Inferring the complex internal structure of dwarf galaxies

 This thesis focuses on modeling the complex internal structure of dwarf galaxies. Dwarf  galaxy is the most frequent type of galaxy in the universe. It is dominated by the dark  matter and thus naturally becomes the best targets for testing the dark matter theories.  The work I did focuses on the observations of the dwarf galaxies and infers the underlying structures using the observational data from several sky surveys.  

In the first project, I infer the 3D shape of the dwarf galaxies around the MW. I com bine the Dark Energy Camera Legacy Survey (DECaLS) DR8 photometry with Gaia photometry to study the 3D structure of Bootes I, Draco, Ursa Minor, Sextans and Sculptor  dwarf galaxies using blue horizontal branch (BHB) stars as distance indicators. I construct a new colour-absolute magnitude of BHB stars that I use to measure the distance  gradients within the bodies of the dwarf galaxies. I detect a statistically significant non zero gradient only in Sextans and Sculptor. Through modelling of the gradient and 2-D  density of the systems using triaxial Plummer models, I find that the distance gradients  in both dwarf galaxies are inconsistent with prolate shape but compatible with oblate or  triaxial shapes. In order to explain the observed gradients, the oblate models of Sextans  and Sculptor need to have asignificant intrinsic ellipticity larger than 0.47 for Sextans and  0.46 for Sculptor. The flattened oblate shape may imply a significant anisotropy in the velocity distribution in order to be consistent with the lack of significant velocity gradients  in these systems.  

In the next project, I look at a specific system: the Sagittarius dwarf galaxy and M54.  I present results from simultaneous modeling of 2D (projected along the line of sight)  position, proper motion and line-of-sight velocity for Gaia- and APOGEE-observed stars  near the centre of the Sagittarius (Sgr) dwarf spheroidal galaxy. I use a mixture model  that allows for independent sub-populations contributed by the Sgr galaxy, its nuclear  star cluster M54, and the Milky Way foreground. I find an offset of 0.295 ± 0.029 degrees between the inferred centroids of Sgr and M54, corresponding to a (projected)  physical separation of 0.135 ± 0.013 kpc. The detected offset might plausibly be driven  by unmodelled asymmetry in Sgr’s stellar configuration; however, standard criteria for  model selection favour our symmetric model over an alternative that allows for bilateral asymmetry. I infer an offset between the proper motion centres of Sgr and M54 of  [∆µα cosδ,∆µδ] = [4.9,−19.7] ± [6.8,6.2] µas yr−1, with magnitude similar to the covariance expected due to spatially-correlated systematic error. I infer an offset of 4.1±1.2 km  s−1 in line-of-sight velocity. Using inferred values for the systemic positions and motions  of Sgr and M54 as initial conditions, I calculate the recent orbital history of a simplified  Sgr/M54 system, which I demonstrate to be sensitive to any line-of-sight distance offset  between M54 and Sgr, and to the distribution of dark matter within Sgr. Considering  the case that the centroids of M54 and Sgr currently are offset by ≲ 0.7 kpc, I find that  if Sgr’s dark matter halo has the central ‘cusp’ that characterises cold dark matter halos,  then M54 is currently approaching apocentre of an orbit (within Sgr) that is gradually de caying due to dynamical friction. If Sgr’s dark matter halo has a ‘core’ of uniform central  density, then M54 is currently near pericentre of an orbit that is, as expected from previous work, relatively unaffected by dynamical friction. Finally, if the centroids currently  are offset by ≳ 0.7 kpc, (thus dominated by an offset in line-of-sight distance), then the  calculated orbits would imply that M54 fell into Sgr within the past 200 Myr.