Photoelectric heating--heating of dust grains by far-ultraviolet photons--has long been recognized as the primary source of heating for the neutral interstellar medium. Simulations of spiral galaxies ...have shown some indication that photoelectric heating could suppress star formation; however, simulations that include photoelectric heating have typically shown that it has little effect on the rate of star formation in either spiral galaxies or dwarf galaxies, which suggests that supernovae are responsible for setting the gas depletion time in galaxies. This result is in contrast with recent work indicating that a star formation law that depends on galaxy metallicity--as is expected with photoelectric heating,but not with supernovae--reproduces the present-day galaxy population better than does a metallicity-independent one. Here we report a series of simulations of dwarf galaxies, the class of galaxy in which the effects of both photoelectric heating and supernovae are expected to be strongest. We simultaneously include space and time-dependent photoelectric heating in our simulations, and we resolve the energy-conserving phase of every supernova blast wave, which allows us to directly measure the relative importance of feedback by supernovae and photoelectric heating in suppressing star formation. We find that supernovae are unable to account for the observed large gas depletion times in dwarf galaxies. Instead, photoelectric heating is the dominant means by which dwarf galaxies regulate their star formation rate at any given time,suppressing the rate by more than an order of magnitude relative to simulations with only supernovae.
We use ∼100 cosmological galaxy formation ‘zoom-in’ simulations using the smoothed particle hydrodynamics code gasoline to study the effect of baryonic processes on the mass profiles of cold dark ...matter haloes. The haloes in our study range from dwarf (M
200 ∼ 1010 M⊙) to Milky Way (M
200 ∼ 1012 M⊙) masses. Our simulations exhibit a wide range of halo responses, primarily varying with mass, from expansion to contraction, with up to factor ∼10 changes in the enclosed dark matter mass at 1 per cent of the virial radius. Confirming previous studies, the halo response is correlated with the integrated efficiency of star formation: ϵSF ≡ (M
star/M
200)/(Ωb/Ωm). In addition, we report a new correlation with the compactness of the stellar system: ϵR ≡ r
1/2/R
200. We provide an analytic formula depending on ϵSF and ϵR for the response of cold dark matter haloes to baryonic processes. An observationally testable prediction is that, at fixed mass, larger galaxies experience more halo expansion, while the smaller galaxies more halo contraction. This diversity of dark halo response is captured by a toy model consisting of cycles of adiabatic inflow (causing contraction) and impulsive gas outflow (causing expansion). For net outflow, or equal inflow and outflow fractions, f, the overall effect is expansion, with more expansion with larger f. For net inflow, contraction occurs for small f (large radii), while expansion occurs for large f (small radii), recovering the phenomenology seen in our simulations. These regularities in the galaxy formation process provide a step towards a fully predictive model for the structure of cold dark matter haloes.
We simulate the build-up of galaxies by spherical gas accretion through dark matter haloes, subject to the development of virial shocks. We find that a uniform cosmological accretion rate turns into ...a rapidly varying disc build-up rate. The generic sequence of events (Shocked-Accretion Massive Burst and Shutdown, SAMBA) consists of four distinct phases: (i) continuous cold accretion while the halo is below a threshold mass Msh∼ 1012 M⊙, (ii) tentative quenching of gas supply for ∼2 Gyr, starting abruptly once the halo is ∼Msh and growing a rapidly expanding shock, (iii) a massive burst due to the collapse of ∼1011 M⊙ gas in ∼0.5 Gyr, when the accumulated heated gas cools and joins new infalling gas and (iv) a long-term shutdown, enhanced by a temporary shock instability in late SAMBAs, those that quench at z∼ 2, burst at z∼ 1 and end up quenched in 1012−13 M⊙ haloes today. The quenching and bursting occur at all redshifts in galaxies of baryonic mass ∼1011 M⊙ and involve a substantial fraction of this mass. They arise from rather smooth accretion, or minor mergers, which, unlike major mergers, may leave the disc intact while being built in a rapid pace. The early bursts match observed maximum starbursting discs at z≳ 2, predicted to reside in ≲1013 M⊙ haloes. The late bursts resemble discy luminous infrared galaxies (LIRGs) at z≲ 1. On the other hand, the tentative quenching gives rise to a substantial population of ∼1011 M⊙ galaxies with a strongly suppressed star formation rate at z∼ 1−3. The predicted long-term shutdown leads to red and dead galaxies in groups. A complete shutdown in more massive clusters requires an additional quenching mechanism, as may be provided by clumpy accretion. Alternatively, the SAMBA bursts may trigger the active galactic nucleus (AGN) activity that couples to the hot gas above Msh and helps the required quenching. The SAMBA phenomenon is predicted based on a spherical model that does not simulate star formation and feedback – it is yet to be investigated using detailed cosmological simulations.
