Feedback shaped the galaxy morphological sequence in presence of mergers

Abstract

Bulges and disks are major structural components that define galaxy morphology. The mass ratios of bulges and disks increase statistically with the galaxy mass, with the high-mass end occupied by elliptical galaxies. Although previous theoretical studies have succeeded in reproducing this morphological sequence, it is not yet fully understood why and how this morphological sequence emerged. Galaxy mergers accompanying dark matter halo mergers have been proposed as the major route for bulge formation. On the other hand, it is observationally known that the mass fraction of galaxies (stars plus cold gas) in dark matter halos attains the peak value at $M_{\rm halo} \sim 10^{12} {\rm M}_\odot$ throughout the cosmic time. Using a simple galaxy evolution model including mergers, we show that this feature is the fundamental cause of the morphological sequence. Halos hosting massive galaxies, which stay more massive than this peak mass for long periods during their growth, merge mostly with satellite halos having larger galaxy mass fractions than themselves. Such mergers increase the bulge mass fraction efficiently. In contrast, host halos of low-mass galaxies evolve under unfavorable condition for bulge growth because they stay below the peak mass and merge with satellite halos with smaller galaxy mass fractions. Previous studies suggest that the peak in galaxy mass fraction is created by feedback processes from active galactic nuclei (AGN) and young massive stars including supernovae (SN), which are considered to suppress star formation in high-mass and low-mass galaxies, respectively. This study thus points to a close relationship between the galaxy morphology and feedback processes which have hitherto been considered unrelated and suggests the importance of further investigation into their causal relationship.

Figures (5)

• Figure 1: Effects of feedback and galaxy-to-halo mass ratios on the bulge mass fraction and age. Models SN+AGN, SN, AGN, and NULL are colored by red, brown, blue and purple, respectively. a and d: The mass ratio, $\gamma$, of the galaxy and its halo is plotted against the halo mass. Gray lines indicate observations by Popping2015 ( a) and guo2023 ( d) for $z=$2 (dark), 1 (medium), and 0 (light), respectively. Each plot is normalized so that its maximum value is unity. b and c: Mass ratio, $B/T$, of bulges and total stellar content of galaxies at present are plotted against the halo mass ( b) and the galaxy stellar mass ( c). Green lines in c denote the data for MaNGA galaxies Bundy2015 (dark), SDSS galaxies sampled by 2011ApJS..196...11S(medium), and SDSS galaxies sampled by 2014ApJS..210....3M (light), respectively. e and f: Same as b and c, respectively, but for ages of bulges. Here, age is the lookback time at which 50$\%$ of stars were formed. Green dots in f indicate the observation for local galaxies by Breda2018.
• Figure 2: Merger characteristics for Galaxy A (upper panels) and Galaxy B (lower panels). a and d: Masses of satellite halos are plotted at the time of mergers, color-coded in accordance with epochs (I, II, III, IV) divided by vertical dashed lines. Solid lines indicate the mass growth of the primary halo (thick: mean, thin: maximum and minimum). Superposed is the gray-scale map for the galaxy-to-halo mass ratio, $\gamma$, for all galaxies in arbitrary units. b and e: Galaxy-to-halo mass ratios of the primary (large dots) and satellites (small dots) are plotted against halo masses, color-coded as in a and d. The dashed vertical lines approximately indicate the peak mass in $\gamma-M_{\rm halo}$ relation. c and f: Cumulative number distributions of $\xi$ (dashed lines) and $\delta$ (solid lines) for different epochs, color-coded as in a and d.
• Figure 3: Star formation history and assembly history for bulge stars in Galaxy A. Three pathways of the bulge growth are shown separately (green: conversion of the primary disk, orange: acquisition from satellite galaxies, cyan: starbursts driven by mergers). Total bulge component is shown in black. a and d show the star formation rates (SFR) and assembly rates, respectively. b and e respectively plot the growth of the formed mass and the assembled mass. The gray-scale map in c shows the formation sites of bulge stars, i.e., when and in what mass of the halo they were formed. Colored contours indicate 20% level of the peak intensity for each pathway. f indicates assembly (i.e., merger) events. Radii of the colored circles are proportional to the assembled mass for three pathways. In c and f, white solid lines indicate the mass growth of the primary halo (thick: mean, thin: maximum and minimum).
• Figure 4: Mass fractions (upper panel) and ages (lower panel) of three components of the present bulge plotted against the present stellar mass of the galaxy. Gray dots in the lower panel indicate the observation for total bulges in local galaxies by Breda2018.
• Figure 5: Upper panels: Halo-mass dependence of bulge formation and assembly. a: Solid (dashed) line indicates the formation (assembly) age, which is the lookback time at which half of the stars contained in the present bulge were formed (assembled). b: Contours indicate the formation sites of the present bulge stars for four mass groups by different colors. Each contour shows the level for half the maximum value. Dashed curves indicate the mean growth of halo mass in each group. The gray-scale map indicates the star formation rate as a function of the halo mass and the lookback time for all galaxies in arbitrary units. c: Same as b but merger events are indicated by circles, the radius of which is proportional to the ratio of the satellite galaxy mass and the present bulge mass. Lower panels: d and e: $B/T$ ratios for model galaxies ( d) and CANDELS galaxies observed by Hubble Space Telescope from Bruce2014 ( e). In d, the contours for $B/T$ = 0.2, 0.4, 0.6 0.8 are drawn with corresponding colors. f: Development of the galaxy morphological sequence. Shaded regions indicate 25 and 75 percentiles for model galaxies at each galaxy stellar mass for different lookback times as given in the diagram. Dots indicate the median in the stellar mass and $B/T$ for CANDELS galaxies while horizontal and vertical bars indicate 25 and 75 percentiles for the galaxy mass and $B/T$, respectively, for two indicated epochs.