Where are the extremely metal-poor stars in the Milky Way and Andromeda? Expectations from TNG50

Li-Hsin Chen, Annalisa Pillepich, Simon C. O. Glover, Ralf S. Klessen

Submitted on 31 October 2022, last revised on 1 December 2022


We analyse the location of extremely metal-poor stars (EMPs, [Fe/H]<3) in 198 Milky Way (MW)/M31-like galaxies at z=0 in the TNG50 simulation. Each system is divided into four kinematically-defined morphological stellar components based on stellar circularity and galactocentric distance, namely bulge, cold disk, warm disk, and stellar halo, in addition to satellites (with stellar mass 5×106M). According to TNG50 and across all simulated systems, the stellar halo of the main galaxy and satellites present the highest frequency of EMPs (largest MEMP,comp-to-Mtot,comp stellar mass ratio), and thus the highest chances of finding them. Such frequency is larger in lower-mass than high-mass satellites. Moreover, TNG50 predicts that the stellar halo of the main galaxy always hosts and thus contributes the majority of the EMPs of the system. Namely, it has the highest mass ratio of EMPs in it to all the EMPs in the system (largest MEMP,comp-to-MEMP(<300kpc)). However, notably, we also find that 33 MW/M31-like galaxies in TNG50 have cold disks that contribute more than 10 per cent to the total EMP mass, each with 106.57M of EMPs in cold circular orbits. These qualitative statements do not depend on the precise definition of EMP stars, i.e. on the adopted metallicity threshold. The results of this work provide a theoretical prediction for the location of EMP stars from both a spatial and kinematic perspective and across an unprecedented number of well-resolved MW/M31-like systems.


Comment: Part of a set of papers based on TNG50 MW/M31-like galaxies. 14 pages, 9 figures. Accepted manuscript by MNRAS

Subjects: Astrophysics - Astrophysics of Galaxies; Astrophysics - Cosmology and Nongalactic Astrophysics


{\bf Stellar metallicity distribution functions (MDFs) of MW/M31-like galaxies in TNG50.} {\it Top panels:} we show the MDFs of all 198 MW/M31 systems, across {\it all} their morphological components: disks, bulge, stellar halo and satellites, colour coded by galaxy stellar mass. The redder the colour, the higher the galaxy stellar mass. On the left, we show the cumulative fraction and on the right, we show the stellar mass. {\it Middle three panels:} MDFs of the subsample TNG50 galaxies with mass more similar to the Milky Way (gray curves) overlaid to results from observations and thus with stars selected by height and radial distance to attempt to account for the surveys' selection functions: we report here the Milky Way's MDF by \citet[][green]{Youakim:2020aa}, \citet[][magenta]{Bonifacio:2021aa}, and by \citet[][orange]{Buder:2021aa}. For the middle left panel, we select stars that have heliocentric distances between 6 and 20 kpc, and at Galactic latitudes between 30 and 78 degrees ($|b| = 30-78$). The probability is then normalised to the total number of stars between $-4. < \feh < -1.05$ to compare with \citet{Youakim:2020aa}. In the middle central panel, we select stars that are < 6\,kpc from the galactic plane and < 14\,kpc from the galactic centre. The probability is again normalised to the total number of stars between $-4. < \feh < -1.05$ to compare with \citet{Bonifacio:2021aa}. For the middle right panel, we select stars that have heliocentric distances $<2$ kpc and that are positioned at Galactic latitudes larger than 10 degrees ($|b| > 10$) to compare with data from GALAH survey Data Release 3 \citep{Buder:2021aa}. In order to take into account the distance-selection criterion from the survey, the solar position in each MW-like galaxy is randomly sampled, at 8.2 kpc from the galactic centre, and we compute the mean MDF over 100 possibilities. The probability is normalised to the total number of stars. {\it Bottom three panels:} MDF comparison to data from \citet[][shades of blue, for three different height selections]{Hayden:2015aa}. We group stars at different distances from the galactic plane to compare with \citet{Hayden:2015aa}, where the probabilities are normalised to total number of stars in each subset. We do not impose any additional selection function to TNG50 star particles but for the aforementioned geometrical spatial cuts that account for the effective footprints of the APOGEE survey Data Release 12. Note that the survey cannot report many metal-poor stars as it hits the detection limit at $\feh \sim -2$.