Globular Clusters In Early-type Galaxies Research Articles

The range of UV upturn strengths in early-type galaxies can be caused by dissolved metal-rich Globular Clusters

AbstractI summarize the scenario by Goudfrooij (2018) in which the bulk of the ultraviolet (UV) upturn of giant early-type galaxies (ETGs) is due to helium-rich stellar populations that formed in massive metal-rich globular clusters (GCs) and subsequently dissolved in the strong tidal field in the central regions of the massive host galaxy. These massive GCs are assumed to show UV upturns similar to those observed recently in M87, the central galaxy in the Virgo cluster of galaxies. Data taken from the literature reveals a strong correlation between the strength of the UV upturn and the specific frequency of metal-rich GCs in ETGs. Adopting a Schechter function parametrization of GC mass functions, simulations of long-term dynamical evolution of GC systems show that this correlation can be explained by variations in the characteristic truncation mass Mc such that Mc increases with ETG luminosity in a way that is consistent with observed GC luminosity functions in ETGs. These findings suggest that the nature of the UV upturn in ETGs and the variation of its strength among ETGs are causally related to that of helium-rich populations in massive GCs, rather than intrinsic properties of field stars in ETGs.

Multiple populations within globular clusters in early-type galaxies exploring their effect on stellar initial mass function estimates

It is now well-established that most (if not all) ancient globular clusters host multiple popula- tions, that are characterised by distinct chemical features such as helium abundance variations along with N-C and Na-O anti-correlations, at fixed [Fe/H]. These very distinct chemical fea- tures are similar to what is found in the centres of the massive early-type galaxies and may influence measurements of the global properties of the galaxies. Additionally, recent results have suggested that M/L variations found in the centres of massive early-type galaxies might be due to a bottom-heavy stellar initial mass function. We present an analysis of the effects of globular cluster-like multiple populations on the integrated properties of early-type galaxies. In particular, we focus on spectral features in the integrated optical spectrum and the global mass-to-light ratio that have been used to infer variations in the stellar initial mass function. To achieve this we develop appropriate stellar population synthesis models and take into account, for the first time, an initial-final mass relation which takes into consideration a varying He abundance. We conclude that while the multiple populations may be present in massive early-type galaxies, they are likely not responsible for the observed variations in the mass- to-light ratio and IMF sensitive line strengths. Finally, we estimate the fraction of stars with multiple populations chemistry that come from disrupted globular clusters within massive ellipticals and find that they may explain some of the observed chemical patterns in the centres of these galaxies.

Dissolved Massive Metal-rich Globular Clusters Can Cause the Range of UV Upturn Strengths Found among Early-type Galaxies

Abstract I discuss a scenario in which the ultraviolet (UV) upturn of giant early-type galaxies (ETGs) is primarily due to helium-rich stellar populations that formed in massive metal-rich globular clusters (GCs), which subsequently dissolved in the strong tidal field in the central regions of the massive host galaxy. These massive GCs are assumed to show UV upturns similar to those observed recently in M87, the central giant elliptical galaxy in the Virgo cluster of galaxies. Data taken from the literature reveal a strong correlation between the strength of the UV upturn and the specific frequency of metal-rich GCs in ETGs. Adopting a Schechter function parameterization of GC mass functions, simulations of long-term dynamical evolution of GC systems show that the observed correlation between UV upturn strength and GC specific frequency can be explained by variations in the characteristic truncation mass such that increases with ETG luminosity in a way that is consistent with observed GC luminosity functions in ETGs. These findings suggest that the nature of the UV upturn in ETGs and the variation of its strength among ETGs are causally related to that of helium-rich populations in massive GCs, rather than intrinsic properties of field stars in massive galactic spheroids. With this in mind, I predict that future studies will find that [N/Fe] decreases with increasing galactocentric radius in massive ETGs, and that such gradients have the largest amplitudes in ETGs with the strongest UV upturns.

THE ACCRETION OF DWARF GALAXIES AND THEIR GLOBULAR CLUSTER SYSTEMS

The question of where the low-metallicity globular clusters in early-type galaxies came from has profound implications for the formation of those galaxies. Our work supports the idea that the metal-poor globular cluster systems of giant early-type galaxies formed in dwarf galaxies that have been subsumed by the giants. To support this hypothesis, two linear relations, one involving globular cluster metallicity versus host galaxy luminosity and one involving metallicity versus velocity dispersion were studied. Tentatively, these relations show that the bright ellipticals do not obey the same trend as the dwarfs, suggesting that the low-metallicity globular clusters did not form within their parent bright ellipticals.

