The Gems of Galaxy Assembly

The main goal of my research is to better understand the processes that shape galaxies by exploring their fossil record of formation and evolution. Fossil records—the very gems of galaxy assembly—reside at various locations within and between galaxies. In my research I use several of these in complementary ways to observationally quantify the mass assembly of galaxies. Below you will find brief summaries of some of the scientific projets that I currently work on or have worked in the past. I am a member of the Fornax3D survey collaboration, the XSL project (to which I was the Project Manager), and the CALIFA survey.

Recent PhD students at ESO

  • 09/2018 – present: Katja Fahrion, "The Build-up of Galactic Nuclei" (ESO-IMPRS)
  • 04/2019 – 04/2020: Adriano Poci, "Combining stellar populations and dynamics" (ESO studentship, home institute: Macquarie University, Sidney, Australia)
  • 06/2019 – 06/2020: Kshama Kurian in co-supervision with Dominika Wylezalek (ESO Studentship, home institution: Indian Institute of Astrophysics, Bangalore, India)

    Research projects

    Looking for the fossil record of galaxy nuclei formation

    Galaxy nuclei’s angular momentum is very similar to N-body simulated NSCs that are formed via the in-spiralling of globular clusters (Lyubenova & Tsatsi 2019).

    Over the past decade we learnt that many galaxies harbour nuclear star clusters (NSCs) in their centres. There is evidence that they can co-exist with super-massive black holes, like in the Milky Way. The relationship between the mass of these central objects and the properties of their host galaxies is likely to be fundamental as it connects quantities that differ by several orders of magnitude. NSCs, unlike black holes, preserve their (and the host galaxy's) evolutionary history imprinted in their stellar populations and kinematics. The goal of my research is to understand whether the scaling relations between galaxy nuclei and host galaxy properties are driven by physical processes, like black hole feedback, or statistical ones, involving many subsequent mergers of galaxies and their NSCs and/or black holes. See below a list with the key publications where I and my current PhD student Katja Fahrion have presented several of our findings.

  • "Diversity of nuclear star cluster formation mechanisms revealed by their star formation histories", Fahrion, Lyubenova, et al. (2021), A&A in press
  • "Nuclear angular momentum of early-type galaxies hosting nuclear star clusters", Lyubenova & Tsatsi (2019), A&A 629, A44
  • "Constraining nuclear star cluster formation using MUSE-AO observations of the early-type galaxy FCC47 , Fahrion, Lyubenova, et al. (2019), A&A 628, A92
  • "The complex nature of the nuclear star cluster in FCC 277", Lyubenova et al. (2013), MNRAS 431, 3364–3372
  • "Central K-band kinematics and line strength maps of NGC 1399", Lyubenova, et al. (2008), A&A 485, 425

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  • The interplay between baryons and dark matter in galaxies

    The double power law IMF shape in early-type galaxies and its systemic variation (Lyubenova, Martín-Navarro et al. 2016).

    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 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 was 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 two-dimensional distribution of IMF in the Fornax cluster galaxy FCC167. (Martín-Navarro, Lyubenova, et al. 2019)

    A spatially varying IMF slope within a galaxy imposes several challenges to the way we measure the mass of its various components — from the central super-massive black hole all the way out to the dark halo. I am currently working on constraining the two-dimensional variation of the IMF slope inside galaxies. Our pilot analysis of FCC167 provided the first two-dimensional distribution of the IMF slope in a galaxy (Martín-Navarro, Lyubenova, et al. 2019). When compared with the stellar orbital distribution of the galaxy, we found that the IMF slope follows the bulge-like orbits. Together with collaborators I am currnetly working on including a variable IMF slope into our orbit-based dynamical modelling, which is essential to robustly determine the galaxies’ baryonic masses. This in turn will allow us to estimate the dark matter distribution without any assumptions on its halo profiles. This precise knowledge of the mass distribution in galaxies will be a true benchmark for galaxy formation models in a cosmological context.

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    Stellar kinematics and dynamics with CALIFA

    Examples of line-of-sight stellar kinematic maps from the CALIFA survey (Falcón-Barroso, Lyubenova, van de Ven et al. 2017).

    The motion of stars within galaxies is a fundamental property set very early on in their life. Thus the galaxy's evolutionary history can be constrained from the dynamical memory imprinted in the orbital distribution of collisionless stellar systems. In the period 2012-2017 I co-led (together with my colleagues Glenn van de Ven (University of Vienna) and Jesus Falcón-Barroso (IAC)) the working group of "Stellar kinematics and Dynamics" of the CALIFA survey. We used this unique 3D spectroscopy data set of about 600 galaxies of all types in the Local Universe to explore various traces of galaxy evolution using stellar kinematics and dynamics. Below some of our key results are listed.

  • "The CALIFA view on stellar angular momentum across the Hubble sequence", Falcón-Barroso, van de Ven, Lyubenova et al. (2019), A&A 632, A59
  • "The stellar orbit distribution in present-day galaxies inferred from the CALIFA survey", Zhu, van den Bosch, van de Ven, Rix, Lyubenova et al. (2018), Nature Astronomy, 2, 233
  • "Orbital decomposition of CALIFA spiral galaxies", Zhu, van den Bosch, van de Ven, Lyubenova et al. (2018), MNRAS, 473, 3000
  • "CALIFA reveals prolate rotation in massive early-type galaxies: A polar galaxy merger origin?", Tsatsi, Lyubenova, van de Ven et al. (2017), A&A 606, A62
  • "Stellar kinematics across the Hubble sequence in the CALIFA survey: general properties and aperture corrections", Falcón-Barroso, Lyubenova, van de Ven et al. (2017), A&A 597, A48
  • "IMF shape constraints from stellar populations and dynamics from CALIFA", Lyubenova, Martín-Navarro, van de Ven, Falcón-Barroso et al. (2016), MNRAS, 463, 3220

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  • The X-shooter Spectral Library

    The X-shooter Spectral Library (XSL) of stellar spectra. (Gonneau, Lyubenova, et al. 2020).

    The X-shooter Spectral Library (XSL) is a stellar spectroscopy library of 666 unique stars observed with the VLT/X-shooter instrument. It provides simultaneous spectra over the ultraviolet, visible, and near-infrared portion of the spectrum. The second data release (or DR2, Gonneau, Lyubenova et al. 2020) contains 816 observations of 666 unique stars. The end user has access to spectra that are corrected for instrument transmission and telluric absorption, and also corrected for wavelength-dependent flux-losses in 85% of the cases.

  • "The X-shooter Spectral Library (XSL): Data release 2", Gonneau, Lyubenova, Lançon, Trager, Peletier, et al. 2020, A&A, 634, A133
  • "Stellar atmospheric parameters for 754 spectra from the X-shooter Spectral Library", Arentsen, Prugniel, Gonneau, et al. 2019, A&A, 627, A138
  • XSL website and data access
  • Access to XSL data via ESO's Science Archive Facility

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  • Calibrating stellar population models in the near-IR

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

    Stellar population synthesis models have the largest uncertainties in the near-IR, due to the lack of large and well calibrated spectral libraries. To tackle this issue, during my PhD I created 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.

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