University of Heidelberg

My Research:
3. Star Cluster Systems


Preamble

The globular clusters Palomar 5 and M3 on the same scale (SDSS) Star clusters cover a wide range of gravitationally bound stellar agglomerates ranging from massive, compact, long-lived globular clusters to low-mass, diffuse, short-lived associations, from metal-poor objects to clusters with super-solar metallicity, and from very old populations to embedded young clusters. It has been suggested that all stars originally formed in clusters or asociations. The study of the many types of clusters is worthwhile in its own right and reveals important information on star formation processes and the impact of galactic environment. A crucial advantage of star clusters relative to field star populations is that they represent, to first order, coeval populations and can be easily age-dated when they are resolved into individual stars. Star clusters have been identified in all galaxy types except for the least massive ones. As ensembles, star clusters can serve as a very useful tracer of galaxian star formation history.

The work presented in this section was done with my past and present graduate students and postdocs Katharina Glatt, Andreas Koch, Geneviève Parmentier, Frederik Schönebeck, Fayezeh Shabani, and Carolin Wittmann.

Subtopics

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3.1 Globular Cluster Systems of Massive Galaxies

The globular cluster system of the elliptical galaxy NGC 4636 Massive elliptical galaxies host the richest globular cluster systems, usually encompassing many thousands of globular clusters. Rich globular cluster systems usually show a shallower density distribution profile than the distribution of the underlying host galaxy's light (e.g., Kissler et al. 1994). It is customary to normalize the number of globular clusters by the absolute visual brightness of their host galaxy, which yields the so-called specific frequency of globular clusters as a quantitative and comparable measure (Harris & van den Bergh 1981). Isolated ellipticals tend to have lower specific frequencies than the very massive elliptical galaxies in galaxy clusters. Globular cluster systems tend to exhibit color bimodalities. It is believed that the redder and more metal-rich globular clusters formed in situ, while the bluer and probably more metal-poor globulars may have been accreted from smaller galaxies.

The radial mass density profile of the old halo globular cluster system in the Milky Way (Parmentier & Grebel 2005) A dichotomy between globular clusters formed in situ and accreted ones has also been suggested for disk galaxies like our Milky Way: As my former postdoc Geneviève Parmentier showed, the radial mass density profile of the old halo globular cluster system may be a relic of the distribution of the cold baryonic material in the protogalaxy. Beyond a Galactocentric distance of 2 to 3 kpc, the initial shape of this mass density profile is preserved, and even the flattening of this profile observed at shorter Galactocentric radii may be in part of primordial origin (Parmentier & Grebel 2005). In contrast, the so-called young halo globular clusters, which show a more extended distribution, are believed to have been accreted.

We found the elliptical galaxy NGC 1404 in the Fornax galaxy cluster to have a comparatively low globular cluster specific frequency. We interpreted this as well as its unsually high radial velocity and high X-ray luminosity as evidence for recent infall into the cluster (Richtler et al. 1992). Our kinematic studies of the globulars around the dominant elliptical in Fornax, NGC 1399, revealed that its red globular cluster system has a lower velocity dispersion than its blue globular clusters (Dirsch et al. 2004; Richtler et al. 2004). Globular clusters have repeatedly been used as dynamical tracers of the underlying galactic mass distribution. For NGC 1399, we found that its dark halo potential can be well-reproduced by a logarithmic potential or by a Navarro, Frenk, & White (1997) profile (Richtler et al. 2004). A wide-field imaging study of the central 0.5 square degrees of the Fornax cluster allowed us to investigate the large-scale distribution of globular clusters in this galaxy cluster (D'Abrusco et al. 2016). We found that the spatial distribution of the globular clusters connects NGC 1399 to its neighboring giant ellipticals, which may imply that NGC 1399 stripped globular clusters from its neighbors. Red globular clusters tend to be more spatially concentrated around the loci of the ellipticals and show relatively small-scale density structures possibly caused by galaxy-galaxy interactions. On the other hand, blue globulars show a much more spatially extended distribution and may have been removed from the halos of massive galaxies in the core of Fornax by this galaxy cluster's substantial gravitational potential (D'Abrusco et al. 2016).

