Mass-size Relation Research Articles

The VMC Survey. XXIX. Turbulence-controlled Hierarchical Star Formation in the Small Magellanic Cloud

Abstract In this paper we report a clustering analysis of upper main-sequence stars in the Small Magellanic Cloud, using data from the VMC survey (the VISTA near-infrared YJK s survey of the Magellanic system). Young stellar structures are identified as surface overdensities on a range of significance levels. They are found to be organized in a hierarchical pattern, such that larger structures at lower significance levels contain smaller ones at higher significance levels. They have very irregular morphologies, with a perimeter–area dimension of 1.44 ± 0.02 for their projected boundaries. They have a power-law mass–size relation, power-law size/mass distributions, and a log-normal surface density distribution. We derive a projected fractal dimension of 1.48 ± 0.03 from the mass–size relation, or of 1.4 ± 0.1 from the size distribution, reflecting significant lumpiness of the young stellar structures. These properties are remarkably similar to those of a turbulent interstellar medium, supporting a scenario of hierarchical star formation regulated by supersonic turbulence.

Must Star-forming Galaxies Rapidly Get Denser before They Quench?

Abstract Using the deepest data yet obtained, we find no evidence preferring compaction-triggered quenching—where rapid increases in galaxy density truncate star formation—over a null hypothesis in which galaxies age at constant surface density ( ). Results from two fully empirical analyses and one quenching-free model calculation support this claim at all z ≤ 3: (1) qualitatively, galaxies’ mean U–V colors at have reddened at rates/times correlated with , implying that there is no density threshold at which galaxies turn red but that sets the pace of maturation; (2) quantitatively, the abundance of red galaxies never exceeds that of the total population a quenching time earlier at any , implying that galaxies need not transit from low to high densities before quenching; (3) applying to a suite of lognormal star formation histories reproduces the evolution of the size–mass relation at . All results are consistent with evolutionary rates being set ab initio by global densities, with denser objects evolving faster than less-dense ones toward a terminal quiescence induced by gas depletion or other ∼Hubble-timescale phenomena. Unless stellar ages demand otherwise, observed thresholds need not bear any physical relation to quenching beyond this intrinsic density–formation epoch correlation, adding to Lilly & Carollo’s arguments to that effect.

Universal formula for the holographic speed of sound

We consider planar hairy black holes in five dimensions with a real scalar field in the Breitenlohner–Freedman window and derive a universal formula for the holographic speed of sound for any mixed boundary conditions of the scalar field. As an example, we numerically construct the most general class of planar black holes coupled to a single scalar field in the consistent truncation of type IIB supergravity that preserves the SO(3)×SO(3) R-symmetry group of the gauge theory. For this particular family of solutions, we find that the speed of sound exceeds the conformal value. From a phenomenological point of view, the fact that the conformal bound can be violated by choosing the right mixed boundary conditions is relevant for the existence of neutron stars with a certain mass-size relationship for which a large value of the speed of sound codifies a stiff equation of state. In the way, we also shed light on a puzzle regarding the appearance of the scalar charges in the first law. Finally, we generalize the formula of the speed of sound to arbitrary dimensional scalar-metric theories whose parameters lie within the Breitenlohner–Freedman window.

The Isophotal Structure of Star-forming Galaxies at 0.5 < z < 1.8 in CANDELS: Implications for the Evolution of Galaxy Structure

Abstract We have measured the radial profiles of isophotal ellipticity (ε) and disky/boxy parameter A 4 out to radii of about three times the semimajor axes for ∼4600 star-forming galaxies (SFGs) between redshift 0.5 and 1.8 in the CANDELS/GOODS-S and UDS fields. Based on the average size–mass relation in each redshift bin, we divide our galaxies at a given mass into Small SFGs (SSFGs; smaller than the average) and Large SFGs (LSFGs; larger than the average). We show that, at low masses ( ), the SSFGs generally have nearly flat ε and A 4 profiles in both edge-on and face-on views, especially at . Moreover, the median A 4 values at all radii are almost zero. In contrast, the highly inclined low-mass LSFGs in the same mass-redshift bins generally have monotonically increasing ε profiles with radius and disky feature dominated in the intermediate regions. These findings imply that at these redshifts, the low-mass SSFGs are not disk-like, whereas the low-mass LSFGs likely harbour disk-like components flattened by significant rotations. At high masses ( ), both highly inclined SSFGs and LSFGs generally exhibit distinct trends in both ε and A 4 profiles, which increase at lower radii, reach maxima, then decrease at larger radii. Such the feature is more prevalent for more massive ( ) galaxies or at lower redshifts ( ). This feature can be simply explained if galaxies possess all three components: central bulges, disks in the intermediate regions, and halo-like stellar components in the outskirts.

