IMF shape constraints from stellar populations and dynamics

Once considered to be universal, nowadays there is growing evidence that the stellar initial mass function (IMF) varies systematically between and within elliptical galaxies. However, the precise kind of this ƒ variation is still a topic of heated debate. Stellar population studies tell us what is the fraction of low mass stars (<0.5 M) in a galaxy, but not what is the functional mass distribution of all the stars. In Lyubenova et al. (2016) we used a homogenous sample of giant elliptical galaxies from the CALIFA survey to study the shape of their IMFs. We compared the galaxies’ dynamical mass-to-light ratios (Υdyn) with their stellar populations based ones (Υ). Our study is the first to use this approach to test the predictions of stellar population models over a homogenous data set. First, we used the stringent constraint Υdyn ≥Υ, as there cannot be less total mass than only the one of the stars. We ruled out a single power-law IMF shape for 75 per cent of the galaxies in our sample. Conversely, we found that a double power-law IMF shape with a varying high-mass end slope is compatible (within 1σ ) with 95 per cent of the galaxies. We also showed that dynamical and stellar IMF mismatch factors give consistent results for the systematic variation of the IMF in these galaxies.

The True Nature of Nuclear Star Clusters in Early-Type Galaxies

FCC277 - a nucleated E5 galaxy in the Fornax Cluster, ACS/FCS

It is widely believed that 70-80% of all galaxies host nuclear star clusters (NSCs). However, only a small amount of studies about their detailed properties exists and they are mainly focused on NSCs in late-type galaxies. Characterising the NSCs in early-type galaxies is a non trivial task, both because of the high surface brightness of the underlying galaxy, and because of their compact sizes. Thus we have little observational insights about the formation processes that shape these objects. However, with the availability of adaptive optics fed IFU instruments, this task is less of a burden. In Lyubenova et al. (2013), we presented our study of the NSC in FCC277, a nucleated elliptical galaxy in the Fornax cluster. We used a combination of adaptive optics assisted near-infrared integral field spectroscopy, Hubble Space Telescope imaging and literature long-slit data. We showed that while the NSC does not appear to rotate within our detection limit of ∼6 km/s, rotation is detected at larger radii, where the isophotes appear to be discy, suggesting the presence of a nuclear disc. We also observed a distinct central velocity dispersion drop that is indicative of a dynamically cold rotating sub-system. Following the results of orbit-based dynamical modelling, corotating as well as counter-rotating stellar orbits are simultaneously needed to reproduce the observed kinematics. We found evidence for varying stellar populations, with the NSC and nuclear disc hosting younger and more metal rich stars than the main body of the galaxy. We argued that gas dissipation and some level of merging have likely played an important role in the formation of the nucleus of this intermediate-mass galaxy. This is in contrast to NSCs in low-mass early-type galaxies, which may have been formed primarily through the infall of star clusters.

The Mass distribution of Galaxies

The total mass of a galaxy is one of its most important parameter when we talk about galaxy evolution. However, measuring galaxy masses is not trivial. A very precise way to obtain the total mass (luminous plus dark) is to fit to the galaxy's 2-dimensional kinematics a dynamical model that will give us a reliable estimate. Unfortunately, such 2-dimensional kinematics information is available in a systematic way only for a few early-type galaxies. The goal of the CALIFA survey is to change that! This unprecedented data set will contain 2-dimensional Integral Field Spectroscopy for about 600 galaxies of all types in the Local Universe. We will be able to reconstruct the mass distribution of a large number of different type of galaxies, by building dynamical models based on their stellar and gas kinematics. Once we connect the intrinsic mass distribution to the properties of the stars and gas, we will be able to better understand the evolution of galaxies from the blue cloud to the red sequence.

Calibrating stellar population models in the near-IR

Near-IR SEDs of our sample of globular clusters, Lyubenova et al., 2012

Current population synthesis models have the largest uncertainties in the near-IR, due to the lack of large and well calibrated spectral libraries. To resolve this issue, I am leading a project to create the first library of integrated near-IR spectra of globular clusters in the Local Group. Our first K-band results (Lyubenova et al., 2010) demonstrated the importance of the near-IR light when exploring 1-3 Gyr old stellar populations, where the presence of carbon-rich AGB stars dramatically changes the integrated spectro-photometric properties of the host stellar population. In Lyubenova et al., 2012 we finalised this project by extending our library to the J- and H-band. We showed that the H-band C2 absorption feature and the overall J-, H-band spectral shape can be used as an age indicator for intermediate age stellar populations in integrated spectra of star clusters and galaxies.

Mariya, Feb 2015


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