The observational indications for a constant specific star formation rate (sSFR) in the redshift range z= 2-7 are puzzling in the context of current galaxy-formation models. Despite the tentative ...nature of the data, their marked conflict with theory motivates a study of the possible implications. The plateau at sSFR ∼ 2 Gyr−1 is hard to reproduce because (a) its level is low compared to the cosmological specific accretion rate at z≥ 6, (b) it is higher than the latter at z∼ 2, (c) the natural correlation between SFR and stellar mass makes it difficult to manipulate their ratio, and (d) a low SFR at a high z makes it hard to produce enough massive galaxies by z∼ 2. Using a flexible semi-analytic model, we explore ad hoc modifications to the standard physical recipes trying to obey the puzzling observational constraints. Successful models involve non-trivial modifications, such as (a) a suppressed SFR at z≥ 4 in galaxies of all masses, by enhanced feedback or reduced SFR efficiency, following an initial active phase at z > 7; (b) a delayed gas consumption into stars, allowing the gas that was prohibited from forming stars or ejected at high z to form stars later in more massive galaxies; and (c) enhanced growth of massive galaxies, in terms of either faster assembly or more efficient starbursts in mergers, or by efficient star formation in massive haloes.
The majority of massive star-forming galaxies at z ∼ 2 have velocity gradients suggestive of rotation, in addition to large amounts of disordered motions. In this paper, we demonstrate that it is ...challenging to distinguish the regular rotation of a disk galaxy from the orbital motions of merging galaxies with seeing-limited data. However, the merger fractions at z ∼ 2 are likely too low for this to have a large effect on measurements of disk fractions. To determine how often mergers pass for disks, we look to galaxy formation simulations. We analyze ∼24,000 synthetic images and kinematic maps of 31 high-resolution simulations of isolated galaxies and mergers at z ∼ 2. We determine if the synthetic observations pass the criteria commonly used to identify disk galaxies and whether the results are consistent with their intrinsic dynamical states. Galaxies that are intrinsically mergers pass the disk criteria for anywhere from 0% to 100% of sightlines. The exact percentage depends strongly on the specific disk criteria adopted and weakly on the separation of the merging galaxies. Therefore, one cannot tell with certainty whether observations of an individual galaxy indicate a merger or a disk. To estimate the fraction of mergers passing as disks in current kinematics samples, we combine the probability that a merger will pass as a disk with theoretical merger fractions from a cosmological simulation. Taking the latter at face value, the observed disk fractions are overestimated by small amounts: at most by 5% at high stellar mass (1010-11 M ) and 15% at low stellar mass (109-10 M ).
Abstract
We model the projected b/a–log a distributions of CANDELS star-forming main-sequence galaxies, where a (b) is the half-light semimajor (semiminor) axis of the galaxy images measured by ...galfit. We find that smaller a galaxies are rounder at all stellar masses M* and redshifts, so we include a when analysing b/a distributions. Approximating intrinsic shapes of the galaxies as triaxial ellipsoids and assuming a multivariate normal distribution of galaxy size and two shape parameters, we construct their intrinsic shape and size distributions to obtain the fractions of elongated (prolate), discy (oblate), and spheroidal galaxies in each redshift and mass bin. We find that galaxies tend to be prolate at low M* and high redshifts, and discy at high M* and low redshifts, qualitatively consistent with van der Wel et al., implying that galaxies tend to evolve from prolate to discy. These results are consistent with the predictions from simulations that the transition from prolate to oblate is caused by a compaction event at a characteristic mass range, making the galaxy centre baryon dominated. We give probabilities of a galaxy’s being elongated, discy, or spheroidal as a function of its M*, redshift, and projected b/a and a, which can facilitate target selections of galaxies with specific shapes at high redshifts.