SDSS-IV MaNGA: global stellar population and gradients for about 2000 early-type and spiral galaxies on the mass–size plane

We perform full spectrum fitting stellar population analysis and Jeans Anisotropic modelling (JAM) of the stellar kinematics for about 2000 early-type galaxies (ETGs) and spiral galaxies from the MaNGA DR14 sample. Galaxies with different morphologies are found to be located on a remarkably tight mass plane which is close to the prediction of the virial theorem, extending previous results for ETGs. By examining an inclined projection (`the mass-size' plane), we find that spiral and early-type galaxies occupy different regions on the plane, and their stellar population properties (i.e. age, metallicity and stellar mass-to-light ratio) vary systematically along roughly the direction of velocity dispersion, which is a proxy for the bulge fraction. Galaxies with higher velocity dispersions have typically older ages, larger stellar mass-to-light ratios and are more metal rich, which indicates that galaxies increase their bulge fractions as their stellar populations age and become enriched chemically. The age and stellar mass-to-light ratio gradients for low-mass galaxies in our sample tend to be positive ($\rm centre<outer$), while the gradients for most massive galaxies are negative. The metallicity gradients show a clear peak around velocity dispersion $\log_{10} \sigma_{\rm e}\approx 2.0$, which corresponds to the critical mass $\sim 3\times 10^{10}M_{\odot}$ of the break in the mass-size relation. Spiral galaxies with large mass and size have the steepest gradients, while the most massive ETGs, especially above the critical mass $M_{\rm crit}\ge 2\times 10^{11} M_{\odot}$, where slow rotator ETGs start dominating, have much flatter gradients. This may be due to differences in their evolution histories, e.g. mergers.

Possible Imprints of Cold-mode Accretion on the Present-day Properties of Disk Galaxies

Abstract Recent theoretical studies suggest that a significant part of the primordial gas accretes onto forming galaxies as narrow filaments of cold gas without building a shock and experiencing heating. Using a simple model of disk galaxy evolution that combines the growth of dark matter halos predicted by cosmological simulations with a hypothetical form of cold-mode accretion, we investigate how this cold-accretion mode affects the formation process of disk galaxies. It is found that the shock-heating and cold-accretion models produce compatible results for low-mass galaxies owing to the short cooling timescale in such galaxies. However, cold accretion significantly alters the evolution of disk galaxies more massive than the Milky Way and puts observable fingerprints on their present properties. For a galaxy with a virial mass , the scale length of the stellar disk is larger by 41% in the cold-accretion model than in the shock-heating model, with the former model reproducing the steep rise in the size–mass relation observed at the high-mass end. Furthermore, the stellar component of massive galaxies becomes significantly redder (0.66 in u − r at ), and the observed color–mass relation in nearby galaxies is qualitatively reproduced. These results suggest that large disk galaxies with red optical colors may be the product of cold-mode accretion. The essential role of cold accretion is to promote disk formation in the intermediate-evolution phase ( ) by providing the primordial gas having large angular momentum and to terminate late-epoch accretion, quenching star formation and making massive galaxies red.

