arXiv subjects

A. D. Mackey

Publications and source records attributed to A. D. Mackey.

67 records · Page 4Linked to original sources

arXiv2006

Stellar kinematics and metallicities in the Leo I dwarf spheroidal galaxy -- wide field implications for galactic evolution

We present low-resolution spectroscopy of 120 red giants in the Galactic satellite dwarf spheroidal (dSph) Leo I, obtained with the GeminiN-GMOS and Keck-DEIMOS spectrographs. We find stars with velocities consistent with membership of Leo I out to 1.3 King tidal radii. By measuring accurate radial velocities with a median measurement error of 4.6 km/s we find a mean systemic velocity of 284.2 km/s with a global velocity dispersion of 9.9 km/s. The dispersion profile is consistent with being flat out to the last data point. We show that a marginally-significant rise in the radial dispersion profile at a radius of 3' is not associated with any real localized kinematical substructure. Given its large distance from the Galaxy, tides are not likely to have affected the velocity dispersion, a statement we support from a quantitative kinematical analysis, as we observationally reject the occurrence of a significant apparent rotational signal or an asymmetric velocity distribution. Mass determinations adopting both isotropic stellar velocity dispersions and more general models yield a M/L ratio of 24, which is consistent with the presence of a significant dark halo with a mass of about 3x10^7 M_sun, in which the luminous component is embedded. This suggests that Leo I exhibits dark matter properties similar to those of other dSphs in the Local Group. Our data allowed us also to determine metallicities for 58 of the targets. We find a mildly metal poor mean of -1.31 dex and a full spread covering 1 dex. In contrast to the majority of dSphs, Leo I appears to show no radial gradient in its metallicities, which points to a negligible role of external influences in this galaxy's evolution.

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arXiv2006

ACS photometry of extended, luminous globular clusters in the outskirts of M31

A new population of extended, luminous globular clusters has recently been discovered in the outskirts of M31. These objects have luminosities typical of classical globular clusters, but much larger half-light radii. We report the first results from deep ACS imaging of four such clusters, one of which is a newly-discovered example lying at a projected distance of ~60 kpc from M31. Our F606W, F814W colour-magnitude diagrams extend ~3 magnitudes below the horizontal branch level, and clearly demonstrate, for the first time, that all four clusters are composed of >10 Gyr old, metal-poor stellar populations. No evidence for multiple populations is observed. From a comparison with Galactic globular cluster fiducials we estimate metallicities in the range -2.2 < [Fe/H] < -1.8. The observed horizontal branch morphologies show a clear second parameter effect between the clusters. Preliminary radial luminosity profiles suggest integrated magnitudes in the range -6.6 < M_V < -7.7, near the median value of the globular cluster luminosity function. Our results confirm that these four objects are bona fide old, metal-poor globular clusters, albeit with combined structures and luminosities unlike those observed for any other globular clusters in the Local Group or beyond.

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arXiv2006

Photometry of Magellanic Cloud clusters with the Advanced Camera for Surveys - II. The unique LMC cluster ESO 121-SC03

We present the results of photometric measurements from images of the LMC cluster ESO 121-SC03 taken with HST/ACS. Our resulting CMD reaches 3 magnitudes below the main-sequence turn-off, representing the deepest observation of this cluster to date. We also present similar photometry from ACS imaging of the accreted Sagittarius dSph cluster Pal. 12, used in this work as a comparison cluster. From analysis of its CMD, we obtain estimates for the metallicity and reddening of ESO 121-SC03: [Fe/H] = -0.97 +/- 0.10 and E(V-I) = 0.04 +/- 0.02, in excellent agreement with previous studies. The observed horizontal branch level in ESO 121-SC03 suggests this cluster may lie 20 per cent closer to us than does the centre of the LMC. ESO 121-SC03 also possesses a significant population of blue stragglers, which we briefly discuss. Our new photometry allows us to undertake a detailed study of the age of ESO 121-SC03 relative to Pal. 12 and the Galactic globular cluster 47 Tuc. Our main result is that ESO 121-SC03 is 73 +/- 4 per cent the age of 47 Tuc, and 91 +/- 5 per cent the age of Pal. 12. Pal. 12 is 79 +/- 6 per cent as old as 47 Tuc, consistent with previous work. Our result corresponds to an absolute age for ESO 121-SC03 in the range 8.3-9.8 Gyr depending on the age assumed for 47 Tuc, therefore confirming ESO 121-SC03 as the only known cluster to lie squarely within the LMC age gap. We briefly discuss a suggestion from earlier work that ESO 121-SC03 may have been accreted into the LMC system.

