J/ApJ/730/88 Star-forming regions in NGC 6822 from UV data (Efremova+, 2011)
The recent star formation in NGC 6822: an ultraviolet study.
Efremova B.V., Bianchi L., Thilker D.A., Neill J.D., Burgarella D.,
Wyder T.K., Madore B.F., Rey S.-C., Barlow T.A., Conrow T., Forster K.,
Friedman P.G., Martin D.C., Morrissey P., Neff S.G., Schiminovich D.,
Seibert M., Small T.
<Astrophys. J., 730, 88 (2011)>
=2011ApJ...730...88E 2011ApJ...730...88E
ADC_Keywords: Galaxies, nearby ; Photometry, ultraviolet ; Photometry, H-alpha ;
Extinction ; H II regions ; Cross identifications
Keywords: galaxies: individual (NGC 6822) - galaxies: stellar content -
Local Group - stars: formation - ultraviolet: stars
Abstract:
We characterize the star formation in the low-metallicity galaxy
NGC 6822 over the past few hundred million years, using GALEX far-UV
(FUV, 1344-1786Å) and near-UV (NUV, 1771-2831Å) imaging, and
ground-based Hα imaging. From the GALEX FUV image, we define 77
star-forming (SF) regions with area >860pc2, and surface brightness
≲26.8 mag (AB) arcsec-2, within 0.2° (1.7kpc) of the center of
the galaxy. We estimate the extinction by interstellar dust in each SF
region from resolved photometry of the hot stars it contains: E(B-V)
ranges from the minimum foreground value of 0.22mag up to
0.66±0.21mag. The integrated FUV and NUV photometry, compared with
stellar population models, yields ages of the SF complexes up to a few
hundred Myr, and masses from 2x102M☉ to 1.5x106M☉. The
derived ages and masses strongly depend on the assumed type of
interstellar selective extinction, which we find to vary across the
galaxy. The total mass of the FUV-defined SF regions translates into
an average star formation rate (SFR) of 1.4x10-2M☉/yr over the
past 100Myr, and SFR=1.0x10-2M☉/yr in the most recent 10Myr.
The latter is in agreement with the value that we derive from the
Hα luminosity, SFR=0.008M☉/yr. The SFR in the most recent
epoch becomes higher if we add the SFR=0.02M☉/yr inferred from
far-IR measurements, which trace star formation still embedded in dust
(age ≲ a few Myr).
Description:
We used GALEX images in FUV (1539Å, FWHM∼270Å), and NUV
(2316Å, FWHM∼615Å) with resolution 4.2" (FUV), and 5.3" (NUV),
corresponding to ∼12pc at the distance of NGC 6822. The images are
sampled with 1.5" pixels. The GALEX images of NGC 6822 were taken on
2005 August 20 as part of the NGS program, with exposure times of
4654s (FUV) and 6198s (NUV). The data were downloaded from the MAST
archive.