THE ADVANCED CAMERA FOR SURVEYS FORNAX CLUSTER SURVEY. VII. HALF-LIGHT RADII OF GLOBULAR CLUSTERS IN EARLY-TYPE GALAXIES

We measure the half-light radii of globular clusters (GCs) in 43 galaxies from the ACS Fornax Cluster Survey (ACSFCS). We use these data to extend previous work in which the environmental dependencies of the half-light radii of GCs in early type galaxies in the ACS Virgo Cluster Survey (ACSVCS) were studied, and a corrected mean half-light radius (corrected for the observed environmental trends) was suggested as a reliable distance indicator. This work both increases the sample size for the study of the environmental dependencies, and adds leverage to the study of the corrected half-light radius as a possible distance indicator (since Fornax lies at a larger distance than the Virgo cluster). We study the environmental dependencies of the size of GCs using both a Principal Component Analysis as well as 2D scaling relations. We largely confirm the environmental dependencies shown in Jordan et al. (2005), but find evidence that there is a residual correlation in the mean half-light radius of GC systems with galaxy magnitude, and subtle differences in the other correlations - so there may not be a universal correction for the half-light radii of lower luminosity galaxy GC systems. The main factor determining the size of a GC in an early type galaxy is the GC color. Red GCs have <r_h> = 2.8+/-0.3 pc, while blue GCs have <r_h> = 3.4+/-0.3 pc. We show that for bright early-type galaxies (M_B < -19 mag), the uncorrected mean half-light radius of the GC system is by itself an excellent distance indicator (with error ~11%), having the potential to reach cosmologically interesting distances in the era of high angular resolution adaptive optics on large optical telescopes.

Globular cluster kinematics and X-ray emission in the early-type galaxy NGC 1399

Context. Of the various observational methods used to address the question of dark matter in early-type galaxies there is the possibility of spatial overlap in using kinematics of stars, clusters and X-ray halos. Aims. To investigate methods for the determination of velocity distribution profiles and deviations from a Gaussian distribution using globular clusters in early-type galaxies, and the implications for dark matter. Methods. The maximum likelihood method together with standard statistical procedures were used to calculate the velocity dispersion. A new “tracer mass estimator” was applied to obtain a mass estimate based on the globular clusters, which is then compared to the mass obtained using X-rays. Results. In NGC 1399 the departures from a Gaussian distribution of the velocities at all radial distances are small. Our conclusion is that in spite of the observations that show that the velocity dispersion decreases between 4 and 10 R e there is evidence that dark matter exists beyond ${\sim} 3\,R_{\rm e}$.

Low Mass X-ray Binaries and Globular Clusters in Early-Type Galaxies

Chandra observations have allowed the detection of a large number of low mass X-ray binaries (LMXBs) in early-type galaxies. Comparison to catalogs of globular clusters (GCs) from Hubble Space Telescope observations have shown that a high fraction of the LMXBs in early-type galaxies are associated with GCs. The fraction of LMXBs associated with globular clusters increases along the Hubble sequence from spiral bulges to S0s to Es to cDs. On the other hand, the fraction of globular clusters which contain X-ray sources appears to be roughly constant ( $\sim$ 4% for $L_X \gtrsim 10^{38}$ ergs/s, $\sim$ 10% for $L_X \gtrsim 10^{37}$ ergs/s). There is a strong tendency for the X-ray sources to be associated with the optically more luminous GCs. There is a trend for the X-ray sources to be found preferentially in redder, more metal-rich GCs, which is independent of optical luminosity correlation. The relative role of formation of LMXBs in GCs and in situ formation in the field is uncertain. One of the best ways to study this is to compare the spatial distribution of GC-LMXBs, field LMXBs, GCs, and optical light in the galaxies. Theoretical models and results of fits to the observed distributions are presented.

Intermediate-age globular clusters in early-type galaxies: Better age determinations by addingU-band observations to theV,I,Kdatasets

This paper represents an extension to a series of publications on combined optical and near-infrared photometry of globular cluster systems in early-type galaxies. Based on color-color diagrams the cumulative age distributions of their globular cluster systems have been derived and revealed intermediate age clusters in two galaxies, namely NGC 4365 and NGC 5846. The extension of this observations towards the blue wavelength range was performed in order to increase the age resolution of photometric studies. In this paper we present the results of U-band observations of NGC 4365 and NGC 5846 and their combination with previously obtained V, I and Ks-band photometry. (U − I )v s. (V − Ks) color-color diagrams are used to derive the cumulative age distribution. The later is compared to simulated globular cluster systems of known age and size composition in order to set constrains on relative ages and size of the sub-populations.