A physically motivated framework for measuring the mass and redshift dependence of galaxy pair fractions across cosmic time
Abstract
Low mass galaxy pair fractions are understudied across cosmic time. In the era of JWST, Roman, and Rubin, a selfconsistent framework is needed to select both low and high mass galaxy pairs to connect observed pair fractions to cosmological merger rates across all mass scales and redshifts. We use the Illustris TNG100 simulation to identify physically associated pairs between $z=04.2$. Our sample includes low mass ($\rm 10^8<M_*<5\times10^9\,M_{\odot}$) and high mass ($\rm 5\times10^9<M_*<10^{11}\,M_\odot$) isolated subhalo pairs, with stellar masses from abundance matching. The low mass pair fraction, i.e. the fraction of galaxies in pairs, increases from $z=02.5$, while the high mass pair fraction peaks at $z=0$ and is constant or slightly decreasing at $z>1$. At $z=0$ the low mass major (1:4 mass ratio) pair fraction is 4$\times$ lower than high mass pairs, consistent with findings for cosmological merger rates. Our results indicate that pair fractions can faithfully reproduce trends in merger rates if galaxy pairs are selected appropriately. Specifically, static pair separation limits applied equivalently to all galaxy pairs do not recover the evolution of low and high mass pair fractions. Instead, we advocate for separation limits that vary with the mass and redshift of the system, such as separation limits scaled by the virial radius of the host halo ($r_{\mathrm{sep}}< 1 R_{\rm vir}$). Finally, we place isolated massanalogs of Local Group galaxy pairs (i.e., MWM31, MWLMC, LMCSMC) in a cosmological context, showing that isolated analogs of LMCSMCmass pairs, and low separation ($<50$kpc) MWLMCmass pairs, are $23\times$ more common at $z\gtrsim23$.
 Publication:

arXiv eprints
 Pub Date:
 September 2023
 DOI:
 10.48550/arXiv.2309.13228
 arXiv:
 arXiv:2309.13228
 Bibcode:
 2023arXiv230913228C
 Keywords:

 Astrophysics  Astrophysics of Galaxies
 EPrint:
 17 pages, 5 figured, submitted to ApJ, comments welcome