Astrophysics

Gas phase Elemental abundances in Molecular CloudS (GEMS) is an IRAM 30m Large Program designed to estimate the S, C, N, and O depletions and gas ionization degree, X(e-), in a set of star-forming filaments of Taurus, Perseus and Orion. Our immediate goal is to build up a complete database of molecular abundances that can serve as an observational basis for estimating X(e-) and the C, O, N, and S depletions through chemical modeling. We observed and derived the abundances of 14 species (13CO, C18O, HCO+, H13CO+, HC18O+, HCN, H13CN, HNC, HCS+, CS, SO, 34SO, H2S, and OCS) in 244 positions, covering the AV 3 to 100 mag, n(H2) a few 10 3 to 10 6 cm ?? , and Tk 10 to 30 K ranges in these clouds, avoiding protostars, HII regions, and outflows. A statistical analysis is carried out to identify general trends between different species and with physical parameters. Relations between molecules reveal strong linear correlations which define three different families: (1) 13CO and C18O; (2) H13CO+, HC18O+, H13CN, and HNC; and (3) the S-bearing molecules. The abundances of the CO isotopologs increase with the gas kinetic temperature until TK 15 K. For higher temperatures, the abundance remains constant with a scatter of a factor of 3. The abundances of H13CO+, HC18O+, H13CN, and HNC are well correlated with each other, and all of them decrease with molecular hydrogen density, following the law n(H2) ??.8±0.2 . The abundances of S-bearing species also decrease with n(H2) at a rate of (S-bearing/H)gas n(H2) ??.6±0.1 . The abundances of molecules belonging to groups 2 and 3 do not present any clear trend with gas temperature. At scales of molecular clouds, the C18O abundance is the quantity that better correlates with the cloud mass. We discuss the utility of the 13CO/C18O, HCO+/H13CO+, and H13CO+/H13CN abundance ratios as chemical diagnostics of star formation in external galaxies.

We study the gravitational fragmentation of circumstellar discs accreting extremely metal-poor ( Z??10 ?? Zsun) gas, performing a suite of three-dimensional hydrodynamic simulations using the adaptive mesh refinement code Enzo. We systematically follow the long-term evolution for 2000 years after the first protostar's birth, for the cases of Z=0 , 10 ?? , 10 ?? , and 10 ?? Zsun. We show that evolution of number of self-gravitating clumps qualitatively changes with Z . Vigorous fragmentation induced by dust cooling occurs in the metal-poor cases, temporarily providing about 10 self-gravitating clumps at Z= 10 ?? and 10 ?? Zsun. However, we also show that the fragmentation is a very sporadic process; after an early episode of the fragmentation, the number of clumps continuously decreases as they merge away in these cases. The vigorous fragmentation tends to occur later with the higher Z , reflecting that the dust-induced fragmentation is most efficient at the lower density. At Z= 10 ?? Zsun, as a result, the clump number stays smallest until the disc fragmentation starts in a late stage. We also show that the clump mass distribution also depends on the metallicity. A single or binary clump substantially more massive than the others appear only at Z= 10 ?? Zsun, whereas they are more evenly distributed in mass at the lower metallicities. We suggest that the disc fragmentation should provide the stellar multiple systems, but their properties drastically change with a tiny amount of metals.

Filamentary molecular clouds are omnipresent in the cold interstellar medium. Observational evidences show that the non-isothermal equations of state describe the filaments properties better than the isothermal one. In this paper we use the logatropic and the polytropic equations of state to study the gravitational instability of the pressure-confined filaments in presence of a uniform axial magnetic field. To fully explore the parameter space we carry out very large surveys of stability analysis that cover filaments with different radii in various magnetic fields. Our results show that for all the equations of state the instability of thinner filaments is more sensitive to the magnetic field variations than the thicker ones. Moreover, for all the equations of state, an intermediate magnetic field can entirely stabilize the thinner filaments. Albeit for the thicker ones this effect is suppressed for the magnetic field stronger than B ' 70 micro G.