We present a simple toy model to understand what sets the scatter in star formation and metallicity of galaxies at fixed mass. According to this model, the scatter ultimately arises from the ...intrinsic scatter in the accretion rate, but may be substantially reduced depending on the time-scale on which the accretion varies compared to the time-scale on which the galaxy loses gas mass. This model naturally produces an anticorrelation between star formation and metallicity at a fixed mass, the basis of the fundamental metallicity relation. We show that observational constraints on the scatter in galaxy scaling relations can be translated into constraints on the galaxy-to-galaxy variation in the mass loading factor at fixed mass, and the time-scales and magnitude of a stochastic component of accretion on to star-forming galaxies. We find a remarkably small scatter in the mass loading factor, ≲ 0.1 dex, and that the scatter in accretion rates is smaller than that expected from N-body simulations.
Although giant clumps of stars are thought to be crucial to galaxy formation and evolution, the most basic demographics of clumps are still uncertain, mainly because the definition of clumps has not ...been thoroughly discussed. In this paper, we carry out a study of the basic demographics of clumps in star-forming galaxies at 0.5 < z < 3, using our proposed physical definition that UV-bright clumps are discrete star-forming regions that individually contribute more than 8% of the rest-frame UV light of their galaxies. Clumps defined this way are significantly brighter than the H II regions of nearby large spiral galaxies, either individually or blended, when physical spatial resolution and cosmological dimming are considered. The clump contribution in the intermediate-mass and massive galaxies is possibly linked to the molecular gas fraction of the galaxies. The clump contribution to the SFR of star-forming galaxies, generally around 4%-10%, also shows dependence on the galaxy M, but for a given galaxy M, its dependence on the redshift is mild.
We present 0 2 resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations at 870 m in a stellar mass-selected sample of 85 massive ( ) star-forming galaxies (SFGs) at in the ...CANDELS/3D-Hubble Space Telescope fields of UDS and GOODS-S. We measure the effective radius of the rest-frame far-infrared (FIR) emission for 62 massive SFGs. They are distributed over wide ranges of FIR size from to . The effective radius of the FIR emission is smaller by a factor of than the effective radius of the optical emission and is smaller by a factor of than the half-mass radius. Taking into account potential extended components, the FIR size would change only by ∼10%. By combining the spatial distributions of the FIR and optical emission, we investigate how galaxies change the effective radius of the optical emission and the stellar mass within a radius of 1 kpc, . The compact starburst puts most of the massive SFGs on the mass-size relation for quiescent galaxies (QGs) at z ∼ 2 within 300 Myr if the current star formation activity and its spatial distribution are maintained. We also find that within 300 Myr, ∼38% of massive SFGs can reach the central mass of , which is around the boundary between massive SFGs and QGs. These results suggest an outside-in transformation scenario in which a dense core is formed at the center of a more extended disk, likely via dissipative in-disk inflows. Synchronized observations at ALMA 870 m and James Webb Space Telescope 3-4 m will explicitly verify this scenario.
We use a series of high-resolution N-body simulations of the concordance cosmology to investigate the redshift evolution since z= 1 of the properties and alignment with the large-scale structure ...(LSS) of haloes in clusters, filaments, sheets and voids. We find that (i) once a rescaling of the halo mass with M*(z), the typical mass scale collapsing at redshift z, is performed, there is no further significant redshift dependence in the halo properties; (ii) the environment influences the halo shape and formation time at all investigated redshifts for haloes with masses M≲M* and (iii) there is a significant alignment of both spin and shape of haloes with filaments and sheets. In detail, at all redshifts up to z= 1: (a) haloes with masses below ∼M* tend to be more oblate when located in clusters than in the other environments; this trend is reversed at higher masses: above about M*, haloes in clusters are typically more prolate than similar massive haloes in sheets, filaments and voids. (b) The haloes with M≳M* in filaments spin more rapidly than similar mass haloes in clusters; haloes in voids have the lowest median spin parameters. (c) Haloes with M≲M* tend to be younger in voids and older in clusters. (d) In sheets, halo spin vectors tend to lie preferentially within the sheet plane independent of halo mass; in filaments, instead, haloes with M≲M* tend to spin parallel to the filament and higher mass haloes perpendicular to it. For halo masses M≳M*, the major axis of haloes in filaments and sheets is strongly aligned with the host filament or the sheet plane, respectively. Such halo–LSS alignments may be of importance in weak lensing analyses of cosmic shear. A question that is opened by our study is why, in the 0 < z < 1 redshift regime that we have investigated, the mass scale for gravitational collapse, M*, sets roughly the threshold below which the LSS environment either begins to affect, or reverses, fundamental properties of dark matter haloes.
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