Is the Initial Mass Function (IMF) stochastic or depends on the initial conditions? One way to test this is to study the mass distributed in the extremely young stellar population of the Magellanic Clouds.


The N159 cloud in the Large Magellanic Cloud [ESA/Hubble & NASA]

 Our project (led by T. Bitsakis, UNAM) utilizes a wide multi-wavelength approach: data are available in broad bands from the UV to the radio frequencies from several instruments. I am involved in the development of the computational technique for the detection of young clusters, and in the estimation of their ages through Bayesian isochrone fitting. I contributed in creating this novel routine which automatically identifies star associations in a completely automated way — superseding previous studies which necessitated strong human intervention — and which yielded the arguably large sample of clusters in the Clouds.

“The distribution and ages of star clusters in the Small Magellanic Cloud: Constraints on the interaction history of the Magellanic Clouds”

Bitsakis T. et al. 2017

“A Novel Method to Automatically Detect and Measure the Ages of Star Clusters in Nearby Galaxies: Application to the Large Magellanic Cloud”

Bitsakis T., Bonfini P. et al. 2017, ApJ, 845, 56B


I focus on the Globular Clusters (GCs) populations of Elliptical galaxies (Es) in order to investigate their formation, and the evolution and dynamics of the host galaxy.  I am also interested in GCs as old stellar populations associated to point X-ray sources.

Globular ClusterNGC 6397 [NASA, ESA and H. Richer — University of British Columbia]

What are GCs? — GCs are compact groups of stars containing 104 to 106 objects. Due to their brightness, extragalactic GCs usually stand out very clearly over the host galaxy background, and they can be used as tracers of the galaxy’s dynamics well beyond the limit where the galaxy surface brightness fades below the background signal. GCs are also excellent labs to explore the host galaxy’s evolution. In fact, to a first approximation, they can be considered as isolated systems of old stars. Therefore, they reflect the metallicity of the host galaxy at the moment of their creation. In particular, is interesting to notice that GC populations of almost all Elliptical galaxies show a clear colour bimodality (explained by a metallicity difference), which addresses to different formation/evolution paths as related to the host galaxy’s history.

Which are the X-ray sources in GCs? —  The X-ray point-source population of elliptical galaxies consists of Low Mass X-ray Binaries (LMXBs) associated with old stellar population. In particular, a large fraction of these LMXBs has GCs counterparts (∼50% for E to ∼70% for cD). LMXBs are associated preferentially to red (metal richer) GCs. On the other hand, roughly 4-5% of the GCs in a galaxy host a LMXB (see Fabbiano 2006, and references therein). Whether GCs are the sole birthplaces of LMXBs (e.g. White, Sarazin, & Kulkarni 2002), or field LMXBs form independently in situ (e.g. Kundu, Maccarone, & Zepf 2007, and references therein), is an old debate.

The first hypothesis is strongly supported by the higher stellar densities in the GCs (which enhance the probability of forming close binaries; e.g. Clark 1975; Fabian, Pringle, & Rees 1975) but requires the existence of an efficient escape mechanism that accounts for the LMXB in the field (possibly supernova kicks or cluster evaporation). The second hypothesis implies that the radial profiles of GC LMXBs and field LMXBs resemble the radial profiles of GC and galaxy light, respectively. As a consequence, the GC LMXBs radial distribution is expected to extend further than that of field LMXBs and flatten faster at small galactic radii. Such behaviour has not been observed so far.

I investigated the case of the elliptical galaxy NGC 4261, which exhibits a peculiar asymmetric 2D distribution of its GC population — despite its overall uniform starlight — and suggested that the origin of the asymmetry could be related to a past (dry) merger or interaction event (e.g. fly-by encounter).

 We found that approximately half of the LMXBs have GC counterparts (preferably red and bright GCs), in agreement with previous studies. However, the low number statistics did not allow to conclusively favor either of the LMXB formation scenarios.

“Studying the asymmetry of the GC population of NGC 4261”

Bonfini P. et al., 2012, MNRAS, 421, 2872

“The Two-dimensional Projected Spatial Distribution of Globular Clusters. I. Method and application to NGC 4261”

 D’Abrusco, R. et al., 2013, ApJ, 773, 87


Other than the formation and evolution of Low Mass X-ray Binaries (LMXBs) in Globular Cluster environments, my research on X-ray binaries (XRBs) is also oriented to the identification and spectral classification of possible optical counterparts of High Mass X-ray Binaries (HMXBs).

I contributed to the identification and spectral classification of the X-ray sources detected in the XMM-Newton major-axis survey of M 31 (Pietsch et al. 2005).

“Spectroscopy of the bright optical counterparts of X-ray sources in the direction of M 31”

Bonfini P. et al 2009, A&A, 507, 705

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