J/AJ/152/45 LAMOST survey of star clusters in M31. II. (Chen+, 2016)
The LAMOST spectroscopic survey of star clusters in M31. II. Metallicities,
ages, and masses.
Chen B., Liu X., Xiang M., Yuan H., Huang Y., Shi J., Fan Z., Huo Z.,
Wang C., Ren J., Tian Z., Zhang H., Liu G., Cao Z., Zhang Y., Hou Y.,
Wang Y.
<Astron. J., 152, 45 (2016)>
=2016AJ....152...45C 2016AJ....152...45C (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, nearby ; Associations, stellar ; Clusters, globular ;
Photometry, ugriz ; Reddening ; Abundances, [Fe/H]
Keywords: galaxies: star clusters: general -
galaxies: star clusters: individual (M31)
Abstract:
We select from Paper I a sample of 306 massive star clusters observed
with the Large Sky Area Multi-Object Fibre Spectroscopic Telescope
(LAMOST) in the vicinity fields of M31 and M33, and determine their
metallicities, ages, and masses. Metallicities and ages are estimated
by fitting the observed integrated spectra with stellar synthesis
population (SSP) models with a pixel-to-pixel spectral fitting
technique. Ages for most young clusters are also derived by fitting
the multi-band photometric measurements with model spectral energy
distributions (SEDs). The estimated cluster ages span a wide range,
from several million years to the age of the universe. The numbers of
clusters younger and older than 1Gyr are, respectively, 46 and 260.
With ages and metallicities determined, cluster masses are then
estimated by comparing the multi-band photometric measurements with
SSP model SEDs. The derived masses range from ∼103 to
∼107M☉, peaking at ∼104.3 and ∼105.7M☉ for young
(<1Gyr) and old (>1Gyr) clusters, respectively. Our estimated
metallicities, ages, and masses are in good agreement with available
literature values. Old clusters richer than [Fe/H]~-0.7dex have a wide
range of ages. Those poorer than [Fe/H]~-0.7dex seem to be composed of
two groups, as previously found for Galactic globular clusters-one of
the oldest ages with all values of metallicity down to ~-2dex and
another with metallicity increasing with decreasing age. The old
clusters in the inner disk of M31 (0-30kpc) show a clear metallicity
gradient measured at -0.038±0.023dex/kpc.
Description:
We select a sample of 306 massive star clusters observed with the
Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) in
the vicinity fields of M31 and M33.
Massive clusters in our sample are all selected from the catalog
presented in Paper I (Chen et al. 2015, Cat. J/other/RAA/15.1392),
including five newly discovered clusters selected with the SDSS
photometry, three newly confirmed, and 298 previously known clusters
from Revised Bologna Catalogue (RBC; Galleti et al. 2012, Cat. V/143;
http://www.bo.astro.it/M31/). Since then another two objects, B341 and
B207, have also been observed with LAMOST, and they are included in
the current analysis. The current sample does not include those listed
in Paper I but is selected from Johnson et al. 2012 (Cat.
J/ApJ/752/95) since most of them are young but not so massive. All
objects are observed with LAMOST between 2011 September and 2014 June.
Table1 lists the name, position, and radial velocity of all sample
clusters analyzed in the current work.
The LAMOST spectra cover the wavelength range 3700-9000Å at a
resolving power of R∼1800. Details about the observations and data
reduction can be found in Paper I. The median signal-to-noise ratio
(S/N) per pixel at 4750 and 7450Å of spectra of all clusters in the
current sample are, respectively, 14 and 37. Essentially all spectra
have S/N(4750Å)>5 except for the spectra of 18 clusters. The latter
have S/N(7540Å)>10.