Spatially associated clump populations in Rosette from CO and dust maps

Spatial association of clumps from different tracers turns out to be a valuable tool to determine the physical properties of molecular clouds. It provides a reliable estimate for the $X$-factors, serves to trace the density of clumps seen in column densities only and allows to measure the velocity dispersion of clumps identified in dust emission. We study the spatial association between clump populations, extracted by use of the GAUSSCLUMPS technique from $^{12}$CO (1-0), $^{13}$CO (1-0) line maps and Herschel dust-emission maps of the star-forming region Rosette, and analyse their physical properties. All CO clumps that overlap with another CO or dust counterpart are found to be gravitationally bound and located in the massive star-forming filaments of the molecular cloud. They obey a single mass-size relation $M_{\rm cl}\propto R_{\rm cl}^\gamma$ with $\gamma\simeq3$ (implying constant mean density) and display virtually no velocity-size relation. We interpret their population as low-density structures formed through compression by converging flows and still not evolved under the influence of self-gravity. The high-mass parts of their clump mass functions are fitted by a power law ${\rm d}N_{\rm cl}/{\rm d}\,\log M_{\rm cl}\propto M_{\rm cl}^{\Gamma}$ and display a nearly Salpeter slope $\Gamma\sim-1.3$. On the other hand, clumps extracted from the dust-emission map exhibit a shallower mass-size relation with $\gamma=2.5$ and mass functions with very steep slopes $\Gamma\sim-2.3$ even if associated with CO clumps. They trace density peaks of the associated CO clumps at scales of a few tenths of pc where no single density scaling law should be expected.

The scaling relationship between baryonic mass and stellar disc size in morphologically late-type galaxies

Abstract Here I report the scaling relationship between the baryonic mass and scale-length of stellar discs for ∼1000 morphologically late-type galaxies. The baryonic mass–size relationship is a single power law $R_\ast \propto M_{\rm b}^{0.38}$ across ∼3 orders of magnitude in baryonic mass. The scatter in size at fixed baryonic mass is nearly constant and there are no outliers. The baryonic mass–size relationship provides a more fundamental description of the structure of the disc than the stellar mass–size relationship. The slope and the scatter of the stellar mass–size relationship can be understood in the context of the baryonic mass–size relationship. For gas-rich galaxies, the stars are no longer a good tracer for the baryons. High-baryonic-mass, gas-rich galaxies appear to be much larger at fixed stellar mass because most of the baryonic content is gas. The stellar mass–size relationship thus deviates from the power-law baryonic relationship, and the scatter increases at the low-stellar-mass end. These extremely gas-rich low-mass galaxies can be classified as ultra-diffuse galaxies based on the structure.

Early science with the Large Millimetre Telescope: fragmentation of molecular clumps in the Galaxy

Sensitive, imaging observations of the 1.1 mm dust continuum emission from a 1 deg^2 area collected with the AzTEC bolometer camera on the Large Millimeter Telescope are presented. A catalog of 1545 compact sources is constructed based on a Wiener-optimization filter. These sources are linked to larger clump structures identified in the Bolocam Galactic Plane Survey. Hydrogen column densities are calculated for all sources and mass and mean volume densities are derived for the subset of sources for which kinematic distances can be assigned. The AzTEC sources are localized, high density peaks within the massive clumps of molecular clouds and comprise 5-15% of the clump mass. We examine the role of the gravitational instability in generating these fragments by comparing the mass of embedded AzTEC sources to the Jeans' mass of the parent BGPS object. For sources with distances less than 6 kpc the fragment masses are comparable to the clump Jeans' mass, despite having isothermal Mach numbers between 1.6 and 7.2. AzTEC sources linked to ultra-compact HII regions have mass surface densities greater than the critical value implied by the mass-size relationship of infrared dark clouds with high mass star formation while AzTEC sources associated with Class II methanol masers have mass surface densities greater than 0.7 g cm^{-2} that approaches the proposed threshold required to form massive stars.

SIDM on fire: hydrodynamical self-interacting dark matter simulations of low-mass dwarf galaxies

We compare a suite of four simulated dwarf galaxies formed in 1010 M☉ haloes of collisionless cold dark matter (CDM) with galaxies simulated in the same haloes with an identical galaxy formation model but a non-zero cross-section for DM self-interactions. These cosmological zoom-in simulations are part of the Feedback In Realistic Environments (fire) project and utilize the fire-2 model for hydrodynamics and galaxy formation physics. We find the stellar masses of the galaxies formed in self-interacting dark matter (SIDM) with σ/m = 1 cm2 g-1 are very similar to those in CDM (spanning M ★ ≈ 105.7-7.0 M☉) and all runs lie on a similar stellar mass-size relation. The logarithmic DM density slope (α = d log ρ/d log r) in the central 250-500 pc remains steeper than α = -0.8 for the CDM-Hydro simulations with stellar mass M ★ ~ 106.6 M☉ and core-like in the most massive galaxy. In contrast, every SIDM hydrodynamic simulation yields a flatter profile, with α > -0.4. Moreover, the central density profiles predicted in SIDM runs without baryons are similar to the SIDM runs that include fire-2 baryonic physics. Thus, SIDM appears to be much more robust to the inclusion of (potentially uncertain) baryonic physics than CDM on this mass scale, suggesting that SIDM will be easier to falsify than CDM using low-mass galaxies. Our fire simulations predict that galaxies less massive than M ★ ≲ 3 × 106 M☉ provide potentially ideal targets for discriminating models, with SIDM producing substantial cores in such tiny galaxies and CDM producing cusps.

