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Register a new userAges and structural and dynamical parameters of two globular clusters in the M81 group
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GC-1 and GC-2 are two globular clusters (GCs) in the remote halo of M81 and M82 in the M81 group discovered by Jang et al. using the {it Hubble Space Telescope} ({it HST}) images. These two GCs were observed as part of the Beijing--Arizona--Taiwan--Connecticut (BATC) Multicolor Sky Survey, using 14 intermediate-band filters covering a wavelength range of 4000--10000 AA. We accurately determine these two clusters ages and masses by comparing their spectral energy distributions (from 2267 to 20000~{AA}, comprising photometric data in the near-ultraviolet of the {it Galaxy Evolution Explorer}, 14 BATC intermediate-band, and Two Micron All Sky Survey near-infrared $JHK_{rm s}$ filters) with theoretical stellar population-synthesis models, resulting in ages of $15.50pm3.20$ for GC-1 and $15.10pm2.70$ Gyr for GC-2. The masses of GC-1 and GC-2 obtained here are $1.77-2.04times 10^6$ and $5.20-7.11times 10^6 rm~M_odot$, respectively. In addition, the deep observations with the Advanced Camera for Surveys and Wide Field Camera 3 on the {it HST} are used to provide the surface brightness profiles of GC-1 and GC-2. The structural and dynamical parameters are derived from fitting the profiles to three different models; in particular, the internal velocity dispersions of GC-1 and GC-2 are derived, which can be compared with ones obtained based on spectral observations in the future. For the first time, in this paper, the $r_h$ versus $M_V$ diagram shows that GC-2 is an ultra-compact dwarf in the M81 group.
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Using high-dispersion spectra from the HIRES echelle spectrograph on the Keck I telescope, we measure velocity dispersions for 4 globular clusters in M33. Combining the velocity dispersions with integrated photometry and structural parameters derived from King-Michie model fits to WFPC2 images, we obtain mass-to-light ratios for the clusters. The mean value is M/LV = 1.53 +/- 0.18, very similar to the M/LV of Milky Way and M31 globular clusters. The M33 clusters also fit very well onto the fundamental plane and binding energy - luminosity relations derived for Milky Way GCs. Dynamically and structurally, the four M33 clusters studied here appear virtually identical to Milky Way and M31 GCs.
In this paper, we present the properties of 10 halo globular clusters with luminosities $Lsimeq 5-7times 10^5{L_odot}$ in the Local Group galaxy M33 using the images of {it Hubble Space Telescope} Wide Field Planetary Camera 2 in the F555W and F814W bands. We obtained ellipticities, position angles and surface brightness profiles for them. In general, the ellipticities of M33 sample clusters are similar to those of M31 clusters. The structural and dynamical parameters are derived by fitting the profiles to three different models combined with mass-to-light ratios ($M/L$ values) from population-synthesis models. The structural parameters include core radii, concentration, half-light radii {bf and} central surface brightness. The dynamical parameters include the integrated cluster mass, integrated binding energy, central surface mass density {bf and} predicted line-of-sight velocity dispersion at the cluster center. The velocity dispersions of four clusters predicted here agree well with the observed dispersions by Larsen et al. The results here showed that the majority of the sample halo globular clusters are well fitted by King model as well as by Wilson model, and better than by Sersic model. In general, the properties of clusters in M33, M31 and the Milky Way fall in the same regions of parameter spaces. The tight correlations of cluster properties indicate a fundamental plane for clusters, which reflects some universal physical conditions and processes operating at the epoch of cluster formation.
We used a proper combination of high-resolution HST observations and wide-field ground based data to derive the radial star density profile of 26 Galactic globular clusters from resolved star counts (which can be all freely downloaded on-line). With respect to surface brightness (SB) profiles (which can be biased by the presence of sparse, bright stars), star counts are considered to be the most robust and reliable tool to derive cluster structural parameters. For each system a detailed comparison with both King and Wilson models has been performed and the most relevant best-fit parameters have been obtained. This is the largest homogeneous catalog collected so far of star count profiles and structural parameters derived therefrom. The analysis of the data of our catalog has shown that: (1) the presence of the central cusps previously detected in the SB profiles of NGC 1851, M13 and M62 is not confirmed; (2) the majority of clusters in our sample are fitted equally well by the King and the Wilson models; (3) we confirm the known relationship between cluster size (as measured by the effective radius) and galactocentric distances; (4) the ratio between the core and the effective radii shows a bimodal distribution, with a peak at ~ 0.3 for about 80% of the clusters, and a secondary peak at ~ 0.6 for the remaining 20%. Interestingly, the main peak turns out to be in agreement with what expected from simulations of cluster dynamical evolution and the ratio between these two radii well correlates with an empirical dynamical age indicator recently defined from the observed shape of blue straggler star radial distribution, thus suggesting that no exotic mechanisms of energy generation are needed in the cores of the analyzed clusters.
In this paper, we present surface brightness profiles for 79 globular clusters in M31, using images observed with {it Hubble Space Telescope}, some of which are from new observations. The structural and dynamical parameters are derived from fitting the profiles to several different models for the first time. The results show that in the majority of cases, King models fit the M31 clusters as well as Wilson models, and better than S{e}rsic models. However, there are 11 clusters best fitted by S{e}rsic models with the S{e}rsic index $n>2$, meaning that they have cuspy central density profiles. These clusters may be the well-known core-collapsed candidates. There is a bimodality in the size distribution of M31 clusters at large radii, which is different from their Galactic counterparts. In general, the properties of clusters in M31 and the Milky Way fall in the same regions of parameter spaces. The tight correlations of cluster properties indicate a fundamental plane for clusters, which reflects some universal physical conditions and processes operating at the epoch of cluster formation.
We obtained spectra of 74 globular clusters in M81. These globular clusters had been identified as candidates in an HST ACS I-band survey. 68 of these 74 clusters lie within 7 of the M81 nucleus. 62 of these clusters are newly spectroscopically confirmed, more than doubling the number of confirmed M81 GCs from 46 to 108. We determined metallicities for our 74 observed clusters using an empirical calibration based on Milky Way globular clusters. We combined our results with 34 M81 globular cluster velocities and 33 metallicities from the literature and analyzed the kinematics and metallicity of the M81 globular cluster system. The mean of the total sample of 107 metallicities is -1.06 +/- 0.07, higher than either M31 or the Milky Way. We suspect this high mean metallicity is due to an overrepresentation of metal-rich clusters in our sample created by the spatial limits of the HST I-band survey. The metallicity distribution shows marginal evidence for bimodality, with metal-rich and metal-poor peaks approximately matching those of M31 and the Milky Way. The GC system as a whole, and the metal-poor GCs alone, show evidence of a radial metallicity gradient. The M81 globular cluster system as a whole shows strong evidence of rotation, with V_r(deprojected) = 108 +/- 22 km/s overall. This result is likely biased toward high rotational velocity due to overrepresentation of metal-rich, inner clusters. The rotation patterns among globular cluster subpopulations are roughly similar to those of the Milky Way: clusters at small projected radii and metal-rich clusters rotate strongly, while clusters at large projected radii and metal-poor clusters show weaker evidence of rotation.
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