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arXiv2005

The properties of Galactic globular cluster sub-systems

In this paper we compare the properties of three sub-systems of Galactic globular clusters, which are defined according to metallicity and HB morphology. We focus on cluster luminosities, structures, surface brightnesses (SB) and ellipticities. It is shown that the so-called "young" halo (YH) clusters, which may have been formed in external satellite galaxies, exhibit characteristics clearly distinct from those of the "old" halo (OH) and bulge/disk (BD) clusters, the majority of which are believed to be Galactic natives. The YH clusters have properties most similar to those of clusters belonging to several present day satellite dwarf galaxies. The OH and BD populations have apparently been modified by destructive tidal forces in the inner Galaxy. We estimate that the present population of Galactic natives may only represent some two-thirds of the original population. Several low SB clusters are highly flattened. We briefly speculate that this ellipticity may represent the intrinsic flattening of dark matter mini-halos in which such optically dim clusters might be embedded. Finally, we examine the distribution of clusters on the size vs.luminosity plane. Three objects fall above the sharp upper envelope of the main distribution on this plane: Omega Cen, M54, and NGC 2419. All three have previously, and independently, been suggested to be the stripped cores of former dwarf galaxies. This suspicion is strengthened by the observation that the massive cluster G1 in M31, plus a number of the most luminous clusters in NGC 5128 also fall in the same region of the plane. All of these objects have also previously been suggested to be the stripped cores of now-defunct dwarf galaxies.

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arXiv2004

Comparing the properties of local globular cluster systems: implications for the formation of the Galactic halo

(Abridged) We investigate the hypothesis that some fraction of the globular clusters presently observed in the Galactic halo formed in external dwarf galaxies, by means of a detailed comparison between Zinn's "old halo", "young halo", and "bulge/disk" sub-systems and the globular clusters in local dwarf galaxies. The Galactic sub-systems possess distinct spatial and age distributions, age-metallicity relationships, and orbital parameters, although we observe some OH clusters with ages and orbits more similar to those of YH objects. Almost all Galactic globulars with large core radii (r_c) fall into the YH sub-system, while the OH and BD ensembles are characterized by compact clusters. The majority of the external clusters are indistinguishable from the Galactic YH objects in terms of HB morphology, but 20-30% of external clusters have HB morphologies most similar to the Galactic OH clusters. In addition, the external clusters have an r_c distribution which very closely matches that for the YH objects. The old halo r_c distribution can be very well represented by a composite distribution formed from 83-85% of objects with structures typical of BD clusters, and 15-17% of objects with structures typical of external clusters. Taken together our results fully support the accretion hypothesis. We conclude that all 30 YH clusters and 15-17% of the OH clusters (10-12 objects) are of external origin. The Galaxy may therefore have experienced ~7 merger events with cluster-bearing dSph-type galaxies during its lifetime, building up 45-50% of the mass of the stellar halo. Finally, we identify a number of OH objects which have properties characteristic of accreted clusters. Several of the clusters associated with the recently proposed CMa dwarf fall into this category.

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arXiv2004

Photometry of Magellanic Cloud clusters with the Advanced Camera for Surveys - I. The old LMC clusters NGC 1928, 1939 and Reticulum

(Abridged) We present the results of photometric measurements from images of the LMC globular clusters NGC 1928, 1939 and Reticulum taken with ACS on HST. F555W and F814W exposures result in high accuracy CMDs for these three clusters. This is the first time that CMDs for NGC 1928 and 1939 have been published. All three clusters possess CMDs with features indicating them to be >10 Gyr old, including main sequence turn-offs at V~23 and well populated HBs. We use the CMDs to obtain metallicity and reddening estimates for each cluster. NGC 1939 is a metal poor cluster, with [Fe/H] = -2.10 +/- 0.19, while NGC 1928 is significantly more metal rich, with [Fe/H] = -1.27 +/- 0.14. Reticulum's abundance lies between the two, at [Fe/H] = -1.66 +/- 0.12. All three clusters are moderately reddened with values ranging from E(V-I) = 0.07 +/- 0.02 for Reticulum and E(V-I) = 0.08 +/- 0.02 for NGC 1928, to E(V-I) = 0.16 +/- 0.03 for NGC 1939. We estimate the HB morphology of each cluster. NGC 1928 and 1939 possess HBs consisting almost exclusively of stars to the blue of the instability strip. In contrast, Reticulum has an intermediate HB morphology, with stars across the instability strip. Using a variety of dating techniques we show that these three clusters are coeval with each other and the oldest Galactic and LMC globular clusters, to within ~2 Gyr. The census of known old LMC globular clusters therefore now numbers 15 plus the unique, somewhat younger cluster ESO121-SC03. The NGC 1939 field contains another cluster in the line-of-sight, NGC 1938. A CMD for this object shows it to be less than ~400 Myr old, and it is therefore unlikely to be physically associated with NGC 1939.

