THE CONTRIBUTION OF STAR FORMATION AND MERGING TO STELLAR MASS BUILDUP IN GALAXIES

摘要

We present a formalism to reveal merging by subtracting the change in the galaxy stellar mass function (MF) due to mass-dependent star formation (SF) from the observed time derivative of the MF. We present the SF rate (SFR) in the FORS Deep Field as a function of stellar mass and time spanning 9 < log M_* < 12 and 0 < z < 5. At z approx > 3, the average SFR is a power law of stellar mass (M_*t ∝ M_*~(0.6)). The average SFR in massive objects at this redshift is 100-500 M_☉ yr~(-1). At z ～ 3, the SFR starts to drop at high masses. The break mass at which the SFR deviates from the power-law form decreases smoothly from log M_* approx> 13 atz ～ 5 to log M_*~1 ～ 10.9 atz ～ 0.5, according to M_*~1(z) = 2.7 × 10~(10)(1 +z)~(2.1) (downsizing). We directly observe a relationship between SF history and mass. More massive galaxies have steeper and earlier onsets of SF, higher peak SFR, followed by a shorter decay time. The SFR in high-mass galaxies at z ～ 4 can explain their rapid increase in abundance. Within large uncertainties, at most 0.8 effective major mergers per Gyr are consistent with the data, yet enough to transform most high-mass objects into ellipticals contemporaneously with their major SF episode. In contrast, atz < 1.5 and at M_* approx> 11, mergers contribute 0.1-0.2 Gyr~(-1) to the increase in number density. This corresponds to ～1 major merger per object a 1.5 > z > 0.At 10< log M_* < 11, galaxies are being preferentially destroyed at early times, while at later times the change in their numbers turns positive. This is an indication of the top-down buildup of the red sequence suggested by other recent observations.