The VIMOS Public Extragalactic Redshift Survey (VIPERS)

We use the full VIPERS redshift survey in combination with SDSS-DR7 to explore the relationships between star-formation history (using d4000), stellar mass and galaxy structure, and how these relationships have evolved since z~1. We trace the extents and evolutions of both the blue cloud and red sequence, by fitting double Gaussians to the d4000 distribution of galaxies in narrow stellar mass bins, for four redshift intervals over 0<z<1. This reveals downsizing in star formation, as the high-mass limit of the blue cloud retreats steadily with time from M*~10^11.2 M_sun at z~0.9 to M*~10^10.7 M_sun by the present day. The number density of massive blue-cloud galaxies (M*>10^11 M_sun, d4000<1.55) drops sharply by a factor five between z~0.8 and z~0.5. These galaxies are becoming quiescent at a rate that largely matches the increase in the numbers of massive passive galaxies seen over this period. We examine the size-mass relation of blue cloud galaxies, finding that its high-mass boundary runs along lines of constant M*/r_e or equivalently inferred velocity dispersion. Larger galaxies can continue to form stars to higher stellar masses than smaller galaxies. As blue cloud galaxies approach this high-mass limit, they start to be quenched, their d4000 values increasing to push them towards the green valley. In parallel, their structures change, showing higher Sersic indices and central stellar mass densities. For these galaxies, bulge growth is necessary for them to reach the high-mass limit of the blue cloud and be quenched by internal mechanisms. The blue cloud galaxies that are being quenched at z~0.8 lie along the same size-mass relation as present day quiescent galaxies, and seem the likely progenitors of today's S0s.

The effects of the cluster environment on the galaxy mass-size relation in MACS J1206.2-0847

The dense environment of galaxy clusters strongly influences the nature of galaxies. Here, we study the cause of the size distribution of a sample of 560 spectroscopic members spanning a wide dynamical range down to 10^8.5 M_sol (log(M)-2) in the massive CLASH cluster MACSJ 1206.2-0847 at z~0.44. We use Subaru SuprimeCam imaging covering the highest-density core out to the infall regions (3 virial radii) to look for cluster-specific effects. We also compare our measurements to a compatible large field study in order to span extreme environmental densities. This paper presents the trends we identified for cluster galaxies divided by their colors into star-forming and quiescent galaxies and into distinct morphological types (using S\'ersic index and bulge/disk decompositions). We observed larger sizes for early type and smaller sizes for massive late type galaxies in clusters in comparison to the field. We attribute this to longer quenching timescales of more massive galaxies in the cluster. Our analysis further revealed an increasing importance of recently quenched transition objects ("red disks"). This is a virialized population found at higher cluster-centric radii with sizes similar to the quiescent, spheroid-dominated population of the cluster center, but with disks still in-tact. The mass-size relation of cluster galaxies may therefore be understood as the consequence of a mix of progenitors formed at different quenching epochs. We also find that galaxy sizes smoothly decreasing as a function of bulge fraction. At same bulge-to-total ratio and same stellar mass, quiescent galaxies are smaller than star-forming galaxies. This is likely because of a fading of the outskirts of the disk, which we saw in comparing sizes of their disk-components. Ram-pressure stripping of the cold gas and other forms of more gradual gas starvation are likely responsible for this observation.

