J/AJ/151/136 LITTLE THINGS dwarf irregular galaxies FUV regions (Hunter+, 2016)
Young star clusters in the outer disks of LITTLE THINGS dwarf irregular
galaxies.
Hunter D.A., Elmegreen B.G., Gehret E.
<Astron. J., 151, 136-136 (2016)>
=2016AJ....151..136H 2016AJ....151..136H (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, nearby ; Clusters, open ; Photometry, ultraviolet ;
Photometry, UBV
Keywords: galaxies: irregular - galaxies: star formation
Abstract:
We examine FUV images of the LITTLE THINGS sample of nearby dwarf
irregular (dIrr) and Blue Compact Dwarf galaxies to identify distinct
young regions in their far outer disks. We use these data, obtained
with the Galaxy Evolution Explorer satellite, to determine the
furthest radius at which in situ star formation can currently be
identified. The FUV knots are found at distances from the center of
the galaxies of 1-8 disk scale lengths and have ages of =<20 Myr and
masses of 20 M☉ to 1x105 M☉. The presence of young
clusters and OB associations in the outer disks of dwarf galaxies
shows that dIrrs do have star formation taking place there in spite of
the extreme nature of the environment. Most regions are found where
the H I surface density is ∼1 M☉/pc2, though both the H I and
dispersed old stars go out much further. This limiting density
suggests a cutoff in the ability to form distinct OB associations and
perhaps even stars. We compare the star formation rates in the FUV
regions to the average rates expected at their radii and beyond from
the observed gas, using the conventional correlation for gas-rich
regions. The localized rates are typically 10% of the expected average
rates for the outer disks. Either star formation in dIrrs at surface
densities <1 M☉/pc2 occurs without forming distinct associations,
or the Kennicutt-Schmidt relation over-predicts the rate beyond this
point. In the latter case, the stellar disks in the far-outer parts of
dIrrs result from scattering of stars from the inner disk.
Description:
The sample of galaxies is taken from LITTLE THINGS (Local Irregulars That
Trace Luminosity Extremes, The H I Nearby Galaxy Survey, Hunter et al.
2012, J/AJ/144/134). This is a multi-wavelength survey of nearby
(<10.3 Mpc) dIrr galaxies and BCDs, which builds on the THINGS project,
whose emphasis was on nearby spirals (Walter et al. 2008, J/AJ/136/2563).
The galaxies and a few key parameters are listed in Table 1.
We used FUV (1516 Å) images obtained by GALEX (Melena et al. 2009,
J/AJ/138/1203; Hunter et al. 2010AJ....139..447H 2010AJ....139..447H, 2011AJ....142..121H 2011AJ....142..121H;
Zhang et al. 2012AJ....143...47Z 2012AJ....143...47Z) to identify knots of emission in the
outer disks of each galaxy. In order to better distinguish knots from
the wide-spread diffuse emission, we subtracted the stellar continuum
from each FUV image using the V-band image.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 82 37 Furthest FUV region
table2.dat 116 37 Photometry, ages and masses of FUV regions
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See also:
II/312 : GALEX-DR5 (GR5) sources from AIS and MIS (Bianchi+ 2011)
J/ApJS/162/49 : UBV imaging of irregular galaxies (Hunter+, 2006)
J/AJ/136/2563 : HI Nearby Galaxy Survey, THINGS (Walter+, 2008)
J/AJ/138/1203 : Star formation regions in nearby dwarf galaxies (Melena+, 2009)
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)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 9 A9 --- ID Galaxy identifier