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arXiv2003

RR Lyrae stars in four globular clusters in the Fornax dwarf galaxy

(Abridged) We have surveyed four globular clusters in the Fornax dwarf galaxy for RR Lyrae stars, using archival HST observations. We identify 197 new RR Lyrae stars in these four clusters. Despite the short observational baseline, we derive periods, light-curves, and photometric parameters for each. The Fornax clusters have exceptionally large RR Lyrae specific frequencies compared with the Galactic globular clusters. Furthermore, the Fornax cluster RR Lyrae stars are unusual in that their characteristics are intermediate between the two Galactic Oosterhoff groups. In this respect the Fornax clusters are similar to the field populations in several dwarf galaxies. We revise previous measurements of the HB morphology in each cluster. The Fornax clusters closely resemble the ``young'' Galactic halo population defined by Zinn. The existence of the second parameter effect among the Fornax clusters is also confirmed. Finally, we determine foreground reddening and distance estimates for each cluster. We find a mean distance modulus to Fornax of (m-M)_0 = 20.66 +/- 0.03 (random) +/- 0.15 (systematic). Our measurements are consistent with a line of sight depth of 8-10 kpc for this galaxy, matching its projected dimensions, and incompatible with tidal model explanations for the observed high velocity dispersions in many dSph galaxies. Dark matter dominance is suggested.

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arXiv2002

Surface Brightness Profiles and Structural Parameters for Globular Clusters in the Fornax and Sagittarius Dwarf Spheroidal Galaxies

We present radial surface brightness profiles for all five globular clusters in the Fornax dwarf spheroidal galaxy, and for the four present members of the Sagittarius dwarf spheroidal galaxy. These profiles are derived from archival Hubble Space Telescope observations, and have been calculated using the same techniques with which we measured profiles in our previous studies of LMC and SMC clusters (astro-ph/0209031 and astro-ph/0209046 respectively), apart from some small modifications. From the surface brightness profiles, we have determined structural parameters for each cluster, including core radii and luminosity and mass estimates. We also provide a brief summary of literature measurements of other parameters for these clusters, including their ages, metallicities and distances. Our core radius measurements are mostly in good agreement with those from previous lower resolution studies, although for several clusters our new values are significantly different. The profile for Fornax cluster 5 does not appear to be well fit by a King-type model and we suggest that it is a post core-collapse candidate. We examine the distribution of cluster core radii in each of the two dwarf galaxy systems, and compare these with the distribution of core radii for old LMC clusters. The three distributions match within the limits of measurement errors and the small sample sizes. We discuss the implications of this in the context of the radius-age trend we have previously highlighted for the Magellanic Cloud clusters.

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arXiv2002

Dynamical Evolution of Globular Clusters in the Magellanic Clouds and Other Local Systems

Observations of rich LMC and SMC clusters reveal an increasing spread in core radius with age. This trend likely represents real physical evolution in these systems. Old Magellanic Cloud clusters appear either extended or compact. Similar core radius distributions are observed for the Fornax and Sagittarius dSph cluster systems and in the ``young'' halo Milky Way subsystem. These observations have implications for both the formation of the Galaxy and the dynamical evolution of globular clusters.

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arXiv2002

Surface Brightness Profiles and Structural Parameters for 10 Rich Stellar Clusters in the Small Magellanic Cloud

As a follow up to our recent study of a large sample of LMC clusters (astro-ph/0209031), we have conducted a similar study of the structures of ten SMC clusters, using archival Hubble Space Telescope snapshot data. We present surface brightness profiles for each cluster, and derive structural parameters, including core radii and luminosity and mass estimates, using exactly the same procedure as for the LMC sample. Because of the small sample size, the SMC results are not as detailed as for the larger LMC sample. We do not observe any post core-collapse clusters (although we did not expect to), and there is little evidence for any double clusters in our sample. Nonetheless, despite the small sample size, we show for the first time that the SMC clusters follow almost exactly the trend in core radius with age observed for the LMC system, including the apparent bifurcation at several hundred Myr. This further strengthens our argument that this relationship represents true physical evolution in these clusters, with some developing significantly expanded cores due to an as yet unidentified physical process. Additional data, both observational and from N-body simulations, is still required to clarify many issues.

