Spectra of globular clusters in the Sombrero galaxy: evidence for spectroscopic metallicity bimodality★

Alves-Brito, Alan; Hau, G. K. T.; Forbes, Duncan A.; Spitler, Lee R.; Strader, Jay; Brodie, Jean P.; Rhode, Katherine L.

doi:10.1111/j.1365-2966.2011.19368.x

Cited by 37 publications

(44 citation statements)

References 46 publications

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“…It is generally accepted that this color bimodality corresponds to an underlying metallicity bimodality, with the red clusters metal-enhanced when compared to the blue clusters. This metallicity bimodality has been spectroscopically confirmed for a limited, but growing, number of systems (Puzia et al 2002;Strader et al 2007;Beasley et al 2008;Alves-Brito et al 2011;Usher et al 2012;Brodie et al 2012).…”

Section: Introductionmentioning

confidence: 83%

THE SLUGGS SURVEY:HST/ACS MOSAIC IMAGING OF THE NGC 3115 GLOBULAR CLUSTER SYSTEM

Jennings

Strader

Romanowsky

et al. 2014

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We present Hubble Space Telescope/Advanced Camera for Surveys (HST/ACS) g and z photometry and half-light radii R h measurements of 360 globular cluster (GC) candidates around the nearby S0 galaxy NGC 3115. We also include Subaru/Suprime-Cam g, r, and i photometry of 421 additional candidates. The well-established color bimodality of the GC system is obvious in the HST/ACS photometry. We find evidence for a "blue tilt" in the blue GC subpopulation, wherein the GCs in the blue subpopulation get redder as luminosity increases, indicative of a mass-metallicity relationship. We find a color gradient in both the red and blue subpopulations, with each group of clusters becoming bluer at larger distances from NGC 3115. The gradient is of similar strength in both subpopulations, but is monotonic and more significant for the blue clusters. On average, the blue clusters have ∼10% larger R h than the red clusters. This average difference is less than is typically observed for early-type galaxies but does match that measured in the literature for the Sombrero Galaxy (M104), suggesting that morphology and inclination may affect the measured size difference between the red and blue clusters. However, the scatter on the R h measurements is large. We also identify 31 clusters more extended than typical GCs, which we term ultra-compact dwarf (UCD) candidates. Many of these objects are actually considerably fainter than typical UCDs. While it is likely that a significant number will be background contaminants, six of these UCD candidates are spectroscopically confirmed as NGC 3115 members. To explore the prevalence of low-mass X-ray binaries in the GC system, we match our ACS and Suprime-Cam detections to corresponding Chandra X-ray sources. We identify 45 X-ray-GC matches: 16 among the blue subpopulation and 29 among the red subpopulation. These X-ray/GC coincidence fractions are larger than is typical for most GC systems, probably due to the increased depth of the X-ray data compared to previous studies of GC systems.

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Section: Introductionmentioning

confidence: 83%

THE SLUGGS SURVEY:HST/ACS MOSAIC IMAGING OF THE NGC 3115 GLOBULAR CLUSTER SYSTEM

Jennings

Strader

Romanowsky

et al. 2014

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“…Beyond the Local Group, there are very few GC systems observed with reasonable numbers of GCs in which to examine the spectroscopic metallicity distribution directly. However, for some that do exist, that is, NGC 4472 (Strader, Beasley & Brodie 2007), NGC 5128 (Woodley et al 2010) and NGC 4594 (Alves‐Brito et al 2011), spectroscopic metallicity bimodality is confirmed. Other galaxies with similar numbers of observed GCs, where metallicities are derived from CaT, do not confirm metallicity bimodality, i.e.…”

Section: Introductionmentioning

confidence: 94%

Wide-field imaging of NGC 4365’s globular cluster system: the third subpopulation revisited

