Speaker
Description
Deriving galaxies’ past merger history is challenging, albeit possible with their 3D internal chemical-dynamical structures illustrated by the MW with resolved single stars. Modern IFU spectrographs offer the ability to map the spatial distribution of the motions, ages, and chemical abundances of stars for many nearby galaxies. But the information is still blended along the line-of-sight. To further uncover the intrinsic 3D chemo-dynamical structures, we developed a novel population-orbit superposition method, which fit all the aforementioned data simultaneously. This allows us to quantify the mass and stellar populations of hot inner stellar halo, a structure as relic of ancient mass mergers like the MW Gaia-Enceladus-sausage, thus uncover the merger history of many nearby galaxies in a comparable way.
By applying the method to the Fornax cluster galaxies observed by MUSE/VLT through the Fornax3D project, we find two of them, NGC1380 and NGC1427, have massive inner stellar halo components. By comparing to analogues from the cosmological galaxy simulation TNG50 and EAGLE, we find that the formation of the inner stellar halo of NGC1380 and NGC1427 requires a merger with stellar mass of ~4e10 Msun and ~1.4e10 Msun, respectively. We further constrain their merger time by the stellar age distributions in different structures, and find the last massive merger should be happened at ~10 Gyr ago for NGC 1380 and <8 Gyr ago for NGC 1427.
On the other hand, we provide decisive evidence for finding the extreme `merger-free’ relic galaxies, which are taken as the survived z~2 red nugget galaxies not experienced any subsequent mergers. Compact ETGs in the nearby universe are usually taken as candidates as they are massive, extremely compact and with old stellar populations, which, however, could also be result of tidal stripping to a normal ETG. By applying our method to a sample of 16 compact ETGs in the nearby universe, we find seven of them have extremely low fraction of hot inner stellar halo, and with a survived cold disk, which strongly support them as true relic galaxies. This is consistent with the high fraction of red disky galaxies found by JWST at hight-z.