Peacock et al. 2010 (Cat. J/MNRAS/402/803) retrieved images of M31
star clusters and candidates from the SDSS archive and extracted ugriz
aperture photometric magnitudes from those objects using the
SExtractor. They present a catalog containing homogeneous ugriz
photometry of 572 star clusters and 373 candidates. Among them, 299
clusters are in our sample.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 97 306 Optical photometry and reddening of massive star
clusters observed with LAMOST
table3.dat 113 306 Derived properties of star clusters in our sample
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See also:
V/143 : Revised Bologna Catalog of M31 clusters, V.5 (Galleti+ 2012)
III/251 : ELODIE library V3.1 (Prugniel+ 2007)
J/other/RAA/15.1392 : LAMOST globular clusters in M 31 and M 33 (Chen+, 2015)
J/ApJS/199/37 : GALEX catalog of star clusters in M31 (Kang+, 2012)
J/ApJ/752/95 : PHAT stellar cluster survey. I. Year 1 (Johnson+, 2012)
J/AJ/139/1438 : SED and age estimates of 104 M31 GCs (Wang+, 2010)
J/ApJ/725/200 : Updated cat. of M31 globular-like clusters (Fan+, 2010)
J/MNRAS/404/1639 : MILES models & new line index system (Vazdekis+, 2010)
J/MNRAS/402/803 : M31 globular cluster system (Peacock+, 2010)
J/A+A/508/1285 : Metallicities of M31 globular clusters (Galleti+, 2009)
J/MNRAS/374/664 : Stellar parameters in MILES library (Cenarro+, 2007)
J/MNRAS/371/703 : MILES library of spectra (Sanchez-Blazquez+, 2006)
J/AJ/123/2490 : RV and [Fe/H] of M31 globular clusters (Perrett+, 2002)
J/AJ/119/727 : M31 globular clusters photometry (Barmby+, 2000)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 10 A10 --- Name Source identifier
12- 19 F8.5 deg RAdeg Right Ascension in decimal degrees (J2000)
21- 28 F8.5 deg DEdeg Declination in decimal degrees (J2000)
30- 33 I4 km/s RV [-725/141] Radial velocity
35- 37 I3 km/s e_RV [1/150]?=9.99 Uncertainty in RV
39- 43 F5.2 mag umag [15.48/21]?=99.99 u band magnitude (1)
45- 48 F4.2 mag e_umag [0.02/0.31]?=9.99 Uncertainty in umag
50- 54 F5.2 mag gmag [14.5/19.45]?=99.99 g band magnitude (1)
56- 59 F4.2 mag e_gmag [0/0.2]?=9.99 Uncertainty in gmag
61- 65 F5.2 mag rmag [13.56/18.55]?=99.99 r band magnitude (1)
67- 70 F4.2 mag e_rmag [0/0.11]?=9.99 Uncertainty in rmag
72- 76 F5.2 mag imag [13.14/18.52]?=99.99 i band magnitude (1)
78- 81 F4.2 mag e_imag [0/0.11]?=9.99 Uncertainty in imag
83- 87 F5.2 mag zmag [12.89/18.71]?=99.99 z band magnitude (1)
89- 92 F4.2 mag e_zmag [0.01/0.21]?=9.99 Uncertainty in zmag
94- 97 F4.2 mag E(B-V) [0.01/1.3] The (B-V) color excess (2)
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Note (1): Peacock et al. 2010 (Cat. J/MNRAS/402/803) retrieved images of M31
star clusters and candidates from the SDSS archive and extracted ugriz
aperture photometric magnitudes from those objects using the SExtractor.
They present a catalog containing homogeneous ugriz photometry of 572 star
clusters and 373 candidates. Among them, 299 clusters are in our sample. Of
those, 280, 289, 289, 287, and 285 are detected in u, g, r, i, and z bands,
respectively. Six objects, including the five newly discovered clusters
reported in Paper I (LAMOST-1-5) and MGC1 (Huxor et al.,
2008MNRAS.385.1989H 2008MNRAS.385.1989H) are not found in Peacock et al. 2010
(Cat. J/MNRAS/402/803) but have been detected by SDSS. For the six objects
we adopt the Petrosian magnitudes available from the SDSS archive.
Note (2): The reddening values are taken from Kang et al. 2012
(Cat. J/ApJS/199/37), compiled from three sources: (1) Determinations
available from the literature (Barmby et al. 2000, Cat. J/AJ/119/727; Fan
et al., 2008MNRAS.385.1973F 2008MNRAS.385.1973F; Caldwell et al. 2009, Cat. J/AJ/137/94;
Caldwell et al. 2011, Cat. J/AJ/141/61). If more than one determinations
are available then the average value is adopted; (2) For clusters that have
no reddening value determinations in the literature, the median value of
clusters within a 2kpc radius of the cluster of concern is adopted; and (3)
For star clusters that fall beyond a galactocentric distance of 22kpc, a
foreground reddening value of E(B-V)=0.13mag is adopted. Some of our sample
clusters are not available in the compilation of Kang et al. 2012
(Cat. J/ApJS/199/37). For those objects we calculate their reddening values
following approaches (2) and (3) above.
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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 --- Name Star cluster identifier
12- 17 F6.2 [Sun] [Fe/H]P [-2.3/0.7]?=-99.99 Metallicity from full
spectral fitting PEGASE-HR model (3)
18 A1 --- f_[Fe/H]P [:] Flag ":" on [Fe/H]P indicates that the
cluster suffers from the degeneracy
20- 23 F4.2 [Sun] e_[Fe/H]P [0.01/0.57]?=9.99 Uncertainty in [Fe/H]P
25- 30 F6.2 [Sun] [Fe/H]V [-2.32/0.22]?=-99.99 Metallicity from full
spectral fitting with the model of Vazdekis
et al. 2010 (Cat. J/MNRAS/404/1639) model (4)
32- 35 F4.2 [Sun] e_[Fe/H]V [0.01/0.87]?=9.99 Uncertainty in [Fe/H]V
37- 42 F6.2 [Sun] [Fe/H]G [-8.21/0.19]?=-99.99 Metallicity from the
[MgFe] index using the relation of Galleti et
al. 2009 (Cat. J/A+A/508/1285) (5)
44- 49 F6.2 [Sun] [Fe/H]C [-2.47/0.07]?=-99.99 Metallicity from the <Fe>
index using the relation of Caldwell et al.