We also used the publicly available CTIO Hα image from the
survey of Massey et al. (2007, Cat. J/AJ/134/2474) to define contours
of Hα emitting regions.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 99 77 FUV-selected star-forming regions
table2.dat 191 39 Hα emission regions
table3.dat 104 77 Ages and masses of the FUV-defined star-forming
regions (SFR) assuming different extinction curves
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See also:
II/312 : GALEX-DR5 (GR5) sources from AIS and MIS (Bianchi+ 2011)
VI/110 : Final Merged Log of IUE Observations (NASA-ESA, 2000)
J/ApJS/192/6 : A GALEX UV imaging survey of nearby galaxies (Lee+, 2011)
J/A+A/495/447 : PN and HII regions in NGC6822 (Hernandez-Martinez+, 2009)
J/AJ/138/1203 : SF regions in nearby dwarf galaxies (Melena+, 2009)
J/ApJ/706/599 : Hα and UV SFR in the local volume (Lee+, 2009)
J/ApJS/178/247 : Hα and [NII] survey in local 11 Mpc (Kennicutt+, 2008)
J/AJ/133/2393 : UBVRI phot. in 7 Local Group dwarfs galaxies (Massey+, 2007)
J/AJ/134/2474 : Ha emission-line stars in 7 dwarfs galaxies (Massey+, 2007)
J/AJ/132/378 : HST and UV photometry of NGC 6822 (Bianchi+, 2006)
J/AJ/128/2170 : Hα imaging of irregular galaxies (Hunter+, 2004)
J/AJ/121/2020 : CTIO and WFPC2 photometry in NGC 6822 (Bianchi+, 2001)
J/AJ/110/2715 : UBV photometry in NGC 6822, M31 and M33 (Massey+ 1995)
J/A+AS/109/341 : UV properties of normal galaxies. II. (Rifatto+, 1995)
Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 6 A6 --- --- [EBFUV-]
8- 9 I2 --- ID [1/77] FUV SF region identification number
(<[EBT2011] EB-FUV NN> in Simbad) (1)
10 A1 --- n_ID [g] FUV source associated to Hα emission
12- 13 I2 h RAh [19] Hour of right ascension (J2000) (G1)
15- 16 I2 min RAm [44/45] Minute of right ascension (J2000)
18- 22 F5.2 s RAs Second of right ascension (J2000)
24 A1 --- DE- [-] Sign of declination (J2000) (G1)
25- 26 I2 deg DEd [14] Degree of declination (J2000) (G1)
28- 29 I2 arcmin DEm [42/58] Arcminute of declination (J2000)
31- 34 F4.1 arcsec DEs Arcsecond of declination (J2000)
36- 39 I4 arcsec2 Area1 [150/5400] SF region area in arcsec2
41- 45 I5 pc2 Area2 Area in parsec2
47- 51 F5.2 mag FUV [15.46/20.55] FUV (1344-1786Å) band
AB magnitude
53- 56 F4.2 mag e_FUV [0.01/0.06] FUV uncertainty (3)
58- 62 F5.2 mag NUV NUV (1771-2831Å) band AB magnitude
64- 67 F4.2 mag e_NUV NUV uncertainty (3)
69- 72 F4.2 --- fFUV [0/1] Background fraction of FUV (4)
74- 77 F4.2 --- fNUV [0/1] Background fraction of NUV (4)
79- 82 F4.2 mag E(B-V) [0.22/0.66] Interstellar reddening (5)
84- 87 F4.2 mag e_E(B-V) 1σ scatter in E(B-V) (5)
89- 99 A11 --- Comm Comment (6)
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Note (1): Star-forming region numbers correspond to the blue labels in Figure 1.
Note (3): Photometric errors are estimated as explained in the text, without
adding the uncertainties of the GALEX calibration zero points.
Note (4): Fraction of background flux over total flux in the aperture.
Note (5): The median of values derived for the massive stars (earlier than B2V)
in each SF region (see Section 3.1).
Note (6): Names of OB associations after Hodge, P.W. (1977ApJS...33...69H 1977ApJS...33...69H).
