THE FATE of A RED NUGGET: In SITU STAR FORMATION of SATELLITES AROUND A MASSIVE COMPACT GALAXY

To study the accretion phase for local massive galaxies, we search accreting satellites around a massive compact galaxy (M∗3.9x10¹⁰M⊙), spectroscopically confirmed (zspec = 1.9213) in the eXtreme Deep Field, which has been originally reported in Szomoru et al. We detect 1369 satellite candidates within the projected virial radius (rvir ∼300 kpc) of the compact galaxy in the all-combined Advanced Camera for Surveys image, with a 5s-limiting magnitude of mACS∼30.6 ABmag, which corresponds to ∼1.6 10⁷ M⊙at the redshift. The photometric redshift measured with 12 multi-band images confirms 34satellites out of the candidates. Most of the satellites are found to have rest-frame colors consistent with star-forming galaxies. We investigate the relation between stellar mass and star formation rate (the star formation main sequence), and find a steeper slope at the lowmass end (>10⁸ M⊙), while more massive satellites are consistently on the sequence reported in previous studies. Within the uncertainties of star formation and photometric redshift, we conjecture possible scenarios for the compact galaxy which evolves to a local massive galaxy by way of significant size and mass growth. While the merging of the existing total stellar mass in the satellites is not enough to explain the mass growth predicted by observations and simulations, the contribution by in situ star formation in the satellites could compensate for the deficit. Provided that most satellites keep the observed in situ star formation and then quench before they accrete by, e.g., environmental quenching, the compact galaxy would become a massive early-type galaxy consistent with the local sizemass relation.