J/ApJ/812/147 Slug analysis of star clusters in NGC 628 & 7793 (Krumholz+, 2015)
Star cluster properties in two LEGUS galaxies computed with stochastic stellar
population synthesis models.
Krumholz M.R., Adamo A., Fumagalli M., Wofford A., Calzetti D., Lee J.C.,
Whitmore B.C., Bright S.N., Grasha K., Gouliermis D.A., Kim H., Nair P.,
Ryon J.E., Smith L.J., Thilker D., Ubeda L., Zackrisson E.
<Astrophys. J., 812, 147 (2015)>
=2015ApJ...812..147K 2015ApJ...812..147K (SIMBAD/NED BibCode)
ADC_Keywords: Clusters, open ; Galaxies, nearby ; Extinction ; Models
Keywords: galaxies: individual: (NGC 628, NGC 7793);
galaxies: star clusters: general; methods: data analysis;
methods: statistical; techniques: photometric
Abstract:
We investigate a novel Bayesian analysis method, based on the
Stochastically Lighting Up Galaxies (slug) code, to derive the masses,
ages, and extinctions of star clusters from integrated light
photometry. Unlike many analysis methods, slug correctly accounts for
incomplete initial mass function (IMF) sampling, and returns full
posterior probability distributions rather than simply probability
maxima. We apply our technique to 621 visually confirmed clusters in
two nearby galaxies, NGC 628 and NGC 7793, that are part of the Legacy
Extragalactic UV Survey (LEGUS). LEGUS provides Hubble Space Telescope
photometry in the NUV, U, B, V, and I bands. We analyze the
sensitivity of the derived cluster properties to choices of prior
probability distribution, evolutionary tracks, IMF, metallicity,
treatment of nebular emission, and extinction curve. We find that
slug's results for individual clusters are insensitive to most of
these choices, but that the posterior probability distributions we
derive are often quite broad, and sometimes multi-peaked and quite
sensitive to the choice of priors. In contrast, the properties of the
cluster population as a whole are relatively robust against all of
these choices. We also compare our results from slug to those derived
with a conventional non-stochastic fitting code, Yggdrasil. We show
that slug's stochastic models are generally a better fit to the
observations than the deterministic ones used by Yggdrasil. However,
the overall properties of the cluster populations recovered by both
codes are qualitatively similar.
Description:
In this paper we use slug, the Stochastically Lighting Up Galaxies
code (da Silva et al. 2012ApJ...745..145D 2012ApJ...745..145D, 2014MNRAS.444.3275D 2014MNRAS.444.3275D;
Krumholz et al. 2015MNRAS.452.1447K 2015MNRAS.452.1447K), and its post-processing tool for
analysis of star cluster properties, cluster_slug, to analyze an
initial sample of clusters from the LEGUS (Calzetti et al.
2015AJ....149...51C 2015AJ....149...51C).
A description of the steps required to produce final cluster catalogs
of the Legacy Extragalactic UV Survey (LEGUS) targets can be found in
Calzetti et al. (2015AJ....149...51C 2015AJ....149...51C), and in A. Adamo et al. (2015,
in preparation). LEGUS is an HST Cycle 21 Treasury program that is
imaging 50 nearby galaxies in five broadbands with the WFC3/UVIS, from
the NUV to the I band.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table3.dat 167 62391 *Summary marginal posterior probability
distribution functions (PDFs)
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Note on table3.dat: The full table includes 645 visually confirmed clusters, of
which 621 are unique and 24 are overlapping between NGC 7793e and NGC 7793w.
It also includes 2326 objects visually classified as unlikely to be clusters.
