Thesis Topic: The origin of the blue stellar halos around Massive Group-Dominant Galaxies

Thesis Supervisors:

Thesis Supervisors: Magda Arnaboldi and Johanna Hartke


The outermost stellar halos around the dominant galaxies in nearby massive groups are found to be very blue in colour. This measurement conflicts with the standard hierarchical mass assembly paradigm: the current late-accretion models predict that the merging satellites that formed these extended halos were relatively massive. Since typical mass ratios are of the order of 1:5, the merging satellites are thus expected to be more metal-rich according to the mass–metallicity relation and to contribute to redder halo colours. One interesting recent line of research in cosmological simulations indicate that mini-mergers (merger mass ratios 1:10 to 1:100; Karademir et al. 2019, MNRAS 487, 318) may be responsible for the formation of very extended blue structures, like diffuse light or very extended disks. This PhD thesis project aims to study the physical properties of the remote outskirts of early-type galaxies primarily using the kinematics of planetary nebulae in these halos. Late-accreted satellites leave tell-tale dynamical substructures at large radii that these observations are uniquely able to detect, even where they are photometrically invisible. This thesis requires active participation to data reduction and analysis and comparison of results with predictions from cosmological simulations provided by computational groups based in the nearby institutes.


  • Hartke, J., Arnaboldi, M., Gerhard, O. et al. ``The halo of M105 and its group environment as traced by planetary nebula populations: I. Wide-field photometric survey of planetary nebulae in the Leo I group'' 2020A&A...642A..46H
  • Hartke, J., Arnaboldi, M., Gerhard, O. et al. ``Three dynamically distinct stellar populations in the halo of M49'', 2018A&A...616A.123H



Figure 1: Field of views for some of the group member galaxies to be studied in this thesis (M60, NGC 4365 and M105). The fields in red are to be surveyed with the Planetary Nebulae Spectrograph (PN.S) mounted on the William Herschel telescope on La Palma, and the whole Field of view with the wide field imager Megacam@CFHT on the Maunakea Observatory.