Reconstructing the Assembly of Massive Galaxies. I. The Importance of the Progenitor Effect in the Observed Properties of Quiescent Galaxies at z ≈ 2
Zhiyuan Ji, Mauro Giavalisco
Abstract
Abstract We study the relationship between the morphology and star formation history (SFH) of 361 quiescent galaxies (QGs) at redshift 〈 z obs 〉 ≈ 2, with stellar mass <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>*</mml:mo> </mml:mrow> </mml:msub> <mml:mo>≥</mml:mo> <mml:mn>10.3</mml:mn> </mml:math> , selected with the UVJ technique. Taking advantage of panchromatic photometry covering the rest-frame UV-to-NIR spectral range ( ≈40 bands), we reconstruct the nonparametric SFH of the galaxies with the fully Bayesian SED fitting code P rospector . We find that the half-light radius R e , observed at z obs , depends on the formation redshift of the galaxies, z form , and that this relationship depends on M * . At <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>*</mml:mo> </mml:mrow> </mml:msub> <mml:mo><</mml:mo> <mml:mn>11</mml:mn> </mml:math> , the relationship is consistent with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>e</mml:mi> </mml:mrow> </mml:msub> <mml:mo>∝</mml:mo> <mml:msup> <mml:mrow> <mml:mfenced close=")" open="("> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>+</mml:mo> <mml:msub> <mml:mrow> <mml:mi>z</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>form</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:mfenced> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> , in line with the expectation that the galaxies’ central density depends on the cosmic density at the time of their formation, i.e., the “progenitor effect.” At <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>*</mml:mo> </mml:mrow> </mml:msub> <mml:mo>></mml:mo> <mml:mn>11</mml:mn> </mml:math> , the relationship between R e and z form flattens, suggesting that mergers become increasingly important for the size growth of more massive galaxies after they quenched. We also find that the relationship between z form and galaxy compactness similarly depends on M * . While no clear trend is observed for QGs with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>*</mml:mo> </mml:mrow> </mml:msub> <mml:mo>></mml:mo> <mml:mn>11</mml:mn> </mml:math> , lower-mass QGs that formed earlier, i.e., with larger z form , have larger central stellar-mass surface densities, both within the R e (Σ e ) and central 1 kpc (Σ 1 kpc ), and also larger M 1 kpc / M * , the fractional mass within the central 1 kpc. These trends between z form and compactness, however, essentially disappear if the progenitor effect is removed by normalizing the stellar density with the cosmic density at z form . Our findings highlight the importance of reconstructing the SFH of galaxies before attempting to infer their intrinsic structural evolution.