describe the data
and our method of identifying HVCs and analyze the overall properties
of the GASS population. We catalog a total of 1693 HVCs at
declinations <0°, including 1111 positive velocity HVCs and 582
negative velocity HVCs. Our catalog also includes 295 anomalous
velocity clouds (AVCs). The cloud line-widths of our HVC population
have a median FWHM of ∼19 km/s, which is lower than that found in
previous surveys. The completeness of our catalog is above 95% based
on comparison with the HIPASS catalog of HVCs upon which we improve by
an order of magnitude in spectral resolution. We find 758 new HVCs and
AVCs with no HIPASS counterpart. The GASS catalog will shed
unprecedented light on the distribution and kinematic structure of
southern sky HVCs, as well as delve further into the cloud populations
that make up the anomalous velocity gas of the Milky Way.
Description:
Our catalog is derived from the second release of Galactic All Sky
Survey (GASS), which surveyed southern sky HI (McClure-Griffiths et
al. 2009ApJS..181..398M 2009ApJS..181..398M; Kalberla et al. 2010A&A...521A..17K 2010A&A...521A..17K),
covering declinations south of δ=1°. We have used all GASS
data south of 0° in the construction of our survey in order to
avoid regions of increased noise near the limits of the survey and to
limit our catalog to a southern sky survey. GASS was undertaken with
the Parkes 64m radio telescope using the 21cm multibeam and consisted
of two sets of scans, one in right ascension and one in declination.
The effective angular resolution of GASS is ∼16' with a spectral
resolution of 1km/s and ∼57mK rms noise. As GASS focuses specifically
on the HI content of the Milky Way, the survey velocity range is
complete from approximately -468km/s to +468km/s.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 178 1693 The GASS high-velocity clouds (HVCs) catalog
table2.dat 169 295 The GASS anomalous velocity clouds (AVCs) catalog
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See also:
J/ApJ/722/367 : HI clouds in Quadrant I of the Milky Way (Ford+, 2010)
J/ApJ/688/290 : GASS HI survey of the lower halo (Ford+, 2008)
J/ApJ/626/887 : HI clouds in Southern Galactic Plane Survey (Kavars+, 2005)
J/MNRAS/339/105 : Western Magellanic Bridge HI observations (Muller+, 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+A/391/159 : A search for Compact High-Velocity Clouds (De Heij+, 2002)
J/AJ/122/3155 : Peculiar HI cloud near galactic plane (Higgs+, 2001)
http://www.atnf.csiro.au/research/GASS/ : GASS home page
Byte-by-byte Description of file: table1.dat table2.dat
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Bytes Format Units Label Explanations
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1- 4 A4 --- Cat [GHAVC] Catalog name, "GHVC" or "GAVC" (1)
6- 20 A15 --- Name HVC/AVC name GLLL.l+BB.b+VVV (1)
22- 23 I2 h RAh Peak Hour of Right Ascension (J2000)
25- 26 I2 min RAm Peak Minute of Right Ascension (J2000)
28- 32 F5.2 s RAs Peak Second of Right Ascension (J2000)
34 A1 --- DE- [-] Sign of the Peak Declination (J2000)
35- 36 I2 deg DEd Peak Degree of Declination (J2000)
38- 39 I2 arcmin DEm Peak Arcminute of Declination (J2000)
41- 42 I2 arcsec DEs Peak Arcsecond of Declination (J2000)
44- 49 F6.1 km/s VLSR [-483/481] Local Standard of Rest velocity
51- 54 F4.1 km/s e_VLSR [1/14] Uncertainty in VLSR (2)
56- 61 F6.1 km/s VGSR [-686/467] Galactic Standard of Rest
velocity (3)
63- 68 F6.1 km/s Vdev [-445/443] Deviation velocity (4)
70- 73 F4.1 km/s FWHM [2.8/58.3] Fitted Full-Width at Half-Maximum (5)
75- 78 F4.1 km/s e_FWHM [2/28] Uncertainty in FWHM
80- 85 F6.2 K TbFit [0.06/120] Fitted peak brightness temperature (6)
87- 93 E7.2 cm-2 NH [1.3e+18/1.1e+22] Peak H I column density (7)
95-101 E7.2 cm-2 e_NH Uncertainty in NH
103-108 F6.1 deg2 Area [0.1/2253] Source area (8)
110-114 F5.1 deg dx [0.1/140] Maximum source x-extent above
2σ0 (9)
116-119 F4.1 deg dy [0.1/73.1] Maximum source y-extent above
2σ0 (9)
121-128 A8 --- Flag [T,H0-9,CA,N -] Source flag(s) (10)
130-155 A26 --- HI HIPASS catalog identifier (11)
157-178 A22 --- WW91 Wakker & van Woerden 1991A&A...250..509W 1991A&A...250..509W
identifier (12)
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Note (1): Each cloud is given the identifier GHVC (GASS High Velocity
Cloud), or GAVC (GASS Anomalous Velocity Cloud) in table 2, followed
by its truncated Galactic coordinates (based on the position) and
fitted LSR (Local Standard of Rest) velocity of the best-fit spectrum.
A GHVC is characterised with a peak velocity deviation >50km/s and a
LSR velocity >90km/s; a GASS deviate significantly from Galactic
rotation without meeting both high-velocity criteria.
