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Register a new userSpectroscopy of M81 Globular Clusters
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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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We perform aperture photometry and profile fitting on 419 globular cluster (GC) candidates with mV leq 23 mag identified in Hubble Space Telescope Advanced Camera for Surveys BVI imaging, and estimate the effective radii of the clusters. We identify 85 previously known spectroscopically-confirmed clusters, and newly identify 136 objects as good cluster candidates within the 3{sigma} color and size ranges defined by the spectroscopically confirmed clusters, yielding a total of 221 probable GCs. The luminosity function peak for the 221 probable GCs with estimated total dereddening applied is V ~(20.26 pm 0.13) mag, corresponding to a distance of ~3.7pm0.3 Mpc. The blue and red GC candidates, and the metal-rich (MR) and metal-poor (MP) spectroscopically confirmed clusters, are similar in half-light radius, respectively. Red confirmed clusters are about 6% larger in median half-light radius than blue confirmed clusters, and red and blue good GC candidates are nearly identical in half-light radius. The total population of confirmed and good candidates shows an increase in half-light radius as a function of galactocentric distance.
We here present the ages of four compact stellar clusters (CSCs) in the nearby spiral galaxy M81, using long-slit optical spectra obtained with the 10.4-m Gran Telescopio Canarias (GTC). All the four CSCs, including the brightest in this galaxy, are found to have ages between 5 to 6 Myr, with one of them showing Wolf-Rayet spectral features. The photometric masses of these clusters, calculated using their spectroscopically-derived ages, lie between 3000 and 18000 Msun. The observed clusters are among the brightest objects, and hence the most massive, in the entire disk of M81. This implies the absence of massive (1.0e5 Msun) compact stellar clusters in M81.
Integrated light (IL) spectroscopy enables studies of stellar populations beyond the Milky Way and its nearest satellites. In this paper, I will review how IL spectroscopy reveals essential information about globular clusters and the assembly histories of their host galaxies, concentrating particularly on the metallicities and detailed chemical abundances of the GCs in M31. I will also briefly mention the effects of multiple populations on IL spectra, and how observations of distant globular clusters help constrain the source(s) of light-element abundance variations. I will end with future perspectives, emphasizing how IL spectroscopy can bridge the gap between Galactic and extragalactic astronomy.
In this paper, we presented metal abundance properties of 144 M81 globular clusters. These globulars consist of the largest globular cluster sample in M81 till now. Our main results are: the distribution of metallicities are bimodal, with metallicity peaks at [Fe/H]sim-1.51 and -0.58, and the metal-poor globular clusters tend to be less spatially concentrated than the metal-rich ones; the metal-rich globular clusters in M81 do not demonstrate a centrally concentrated spatial distribution as the metal-rich ones in M31 do; like our Galaxy and M31, the globular clusters in M81 have a small radial metallicity gradient. These results are consistent with those obtained based on a small sample of M81 globular clusters. In addition, this paper showed that there is evidence that a strong rotation of the M81 globular cluster system around the minor axis exists, and that rotation is present in the metal-rich globular cluster subsample, while the metal-poor globular cluster subsample shows no evidence for rotation. The most significant difference between the rotation of the metal-rich and metal-poor globular clusters occurs at intermediate projected galactocentric radii. The results of this paper confirm the conclusion of Schroder et al. that M81s metal-rich globular clusters at intermediate projected radii were associated with a thick disk of M81.
We analyse the photometric, chemical, star formation history and structural properties of the brightest globular cluster (GC) in M81, referred as GC1 in this work, with the intention of establishing its nature and origin. We find that it is a metal-rich ([Fe/H]=-0.60+/-0.10), alpha-enhanced ([Alpha/Fe]=0.20+/0.05), core-collapsed (core radius r_c=1.2 pc, tidal radius r_t = 76r_c), old (>13 Gyr) cluster. It has an ultraviolet excess equivalent of ~2500 blue horizontal branch stars. It is detected in X-rays indicative of the presence of low-mass binaries. With a mass of 10 million solar masses, the cluster is comparable in mass to M31-G1 and is four times more massive than Omega Cen. The values of r_c, absolute magnitude and mean surface brightness of GC1 suggest that it could be, like massive GCs in other giant galaxies, the left-over nucleus of a dissolved dwarf galaxy.
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