High <i>M/L</i> ratios of UCDs: A variation of the IMF?

Mieske, S.; Dabringhausen, J.; Kroupa, Pavel; Hilker, M.; Baumgardt, Holger

doi:10.1002/asna.200811073

Cited by 11 publications

(13 citation statements)

References 29 publications

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“…Drinkwater et al 2003;Mieske et al 2008a) could be explained by a non-standard stellar initial mass function (IMF). The IMF could either be bottom heavy (Mieske & Kroupa 2008), with a large fraction of the mass in low-luminosity stars, or top heavy (Mieske et al 2008c, Dabringhausen et al 2009, 2012Murray 2009), with much of the mass now locked up in stellar remnants. However, the latter is predicted to increase the population of LMXBs, contrary to our present findings.…”

Section: Discussionmentioning

confidence: 99%

X-ray sources in compact stellar systems in the Fornax Cluster

Phillipps

Young

Drinkwater

et al. 2013

Monthly Notices of the Royal Astronomical Society

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Compact stellar systems (CSS) range in mass from globular clusters (GCs), through the more recently discovered ultracompact dwarfs (UCDs), to the rare compact elliptical galaxies. Given their intermediate position, the origin of UCDs, and hence whether they should be counted as galaxies or star clusters, remains unclear. In this paper, we compare the population of X-ray point sources (expected to be low-mass X-ray binaries; LMXBs) in UCD candidates, that is, relatively high-luminosity compact systems, with that known to exist in lower mass GCs. Any difference in LMXB populations may then indicate physical differences between GCs and UCDs. We find that, despite their much larger masses and hence numbers of stars, UCDs and UCD candidates are, if anything, less likely to contain bright X-ray point sources than are GCs. The LMXB content per unit stellar mass is therefore much lower for the UCD candidates, intermediate between that for GCs and for elliptical galaxies. This difference between UCDs and GCs can be explained if UCDs have significantly lower central densities than lower mass GCs, as this will reduce the stellar encounter rate and therefore the production of suitable binary systems. This supports the previously proposed idea of structural differences between UCDs and GCs, though not necessarily ruling out a close relationship between these two types of CSS.

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Section: Discussionmentioning

confidence: 99%

X-ray sources in compact stellar systems in the Fornax Cluster

Phillipps

Young

Drinkwater

et al. 2013

Monthly Notices of the Royal Astronomical Society

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“…In passing we note that if NCs are related to ultra compact dwarf galaxies (e.g. Kroupa et al 2010) they may have a high stellar M/L due to either a bottom-heavy (Mieske et al 2008) or topheavy initial mass function (Dabringhausen et al 2009).…”

Section: Sample and Datamentioning

confidence: 94%

Updated Mass Scaling Relations for Nuclear Star Clusters and a Comparison to Supermassive Black Holes

Scott

Graham

2013

ApJ

108

100

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We investigate whether nuclear star clusters and supermassive black holes follow a common set of mass scaling relations with their host galaxy's properties, and hence can be considered to form a single class of central massive object. We have compiled a large sample of galaxies with measured nuclear star cluster masses and host galaxy properties from the literature and fit log-linear scaling relations. We find that nuclear star cluster mass, M NC , correlates most tightly with the host galaxy's velocity dispersion: log M NC = (2.11 ± 0.31) log(σ/54) + (6.63 ± 0.09), but has a slope dramatically shallower than the relation defined by supermassive black holes. We find that the nuclear star cluster mass relations involving host galaxy (and spheroid) luminosity and stellar and dynamical mass, intercept with but are in general shallower than the corresponding black hole scaling relations. In particular M NC ∝ M 0.55±0.15Gal,dyn ; the nuclear cluster mass is not a constant fraction of its host galaxy or spheroid mass. We conclude that nuclear stellar clusters and supermassive black holes do not form a single family of central massive objects.

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“…This problem is compounded by the fact that the correct choice of IMF for UCDs is still a matter of debate (see e.g. Mieske et al 2008a; Chilingarian et al 2011). We therefore caution that UCD stellar mass estimates are subject to systematic uncertainties in overall normalization at a factor of ∼3 level, which should not strongly affect comparisons within a given sample analysed uniformly.…”

Section: Observations and Data Reductionmentioning

confidence: 99%

The ubiquity and dual nature of ultra-compact dwarfs

Norris

Kannappan

2011

Monthly Notices of the Royal Astronomical Society

187

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We present the discovery of several ultra‐compact dwarfs (UCDs) located in field/group environments. Examination of these objects, plus literature UCDs, confirms the existence of two distinct formation channels for these compact stellar systems. We find that the UCDs we have discovered around the group elliptical NGC 3923 (and most UCDs in general) have properties consistent with their being the most luminous members of the host galaxy’s globular cluster (GC) system. As with GCs they are therefore likely to be the product of the most violent epochs of galaxy formation. We describe UCDs of this type as giant GCs (GGCs). In contrast, the UCD we have found associated with the isolated S0 NGC 4546 is clearly the result of the stripping of a nucleated companion galaxy. The young age (∼3.4 Gyr) of the UCD, the lack of a correspondingly young GC population, the apparently short dynamical friction decay time‐scale (∼0.5 Gyr) of the UCD and the presence of a counter‐rotating gas disc in the host galaxy (corotating with respect to the UCD) together suggest that this UCD is the liberated nucleus remaining after the recent stripping of a companion by NGC 4546. We infer that the presence of UCDs of either category (GGCs formed in major star‐forming events, or stripped nuclei formed in minor mergers) can provide a useful probe of the assembly history of the host galaxy. We suggest a general scheme that unifies the formation of GCs, UCDs and galaxy nuclei. In this picture, ‘normal’ GCs are a composite population, composed of GCs formed in situ, GCs acquired from accreted galaxies and a population of lower mass stripped dwarf nuclei masquerading as GCs. Above a ‘scaling onset mass’ of 2 × 106 M⊙ (MV∼−10), UCDs emerge together with a mass–size relation and a likely mass–metallicity relation (the ‘blue tilt’). In the mass range up to 7 × 107 M⊙ (MV∼−13) UCDs comprise a composite population of GGCs and stripped nuclei. Interestingly, dwarf nuclei have similar colours to blue GCs and UCDs across the scaling onset mass, smoothly extending the blue tilt, while nuclei of more massive galaxies and a prominent minority of UCDs extend the red locus of GCs. Above 7 × 107 M⊙, UCDs must be almost exclusively stripped nuclei, as no sufficiently rich GC systems exist to populate such an extreme of the GCLF.

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High M/L ratios of UCDs: A variation of the IMF?

Cited by 11 publications

References 29 publications

X-ray sources in compact stellar systems in the Fornax Cluster

X-ray sources in compact stellar systems in the Fornax Cluster

Updated Mass Scaling Relations for Nuclear Star Clusters and a Comparison to Supermassive Black Holes

The ubiquity and dual nature of ultra-compact dwarfs

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