Testing the Recovery of Intrinsic Galaxy Sizes and Masses of z ∼ 2 Massive Galaxies Using Cosmological Simulations

Abstract Accurate measurements of galaxy masses and sizes are key to tracing galaxy evolution over time. Cosmological zoom-in simulations provide an ideal test bed for assessing the recovery of galaxy properties from observations. Here, we utilize galaxies with at z ∼ 1.7–2 from the MassiveFIRE cosmological simulation suite, part of the Feedback in Realistic Environments (FIRE) project. Using mock multi-band images, we compare intrinsic galaxy masses and sizes to observational estimates. We find that observations accurately recover stellar masses, with a slight average underestimate of and scatter. Recovered half-light radii agree well with intrinsic half-mass radii when averaged over all viewing angles, with a systematic offset of (with the half-light radii being larger) and a scatter of . When using color gradients to account for mass-to-light variations, recovered half-mass radii also exceed the intrinsic half-mass radii by . However, if not properly accounted for, aperture effects can bias size estimates by . No differences are found between the mass and size offsets for star-forming and quiescent galaxies. Variations in viewing angle are responsible for ∼25% of the scatter in the recovered masses and sizes. Our results thus suggest that the intrinsic scatter in the mass–size relation may have previously been overestimated by ∼25%. Moreover, orientation-driven scatter causes the number density of very massive galaxies to be overestimated by at .

The Spectroscopy and H-band Imaging of Virgo Cluster Galaxies (SHIVir) Survey: Scaling Relations and the Stellar-to-total Mass Relation

Abstract We present parameter distributions and fundamental scaling relations for 190 Virgo cluster galaxies in the SHIVir survey. The distribution of galaxy velocities is bimodal about V circ ∼ 125 km s−1, hinting at the existence of dynamically unstable modes in the inner regions of galaxies. An analysis of the Tully-Fisher relation (TFR) of late-type galaxies (LTGs) and the fundamental plane (FP) of early-type galaxies (ETGs) is presented, yielding a compendium of galaxy scaling relations. The slope and zero-point of the Virgo TFR match those of field galaxies, while scatter differences likely reflect distinct evolutionary histories. The velocities minimizing scatter for the TFR and FP are measured at large apertures where the baryonic fraction becomes subdominant. While TFR residuals remain independent of any galaxy parameters, FP residuals (i.e., the FP “tilt”) correlate strongly with the dynamical-to-stellar mass ratio, yielding stringent galaxy formation constraints. We construct a stellar-to-total mass relation (STMR) for ETGs and LTGs and find linear but distinct trends over the range M * = 108–11 M ⊙. Stellar-to-halo mass relations (SHMRs), which probe the extended dark matter halo, can be scaled down to masses estimated within the optical radius, showing a tight match with the Virgo STMR at low masses; possibly inadequate halo abundance matching prescriptions and broad radial scalings complicate this comparison at all masses. While ETGs appear to be more compact than LTGs of the same stellar mass in projected space, their mass-size relations in physical space are identical. The trends reported here may soon be validated through well-resolved numerical simulations.

Stellar streams as gravitational experiments

Tidal streams of disrupting dwarf galaxies orbiting around their host galaxy offer a unique way to constrain the shape of galactic gravitational potentials. Such streams can be used as “leaning tower” gravitational experiments on galactic scales. The most well-motivated modification of gravity proposed as an alternative to dark matter on galactic scales is Milgromian dynamics (MOND), and we present here the first ever N-body simulations of the dynamical evolution of the disrupting Sagittarius dwarf galaxy in this framework. Using a realistic baryonic mass model for the Milky Way, we attempt to reproduce the present-day spatial and kinematic structure of the Sagittarius dwarf and its immense tidal stream that wraps around the Milky Way. With very little freedom on the original structure of the progenitor, constrained by the total luminosity of the Sagittarius structure and by the observed stellar mass-size relation for isolated dwarf galaxies, we find reasonable agreement between our simulations and observations of this system. The observed stellar velocities in the leading arm can be reproduced if we include a massive hot gas corona around the Milky Way that is flattened in the direction of the principal plane of its satellites. This is the first time that tidal dissolution in MOND has been tested rigorously at these mass and acceleration scales.