The distribution of red and blue globular clusters in the Fornax galaxy cluster (D'Abrusco et al. 2016)


3.2 Globular Clusters in Dwarf Galaxies

Color-magnitude diagram of the globular cluster NGC 121 in the SMC with Dartmouth isochrones (Glatt et al. 2008). Only the more massive dwarf galaxies contain globular clusters. The number of globular clusters tends to be small but owing to the low absolute luminosities of the dwarfs the resulting specific frequencies may be large (Grebel 2002, 2016). Only four of the Milky Way companions - the Large Magellanic Cloud (LMC), the Small Magellanic Cloud (SMC), and the dwarf spheroidal galaxies Fornax and Sagittarius contain globular clusters. LMC, Fornax, and Sagittarius have globular clusters that are as old as the oldest Galactic globulars (e.g., Grebel & Gallagher 2004). But as my graduate student Katharina Glatt confirmed based on deep color-magnitude diagrams obtained with the Hubble Space Telescope, the only globular cluster in the SMC, NGC 121, is two to three Gyr younger than the oldest Galactic globular clusters (Glatt et al. 2008). With its red horizontal branch, the moderately metal-poor NGC 121 resembles the so-called "young halo globular clusters". It is unclear why the SMC formed only one surviving globular and why this happened a few Gyr later than in the other nearby dwarf galaxies containing globular clusters.

The globular cluster GC1 in the Sculptor group dwarf elliptical galaxy Scl-dE1 (Da Costa et al. 2009) The half-light radii of globular clusters in the Milky Way generally increase with Galactocentric distance although there is a considerable amount of scatter (van den Bergh, Morbey, & Pazder 1991). The "vital diagram" of Gnedin & Ostriker (1997) suggests that extended globular clusters would not survive at small Galactocentric radii. Also globular clusters in the Large Magellanic Cloud show a tendency of increasing sizes with increasing galactocentric distances (Hodge 1962). The mean half-light radii of galactic globular cluster systems increase with decreasing galaxy luminosity (Jordán et al. 2005). Dwarf galaxies contain globular clusters with a range of half-light radii including faint, extended globulars that resemble the properties of outer halo globular clusters (van den Bergh & Mackey 2004). It has been repeatedly suggested that the outer halo globular clusters where primarily accreted from since disrupted dwarf galaxies. This holds in particular for the so-called young halo globular clusters with their predominantly red horizontal branches, although it is important to remember that dwarf galaxies host globulars with the full range of horizontal branch morphologies including second-parameter globular clusters as well as with a variety of half-light radii (e.g., Grebel 2016).

We discovered one of the most extended globular clusters known in a dwarf galaxy in the dwarf elliptical Scl-dE1 in the Sculptor group (Da Costa et al. 2009). While luminous, compact globular clusters typically have half-light radii around 3 pc, this globular cluster Scl-dE1 GC1 has an effective radius of 22 pc. Evaluating the known half-light radii of globular clusters in dwarf galaxies, we find their distribution to be bimodal, including a substantial contribution from extended globular clusters. When considering data of globular clusters in massive galaxies (the Milky Way, M31, and Centaurus A) the fraction of globular clusters with small half-light radii strongly increases, but the overall distribution remains bimodal despite tidal effects that presumably destroyed many of the extended clusters that once formed at shorter galactocentric distances (see figure below; Da Costa et al. 2009). Due to this persistent dichotomy in half-light radii, we suggest that there may be two modes of globular cluster formation with the extended cluster mode (and the survival of such clusters) being more common in environments with lower tidal forces such as dwarf galaxies. In contrast, high-pressure environments such as massive galaxies form preferentially globular clusters with small effective radii (Da Costa et al. 2009).

Bimodal distribution of globular cluster half-light radii in dwarf and massive galaxies (Da Costa et al. 2009).


