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Register a new userDisk Growth in Bulge-Dominated Galaxies: Molecular Gas and Morphological Evolution
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Lisa Young
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Substantial numbers of morphologically regular early-type (elliptical and lenticular) galaxies contain molecular gas, and the quantities of gas are probably sufficient to explain recent estimates of the current level of star formation activity. This gas can also be used as a tracer of the processes that drive the evolution of early-type galaxies. For example, in most cases the gas is forming dynamically cold stellar disks with sizes in the range of hundreds of pc to more than one kpc, although there is typically only 1% of the total stellar mass currently available to form young stars. The numbers are still small, but the molecular kinematics indicate that some of the gas probably originated from internal stellar mass loss while some was acquired from outside. Future studies will help to quantify the role of molecular gas (dissipational processes) in the formation of early-type galaxies and their evolution along the red sequence.
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To break the degeneracy among galactic stellar components, we extract kinematic structures using the framework described in Du et al. (2019, 2020). For example, the concept of stellar halos is generalized to weakly-rotating structures that are composed of loosely bound stars, which can hence be associated to both disk and elliptical type morphologies. By applying this method to central galaxies with stellar mass $10^{10-11.5} M_odot$ from the TNG50 simulation, we identify three broadly-defined types of galaxies: ones dominated by disk, by bulge, or by stellar halo structures. We then use the simulation to infer the underlying connection between the growth of structures and physical processes over cosmic time. Tracing galaxies back in time, we recognize three fundamental regimes: an early phase of evolution ($zgtrsim2$), and internal and external (mainly mergers) processes that act at later times. We find that disk- and bulge-dominated galaxies are not significantly affected by mergers since $zsim2$; the difference in their present-day structures originates from two distinct evolutionary pathways, extended vs. compact, that are likely determined by their parent dark matter halos; i.e., nature. On the other hand, slow rotator elliptical galaxies are typically halo-dominated, forming by external processes (e.g. mergers) in the later phase, i.e., nurture. This picture challenges the general idea that elliptical galaxies are the same objects as classical bulges. In observations, both bulge- and halo-dominated galaxies are likely to be classified as early-type galaxies with compact morphology and quiescent star formation. However, here we find them to have very different evolutionary histories.
We present a study of large-scale bars in the local Universe, based on a large sample of ~3692 galaxies, with -18.5 <= M_g < -22.0 mag and redshift 0.01 <= z < 0.03, drawn from the Sloan Digitized Sky Survey. Our sample includes many galaxies that are disk-dominated and of late Hubble types. Both color cuts and Sersic cuts yield a similar sample of ~2000 disk galaxies. We characterize bars and disks by ellipse-fitting r-band images and applying quantitative criteria. After excluding highly inclined ($>60^{circ}$) systems, we find the following results. (1) The optical r-band fraction (f_opt-r) of barred galaxies, when averaged over the whole sample, is ~48%-52%. (2) When galaxies are separated according to half light radius (r_e), or normalized r_e/R_24, which is a measure of the bulge-to-disk (B/D) ratio, a remarkable result is seen: f_opt-r rises sharply, from ~40% in galaxies that have small r_e/R_24 and visually appear to host prominent bulges, to ~70% for galaxies that have large r_e/R_24 and appear disk-dominated. (3) $f_{rm opt-r}$ rises for galaxies with bluer colors (by ~30%) and lower masses (by ~15%-20%). (4) While hierarchical $Lambda$CDM models of galaxy evolution models fail to produce galaxies without classical bulges, our study finds that ~20% of disk galaxies appear to be ``quasi-bulgeless. (5) After applying the same cutoffs in magnitude (M_V<-19.3 mag), bar size (a_bar >= 1.5 kpc), and bar ellipticity (e_bar >=~0.4) that studies out to z~1 apply to ensure a complete sample, adequate spatial resolution, and reliable bar identification, we obtain an optical r-band bar fraction of 34%. This is comparable to the value reported at z~0.2-1.0, implying that the optical bar fraction does not decline dramatically by an order of magnitude in bright galaxies out to z~1. (abridged)
