The LAMOST spectroscopic survey of globular clusters in M31and M33.I.catalog and new identifications

We present a catalog of 908 objects observed with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope（LAMOST） in fields in the vicinity of M31 and M33, targeted as globular clusters（GCs） and candidates. The targets include known GCs and candidates selected from the literature, as well as new candidates selected from the Sloan Digital Sky Survey（SDSS）. Analysis shows that 356 of them are likely GCs with various confidence levels, while the remaining ones turn out to be background galaxies and quasars, stars and H II regions in M31 or foreground Galactic stars. The 356 likely GCs include 298 bona fide GCs and 26 candidates known in the literature. Three candidates, selected from the Revised Bologna Catalog of M31 GCs and candidates（RBC） and one possible cluster from Johnson et al., are confirmed to be bona fide clusters. We search for new GCs in the halo of the M31 among the new candidates selected from the SDSS photometry. Based on radial velocities yielded by LAMOST spectra and visual examination of the SDSS images, we find 28 objects, 5bona fide and 23 likely GCs. Among the five bona fide GCs, three have been recently discovered independently by others, and the remaining 25 are our new identifications,including two bona fide ones. The newly identified objects fall at projected distances ranging from 13 to 265 kpc from M31. Of the two newly discovered bona fide GCs,one is located near M33, probably a GC belonging to M33. The other bona fide GC falls on the Giant Stream with a projected distance of 78 kpc from M31. Of the 23 newly identified likely GCs, one has a projected distance of about 265 kpc from M31 and could be an intergalactic cluster.

Age and Mass Estimates for 41 Star Clusters in M33

In this second paper of our series, we estimate the age of 41 star clusters, which were detected by Melnick & D'odorico in the nearby spiral galaxy M33, by comparing the integrated photometric measurements with theoretical stellar population synthesis models of Bruzual & Charlot. Also, we calculate the mass of these star clusters using the theoretical M/LV ratio. The results show that, these star clusters formed continuously in M33 from ~ 7 × 106–1010 years and have masses between ~ 103 and 2 × 106M☉. M33 frames were observed as a part of the BATC Multicolor Survey of the sky in 13 intermediate-band filters from 3800 to 10 000?. The relation between age and mass confirms that the sample star cluster masses systematically decrease from the oldest to the youngest.

Estimating the Metallicity of Old Star Clusters in M33

Based on the method proposed by Kong et al. and on the multi-color spectrophotometry by Ma et al., we estimate the metallicities of 16 old star clusters in the nearby spiral galaxy M33, ten of which are halo globular clusters. These old clusters are all metal poor, the range of metallicity ([Fe/H]) is from-0.14 to-2.12. In general, our results are consistent with those derived by other methods, such as integrated spectra and photometry, and our study confirms the reliability of the method of Kong et al.

Positions and Spectral Energy Distributions of 41 Star Clusters in M33

We present accurate positions and multi-color photometry for 41 star clusters detected by Melnick & D'odorico in the nearby spiral galaxy M33 as a part of the BATC Color Survey of the sky in 13 intermediate-band filters from 3800 to 10 000 A. The coordinates of the clusters are found from the HST Guide Star Catalog. By aperture photometry, we obtain the spectral energy distributions of the clusters. Using the relations between the BATC intermediate-band system and UBVRI broadband system, we derive their V magnitudes and B - V colors and find that most of them are blue, which is consistent with previous findings.

Multicolor photometric study of M31 globular clusters

We present the photometry of 30 globular clusters （GCs） and GC candidates in 15 intermediate-band filters covering the wavelength region from - 3000 to 10000A using the archival CCD images of M31 observed as part of the Beijing - Arizona - Taiwan - Connecticut （BATC） Multicolor Sky Survey. We transform these intermediate-band photometric data into the photometry in the standard U BV RI broadbands. These M31 GC candidates are selected from the Revised Bologna Catalog （RBC V.3.5）, and most of these candidates do not have any photometric data. Therefore, the presented photometric data are a supplement to the RBC V.3.5. We find that 4 out of 61 GCs and GC candidates in RBC V.3.5 do not show any signal on the BATC images at their locations. By applying a linear fit of the distribution in the color-magnitude diagram of blue GCs and GC candidates using data from the RBC V.3.5, in this study, we find the ＂blue-tilt＂ of blue M31 GCs with a high confidence at 99.95% or 3.47cr for the confirmed GCs, and 〉 99.99% or 4.87e for GCs and GC candidates.

