Astrophysics

Context. The traditional approach to characterize the structure of molecular clouds is to map their line emission. Aims. We aim to test and apply a stratified random sampling technique that can characterize the line emission from molecular clouds more efficiently than mapping. Methods. We sampled the molecular emission from the Perseus cloud using the H2 column density as a proxy. We divided the cloud into ten logarithmically spaced column density bins, and we randomly selected ten positions from each bin. The resulting 100 cloud positions were observed with the IRAM 30m telescope, covering the 3mm-wavelength band and parts of the 2 and 1mm bands. Results. We focus our analysis on 11 molecular species detected toward most column density bins. In all cases, the line intensity is tightly correlated with the H2 column density. For the CO isotopologs, the trend is relatively flat, while for high-dipole moment species such as HCN, CS, and HCO+ the trend is approximately linear. We reproduce this behavior with a cloud model in which the gas density increases with column density, and where most species have abundance profiles characterized by an outer photodissociation edge and an inner freeze-out drop. The intensity behavior of the high-dipole moment species arises from a combination of excitation effects and molecular freeze out, with some modulation from optical depth. This quasi-linear dependence with the H2 column density makes the gas at low column densities dominate the cloud-integrated emission. It also makes the emission from most high-dipole moment species proportional to the cloud mass inside the photodissociation edge. Conclusions. Stratified random sampling is an efficient technique for characterizing the emission from whole molecular clouds. It shows that despite the complex appearance of Perseus, its molecular emission follows a relatively simple pattern.

In their recent paper, Chan and Lee discuss an interesting possibility: radio continuum emission from a dwarf irregular galaxy may be used to constrain upper limits on the cross section of annihilating dark matter. They claim that the contributions from nonthermal and thermal emission can be estimated with such accuracy that one can place new upper limits on the annihilation cross section. We argue that the observations presented can be explained entirely with a standard spectrum and no contribution from dark matter. As a result, the estimated upper limits of Chan and Lee are atleast by a factor of 100 too low.

Compact groups (CGs) of galaxies appear to be the densest galaxy systems containing a few luminous galaxies in close proximity to each other, which have a typical size of a few tens kilopacsec in observation. On the other hand, in the modern hierarchical structure formation paradigm, galaxies are assembled and grouped in dark matter haloes, which have a typical size of a few hundreds of kiloparsec. Few studies have explored the physical connection between the observation based CGs and halo model based galaxy groups to date. In this study, by matching the largest local CG catalog of Zheng & Shen (2020) to the halo based group catalog of Yang et al. (2007), we find that the CGs are physically heterogenous systems and can be mainly separated into two categories, the isolated systems and those embedded in rich groups or clusters. By examining the dynamical features of CGs, we find that the isolated CGs have systematically lower dynamical masses than that of non-compact ones at the same group luminosity, indicating a more evolved stage of isolated CGs. On the other hand, the embedded CGs are mixtures of chance alignments in poor clusters and recent infalling groups (sub-structures) of rich clusters.

We use Atacama Large Millimeter/submillimeter Array CO(2-1) observations of six low-redshift Palomar-Green quasars to study the distribution and kinematics of the molecular gas of their host galaxies at kpc-scale resolution. While the molecular gas content, molecular gas fraction, and star formation rates are similar to those of nearby massive, star-forming galaxies, the quasar host galaxies possess exceptionally compact, disky molecular gas distributions with a median half-light radius of 1.8 kpc and molecular gas mass surface densities ??2 M ??pc ?? . While the overall velocity field of the molecular gas is dominated by regular rotation out to large radii, with rotation velocity-to-velocity dispersion ratio ?? , the nuclear region displays substantial kinematic complexity associated with small-scale substructure in the gas distribution. A tilted-ring analysis reveals that the kinematic and photometric position angles are misaligned on average by ??4± 26 ??, and provides evidence of kinematic twisting. These observations provide tantalizing clues to the detailed physical conditions of the circumnuclear environments of actively accreting supermassive black holes.