2011 (Cat. J/AJ/141/61) (5)
51- 56 F6.2 [Sun] [Fe/H]E [-1.28/0.08]?=-99.99 Metallicity from the
EZ_Ages package (6)
58- 63 F6.2 [yr] logAP [9/10.3]?=-99.99 Log age from full spectral
fitting with the PEGASE-HR model (3)
65- 68 F4.2 [yr] e_logAP [0.01/0.65]?=9.99 Uncertainty in logAP
70- 75 F6.2 [yr] logAV [9/10.25]?=-99.99 Log age from full spectral
fitting with Vazdekis et al. 2010
(Cat. J/MNRAS/404/1639) model (4)
77- 80 F4.2 [yr] e_logAV [0.01/0.9]?=9.99 Uncertainty in logAV
82- 87 F6.2 [yr] logAS [6.4/9]?=-99.99 Log age from SED fitting (7)
89- 92 F4.2 [yr] e_logAS [0.14/0.69]?=9.99 Uncertainty in logAS
94- 99 F6.2 [yr] logAE [9/10.2]?=-99.99 Log age from EZ_Ages
package (6)
101 I1 --- f_logM [1/2] Flag for note on logM (1 or 2) (8)
103-108 F6.2 [Msun] logM [2.61/7.8]?=-99.99 Log estimated mass (9)
110-113 F4.2 [Msun] e_logM [0/0.51]?=9.99 Uncertainty in logM
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Note (3): Le Borgne et al. (2004A&A...425..881L 2004A&A...425..881L) present Stellar Synthesis
Population (SSP) models PEGASE-HR (http://www2.iap.fr/pegase/pegasehr/)
covering the wavelength range 4000-6800Å at a resolution of
FWHM∼0.55Å. The models are constructed using the empirical spectral
library ELODIE (Prugniel & Soubiran 2001, Cat. III/218; Prugniel et al.
2007, Cat. III/251). ELODIE spectra were collected using an echelle
spectrograph with a very high spectral resolution (R∼42000). See Sect.3.1
for further details about metallicities from full spectral fitting and
Section4.1 for details about ages from LAMOST spectra.
Note (4): Vazdekis et al. 2010 (Cat. J/MNRAS/404/1639) present Stellar Synthesis
Population (SSP) models (http://miles.iac.es/pages/ssp-models.php) covering
an optical wavelength range of 3540.5-7409.6Å at a nominal resolution of
full width at half-maximum (FWHM) of 2.3Å. The models are based on the
empirical stellar spectral library, Medium resolution INT Library of
Empirical Spectra (MILES, Sanchez-Blazquez et al. 2006,
Cat. J/MNRAS/371/703; Cenarro et al. 2007, Cat. J/MNRAS/374/664). Please
refer to Section 3.1 in the paper for additional details about
metallicities from full spectral fitting and Section 4.1 for details about
ages from LAMOST spectra.
Note (5): See Section 3.2 for more details.
Note (6): Ages and metallicities of clusters in M31 can also be determined from
the Lick indices using code EZ_Ages, developed by Graves & Schiavon
(2008ApJS..177..446G 2008ApJS..177..446G) for automatic stellar population analysis. See
Section 3.2 for further details.
Note (7): The ages of young clusters can be estimated by the SED fitting of
multi-band photometry (Fan et al. 2010, Cat. J/ApJ/725/200; Wang et al.
2010, Cat. J/AJ/139/1438; Kang et al. 2012, Cat. J/ApJS/199/37, and
references therein). See Section 4.2 for more details.
Note (8): Flag defined as below:
1 = Mass estimated using the PEGASE-HR age;
2 = Mass estimated using age from SED fitting.
Note (9): Once the age and metallicity of a star cluster have been determined,
its mass can be estimated by comparing the photometric measurements with
the Stellar Synthesis Population (SSP) models. In the current work we use
the SDSS optical ugriz photometry and the Bruzual and Charlot
(2003MNRAS.344.1000B 2003MNRAS.344.1000B) models for this purpose. See Sect.5 in the paper for
more details.
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History:
From electronic version of the journal
References:
Chen et al., Paper I 2015RAA....15.1392C 2015RAA....15.1392C, Cat. J/other/RAA/15.1392
(End) Prepared by [AAS]; Sylvain Guehenneux [CDS] 15-Sep-2016