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arXiv2002

Surface Brightness Profiles and Structural Parameters for 53 Rich Stellar Clusters in the Large Magellanic Cloud

We have compiled a pseudo-snapshot data set of two-colour observations from the Hubble Space Telescope archive for a sample of 53 rich LMC clusters with ages 10^6-10^10 yr. We present surface brightness profiles for the entire sample, and derive structural parameters for each cluster, including core radii, and luminosity and mass estimates. Because we expect the results presented here to form the basis for several further projects, we describe in detail the data reduction and surface brightness profile construction processes, and compare our results with those of previous ground-based studies. The surface brightness profiles show a large amount of detail, including irregularities in the profiles of young clusters (such as bumps, dips, and sharp shoulders), and evidence for both double clusters and post core-collapse (PCC) clusters. In particular we find power-law profiles in the inner regions of several candidate PCC clusters, with slopes of approximately -0.7, but showing considerable variation. We estimate that 20 +/- 7 % of the old cluster population of the LMC has entered PCC evolution, a similar fraction to that for the Galactic globular cluster system. In addition, we examine the profile of R136 in detail and show that it is probably not a PCC cluster. We also observe a trend in core radius with age that has been discovered and discussed in several previous publications by different authors. Our diagram has better resolution however, and appears to show a bifurcation at several hundred Myr. We argue that this observed relationship reflects true physical evolution in LMC clusters, with some experiencing small scale core expansion due to mass loss, and others large scale expansion due to some unidentified characteristic or physical process.

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arXiv2002

Mass segregation in rich clusters in the Large Magellanic Cloud - III. Implications for the initial mass function

The distribution of core radii of rich clusters in the Large Magellanic Cloud (LMC) systematically increases in both upper limit and spread with increasing cluster age. Cluster-to-cluster variations in the stellar initial mass function (IMF) have been suggested as an explanation. We discuss the implications of the observed degree of mass segregation in our sample clusters for the shape of the initial mass function. Our results are based on Hubble Space Telescope/WFPC2 observations of six rich star clusters in the LMC, selected to include three pairs of clusters of similar age, metallicity, and distance from the LMC centre, and exhibiting a large spread in core radii between the clusters in each pair. All clusters show clear evidence of mass segregation: (i) their luminosity function slopes steepen with increasing cluster radius, and (ii) the brighter stars are characterized by smaller core radii. For all sample clusters, both the slope of the luminosity function in the cluster centres and the degree of mass segregation are similar to each other, within observational errors of a few tenths of power-law slope fits to the data. This implies that their INITIAL mass functions must have been very similar, down to ~0.8 - 1.0 M(sun). We therefore rule out variations in the IMF of the individual sample clusters as the main driver of the increasing spread of cluster core radii with cluster age.

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arXiv2001

Mass segregation in young compact star clusters in the Large Magellanic Cloud: II. Mass Functions

We review the complications involved in the conversion of stellar luminosities into masses and apply a range of mass-to-luminosity relations to our Hubble Space Telescope observations of the young LMC star clusters NGC 1805 and NGC 1818. Both the radial dependence of the mass function (MF) and the dependence of the cluster core radii on mass indicate clear mass segregation in both clusters at radii r <= 20-30'', for masses in excess of ~1.6-2.5 Msun. This result does not depend on the mass range used to fit the slopes or the metallicity assumed. It is clear that the cluster MFs, at any radius, are not simple power laws. The global and the annular MFs near the core radii appear to be characterised by similar slopes in the mass range (-0.15 <= log m/Msun <= 0.85), the MFs beyond r >= 30'' have significantly steeper slopes. We estimate that while the NGC 1818 cluster core is between ~5 and ~30 crossing times old, the core of NGC 1805 is likely $\lesssim 3-4$ crossing times old. However, since strong mass segregation is observed out to ~6 Rcore and ~3 Rcore in NGC 1805 and NGC 1818, respectively, it is most likely that significant primordial mass segregation was present in both clusters, particularly in NGC 1805.

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