Whereas cold dark matter (CDM) simulations predict central dark matter cusps with densities that diverge as (r) ∼ 1/r, observations often indicate constant-density cores with finite central densities ...0 and a flat density distribution within a core radius r0. This paper investigates whether this core-cusp problem can be solved by fuzzy dark matter (FDM), a hypothetical particle with a mass of the order of m 10−22 eV and a corresponding de Broglie wavelength on astrophysical scales. We show that galaxies with CDM halo virial masses Mvir ≤ 1011M follow two core-scaling relations. In addition to the well-known universal core column density 0 0 × r0 = 75 pc−2, core radii increase with virial masses as r0 ∼ with γ of order unity. Using the simulations by Schive et al. we demonstrate that FDM can explain the r0-Mvir scaling relation if the virial masses of the observed galaxy sample scale with the formation redshift z as Mvir ∼ (1 + z)−0.4. The observed constant 0 is however in complete disagreement with FDM cores which are characterized by a steep dependence 0 ∼ r , independent of z. More high-resolution simulations are now required to confirm the simulations of Schive et al. and explore the transition region between the soliton core and the surrounding halo. If these results hold, FDM can be ruled out as the origin of observed dark matter cores and other physical processes are required to account for their formation.
The geometry and intrinsic ellipticity distribution of ultra-diffuse galaxies (UDG) is determined from the line-of-sight distribution of axial ratios q of a large sample of UDGs, detected by Koda et ...al. in the Coma cluster. With high significance, the data rules out an oblate, disk-like geometry, characterized by major axes a = b > c. The data is, however, in good agreement with prolate shapes, corresponding to a = b < c. This indicates that UDGs are not thickened, rotating, axisymmetric disks, puffed up by violent processes. Instead, they are anisotropic elongated cigar- or bar-like structures, similar to the prolate dwarf spheroidal galaxy population of the Local Group. The intrinsic distribution of axial ratios of the Coma UDGs is flat in the range of 0.4 ≤ a/c ≤ 0.9 with a mean value of . This might provide important constraints for theoretical models of their origin. Formation scenarios that could explain the extended prolate nature of UDGs are discussed.
We study the origin of high-redshift, compact, quenched spheroids (red nuggets) through the dissipative shrinkage of gaseous discs into compact star-forming systems (blue nuggets). The discs, fed by ...cold streams, undergo violent disc instability that drives gas into the centre (along with mergers). The inflow is dissipative when its time-scale is shorter than the star formation time-scale. This implies a threshold of ∼0.28 in the cold-to-total mass ratio within the disc radius. For the typical gas fraction ∼0.5 at z ∼ 2, this threshold is traced back to a maximum spin parameter of ∼0.05, implying that ∼half the star-forming galaxies
contract to blue nuggets, while the rest form extended stellar discs. Thus, the surface density of blue galaxies is expected to be bimodal about ∼109 M⊙ kpc−2, slightly increasing with mass. The blue nuggets are expected to be rare at low z when the gas fraction is low. The blue nuggets quench to red nuggets by complementary internal and external mechanisms. Internal quenching by a compact bulge, in a fast mode and especially at high z, may involve starbursts, stellar and active galactic nucleus feedback, or Q-quenching. Quenching due to hot-medium haloes above 1012 M⊙ provides maintenance and a slower mode at low redshift. These predictions are confirmed in simulations and are consistent with observations at z = 0-3.
ABSTRACT The structure and dark matter halo core properties of dwarf spheroidal galaxies (dSphs) are investigated. A double-isothermal (DIS) model of an isothermal, non-self-gravitating stellar ...system embedded in an isothermal dark halo core provides an excellent fit to the various observed stellar surface density distributions. The stellar core scale length a* is sensitive to the central dark matter density 0,d. The maximum stellar radius traces the dark halo core radius . The concentration c* of the stellar system, determined by a King profile fit, depends on the ratio of the stellar-to-dark-matter velocity dispersion . Simple empirical relationships are derived that allow us to calculate the dark halo core parameters 0,d, , and d given the observable stellar quantities *, a*, and c*. The DIS model is applied to the Milky Way's dSphs. All dSphs closely follow the same universal dark halo scaling relations M pc−2 that characterize the cores of more massive galaxies over a large range in masses. The dark halo core mass is a strong function of core radius, . Inside a fixed radius of ∼400 pc the total dark matter mass is, however, roughly constant with M , although outliers are expected. The dark halo core densities of the Galaxy's dSphs are very high, with 0.2 M pc−3. dSphs should therefore be tidally undisturbed. Evidence for tidal effects might then provide a serious challenge for the CDM scenario.