11- 14 F4.1 Mpc Dist [0.5/10.3] Distance
16- 19 F4.2 kpc RD [0.16/3.16] Disk scale length (1)
21- 24 F4.2 kpc e_RD [0/0.25] Uncertainty in RD
26- 29 F4.2 kpc RBr [0.13/2.80]? Radius at which V-band surface
brightness profile changes slope (1)
31- 34 F4.2 kpc e_RBr [0/1.35]? Uncertainty in RBr
36- 37 I2 h RAh Hour of Right Ascension (J2000) (2)
39- 40 I2 min RAm Minute of Right Ascension (J2000) (2)
42- 45 F4.1 s RAs Second of Right Ascension (J2000) (2)
47 A1 --- DE- Sign of the Declination (J2000)(2)
48- 49 I2 deg DEd Degree of Declination (J2000) (2)
51- 52 I2 arcmin DEm Arcminute of Declination (J2000) (2)
54- 55 I2 arcsec DEs Arcsecond of Declination (J2000) (2)
57- 60 F4.2 kpc Rk [0.29/6.79] Radius of furthest FUV knot (3)
62- 65 F4.2 kpc e_Rk [0/0.10] Uncertainty in Rk
67- 69 F3.1 --- Rk/RD [0.6/7.7] Ratio of Rk to RD
71- 73 F3.1 --- e_Rk/RD [0/0.9] Uncertainty in Rk/RD
75- 78 F4.1 --- Rk/RBr [0.3/10.3]? Ratio of Rk to RBr
80- 82 F3.1 --- e_Rk/RBr [0.1/5.5]? Uncertainty in Rk/RBr
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Note (1): As given by Herrmann et al. (2013, J/AJ/146/104).
Note (2): Of the furthest FUV region.
Note (3): From the center of the galaxy determined in the plane of the galaxy.
The galaxy disk geometry was determined from V-band images by
Hunter & Elmegreen (2006, J/ApJS/162/49). The uncertainty is assumed
to be 2".
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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- 9 A9 --- ID Galaxy identifier
11- 15 F5.3 mag E(B-V) [0.050/0.558] The (B-V) color excess
17- 21 F5.1 arcsec Rad [4.2/120] Radius of photometry aperture
23- 28 F6.3 mag FUVmag [16.175/23.321] GALEX FUV band AB magnitude
30- 36 F7.3 mag FUVMag [-12.276/-0.905] GALEX FUV band absolute AB
magnitude (1)
38- 42 F5.3 mag e_FUVMag [0.008/0.395] Uncertainty in FUVMag
44- 49 F6.3 mag (F-N) [-1.178/0.176] GALEX (FUV-NUV) band color (1)
51- 55 F5.3 mag e_(F-N) [0.009/0.611] Uncertainty in (F-N)
57- 63 F7.3 mag VMag [-13.063/-2.787]? Absolute V band magnitude (1)
65- 69 F5.3 mag e_VMag [0.001/0.832]? Uncertainty in VMag
71- 76 F6.3 mag (B-V) [-1.063/0.927]? The (B-V) color (1)
78- 82 F5.3 mag e_(B-V) [0.002/0.885]? Uncertainty in (B-V)
84- 89 F6.3 mag (U-B) [-1.730/0.725]? The (U-B) color (1)
91- 95 F5.3 mag e_(U-B) [0.002/0.713]? Uncertainty in (U-B)
97 I1 --- Nfit [0/3] Number of colors used in age
determination (2)
99-100 I2 Myr Age [2/20]? Cluster age
102-103 I2 Myr e_Age [1/12]? Uncertainty in Age (3)
105-110 I6 Msun Mass [16/130000]? Cluster mass
112-116 I5 Msun e_Mass [1/87000]? Uncertainty in Mass
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Note (1): Corrected for extinction. FUV magnitudes are AB magnitudes. The UBV
magnitudes are on the standard Landolt (1992AJ....104..340L 1992AJ....104..340L) system.
Note (2): Number of colors (FUV-NUV, B-V, U-B) used in the age determination.
An Nfit of 1 means the FUV-NUV color was used, a value of 2 means
either U-B or B-V was also used, and a value of 3 means a consistent
age was determined for all three colors.
Note (3): Uncertainties represent the range of ages allowed by the colors.
Derivations of ages consistent with the observed colors were not
possible for LGS3, WLM, and NGC 6822.
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History:
From electronic version of the journal
(End) Prepared by [AAS], Tiphaine Pouvreau [CDS] 15-Mar-2018