We analyze the SDSS data to classify the galaxies based on their colour using a fuzzy set-theoretic method and quantify their environments using the local dimension. We find that the fraction of the green galaxies does not depend on the environment and 10%??0% of the galaxies at each environment are in the green valley depending on the stellar mass range chosen. Approximately 10% of the green galaxies at each environment host an AGN. Combining data from the Galaxy Zoo, we find that ??5% of the green galaxies are spirals and ??% are ellipticals at each environment. Only ??% of green galaxies exhibit signs of interactions and mergers, ??% have dominant bulge, and ??% host a bar. We show that the stellar mass distributions for the red and green galaxies are quite similar at each environment. Our analysis suggests that the majority of the green galaxies must curtail their star formation using physical mechanism(s) other than interactions, mergers, and those driven by bulge, bar and AGN activity. We speculate that these are the massive galaxies that have grown only via smooth accretion and suppressed the star formation primarily through mass driven quenching. Using a Kolmogorov-Smirnov test, we do not find any statistically significant difference between the properties of green galaxies in different environments. We conclude that the environmental factors play a minor role and the internal processes play the dominant role in quenching star formation in the green valley galaxies.

The effect of H 2 ro-vibrational excitation on the chemistry of protoplanetary disks is studied using a framework that solves for the disk physical and chemical structure and includes a detailed calculation of H 2 level populations. Chemistry with ro-vibrationally excited H 2 is found to be important for the formation of several commonly observed species in disks and this work demonstrates the need to accurately treat PDR chemistry in disks if we are to make inferences on the chemical state of the disk during planet formation epochs. This is found to be even more critical for molecules like C 2 H, CN or HCN that are commonly used to infer changes in the elemental disk C/O and N/O ratios, with implications for planetesimal formation and the composition of exoplanet atmospheres. Computed vertical column densities with the full H 2 population calculation are increased by ???? orders of magnitude for molecules such as CN, HCN/HNC compared to calculations with no treatment of excited H 2 . For the commonly used pseudo-level approximation, the computed columns of these molecules are overestimated by a factor of ???? when compared to the full model. We further note that the computed abundance for these molecules strongly depends on the strength of the FUV photons at energies that pump H 2 (i.e. 11-13.6 eV), which is not well constrained in disks, and that rate constants as a function of H 2 ro-vibrational levels for the key reaction N + H 2 ??NH are needed for a more accurate assessment of CN/HCN chemistry but are currently unavailable.

We present the second data release for the HI-MaNGA programme of HI follow-up observations for the SDSS-IV MaNGA survey. This release contains measurements for 3669 unique galaxies, combining 2108 Green Bank Telescope observations with an updated crossmatch of the MaNGA sample with the ALFALFA survey. We combine these data with MaNGA spectroscopic measurements to examine relationships between HI-to-stellar mass ratio (M_HI/M_*) and average ISM/star formation properties probed by optical emission lines. M_HI/M_* is very weakly correlated with the equivalent width of Halpha, implying a loose connection between the instantaneous star formation rate and the HI reservoir, although the link between M_HI/M_* and star formation strengthens when averaged even over only moderate timescales (~30 Myrs). Galaxies with elevated HI depletion times have enhanced [OI]/Halpha and depressed Halpha surface brightness, consistent with more HI residing in a diffuse and/or shock heated phase which is less capable of condensing into molecular clouds. Of all optical lines, M_HI/M_* correlates most strongly with oxygen equivalent width, EW(O), which is likely a result of the existing correlation between M_HI/M_* and gas-phase metallicity. Residuals in the M_HI/M_*-EW(O) relation are again correlated with [OI]/Halpha and Halpha surface brightness, suggesting they are also driven by variations in the fraction of diffuse and/or shock-heated gas. We recover the strong anti-correlation between M_HI/M_* and gas-phase metallicity seen in previous studies. We also find a relationship between M_HI/M_* and [OI]/Halpha, suggesting that higher fractions of diffuse and/or shock-heated gas are more prevalent in gas-rich galaxies.

We present measurements of f h , the ratio of the aligned components of the projected halo and galaxy ellipticities, for a sample of central galaxies using weak gravitational lensing data from the Kilo-Degree Survey (KiDS). Using a lens galaxy shape estimation that is more sensitive to outer galaxy regions, we find f h =0.50±0.20 for our full sample and f h =0.55±0.19 for an intrinsically red (and therefore higher stellar-mass) sub-sample, rejecting the hypothesis of round halos and/or galaxies being un-aligned with their parent halo at 2.5? and 2.9? , respectively. We quantify the 93.4% purity of our central galaxy sample using numerical simulations and overlapping spectroscopy from the Galaxy and Mass Assembly survey. This purity ensures that the interpretation of our measurements is not complicated by the presence of a significant fraction of satellite galaxies. Restricting our central galaxy ellipticity measurement to the inner isophotes, we find f h =0.34±0.17 for our red sub-sample, suggesting that the outer galaxy regions are more aligned with their dark matter halos compared to the inner regions. Our results are in agreement with previous studies and suggest that lower mass halos are rounder and/or less aligned with their host galaxy than samples of more massive galaxies, studied in galaxy groups and clusters.