Context. Since their first detection in the interestellar medium, (sub-)millimeter line observations of different CO isotopic variants have routinely been employed to characterize the kinematic ...properties of the gas in molecular clouds. Many of these lines exhibit broad linewidths that greatly exceed the thermal broadening expected for the low temperatures found within these objects. These observed suprathermal CO linewidths are assumed to originate from unresolved supersonic motions inside clouds. Aims. The lowest rotational J transitions of some of the most abundant CO isotopologues, 12CO and 13CO, are found to present large optical depths. In addition to well-known line saturation effects, these large opacities present a non-negligible contribution to their observed linewidths. Typically overlooked in the literature, in this paper we aim to quantify the impact of these opacity broadening effects on the current interpretation of the CO suprathermal line profiles. Methods. Combining large-scale observations and LTE modeling of the ground J = 1−0 transitions of the main 12CO, 13CO, C18O isotopologues, we have investigated the correlation of the observed linewidths as a function of the line opacity in different regions of the Taurus molecular cloud. Results. Without any additional contributions to the gas velocity field, a large fraction of the apparently supersonic (ℳ ~ 2–3) linewidths measured in both 12CO and 13CO (J = 1−0) lines can be explained by the saturation of their corresponding sonic-like, optically thin C18O counterparts assuming standard isotopic fractionation. Combined with the presence of multiple components detected in some of our C18O spectra, these opacity effects also seem to be responsible for most of the highly supersonic linewidths (ℳ > 8–10) detected in some of the broadest 12CO and 13CO spectra in Taurus. Conclusions. Our results demonstrate that most of the suprathermal 12CO and 13CO linewidths reported in nearby clouds like Taurus could be primarily created by a combination of opacity broadening effects and multiple gas velocity components blended in these saturated emission lines. Once corrected by their corresponding optical depth, each of these gas components present transonic intrinsic linewidths consistently traced by the three isotopologues, 12CO, 13CO, and C18O, with differences within a factor of 2. Highly correlated and velocity-coherent at large scales, the largest and highly supersonic velocity differences inside clouds are generated by the relative motions between individual gas components. In contrast to the classical interpretation within the framework of microscopic turbulence, this highly discretized structure of the molecular gas traced in CO suggest that the gas dynamics inside molecular clouds could be better described by the properties of a fully resolved macroscopic turbulence.
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ABSTRACT Giant clumps are a characteristic feature of observed high-redshift disk galaxies. We propose that these kiloparsec-sized clumps have a complex substructure and are the result of many ...smaller clumps self-organizing themselves into clump clusters (CCs). This bottom-up scenario is in contrast to the common top-down view that these giant clumps form first and then sub-fragment. Using a high-resolution hydrodynamical simulation of an isolated, fragmented massive gas disk and mimicking the observations from Genzel et al. at z ∼ 2, we find remarkable agreement in many details. The CCs appear as single entities of sizes 0.9-1.4 kpc and masses ∼(1.5-3) , representative of high-z observations. They are organized in a ring around the center of the galaxy. The origin of the observed clumps' high intrinsic velocity dispersion 50-100 is fully explained by the internal irregular motions of their substructure in our simulation. No additional energy input, e.g., via stellar feedback, is necessary. Furthermore, in agreement with observations, we find a small velocity gradient 8-27 along the CCs in the beam-smeared velocity residual maps, which corresponds to net prograde and retrograde rotation with respect to the rotation of the galactic disk. The CC scenario could have strong implications for the internal evolution, lifetimes, and the migration timescales of the observed giant clumps, bulge growth, and active galactic nucleus activity, stellar feedback, and the chemical enrichment history of galactic disks.
In this study, we present a detailed, statistical analysis of black hole growth and the evolution of active galactic nuclei (AGN) using cosmological hydrodynamic simulations run down to z = 0. The ...simulations self-consistently follow radiative cooling, star formation, metal enrichment, black hole growth and associated feedback processes from both Type II/Ia supernovae and AGN. We consider two simulation runs, one with a large comoving volume of (500 Mpc)3 and one with a smaller volume of (68 Mpc)3 but with a factor of almost 20 higher mass resolution. We compare the predicted statistical properties of AGN with results from large observational surveys. Consistently with previous results, our simulations can widely match observed black hole properties of the local Universe. Furthermore, our simulations can successfully reproduce the evolution of the bolometric AGN luminosity function for both the low-luminosity and the high-luminosity end up to z = 3.0, only at z = 1.5–2.5, the low-luminosity end is overestimated by up to 1 dex. In addition, the smaller but higher resolution run is able to match the observational data of the low bolometric luminosity end at higher redshifts z = 3–4. We also perform a direct comparison with the observed soft and hard X-ray luminosity functions of AGN, including an empirical correction for a torus-level obscuration, and find a similarly good agreement. These results nicely demonstrate that the observed ‘antihierarchical’ trend in the AGN number density evolution (i.e. the number densities of luminous AGN peak at higher redshifts than those of faint AGN) is self-consistently predicted by our simulations. Implications of this downsizing behaviour on active black holes, their masses and Eddington ratios are discussed. Overall, the downsizing behaviour in the AGN number density as a function of redshift can be mainly attributed to the evolution of the gas density in the resolved vicinity of a (massive) black hole (which is depleted with evolving time as a consequence of star formation and AGN feedback).
