Kavli Affiliate: Luis C. Ho
| First 5 Authors: Yuan Bian, Min Du, Victor P. Debattista, Dylan Nelson, Mark A. Norris
| Summary:
Most galaxies follow well-defined scaling relations of metallicity and
stellar mass; however, some outliers at the low mass end of the observed galaxy
population exhibit unusually high metallicity for their mass. Understanding how
these objects get to be so metal-rich is vital for understanding the role of
feedback in galaxy formation. Using the TNG50 simulation, we explore the
origins of this phenomenon. We identify 227 metal-rich, Compact Stellar Systems
(CSSs) that deviate significantly from this scaling relation. These CSSs are
satellites located in the vicinity of massive host galaxies, with stellar
masses ranging from $10^{8} M_{odot}$ to $10^{10} M_{odot}$ (including six
systems that are close analogs of the M31-M32 system). Contrary to the
previously assumed scenario that such objects are predominantly products of
tidal stripping, our results suggest a more prevalent role for ram pressure in
their formation. Indeed, 76% (173) of these CSSs are formed through a burst of
star formation occurring around the time of the first pericentric passage,
typically at redshifts $zlesssim1$, aided by strong ram pressure and tidal
forces. The high ram pressure, resulting from the CSSs’ rapid motion near the
halo center, facilitates metal enrichment, producing high-metallicity CSSs by
confining the metal-rich gas from bursty star formation, which leads to
distinct stellar populations characterized by enhanced metallicity as well as
high $alpha$-abundance. Only the remaining 24% (54) of metal-rich CSSs are
generated through the tidal stripping of massive progenitors. Our results
further indicate that M32 is more likely to have formed through intense star
formation events rather than through gradual, tidal stripping, thereby
providing crucial insights into the nature of low mass, compact galaxy
formation.
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