We present multi-band Hubble Space Telescope imaging that spans rest-frame near-ultraviolet through near-infrared wavelengths (0.3-1.1 μ m) for 12 compact starburst galaxies at z=0.4-0.8. These massive galaxies (M_stellar ~ 10^11 M_Sun) are driving very fast outflows ( v max =1000-3000 km/s), and their light profiles are dominated by an extremely compact starburst component (half-light radius ~ 100 pc). Our goal is to constrain the physical mechanisms responsible for launching these fast outflows by measuring the physical conditions within the central kiloparsec. Based on our stellar population analysis, the central component typically contributes ??25% of the total stellar mass and the central escape velocities v esc,central ??00 km/s are a factor of two smaller than the observed outflow velocities. This requires physical mechanisms that can accelerate gas to speeds significantly beyond the central escape velocities, and it makes clear that these fast outflows are capable of traveling into the circumgalactic medium, and potentially beyond. We find central stellar densities comparable to theoretical estimates of the Eddington limit, and we estimate Σ 1 surface densities within the central kpc comparable to those of compact massive galaxies at 0.5<z<3.0 . Relative to "red nuggets" and "blue nuggets" at z?? , we find significantly smaller r e values at a given stellar mass, which we attribute to the dominance of a young stellar component in our sample and the better physical resolution for rest-frame optical observations at z??.6 versus z?? . We compare to theoretical scenarios involving major mergers and violent disc instability, and we speculate that our galaxies are progenitors of power-law ellipticals in the local universe with prominent stellar cusps.

Gaia DR2 published positions, parallaxes and proper motions for an unprecedented 1,331,909,727 sources, revolutionising the field of Galactic dynamics. We complement this data with the Astrometry Spread Function (ASF), the expected uncertainty in the measured positions, proper motions and parallax for a non-accelerating point source. The ASF is a Gaussian function for which we construct the 5D astrometric covariance matrix as a function of position on the sky and apparent magnitude using the Gaia DR2 scanning law and demonstrate excellent agreement with the observed data. This can be used to answer the question `What astrometric covariance would Gaia have published if my star was a non-accelerating point source?'. The ASF will enable characterisation of binary systems, exoplanet orbits, astrometric microlensing events and extended sources which add an excess astrometric noise to the expected astrometry uncertainty. By using the ASF to estimate the unit weight error (UWE) of Gaia DR2 sources, we demonstrate that the ASF indeed provides a direct probe of the excess source noise. We use the ASF to estimate the contribution to the selection function of the Gaia astrometric sample from a cut on astrometric_sigma5d_max showing high completeness for G<20 dropping to <1% in underscanned regions of the sky for G=21 . We have added an ASF module to the Python package SCANNINGLAW (this https URL) through which users can access the ASF.

We investigate the connection between galactic outflows and star formation using two independent data sets covering a sample of 22 galaxies between 1?�z??.5 . The HST WFC3/G141 grism provides low spectral resolution, high spatial resolution spectroscopy yielding H α emission line maps from which we measure the spatial extent and strength of star formation. In the rest-frame near-UV, Keck/DEIMOS observes Fe II and Mg II interstellar absorption lines, which provide constraints on the intensity and velocity of the outflows. We compare outflow properties from individual and composite spectra with the star formation rate (SFR) and SFR surface density (SigmaSFR), as well as the stellar mass and specific star formation rate (sSFR). The Fe II and Mg II equivalent widths (EWs) increase with both SFR and SigmaSFR at ??? significance, while the composite spectra show larger Fe II EWs and outflow velocities in galaxies with higher SFR, SigmaSFR, and sSFR. Absorption line profiles of the composite spectra further indicate that the differences between subsamples are driven by outflows rather than the ISM. While these results are consistent with those of previous studies, the use of H α images makes them the most direct test of the relationship between star formation and outflows at z>1 to date. Future facilities such as the James Webb Space Telescope and the upcoming Extremely Large Telescopes will extend these direct, H α -based studies to lower masses and star formation rates, probing galactic feedback across orders of magnitude in galaxy properties and augmenting the correlations we find here.