Blom¹,

Spitler²,

Forbes³

2011

Monthly Notices of the Royal Astronomical Society

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121

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Analysis of the globular cluster (GC) system of the giant elliptical (E3) galaxy NGC 4365, from eight Hubble Space Telescope/Advanced Camera for Surveys pointings and a wide‐field Subaru/Suprime‐Cam image, is presented. Using magnitude, colour and size criteria, we obtain a catalogue of GC candidates. We also measure the photometric properties of the galaxy starlight, including a new measure of the effective radius. We determine the lower limit on the number of GCs to be and show that the GC system extends beyond 134 kpc (9.5 galaxy effective radii). We revisit the question of whether NGC 4365 has a bimodal or trimodal GC colour distribution and find support for three distinct GC colour subpopulations (i.e. blue, green and red). Sérsic profile fits to the radial surface density of each subpopulation reveal that the blue GCs are more extended than either the red or the green GCs. The median half‐light radii for GCs in the blue, green and red subpopulations are , and pc, respectively. The estimated subpopulation ellipticities are , and for the blue, green and red GCs, respectively, where alignment with the photometric position angle of the galaxy () is assumed. A Kolmogorov–Smirnov test on the mass functions shows a 98 per cent probability that all three subpopulations are distinct from one another. We also find radial gradients of GC size and colour (metallicity) and a blue tilt. The properties, including surface density profile, position angle, ellipticity and radial colour gradient, of the red GC subpopulation are very similar to the properties of NGC 4365’s starlight. This result supports the hypothesis that red GCs are formed along with the bulk of the diffuse starlight in the galaxy. NGC 4365 has a kinematically distinct core and a significant misalignment between the photometric and kinematic major‐axes. We discuss the possibility that these kinematic features are related to the presence of the distinct third GC subpopulation. We briefly discuss implications for the formation of NGC 4365, finding that major‐merger, multiphase‐collapse and accretion formation scenarios could all account for the existence of the third GC subpopulation.

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“…While NGC 5128 presents an obvious R − [3.6] bimodal distribution, more compatible with the multimodal peaks that the spectroscopic work of Beasley et al (2008) find (see also Woodley et al 2010), NGC 4594 has a less clear bimodal distribution in this baseline. In contrast, Alves-Brito et al (2011) present spectra for over 200 GCs in the Sa NGC 4594 and find a clear bimodal distribution. More recently, Foster et al (2010Foster et al ( , 2011 find very similar Calcium triplet values for red and blue GCs in NGC 1407 and NGC 4494 despite their large colour difference.…”

Section: Introductionmentioning

confidence: 95%

An optical/NIR survey of globular clusters in early-type galaxies

Chies-Santos

Larsen

Cantiello

et al. 2012

A&A

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Context. The interpretation that bimodal colour distributions of globular clusters (GCs) reflect bimodal metallicity distributions has been challenged. Non-linearities in the colour to metallicity conversions caused for example by the horizontal branch (HB) stars may be responsible for transforming a unimodal metallicity distribution into a bimodal (optical) colour distribution. Aims. We study optical/near-infrared (NIR) colour distributions of the GC systems in 14 E/S0 galaxies. Methods. We test whether the bimodal feature, generally present in optical colour distributions, remains in the optical/NIR ones. The latter colour combination is a better metallicity proxy than the former. We use KMM and GMM tests to quantify the probability that different colour distributions are better described by a bimodal, as opposed to a unimodal distribution. Results. We find that double-peaked colour distributions are more commonly seen in optical than in optical/NIR colours. For some of the galaxies where the optical (g − z) distribution is clearly bimodal, a bimodal distribution is not preferred over a unimodal one at a statistically significant level for the (g − K) and (z − K) distributions. The two most cluster-rich galaxies in our sample, NGC 4486 and NGC 4649, show some interesting differences. The (g − K) distribution of NGC 4649 is better described by a bimodal distribution, while this is true for the (g − K) distribution of NGC 4486 GCs only if restricted to a brighter sub-sample with small K-band errors (<0.05 mag). Formally, the K-band photometric errors cannot be responsible for blurring bimodal metallicity distributions to unimodal (g − K) colour distributions. However, simulations including the extra scatter in the colour-colour diagrams (not fully accounted for in the photometric errors) show that such scatter may contribute to the disappearance of bimodality in (g − K) for the full NGC 4486 sample. For the less cluster-rich galaxies results are inconclusive due to poorer statistics. Conclusions. A bimodal optical colour distribution is not necessarily an indication of an underlying bimodal metallicity distribution. Horizontal branch morphology may play an important role in shaping some of the optical GC colour distributions. However, we find tentative evidence that the (g − K) colour distributions remain bimodal in the two cluster-rich galaxies in our sample (NGC 4486 and NGC 4649) when restricted to clusters with small K-band photometric errors. This bimodality becomes less pronounced when including objects with larger errors, or for the (z − K) colour distributions. Deeper observations of large numbers of GCs will be required to reach more secure conclusions.

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Spectra of globular clusters in the Sombrero galaxy: evidence for spectroscopic metallicity bimodality★

Cited by 37 publications

References 46 publications

THE SLUGGS SURVEY:HST/ACS MOSAIC IMAGING OF THE NGC 3115 GLOBULAR CLUSTER SYSTEM

THE SLUGGS SURVEY:HST/ACS MOSAIC IMAGING OF THE NGC 3115 GLOBULAR CLUSTER SYSTEM

Wide-field imaging of NGC 4365’s globular cluster system: the third subpopulation revisited

An optical/NIR survey of globular clusters in early-type galaxies

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