Note (2): The uncertainty in the peak velocity is the theoretical estimate
of Taylor et al. (1999ASPC..180.....T 1999ASPC..180.....T), which is given by
σ*FWHM/(2*Tpeak).
Note (3): The GSR velocity is calculated using the standard
VGSR=220cos(b).sin(l)+VLSR. The uncertainty in LSR velocity also
applies to the error in the velocity in GSR.
Note (4): We estimate the deviation velocity based on the same model of
Galactic rotation used to mask low deviation velocities in Section 2.1.
Note (5): Measured using a least-squares Gaussian fitting routine. As this
was performed using a single component fit for all sources to ensure
consistency, the fitted FWHM should be treated with caution for all
sources flagged in column Flag as having two or more spectral
components. The theoretical error of Taylor et al. (1999ASPC..180.....T 1999ASPC..180.....T)
is given for the FWHM, and is defined as σ*FWHM/(2*Tpeak).
Note (6): The fitted peak is given to account for the possible effect of
noise, noting that this is based on the assumption of the source as a
single-component Gaussian and as such may underestimate the true peak
in cases of complex spectra. The error on the peak value is taken to
be the RMS of the GASS data, which is in all cases assumed to be 57mK.
Note (7): This is determined on the basis of measuring the peak integrated
intensity from the 0th moment map, and using the standard conversion
factor of 1.8x1018 to convert from K.km/s to cm-2 of column
density (assuming negligible optical depth which may not be true in
all cases). The error on this value is assumed to be the nominal GASS
noise of 57 mK multiplied by the FWHM of each source.
Note (8): This area is determined starting from the rough area of the
source on the sky, shown as rectangles in Figure 6. We estimate the
noise level σ0 in the 0th moment map as the spectral width of
the cube containing the cloud multiplied by 57 mK, and count all
pixels above 2σ0. The area is the total number of pixels
multiplied by the pixel area of 0.08x0.08°. It is not practical
in our case to perform angular fitting for the majority of sources, as
many are either complexes of several clouds or resolved with structure
(and hence not well approximated as Gaussians).
Note (9): These dimensions correspond to the x-extent and y-extent of each
source in the original zenith-equal-area (ZEA) coordinate system, and
are included to give an idea of the source dimensions. The area given
in column Area will generally be smaller than the combination of dx
and dy because it only considers the pixels above 2σ0 as part
of the source.
Note (10): The flags are one or more of ("-" stands for empty flags):
* A flag "T" indicates a two-component cloud.
A cloud is determined to be better represented by two components if
the standard deviation of the single-component residual is >10% higher
than the standard deviation of the two-component residual. These
two-component sources were also checked to make sure they were
accurately classified, and removed from the group if their spectrum
did not show convincing two component structure.
* A flag of "HNNN" indicates that the region the source occupies
contains NNN HIPASS sources.
* Flags "C" and "A" characterize sources near the edge of the deviation
velocity (Cutoff or Anomalous).
Sources which sit near the edge of our 30km/s deviation velocity
cutoff are required to show a well-defined maximum in their spectra,
but due to increased uncertainty in their properties (such as FWHM and
NH) we also flag any partially-cutoff sources with a "C". If the
source does not meet the velocity cutoff criteria of an HVC, it is
thus classified as an anomalous velocity cloud (AVC) and is labelled
with an "A" as well as its name changing from GHVC to GAVC.
The "A" flag is not the same as the "C" flag because it is possible
for a source to not meet the HVC velocity criteria and still be
significantly away from the deviation velocity mask (and hence not
cutoff), however any source with a "C" flag will always have an "A"
flag.
* A flag "N" indicates a cloud found in the targeted narrow cloud
search rather than in the main source finding process.
Note (11): For GASS sources containing HIPASS clouds the name of the "best
match" in HIPASS is provided. This was done using the results of
cross-matching with the HIPASS catalog of HVCs (Putman et al. 2002,
Cat. J/AJ/123/873) in Section 4. The best match HIPASS cloud indicates
that the corresponding GASS cloud is the closest in spatial and
spectral coordinates to the specific HIPASS cloud. For the largest
GASS sources which contain many HIPASS sources, we list the name of
the region (e.g. Magellanic Clouds) rather than the best match cloud.
If the flag column indicates a HIPASS source but the HIPASS ID is
blank, this means that although the GASS cloud contains the HIPASS
source, a different GASS cloud is a closer match.
Note (12): The complexes and populations of Wakker & van Woerden
(1991A&A...250..509W 1991A&A...250..509W, table 3) that the particular GASS cloud may be
associated with based on its Galactic longitude, Galactic latitude and
VLSR velocity. If a cloud is listed to be potentially part of several
complexes, this is due to overlap in the longitude/latitude/velocity
definitions of regions. Abbreviations are:
MS = Magellanic Stream;
GCN = Negative-velocity clouds near the Galactic center;
GCP = positive-velocity clouds near the Galactic center;
ACVHV = anti center complex
ACHV = anti center high-velocity complex
EN = extreme negative velocity clouds
EP = extreme positive velocity clouds
N = negative-velocity clouds with Vlsr≳-200km/s
P = positive-velocity clouds with Vlsr≲200km/s.
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History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Emmanuelle Perret [CDS] 20-Nov-2013