3.3 The Recent Star Formation History of the Magellanic Clouds from Star Clusters

LMC and SMC young cluster age distribution If star clusters cover a sufficiently wide range of ages and are sufficiently numerous, they can be used as tracers of galactic star formation histories. Unlike the Milky Way, the Magellanic Clouds host a large number of massive, young, compact star clusters. In order to differentiate them from the low-mass, loose open clusters commonly found in the Milky Way as well as from old globular clusters, the term "populous clusters" was introduced for this cluster type (Hodge 1961. Owing to their proximity, young Magellanic Cloud clusters can easily be resolved into individual stars via ground-based observations.

In the framework of the Magellanic Clouds Photometric Survey (MCPS; Zaritsky et al. 2002, 2004), we carried out a multi-color CCD drift scan survey of the central 64 deg² of the Large Magellanic Cloud (LMC) and of the central 18 deg² of the Small Magellanic Cloud (SMC). Using the resulting point-source photometry catalogs, my graduate student Katharina Glatt age-dated 1193 populous star clusters in the LMC and 324 SMC clusters via isochrone fits to their resolved color-magnitude diagrams (Glatt, Grebel, & Koch 2010). Recent SMC star formation history based on its star clusters We found indications for two periods of enhanced star cluster formation in both galaxies some 100 to 200 Myr and 600 to 800 Myr ago, which may have been caused by past close encounters between the Clouds and the Milky Way. Moreover, we obtained the spatially resolved recent star formation history of both Clouds based on their young star clusters. The youngest clusters reside in supergiant and giant shells, in the compressed regions between two adjacent shells, and toward regions with a high Hα content, suggesting that their formation is related to shell expansion and shell-shell interaction. Most of the clusters, however, are older than the dynamical age of the supergiant shells (Glatt et al. 2010).

In the SMC, clusters younger than about 15 Myr are located along the bar, particularly toward the eastern and western rims of the HI supershells 37A and 304A. Clusters with ages of about 16 to 60 Myr are still concentrated around the bar with some outliers in the SMC wing. In the age range of 60 to 300 Myr, clusters are seen throughout the main body and the northern part of the SMC. The oldest age-dated clusters with ages between 300 Myr to 1 Gyr populate mainly the western part of the SMC's main body (Glatt et al. 2010).

In the LMC, clusters younger than 20 Myr are mainly found in the 30 Doradus region, in supershells LMC 3, 4, 8 (especially along the rims), in many of the giant shells, and in the western part of the bar. For ages of 20 to 50 Myr, the star clusters are more widely distributed across 30 Dor, LMC 3, 4, 5, 8, and many giant shells. Also, they are scattered throughout the western part of the LMC and show an increased concentration along the western and central parts of the bar. In the age range of 50 to 100 Myr, the cluster distribution resembles a slightly tilted capital Greek Θ with a high concentration of clusters in the northeast and along most of the bar (except for its easternmost parts). For ages from 100 to 250 Myr the similarity to a tilted Θ persists, and star clusters now occupy the entire length of the bar. In the age range of 250 to 500 Myr, star clusters populate mostly the LMC bar, but many are also distributed along the western and northwestern LMC rim as well as in the northeast. 30 Dor and LMC 3 are now almost devoid of clusters, and very few are seen in LMC 4. At yet higher ages the concentration of clusters along the bar shows a pronounced decline (Glatt et al. 2010). These results demonstrate how massive star formation migrated with time across the disk and gradually propagated along the bar of the LMC.