We have carried out a joint photometric and structural analysis of red sequence galaxies in four clusters at a mean redshift of z ~ 1.25 using optical and near-IR HST imaging reaching to at least 3 magnitudes fainter than $M^*$. As expected, the photometry and overall galaxy sizes imply purely passive evolution of stellar populations in red sequence cluster galaxies. However, the morphologies of red sequence cluster galaxies at these redshifts show significant differences to those of local counterparts. Apart from the most massive galaxies, the high redshift red sequence galaxies are significantly diskier than their low redshift analogues. These galaxies also show significant colour gradients, again not present in their low redshift equivalents, most straightforwardly explained by radial age gradients. A clear implication of these findings is that red sequence cluster galaxies originally arrive on the sequence as disk-dominated galaxies whose disks subsequently fade or evolve secularly to end up as high Sersic index early-type galaxies (classical S0s or possibly ellipticals) at lower redshift. The apparent lack of growth seen in a comparison of high and low redshift red sequence galaxies implies that any evolution is internal and is unlikely to involve significant mergers. While significant star formation may have ended at high redshift, the cluster red sequence population continues to evolve (morphologically) for several Gyrs thereafter.
The role of disk instabilities, such as bars and spiral arms, and the associated resonances, in growing bulges in the inner regions of disk galaxies have long been studied in the low-redshift nearby Universe. There it has long been probed observationally, in particular through peanut-shaped bulges. This secular growth of bulges in modern disk galaxies is driven by weak, non-axisymmetric instabilities: it mostly produces pseudo-bulges at slow rates and with long star-formation timescales. Disk instabilities at high redshift (z>1) in moderate-mass to massive galaxies (10^10 to a few 10^11 Msun of stars) are very different from those found in modern spiral galaxies. High-redshift disks are globally unstable and fragment into giant clumps containing 10^8-10^9 Msun of gas and stars each, which results in highly irregular galaxy morphologies. The clumps and other features associated to the violent instability drive disk evolution and bulge growth through various mechanisms, on short timescales. The giant clumps can migrate inward and coalesce into the bulge in a few 10^8 yr. The instability in the very turbulent media drives intense gas inflows toward the bulge and nuclear region. Thick disks and supermassive black holes can grow concurrently as a result of the violent instability. This chapter reviews the properties of high-redshift disk instabilities, the evolution of giant clumps and other features associated to the instability, and the resulting growth of bulges and associated sub-galactic components.
Spiral galaxies have most of their stellar mass in a large rotating disk, and only a modest fraction in a central spheroidal bulge. This poses a major challenge for cosmological models of galaxy formation. Galaxies form at the centre of dark matter halos through a combination of hierarchical merging and gas accretion along cold streams, and should rapidly grow their bulge through mergers and instabilities. Cosmological simulations predict galaxies to have most of their mass in the central bulge, and therefore an angular momentum much below the observed level, except in dwarf galaxies. We propose that the continuous return of fresh gas by stellar populations over cosmic times could solve this issue. A population of stars formed at a given instant typically returns half of its initial mass in the form of gas over 10 billion years, and the process is not dominated by rapid supernovae explosions but by the long-term mass-loss from low- and intermediate-mass stars. Using simulations of galaxy formation, we show that this recycling of gas can strongly affect the structural evolution of massive galaxies, potentially solving the bulge fraction issue: we find that the bulge-to-disk ratio of a massive galaxy can be divided by a factor of 3. The continuous recycling of baryons through star formation and stellar mass loss helps the growth of disks and their survival to interactions and mergers. Instead of forming only early-type, spheroid-dominated galaxies (S0 and ellipticals), the standard cosmological model can then successfully account for massive late-type, disk-dominated spiral galaxies (Sb-Sc).
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