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Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 9 A9 --- --- [EBHalpha- ]
11- 12 I2 --- ID [1/39] Hα region identification number
(<[EBT2011] EB-Ha NN> in Simbad)
14- 15 I2 h RAh [19] Hour of right ascension (J2000) (G1)
17- 18 I2 min RAm [44/45] Minute of right ascension (J2000)
20- 24 F5.2 s RAs Second of right ascension (J2000)
26 A1 --- DE- [-] Sign of declination (J2000) (G1)
27- 28 I2 deg DEd [14] Degree of declination (J2000) (G1)
30- 31 I2 arcmin DEm Arcminute of declination (J2000)
33- 36 F4.1 arcsec DEs Arcsecond of declination (J2000)
38- 41 I4 arcsec2 Area1 [150/9500] Area in arcsec2
43- 47 I5 pc2 Area2 Area in pc2
49- 53 F5.2 10-16W/m2 FHa [0.03/50.74] Hα flux in
units of 10-13erg/s/cm2
55- 58 F4.2 10-16W/m2 e_FHa [0.02/3.49] FHa uncertainty
60-191 A132 --- Comm Cross-identifications and comments (1)
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Note (1): Cross-identifications with Hα sources previously defined:
* "H" followed by a roman number: Hubble (1925ApJ....62..409H 1925ApJ....62..409H),
<[H25] R> in Simbad
* "Ho" followed by an arabic number: Hodge (1977ApJS...33...69H 1977ApJS...33...69H)
* "K" followed by a Greek letter: Kinman et al. (1979PASP...91..749K 1979PASP...91..749K),
<[KGM79] aaa> in Simbad
* "KD" followed by an arabic number: Killen & Dufour (1982PASP...94..444K 1982PASP...94..444K),
in Simbad
* "HK" followed by an arabic number: Hodge et al. (1988PASP..100..917H 1988PASP..100..917H),
<[HKL88] NNN> in Simbad
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Byte-by-byte Description of file: table3.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 6 A6 --- --- [EBFUV-]
8- 9 I2 --- ID [1/77] FUV SF region identification number
(<[EBT2011] EB-FUV NN> in Simbad)
11- 13 I3 Myr Age0 [2/374] Age with Milky-Way type Rv=3.1 (2)
15- 16 I2 Myr E_Age0 Positive error on Age0
18- 19 I2 Myr e_Age0 Negative error on Age0
21- 26 F6.1 kMsun M0 [0.2/1500] Mass with Milky-Way type Rv=3.1 (2)
28- 33 F6.1 kMsun E_M0 Positive error on M0
35- 40 F6.1 kMsun e_M0 Negative error on M0
42- 44 I3 Myr Age1 ? Age with LMC exinction (2)
46- 48 I3 Myr E_Age1 ? Positive error on Age1
50- 51 I2 Myr e_Age1 ? Negative error on Age1
53- 57 F5.1 kMsun M1 ? Mass with LMC exinction (2)
59- 64 F6.1 kMsun E_M1 ? Positive error on M1
66- 70 F5.1 kMsun e_M1 ? Negative error on M1
72- 74 I3 Myr Age2 ? Age with LMC2 extinction(2)
76- 78 I3 Myr E_Age2 ? Positive error on Age2
80- 81 I2 Myr e_Age2 ? Negative error on Age2
83- 87 F5.1 kMsun M2 ? Mass with LMC2 extinction (2)
89- 93 F5.1 kMsun E_M2 ? Positive error on M2
95- 98 F4.1 kMsun e_M2 ? Negative error on M2
100-104 A5 --- EC Extinction curve type adopted (2)
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Note (2): For all sources, a foreground extinction of E(B-V)=0.22 was applied
with MW-type dust, and the additional internal extinction (from the
total "E(B-V)" given in Table 1) using different extinction types:
Rv3.1 = Milky-Way type extinction with extinction ratio RV=3.1. In this
case, the GALEX (FUV-NUV) color is basically reddening free
(Bianchi et al. 2011MNRAS.411.2770B 2011MNRAS.411.2770B)
LMC = Average extinction curve derived by Misselt et al.
(1999ApJ...515..128M 1999ApJ...515..128M) for LMC stars outside the 30 Doradus region,
which gives an average color excess ratio for hot stars
(Teff>10000K) of E(FUV-NUV)/E(B-V)∼1
LMC2 = Extinction curve in the LMC 30 Doradus region derived by Misselt et
al. (1999ApJ...515..128M 1999ApJ...515..128M), yielding E(FUV-NUV)/E(B-V)∼2
SMC = Extremely UV-steep extinction curve derived by Gordon & Clayton
(1998ApJ...500..816G 1998ApJ...500..816G) for SMC stars, which yields E(FUV-NUV)/E(B-V)∼5
(used only for source#6)
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Global notes:
Note (G1): Coordinates of the centroids of the contours (see Section 2.2).
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(End) Emmanuelle Perret [CDS] 22-May-2013