The recent SFB of the LMC traced by its star clusters

We used a subset of 307 age-dated clusters with masses > 5000 M⊙ and ages > 10 Myr in order to investigate the cluster age distribution and dissolution rate in the LMC (Baumgardt et al. 2013). We found the frequency of massive clusters to be roughly constant between 10 and 200 Myr. In other words, there is no significant influence of cluster dissolution for clusters younger than about 200 Myr, since both the ratio of the number of clusters to the absolute field star formation rate and the cluster mass function are independent of time for these clusters. LMC cluster dissolution: Cluster frequency vs. age If residual gas expulsion is an important dissolution mechanism for star clusters ("infant mortality"), its influence seems to be restricted to the first 10 Myr of cluster evolution or to low-mass clusters with masses below 5000 M⊙. Young star clusters in the LMC form with a power-law mass function of slope α = 2.3. Extrapolating this mass function down to 100 M⊙ and comparing it with the absolute star formation rate indicates that about 15% of all stars in the LMC form in bound star clusters that survive for at least 10 Myr (Baumgardt et al. 2013).

For ages older than 200 Myr, the cluster frequency starts to drop. The ratio of cluster frequency to star formation rate is approximately a factor of 40 smaller at 4 Gyr than what it was at 200 Myr. Also, the cluster mass function exhibits a much shallower slope for clusters older than 1 Gyr. The number of missing clusters in our catalog needed to explain this flattening seems to be too large to be explained by incompleteness, suggesting that most of the flattening is due to cluster dissolution. In fact, for ages higher than 200 Myr about 90% of all clusters are lost per dex of lifetime (Baumgardt et al. 2013).


3.4 Young Massive Clusters in Star-Forming Galaxies

Legacy ExtraGalactic UV Survey (LEGUS) The Legacy ExtraGalactic Ultraviolet Survey (LEGUS) is a Hubble Space Telescope Treasury Program that has imaged 50 star-forming galaxies of different Hubble types within 12 Mpc in UV and optical broadband filters (Calzetti et al. 2015a). A key data product of LEGUS are catalogs of the numerous young star clusters in the target galaxies as well as the physical characteristics of these clusters (Adamo et al., in prep.). Apart from studies of the resolved massive stellar populations, these unique data permit us to explore the young star cluster systems as a function of environment and host galaxy properties, to investigate the evolution of the clustering properties of intense massive star formation in space and time, cluster mass functions and their evolution, cluster dissolution time scales, and more. The analysis of the LEGUS data is ongoing, involving my graduate student Fayezeh Shabani. Below a few of the results are highlighted, focussing on star clusters.

LEGUS cluster age dating Thanks to the good wavelength coverage of the LEGUS HST data, the star clusters identified in the images can be age-dated via two-color diagrams and simple stellar population isochrones, via spectral energy distribution (SED) fits, and more complex modeling of stellar and nebular emission contributions, the stochastic sampling of the stellar mass function, etc. (see Calzetti et al. 2015a, 2015b; Krumholz et al. 2015; Wofford et al. 2016; and Adamo et al., in prep.). We found that applying different spectral synthesis models (from the Auckland, Geneva, and Padova groups) on average only leads to small differences in the resulting reddening, age, and mass. Models with interacting binaries provide the best fits while models with single rotating stars reproduce the SEDs less well (Wofford et al. 2016).

In the star-forming spiral galaxy NGC 628 we used 1392 cluster candidates younger than approximately 100 Myr to study their clustering properties via a two-point correlation function analysis (Grasha et al. 2015). We found that the strength of the clustering decreases with both increasing spatial scale and increasing age. For clusters older than 40 Myr we observe a pronounced decrease of the clustering strength - these clusters are largely randomly distributed. In terms of spatial scales, clusters beyond a scale length of about 160 pc show a fairly homogeneous distribution. Clusters with centrally concentrated light profiles ("compact clusters") tend to have the highest masses and oldest ages and show little clustering across all spatial scales, whereas extended, asymmetrical clusters ("associations") include those with the lowest average masses and youngest ages. They show the steepest clustering slopes. Generally though the mass of the star clusters turns out to have little effect on the clustering strength; it is primarily governed by age. Overall, the hierarchy in stellar aggregate clustering resembles that of the turbulent interstellar medium (Grasha et al. 2015).

NGC 628: Cluster age-mass diagram and clustering properties.

References



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