We explore the possibility that the G2 gas cloud falling in toward SgrA* is the mass-loss envelope of a young T Tauri star. As the star plunges to smaller radius at 1000-6000 km s super(-1), a strong ...bow shock forms where the stellar wind is impacted by the hot X-ray emitting gas in the vicinity of SgrA*. For a stellar mass-loss rate of 4 X 10 super(-8) M sub(middot in circle) yr super(-1) and wind velocity 100 km s super(-1) the bow shock will have an emission measure (EM = n super(2)vol) at a distance ~10 super(16) cm, similar to that inferred from the IR emission lines. The ionization of the dense bow shock gas is potentially provided by collisional ionization at the shock front and cooling radiation (X-ray and UV) from the post shock gas. The former would predict a constant line flux as a function of distance from SgrA*, while the latter will have increasing emission at lesser distances. In this model, the star and its mass-loss wind should survive pericenter passage since the wind is likely launched at 0.2 AU and this is much less than the Roche radius at pericenter (~3 AU for a stellar mass of 2 M sub(middot in circle)). In this model, the emission cloud will probably survive pericenter passage, discriminating this scenario from others.
ABSTRACT
We investigate the contribution of clumps and satellites to the galaxy mass assembly. We analysed spatially resolved HubbleSpace Telescope observations (imaging and slitless spectroscopy) of ...53 star-forming galaxies at z ∼ 1–3. We created continuum and emission line maps and pinpointed residual ‘blobs’ detected after subtracting the galaxy disc. Those were separated into compact (unresolved) and extended (resolved) components. Extended components have sizes ∼2 kpc and comparable stellar mass and age as the galaxy discs, whereas the compact components are 1.5 dex less massive and 0.4 dex younger than the discs. Furthermore, the extended blobs are typically found at larger distances from the galaxy barycentre than the compact ones. Prompted by these observations and by the comparison with simulations, we suggest that compact blobs are in situ formed clumps, whereas the extended ones are accreting satellites. Clumps and satellites enclose, respectively, ∼20 per cent and ≲80 per cent of the galaxy stellar mass, ∼30 per cent and ∼20 per cent of its star formation rate. Considering the compact blobs, we statistically estimated that massive clumps (M⋆ ≳ 109 M⊙) have lifetimes of ∼650 Myr, and the less massive ones (108 < M⋆ < 109 M⊙) of ∼145 Myr. This supports simulations predicting long-lived clumps (lifetime ≳ 100 Myr). Finally, ≲30 per cent (13 per cent) of our sample galaxies are undergoing single (multiple) merger(s), they have a projected separation ≲10 kpc, and the typical mass ratio of our satellites is 1:5 (but ranges between 1:10 and 1:1), in agreement with literature results for close pair galaxies.
ABSTRACT
We extend our previous work on simulations with the code ramses on accretion-driven turbulence by including self-gravity and study the effects of core formation and collapse. We show that ...radial accretion on to filaments drives turbulent motions which are not isotropic but radially dominated. In contrast to filaments without gravity, the velocity dispersion of self-gravitating filaments does not settle in an equilibrium. Despite showing similar amounts of driven turbulence, they continually dissipate their velocity dispersion until the onset of core formation. This difference is connected to the evolution of the radius as it determines the dissipation rate. In the non-gravitational case filament growth is not limited and its radius grows linearly with time. In contrast, there is a maximum extent in the self-gravitational case resulting in an increased dissipation rate. Furthermore, accretion-driven turbulence shows a radial profile which is anticorrelated with density. This leads to a constant turbulent pressure throughout the filament. As the additional turbulent pressure does not have a radial gradient it does not contribute to the stability of filaments and does not increase the critical line-mass. However, this radial turbulence does affect the radius of a filament, adding to the extent and setting its maximum value. Moreover, the radius evolution also affects the growth time-scale of cores which compared to the time-scale of collapse of an accreting filament limits core formation to high line-masses.
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