J/AJ/157/241 HI clouds in LITTLE THINGS dwarf irregular galaxies (Hunter+, 2019)
H I clouds in LITTLE THINGS dwarf irregular galaxies.
Hunter D.A., Elmegreen B.G., Berger C.L.
<Astron. J., 157, 241 (2019)>
=2019AJ....157..241H 2019AJ....157..241H (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, nearby ; Molecular clouds ; H I data
Keywords: galaxies: dwarf - galaxies: irregular - galaxies: ISM -
galaxies: star formation
Abstract:
We identify 814 discrete H I clouds in 40 dwarf irregular galaxies from
the LITTLE THINGS survey using an automated cloud-finding algorithm.
The cloud masses range from ∼103 to 107 M☉, have a surface
density averaged over all of the clouds of ∼9.65 M☉/pc2, and
constitute 2%-53% of the total H I mass of the host galaxy. For individual
clouds, the mass including He varies with cloud radius as
logMgas=(2.11±0.04)xlogRcl+(0.78±0.08) and the internal velocity
dispersion varies as logVdisp=0.5xlogRcl-0.57±0.21. The H I clouds
tend to be in the outer regions of the galaxies, with 72% of the galaxies
having more than 70% of their clouds outside one disk scale length and 32%
of the galaxies having more than 50% of their clouds outside the radius
encircling the H II emission. Thirty-six percent of the clouds are
essentially non-self-gravitating from H I alone, with a virial parameter
that exceeds αvir∼10, and 5% have αvir=<2. We estimate
the missing molecular mass, based on the total star formation rate and
a typical molecular consumption time of 2 Gyr, as observed in CO-rich
galaxies. The resulting molecular fraction has a value averaged over
the galaxies of 0.23 and correlates with both the surface density of star
formation and the fraction of H I clouds in the outer regions. We conclude
that a significant fraction of the inner parts of these dwarf galaxy disks
is in the form of dark molecular gas, and that this fraction could be
high enough to make the inner disks mildly gravitationally unstable as
a precursor to star formation.
Description:
Here, we present a study of the H I clouds in a sample of 40 nearby dwarf
irregular galaxies. The clouds are found from radio interferometric
position-velocity H I data cubes using automatic cloud detection software
made publicly available by Rosolowsky & Leroy (2006PASP..118..590R 2006PASP..118..590R).
We used the LITTLE THINGS (Local Irregulars That Trace Luminosity Extremes,
The H I Nearby Galaxy Survey; Hunter et al. 2012, J/AJ/144/134) sample of
dIrr galaxies. LITTLE THINGS is a multi-wavelength survey of 37 dIrr
galaxies and 4 Blue Compact Dwarfs aimed at understanding what drives star
formation in tiny systems. The LITTLE THINGS galaxies were chosen to be
nearby (=<10.3 Mpc), contain gas so they could be forming stars, and cover
a large range in dwarf galactic properties, such as rate of star formation.
The LITTLE THINGS data sets include H I-line maps obtained with the
National Science Foundation's Karl G. Jansky Very Large Array (VLA).
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 70 40 Sample galaxies
table3.dat 49 40 Galaxy cloud properties
table2.dat 109 814 Cloud catalog
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See also:
J/AJ/128/2170 : Hα imaging of irregular galaxies (Hunter+, 2004)
J/ApJ/636/214 : HII regions in dwarf irregular galaxies (van Zee+, 2006)
J/AJ/136/2563 : HI Nearby Galaxy Survey, THINGS (Walter+, 2008)
J/AJ/136/2782 : Star formation efficiency in nearby galaxies (Leroy+, 2008)
J/AJ/140/1194 : FUV/HI relations in nearby galaxies (Bigiel+, 2010)
J/AJ/144/134 : LITTLE THINGS survey of nearby dwarf galaxies (Hunter+, 2012)
J/AJ/146/104 : Dwarf galaxies surface brightness profiles. I.