2MASS photometry and age estimation of the globular clusters in the outer halo of M31

We present the first photometric results in J, H and Ks from 2MASS （Two Micron All Sky Survey） imaging of 10 classical globular clusters （GCs） in the far outer regions of M31. Combined with the V and I photometric data from previous literature, we constructed a color-color diagram between J - Ks and V - Ⅰ. By comparing the integrated photometric measurements with evolutionary models, we estimate the ages of these clusters. The results showed that all of these clusters are older than 3 × 109 yr, and among them four are older than 10 Gyr and the other six have intermediate ages between 3 - 8 Gyr. The masses for these outer-halo GCs are from 7.0 × 104 M to 1.02 × 10^6 M. We argued that GC2 and GC3, whose ages, metallicities and distance moduli are almost the same, were accreted from the same satellite galaxy, if they did not form in situ. The statistical results show that the ages and metallicities of these 10 M31 outer-halo GCs do not vary with projected radial position, and a relationship between age and metallicity does not exist.

Spectroscopic study of globular clusters in the halo of M31 with the Xinglong 2.16 m telescope Ⅱ: dynamics, metallicity and age

In Paper I, we performed spectroscopic observations on 11 confirmed glob- ular clusters （GCs） in M31 with the Xinglong 2.16 m telescope. We mainly focused on the fitting method and the metallicity gradient for the M31 GC sample. Here, we analyze and further discuss the dynamics, metallicity and age, and their distributions, as well as the relationships between these parameters. In our work, eight more con- firmed GCs in the halo of M31 were observed, most of which lack previous spectro- scopic information. These star clusters are located far from the galactic center at a projected radius of ~ 14 to ~ 117 kpc, which is more spatially extended than that in the previous work. Firstly, we measured the Lick absorption-line indices and the ra- dial velocities. Then the ages and metallicity values of [Fe/H] and [a/Fe] were fitted by comparing the observed spectral feature indices and the Single Stellar Population model of Thomas et al. in the Cassisi and Padova stellar evolutionary tracks, respec- tively. Our results show that most of the star clusters in our sample are older than 10 Gyr except B290, which is ~ 5.5 Gyr, and most of them are metal-poor with metallicity [Fe/H] 〈 - 1, suggesting that these clusters were born at the early stage of the galaxy＇s formation. We find that the metallicity gradient for the outer halo clusters with rp 〉 25 kpc may have an insignificant slope of -0.005 ＋ 0.005 dex kpc-1 and if the outliers G001 and H11 are excluded, the slope does not change significantly, with a value of -0.002 ~ 0.003 dex kpc-1. We also find that the metallicity is not a function of age for the GCs with age 〈 7 Gyr, but for the old GCs with age 〉 7 Gyr, there seems to be a trend that the older ones have lower metallicity. Additionally, we plot metallicity distributions with the largest sample of M31 GCs so far and show the bimodality is not significant, and the number of metal-poor and metal-rich groups be- comes comparable. The spatial distributions show that the metal-rich group is more centrally concentrated but the metal-poor group occupies a more extended halo. In ad- dition, the young population is centrally concentrated but the old population is more spatially extended towards the outer halo.

Old stellar population synthesis:new age and mass estimates for Mayall II = G1

Mayall Ⅱ = G1 is one of the most luminous globular clusters （GCs） in M31. Here, we determine its age and mass by comparing multicolor photometry with theoretical stellar population synthesis models. Based on far- and near-ultraviolet GALEX photometry, broad-band UBVRI, and infrared JHKs 2MASS data, we construct the most extensive spectral energy distribution of G 1 to date, spanning the wavelength range from 1538 to 20 000A. A quantitative comparison with a variety of simple stellar population （SSP） models yields a mean age which is consistent with G1 being among the oldest building blocks of M31 and having formed within ～1.7 Gyr after the Big Bang. Irrespective of the SSP model or stellar initial mass function adopted, the resulting mass estimates （of order 10^7M⊙） indicate that GI is one of the most massive GCs in the Local Group. However, we speculate that the cluster＇s exceptionally high mass suggests that it may not be a genuine GC. Our results also suggest that G1 may contain, on average, （1.65±0.63） × 10^2L⊙ far-ultraviolet-bright, hot, extreme horizontal-branch stars, depending on the adopted SSP model. In addition, we demonstrate that extensive multi-passband photometry coupled with SSP analysis enables one to obtain age estimates for old SSPs that have similar accuracies as those from integrated spectroscopy or resolved stellar photometry, provided that some of the free parameters can be constrained independently.