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Register a new userHubble Space Telescope Observations of Globular Clusters in M31 II: Structural Parameters
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C.J. Grillmair
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We analyze post-refurbishment Hubble Space Telescope images of four globular clusters in M31. The ability to resolve stars to below the horizontal branch permits us to use star counts to extend the surface brightness profiles determined using aperture photometry to almost 5 orders of magnitude below the central surface density. Three of the resulting cluster profiles are reasonably well-fit using single-mass King models, with core and tidal radii typical of those seen in Galactic globular clusters. We confirm an earlier report of the discovery of a cluster which has apparently undergone core collapse. Three of the four clusters show departures in their outskirts from King model behavior which, based on recent results for Galactic globulars, may indicate the presence of tidal tails.
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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 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.
Mayall II = G1 is one of the most luminous globular clusters (GCs) known in M31. New deep, high-resolution observations with the Advanced Camera for Surveys on the {sl Hubble Space Telescope} are used to provide accurate photometric data to the smallest radii yet. In particular, we present the precise variation of ellipticity and position angle, and of surface brightness for the core of the object. Based on these accurate photometric data, we redetermine the structural parameters of G1 by fitting a single-mass isotropic King model. We derive a core radius, $r_c=0.21pm0.01arcsec (=0.78pm0.04 rm{pc})$, a tidal radius, $r_t=21.8pm1.1arcsec (=80.7pm3.9 rm{pc})$, and a concentration index $c=log (r_t/r_c)=2.01pm0.02$. The central surface brightness is 13.510 mag arcsec$^{-2}$. We also calculate the half-light radius, at $r_h=1.73pm0.07arcsec(=6.5pm0.3 rm{pc})$. The results show that, within 10 core radii, a King model fits the surface brightness distribution well. We find that this object falls in the same region of the $M_V$ vs. $log R_h$ diagram as $omega$ Centauri, M54 and NGC 2419 in the Milky Way. All three of these objects have been claimed to be the stripped cores of now defunct dwarf galaxies. We discuss in detail whether GCs, stripped cores of dwarf spheroidals and normal dwarf galaxies form a continuous distribution in the $M_V$ versus $log R_h$ plane, or if GCs and dwarf spheroidals constitute distinct classes of objects; we present arguments in favour of this latter view.
With the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope, we have discovered in M4 (NGC 6121, C 1620-264) the first extensive sequence of cooling white dwarfs seen in a globular cluster. Adopting a distance modulus of (m-M)_V = 12.65 and a reddening of E(B-V) = 0.37, we show that the sequence, which extends over 9 < M_U < 13, is comprised of white dwarfs of mass sim 0.5 M_{odot}. The total mass loss from the present turnoff to the white dwarf sequence is 0.31 M_{odot} and the intrinsic dispersion in the mean mass appears to be < 0.05 M_{odot}. Both the location of the white dwarf cooling sequence in the cluster color-magnitude diagram and the cumulative luminosity function attest to the basic correctness and completeness of the physics in theoretical models for the upper three magnitudes of the observed white dwarf cooling sequence. To test the theory in globular clusters at cooling ages beyond sim 3 times 10^8 years will require deeper and more complete data.
The giant elliptical galaxy M87 has been imaged over 30 consecutive days in 2001, 60 consecutive days in 2005-2006, and every 5 days over a 265 day span in 2016-2017 with the Hubble Space Telescope, leading to the detection of 137 classical novae throughout M87. We have identified 2134 globular clusters (GC) in M87 in these images, and carried out searches of the clusters for classical novae erupting in or near them. One GC CN was detected in the 2001 data, while zero novae were found during the 2005-2006 observations. Four candidate GC novae were (barely) detected in visible light during the 2016-2017 observations, but none of the four were seen in near-ultraviolet light, leading us to reject them. Combining these results with our detection of one M87 GC nova out of a total of 137 detected CN, we conclude that such novae may be overabundant relative to the field, but small number statistics dominate this (and all other) searches. A definitive determination of GC CN overabundance (or not) will require much larger samples which LSST should provide in the coming decade.
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