(Herrmann+, 2013)
J/AJ/151/136 : LITTLE THINGS dwarf irregular galaxies FUV regions
(Hunter+, 2016)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 10 A10 --- Galaxy Galaxy identifier
12- 15 F4.1 Mpc Dist [0.7/10.3] Distance
17- 19 I3 pc Beam [19/424] Beam size (1)
21- 25 F5.2 [Msun/yr/kpc2] logSFR [-3.88/-0.01]? Integrated star formation
rate (2)
27- 30 F4.2 [Msun/yr/kpc2] e_logSFR [0/0.06]? Uncertainty in logSFR
32- 35 F4.2 [Msun] logMHI [5.19/8.85] Total H I mass of the galaxy
from Hunter et al. (2012, J/AJ/144/134)
37- 40 F4.2 kpc RBr [0.13/2.8]? Radius at which the V-band
surface brightness profile changes
slope (Herrmann et al. 2013,
J/AJ/146/104)
42- 45 F4.2 kpc e_RBr [0/1.35]? Uncertainty in RBr
47- 50 F4.2 kpc RD [0.15/2.24] Disk scale length determined
from the V-band (Herrmann et al. 2013,
J/AJ/146/104)
52- 55 F4.2 kpc e_RD [0/0.25] Uncertainty in RD
57- 60 F4.2 kpc RHa [0.3/5.58]? Extent of Hα emission,
RHα (Hunter & Elmegreen 2004,
J/AJ/128/2170)
62- 65 F4.2 kpc RFUV [0.29/6.79]? Radius of the furthest out
FUV knot identified on GALEX images
(Hunter et al. 2016, J/AJ/151/136)
67- 70 F4.2 kpc e_RFUV [0.01/0.1]? Uncertainty in RFUV
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Note (1): Beam FWHM determined from the square-root of the major axis times
the minor axis, for robust-weighted maps (Hunter et al. 2012, J/AJ/144/134).
Note (2): Integrated star formation rate (SFR) determined from the Galaxy
Evolution Explorer (GALEX; Martin et al. 2005ApJ...619L...1M 2005ApJ...619L...1M) far-ultraviolet
(FUV) luminosity (Hunter et al. 2010AJ....139..447H 2010AJ....139..447H). The SFR is normalized
by dividing by the area within one disk scale length RD.
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Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 10 A10 --- Galaxy Galaxy identifier
12- 14 I3 --- Ncl [1/161] Number of clouds identified in galaxy
16- 19 F4.2 [Msun] logMcl [3.89/7.73] Log cloud mass
21- 24 F4.2 --- Mcl/MHI [0.02/0.53] Ratio of cloud mass to total H I mass
26- 29 F4.2 [Msun] logM3rd [3.24/6.59]? H I mass of third most massive cloud
for galaxies with at least three clouds
31- 34 F4.2 --- Fr-RBr [0/1]? Fraction of clouds beyond the break radius
RBr
36- 39 F4.2 --- Fr-RD [0/1] Fraction of clouds beyond one disk scale
length RD
41- 44 F4.2 --- Fr-RHa [0/1]? Fraction of clouds beyond the extent
of Hα emission RHα
46- 49 F4.2 --- Fr-RFUV [0/1]? Fraction of clouds beyond the furthest FUV
knot RFUV
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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- 8 A8 --- Galaxy Galaxy identifier
10- 12 I3 --- Cloud [1/161] Cloud identifier
14- 15 I2 h RAh Hour of Right Ascension (J2000)
17- 18 I2 min RAm Minute of Right Ascension (J2000)
20- 23 F4.1 s RAs Second of Right Ascension (J2000)
25 A1 --- DE- Sign of the Declination (J2000)
26- 27 I2 deg DEd Degree of Declination (J2000)
29- 30 I2 arcmin DEm Arcminute of Declination (J2000)
32- 33 I2 arcsec DEs Arcsecond of Declination (J2000)
35- 40 F6.1 km/s Vcen [-395.1/632] Central velocity
42- 46 F5.1 deg PA [-90.0/89.7] Position angle
48- 50 I3 pc Rcl [15/817] Cloud major axis radius
52- 53 I2 pc e_Rcl [2/88] Uncertainty in Rcl
55- 58 F4.2 --- Ratio [0.15/0.96] Minor-to-major axis ratio
60- 63 F4.1 --- S/N [4.3/61.2] Peak signal-to-noise ratio
65- 68 F4.1 km/s Vdisp [0/12.8] Velocity dispersion
70- 72 F3.1 km/s e_Vdisp [0/1.5] Uncertainty in Vdisp
74- 77 F4.2 [Msun] logMg [3.24/7.22] Log gas mass; H I + He
79- 82 F4.2 [Msun] e_logMg [0.01/0.16] Uncertainty in logMg
84- 88 F5.1 Msun/pc2 Sgas [0.4/927.1] Gas surface density Σgaz
90- 94 F5.1 Msun/pc2 e_Sgas [0.1/168.8] Uncertainty in Sgas
96- 98 F3.1 kpc Rgal [0.1/6.9] Radius from galaxy center
100-104 F5.1 --- alpha [0.5/222.8]? Virial ratio αvir
106-109 F4.1 --- e_alpha [0/30.7]? Uncertainty in alpha
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History:
From electronic version of the journal
(End) Prepared by [AAS], Tiphaine Pouvreau [CDS] 21-Aug-2019