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See also:
B/hst : HST Archived Exposures Catalog (STScI, 2007)
J/MNRAS/445/899 : LVL SEDs and physical properties (Cook+, 2014)
J/ApJ/752/95 : PHAT stellar cluster survey. I. Year 1 (Johnson+, 2012)
J/ApJ/751/122 : Ages and masses for 920 LMC clusters (Popescu+, 2012)
J/A+A/537/A146 : Stellar models with rotation Z=0.014 (Ekstrom+, 2012)
J/MNRAS/417/L6 : Catalog of stellar cluster properties in M83 (Bastian+, 2011)
J/A+A/532/A131 : New Galactic star clusters in VVV survey (Borissova+, 2011)
J/MNRAS/405/857 : Star clusters in NGC 3256 (Goddard+, 2010)
J/ApJ/706/599 : Hα and UV SFR in the local volume (Lee+, 2009)
J/A+A/443/41 : NGC 4038/4039 broad & narrow band photometry (Mengel+, 2005)
J/AJ/129/2701 : UBVRI photometry of SMC star clusters (Rafelski+, 2005)
J/A+A/397/473 : HST photometry of M51 cluster (Bik+, 2003)
J/AJ/124/1418 : Radio-opt. identification of star clusters (Whitmore+, 2002)
J/A+AS/141/371 : Low-mass stars evolutionary tracks+isochrones (Girardi+, 2000)
http://legus.stsci.edu/ : LEGUS (HST Legacy Extragalactic UV Survey) home page
http://www.slugsps.com/ : SLUG home page
Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 11 A11 --- Field Field identifier (1)
13- 16 I4 --- ID [1/1753] ID in the LEGUS photometric catalog
18- 21 F4.2 --- Mode Visual classification, mode of classifiers (2)
23- 26 F4.2 --- Mean Visual classification, mean of classifiers (2)
28- 50 A23 --- Lib Name of the cluster_slug library (3)
52- 55 F4.1 --- beta [-2/-2] Slope of the mass prior, dN/dM∼Mbeta
57- 60 F4.1 --- gamma [-1/-0] Slope of the age prior, dN/dT∼Tgamma
62- 66 F5.3 [Msun] logM-14 [1.9/6.8] The 14th percentile of log mass
68- 72 F5.3 [Msun] logM-25 The 25th percentile of log mass
74- 78 F5.3 [Msun] logM-50 The 50th percentile of log mass
80- 84 F5.3 [Msun] logM-75 The 75th percentile of log mass
86- 90 F5.3 [Msun] logM-84 [2.3/7.1] The 84th percentile of log mass
92- 96 F5.3 [yr] logT-14 [5.2/9.7] The 14th percentile of log age
98-102 F5.3 [yr] logT-25 The 25th percentile of log age
104-109 F6.3 [yr] logT-50 The 50th percentile of log age
111-116 F6.3 [yr] logT-75 The 75th percentile of log age
118-123 F6.3 [yr] logT-84 [5.8/10.2] The 84th percentile of log age
125-129 F5.3 mag AV-14 [0.02/2.8] The 14th percentile of
V band extinction
131-135 F5.3 mag AV-25 The 25th percentile of V band extinction
137-141 F5.3 mag AV-50 The 50th percentile of V band extinction
143-147 F5.3 mag AV-75 The 75th percentile of V band extinction
149-153 F5.3 mag AV-84 [0.1/3] The 84th percentile of
V band extinction
155-160 F6.3 mag D5 [0/76] The 5th nearest neighbor distance
in library
162-167 F6.3 --- Dnorm5 [0.003/73] The 5th nearest neighbor distance
in library normalized to photometric errors
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Note (1): Field identifier as follows:
NGC_0628 = NGC0628 high confidence clusters (5439 rows for 259 unique IDs)
NGC_7793e = NGC7793 East high confidence clusters (3717 rows; 177 unique IDs)
NGC_7793w = NGC7793 West high confidence clusters (4389 rows; 209 unique IDs)
NGC0628l = NGC0628 low confidence clusters (7014 rows for 334 unique IDs)
NGC7793el = NGC7793 East low confidence clusters (9408 rows; 448 unique IDs)
NGC7793wl = NGC7793 West low confidence clusters (32424 rows;
1544 unique IDs).
Note (2): Visual classification code as follows:
0 = source was not visually classified (too faint);
1 = symmetric, compact cluster;
2 = concentrated object with some degree of asymmetry or color gradient;
3 = diffuse or multiple peak system, possibly spurious alignment;
4 = spurious detection (foreground/background source, single
bright star, artifact).
Note (3): The cluster_slug libraries name format is NNNZZZIII_EE, where
NNN is pad for Padova tracks, gen for Geneva tracks; ZZZ indicates
metallicity (020 = 0.02, 004 = 0.004, etc.), III = kroupa for Kroupa
IMF, chabrier for chabrier IMF, EE = MW for Milky Way extinction law,
SB for starburst extinction law. See Table 2 for additional details.
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History:
From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 05-Feb-2016