On the galaxy–halo connection in the EAGLE simulation

Abstract Empirical models of galaxy formation require assumptions about the correlations between galaxy and halo properties. These may be calibrated against observations or inferred from physical models such as hydrodynamical simulations. In this Letter, we use the EAGLE simulation to investigate the correlation of galaxy size with halo properties. We motivate this analysis by noting that the common assumption of angular momentum partition between baryons and dark matter in rotationally supported galaxies overpredicts both the spread in the stellar mass–size relation and the anticorrelation of size and velocity residuals, indicating a problem with the galaxy–halo connection it implies. We find the EAGLE galaxy population to perform significantly better on both statistics, and trace this success to the weakness of the correlations of galaxy size with halo mass, concentration and spin at fixed stellar mass. Using these correlations in empirical models will enable fine-grained aspects of galaxy scalings to be matched.

Evaluation of radar reflectivity factor simulations of ice crystal populations from in situ observations for the retrieval of condensed water content in tropical mesoscale convective systems

Abstract. This study presents the evaluation of a technique to estimate cloud condensed water content (CWC) in tropical convection from airborne cloud radar reflectivity factors at 94 GHz and in situ measurements of particle size distributions (PSDs) and aspect ratios of ice crystal populations. The approach is to calculate from each 5 s mean PSD and flight-level reflectivity the variability of all possible solutions of m(D) relationships fulfilling the condition that the simulated radar reflectivity factor (T-matrix method) matches the measured radar reflectivity factor. For the reflectivity simulations, ice crystals were approximated as oblate spheroids, without using a priori assumptions on the mass–size relationship of ice crystals. The CWC calculations demonstrate that individual CWC values are in the range ±32 % of the retrieved average CWC value over all CWC solutions for the chosen 5 s time intervals. In addition, during the airborne field campaign performed out of Darwin in 2014, as part of the international High Altitude Ice Crystals/High Ice Water Content (HAIC/HIWC) projects, CWCs were measured independently with the new IKP-2 (isokinetic evaporator probe) instrument along with simultaneous particle imagery and radar reflectivity. Retrieved CWCs from the T-matrix radar reflectivity simulations are on average 16 % higher than the direct CWCIKP measurements. The differences between the CWCIKP and averaged retrieved CWCs are found to be primarily a function of the total number concentration of ice crystals. Consequently, a correction term is applied (as a function of total number concentration) that significantly improves the retrieved CWC. After correction, the retrieved CWCs have a median relative error with respect to measured values of only −1 %. Uncertainties in the measurements of total concentration of hydrometeors are investigated in order to calculate their contribution to the relative error of calculated CWC with respect to measured CWCIKP. It is shown that an overestimation of the concentration by about +50 % increases the relative errors of retrieved CWCs by only +29 %, while possible shattering, which impacts only the concentration of small hydrometeors, increases the relative error by about +4 %. Moreover, all cloud events with encountered graupel particles were studied and compared to events without observed graupel particles. Overall, graupel particles seem to have the largest impact on high crystal number-concentration conditions and show relative errors in retrieved CWCs that are higher than for events without graupel particles.

Estimating radar reflectivity - Snowfall rate relationships and their uncertainties over Antarctica by combining disdrometer and radar observations

Snowfall rate (SR) estimates over Antarctica are sparse and characterised by large uncertainties. Yet, observations by precipitation radar offer the potential to get better insight in Antarctic SR. Relations between radar reflectivity (Ze) and snowfall rate (Ze-SR relations) are however not available over Antarctica. Here, we analyse observations from the first Micro Rain Radar (MRR) in Antarctica together with an optical disdrometer (Precipitation Imaging Package; PIP), deployed at the Princess Elisabeth station. The relation Ze=A*SRB was derived using PIP observations and its uncertainty was quantified using a bootstrapping approach, randomly sampling within the range of uncertainty. This uncertainty was used to assess the uncertainty in snowfall rates derived by the MRR. We find a value of A = 18 [11–43] and B = 1.10 [0.97–1.17]. The uncertainty on snowfall rates of the MRR based on the Ze-SR relation are limited to 40%, due to the propagation of uncertainty in both Ze as well as SR, resulting in some compensation. The prefactor (A) of the Ze-SR relation is sensitive to the median diameter of the snow particles. Larger particles, typically found closer to the coast, lead to an increase of the value of the prefactor (A=44). Smaller particles, typical of more inland locations, obtain lower values for the prefactor (A=7). The exponent (B) of the Ze-SR relation is insensitive to the median diameter of the snow particles. In contrast with previous studies for various locations, shape uncertainty is not the main source of uncertainty of the Ze-SR relation. Parameter uncertainty is found to be the most dominant term, mainly driven by the uncertainty in mass-size relation of different snow particles. Uncertainties on the snow particle size distribution are negligible in this study as they are directly measured. Future research aiming at reducing the uncertainty of Ze-SR relations should therefore focus on obtaining reliable estimates of the mass-size relations of snow particles.