Outflows driven by active galactic nuclei (AGN) are expected to have a significant impact on the host galaxy evolution, but it is still debated how they are accelerated and propagate on galaxy-wide scales. This work addresses these questions by studying the link between X-ray, nuclear ultra-fast outflows (UFOs) and extended ionised outflows, for the first time in two quasars close to the peak of AGN activity ( z?? ), where AGN feedback is expected to be more effective. As targets, we selected two multiple-lensed quasars at z??.5 , HS 0810+2554 and SDSS J1353+1138, known to host UFOs and observed with the near-IR integral field spectrometer SINFONI at the VLT. We performed a kinematical analysis of the [O III] λ 5007 optical emission line, in order to trace the presence of ionised outflows. We detected spatially resolved ionised outflows in both galaxies, extended more than 8 kpc and moving up to v>2000 km/s. We derived mass outflow rates of ??12 M sun /yr and ??2 M sun /yr for HS 0810+2554 and SDSS J1353+1138. Comparing with the co-hosted UFO energetics, the ionised outflow energetics in HS 0810+2554 is broadly consistent with a momentum-driven regime of wind propagation, while in SDSS J1353+1138 it differs by a factor of ??100 from theoretical predictions, requiring either a massive molecular outflow or a high variability of the AGN activity to account for such a discrepancy. By additionally considering our results with those from the small sample of well-studied objects (all local but one), with both UFO and extended (ionised/atomic/molecular) outflow detections, we found that in 10 out of 12 galaxies the large-scale outflow energetics is consistent with the theoretical predictions of either a momentum- or an energy-driven scenario. This suggests that such models explain relatively well the acceleration mechanism of AGN-driven winds on large scales.

Cloud-cloud collision (CCC) has been suggested as a mechanism to induce massive star formation. Recent simulations suggest that a CCC speed is different among galactic-scale environments, which is responsible for observed differences in star formation activity. In particular, a high-speed CCC is proposed as a cause of star formation suppression in the bar regions in barred spiral galaxies. Focusing on the strongly barred galaxy NGC1300, we investigate the CCC speed. We find the CCC speed in the bar and bar-end tend to be higher than that in the arm. The estimated CCC speed is ??0 km s ?? , ??6 km s ?? , and ??1 km s ?? in the bar, bar-end, and arm, respectively. Although the star formation activity is different in the bar and bar-end, the CCC speed and the number density of high-speed CCC with >20 km s ?? are high in both regions, implying the existence of other parameters that control the star formation. The difference in molecular gas mass (average density) of the giant molecular clouds (GMCs) between the bar (lower mass and lower density) and bar-end (higher mass and higher density) may be cause for the different star formation activity. Combining with our previous study (Maeda et al.), the leading candidates of causes for the star formation suppression in the bar in NGC1300 are the presence of a large amount of diffuse molecular gases and high-speed CCCs between low mass GMCs.

In this paper, we present a new method to constrain the metallicities of high redshift damped Ly α (DLA) absorbers using observed extinction curves. This is the first time such an approach is employed to constrain the metallicities of extragalactic absorbers. To demonstrate our method, we use the spectra of 13 quasars and one GRB with DLA absorbers detected along their sightlines. By using the Kramers-Kronig (KK) relation, which relates the wavelength-integrated extinction to the total volume occupied by dust per hydrogen nucleon, we set some robust lower limits on the metallicity of the DLAs. The resulting lower limits are all consistent with the DLA metallicities from the literature. The GRB extinction curve exhibits a very strong 2175 A extinction bump. We try to constrain the metallicity of the GRB DLA by modeling the GRB extinction curve using dust models with two (graphite and silicates) and three (PAH, hydrogenated amorphous carbon, and silicates) dust components. The two-component model resulted in a metallicity of Z????0.45 while the three-component model gives Z????0.50. On the other hand, the lower limit from the KK approach for this DLA is Z????0.60. Modeling a large sample of extinction curves with 2175 A extinction bump and measured DLA metallicities would allow a thorough comparison between the KK and the model-dependent approach. In cases where the precise measurement of the metallicity of a DLA is not possible (e.g. due to the saturation of important absorption lines), the approach presented in this paper can be used to constrain the metallicity.