Employing the the stellar evolution code (Modules for Experiments in Stellar Astrophysics), we calculate yields of heavy elements from massive stars via stellar wind and core-collapse supernovae (CCSN) ejecta to interstellar medium (ISM). In our models, the initial masses ( M ini ) of massive stars are taken from 13 to 80 M ??, their initial rotational velocities (V) are 0, 300 and 500 km s ?? , and their metallicities are [Fe/H] = -3, -2, -1, and 0. The yields of heavy elements coming from stellar winds are mainly affected by the stellar rotation which changes the chemical abundances of stellar surfaces via chemically homogeneous evolution, and enhances mass-loss rate. We estimate that the stellar wind can produce heavy element yields of about 10 ?? (for low metallicity models) to several M ??(for low metallicity and rapid rotation models) mass. The yields of heavy element produced by CCSN ejecta also depend on the remnant mass of massive mass which is mainly determined by the mass of CO-core. Our models calculate that the yields of heavy elements produced by CCSN ejecta can get up to several M ??. Compared with stellar wind, CCSN ejecta has a greater contribution to the heavy elements in ISM. We also compare the 56 Ni yields by calculated in this work with observational estimate. Our models only explain the 56 Ni masses produced by faint SNe or normal SNe with progenitor mass lower than about 25 M ??, and greatly underestimate the 56 Ni masses produced by stars with masses higher than about 30 M ??.

The origin and fate of the most extended extragalactic neutral cloud known in the local Universe, the Leo ring, is still debated 38 years after its discovery. Its existence is alternatively attributed to leftover primordial gas with some low level of metal pollution versus enriched gas stripped during a galaxy-galaxy encounter. Taking advantage of MUSE (Multi Unit Spectroscopic Explorer) operating at the VLT, we performed optical integral field spectroscopy of 3 HI clumps in the Leo ring where ultraviolet continuum emission has been found. We detected, for the first time, ionized hydrogen in the ring and identify 4 nebular regions powered by massive stars. These nebulae show several metal lines ([OIII],[NII],[SII]) which allowed reliable measures of metallicities, found to be close to or above the solar value. Given the faintness of the diffuse stellar counterparts, less than 3 percent of the observed heavy elements could have been produced locally in the main body of the ring and not much more than 15 percent in the HI clump towards M96. This inference, and the chemical homogeneity among the regions, convincingly demonstrates that the gas in the ring is not primordial, but has been pre-enriched in a galaxy disk, then later removed and shaped by tidal forces and it is forming a sparse population of stars.

We have measured the Faraday rotation of 62 extra-galactic background sources in 58 fields using the CSIRO Australia Telescope Compact Array (ATCA) with a frequency range of 1.1 - 3.1 GHz with 2048 channels. Our sources cover a region ??2deg?12deg ( ?? kpc) around the Galactic Centre region. We show that the Galactic Plane for |l|< 10 ??exhibits large Rotation Measures (RMs) with a maximum |RM| of 1691.2±4.9rad m ?? and a mean |RM|=219±42rad m ?? . The RMs decrease in magnitude with increasing projected distance from the Galactic Plane, broadly consistent with previous findings. We find an unusually high fraction (95\%) of the sources show Faraday complexity consistent with multiple Faraday components. We attribute the presences of multiple Faraday rotating screens with widely separated Faraday depths to small-scale turbulent RM structure in the Galactic Centre region. The second order structure function of the RM in the Galactic Centre displays a line with a gradient of zero for angular separations spanning 0.83 ????11 ??( ??20??500 pc), which is expected for scales larger than the outer scale (or driving scale) of magneto-ionic turbulence. We place an upper limit on any break in the SF gradient of 66'', corresponding to an inferred upper limit to the outer scale of turbulence in the inner 1 kpc of the Galactic Centre of 3 pc. We propose stellar feedback as the probable driver of this small-scale turbulence.