Constraints on Quenching of Z ≲ 2 Massive Galaxies from the Evolution of the Average Sizes of Star-forming and Quenched Populations in COSMOS

Abstract We use &gt;9400 quiescent and star-forming galaxies at z ≲ 2 in COSMOS/UltraVISTA to study the average size evolution of these systems, with focus on the rare ultra-massive population at . The large 2 square degree survey area delivers a sample of ∼400 such ultra-massive systems. Accurate sizes are derived using a calibration based on high-resolution images from the Hubble Space Telescope. We find that at these very high masses, the size evolution of star-forming and quiescent galaxies is almost indistinguishable in terms of normalization and power-law slope. We use this result to investigate possible pathways of quenching massive m &gt; M* galaxies at z &lt; 2. We consistently model the size evolution of quiescent galaxies from the star-forming population by assuming different simple models for the suppression of star formation. These models include an instantaneous and delayed quenching without altering the structure of galaxies and a central starburst followed by compaction. We find that instantaneous quenching reproduces the observed mass-size relation of massive galaxies at z &gt; 1 well. Our starburst+compaction model followed by individual growth of the galaxies by minor mergers is preferred over other models without structural change for galaxies at z &gt; 0.5. None of our models is able to meet the observations at m &gt; M* and z &lt; 1 without significant contribution of post-quenching growth of individual galaxies via mergers. We conclude that quenching is a fast process in galaxies with m ≥ 1011 M ⊙, and that major mergers likely play a major role in the final steps of their evolution.

SOFIA/GREAT [C ii] observations in nearby clouds near the lines of sight towards B0355+508 and B0212+735

We present SOFIA/GREAT [C ii] 2 P 3/2 → 2 P 1/2 (1.9005369 THz) observations of nearby clouds near the lines of sight towards the quasars B0355+508 and B0212+735. These clouds have previously been identified as warm non-LTE diffuse clouds with a temperature of T ≳ 30 K and sub-thermally excited CO lines. They are highly structured in CO with small-scale bright spots ( I (CO J = 1–0)~ 5–20 K km s -1 ), both spectrally and spatially. This small-scale structure has been interpreted as small-scale variations in the chemistry, not as density and column-density structure. We did not detect [C ii] 158 μ m emission within the rms noise of ~0.1–0.3 K. Our non-detection in [C ii] contradicts the above scenario. In diffuse clouds, the efficiency of photo-electric heating is highest. Under the assumption that [C ii] is the dominant coolant in diffuse clouds, we can calculate the predicted [C ii] emission arising from these clouds. Based on the derived hydrogen column densities for the diffuse clouds, a line width in [C ii] of similar order to that of CO, and when at least the minimum amount of heating in the clouds is due to cosmic-ray heating, [C ii] line intensities ≳1.5 to 5 K km s -1 are expected, which is a factor ~3 to 15 above the upper limits of the observations. The upper limits of the [C ii] and the observed CO line intensities are, however, consistent with the intensities predicted for photon-dominated region (PDR) surfaces on regular cold ( T ~ 15 K) low-density cloud fragments. Lower temperatures lead to less excited [C ii]. The assumption of lower densities, which would equally lower the [C ii] excitation, contradicts the observed cloud sizes and column densities. The CO(2–1)/CO(1–0) line ratios observed in these clouds are consistent with cloud temperatures of T ~ 15 K. The KOSMA- τ PDR-model of a cold moderate-density clump or an ensemble of such clumps with a canonical mass-size relation and mass spectrum, consistent with the total column densities derived for low density, shows that cold PDRs can reproduce the observed CO intensities, the observed CO(1–0) and CO(2–1) ratio, and the observed upper limits for [C ii].