The interstellar chemistry of C3H and C3H2 isomers [GA]

We report the detection of linear and cyclic isomers of C3H and C3H2 towards various starless cores and review the corresponding chemical pathways involving neutral (C3Hx with x=1,2) and ionic (C3Hx+ with x = 1,2,3) isomers. We highlight the role of the branching ratio of electronic Dissociative Recombination (DR) reactions of C3H2+ and C3H3+ isomers showing that the statistical treatment of the relaxation of C3H* and C3H2* produced in these DR reactions may explain the relative c,l-C3H and c,l-C3H2 abundances. We have also introduced in the model the third isomer of C3H2 (HCCCH). The observed cyclic-to-linear C3H2 ratio vary from 110 + or – 30 for molecular clouds with a total density around 1e4 to 30 + or – 10 for molecular clouds with a total density around 4e5, a trend well reproduced with our updated model. The higher ratio for low molecular cloud densities is mainly determined by the importance of the H + l-C3H2 -> H + c-C3H2 and H + t-C3H2 -> H + c-C3H2 isomerization reactions.

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J. Loison, M. Agundez, V. Wakelam, et. al.
Wed, 26 Jul 17

Comments: Accepted for publication in MNRAS

The Second Data Release of the KODIAQ Survey [GA]

We present and make publicly available the second data release (DR2) of the Keck Observatory Database of Ionized Absorption toward Quasars (KODIAQ) survey. KODIAQ DR2 consists of a fully-reduced sample of 300 quasars at 0.07 < z_em < 5.29 observed with HIRES at high resolution (36,000 <= R <= 103,000). DR2 contains 831 spectra available in continuum normalized form, representing a sum total exposure time of ~4.9 megaseconds on source. These co-added spectra arise from a total of 1577 individual exposures of quasars taken from the Keck Observatory Archive (KOA) in raw form and uniformly processed. DR2 extends DR1 by adding 130 new quasars to the sample, including additional observations of QSOs in DR1. All new data in DR2 were obtained with the single-chip Tektronix TK2048 CCD configuration of HIRES in operation between 1995 and 2004. DR2 is publicly available to the community, housed as a higher level science product at the KOA and in the igmspec database (v03).

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J. OMeara, N. Lehner, J. Howk, et. al.
Wed, 26 Jul 17

Comments: Accepted for publication in the Astronomical Journal. 4 pages, 3 tables, 7 figures

The vertical structure of gaseous galaxy discs in cold dark matter halos [GA]

We study the vertical structure of polytropic, $P\propto \rho^\Gamma$, centrifugally-supported gaseous discs embedded in cold dark matter (CDM) halos. At fixed radius $R$, the shape of the vertical density profile depends only weakly on whether the disc is self-gravitating (SG) or not (NSG). The disc thickness, set by the midplane sound speed and circular velocity, $(c_s/V_c)R$, in the NSG case, and by the sound speed and surface density, $c_s^2/G\Sigma$, in SG discs, is smaller than either of these scales. SG discs are typically Toomre unstable, NSG discs are stable. Exponential discs in CDM halos with roughly flat circular velocity curves generally “flare” outwards. For the polytropic equation of state of the EAGLE simulations, discs whose mass and size match observational constraints are stable (NSG) for $M_d< 3\times 10^9\, M_\odot$ and unstable (SG) at higher masses, if fully gaseous. We test these analytic results using a set of idealized SPH simulations and find excellent agreement. Our results clarify the role of the gravitational softening on the thickness of simulated discs, and on the onset of radial instabilities. EAGLE low-mass discs are non-self-gravitating so the softening plays no role in their vertical structure. High-mass discs, on the other hand, are expected to be self-gravitating and unstable, and may be artificially thickened and stabilized unless gravity is well resolved. Simulations with spatial resolution high enough to not compromise the vertical structure of a disc also resolve the onset of their instabilities, but the converse is not true: resolving instabilities does not guarantee that the vertical structure is resolved.

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A. Benitez-Llambay, J. Navarro, C. Frenk, et. al.
Wed, 26 Jul 17

Comments: Submitted to MNRAS

On the two main classes of Active Galactic Nuclei [GA]

Active Galactic Nuclei (AGN) are traditionally divided empirically into two main classes: “radio-loud” and “radio-quiet” sources. These labels, which are more than fifty years old, are obsolete, misleading, and wrong. I argue that AGN should be classified based on a fundamentally physical rather than just an observational difference, namely the presence (or lack) of strong relativistic jets, and that we should use the terms “jetted” and “non-jetted” AGN instead.

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P. Padovani
Wed, 26 Jul 17

Comments: Author’s version of Nature Astronomy Comment. 5 pages, 1 figure. See this https URL

How fast is mass-segregation happening in hierarchical formed embedded star clusters? [GA]

We investigate the evolution of mass segregation in initially sub-structured young embedded star clusters with two different background potentials mimicking the gas. Our clusters are initially in virial or sub-virial global states and have different initial distributions for the most massive stars: randomly placed, initially mass segregated or even inverse segregation. By means of N-body simulation we follow their evolution for 5 Myr. We measure the mass segregation using the minimum spanning tree method Lambda_MSR and an equivalent restricted method. Despite this variety of different initial conditions, we find that our stellar distributions almost always settle very fast into a mass segregated and more spherical configuration, suggesting that once we see a spherical or nearly spherical embedded star cluster, we can be sure it is mass segregated no matter what the real initial conditions were. We, furthermore, report under which circumstances this process can be more rapid or delayed, respectively.

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R. Domiguez, M. Fellhauer, M. Blana, et. al.
Wed, 26 Jul 17

Comments: 11 pages, 7 figures, MNRAS accepted

A Spatially Resolved Study of the GRB 020903 Host Complex [HEAP]

GRB 020903 is a long-duration gamma ray burst (LGRB) with a host galaxy close enough and extended enough for spatially-resolved observations, making it one of less than a dozen GRBs where such host studies are possible. GRB 020903 lies in a galaxy host complex that appears to consist of four interacting components. Here we present the results of spatially-resolved spectroscopic observations of the GRB 020903 host. By taking observations at two different position angles we were able to obtain optical spectra (3600-9000{\AA}) of multiple regions in the host complex. After examining the data we conclude that the GRB 020903 host is not, as previously believed, composed of four interacting regions – two are star-forming regions at the same redshift as the GRB explosion site (z=0.251), while two others appear to comprise a background star-forming galaxy at z=0.662. We also measure the metallicities of the two regions at the redshift of GRB 020903, and find that the explosion site and the nearby star-forming region both have metallicities of log(O/H)+12 ~ 8.1 +/- 0.2. We conclude that, in agreement with past spatially-resolved studies of GRBs, the GRB explosion site is representative of the host galaxy as a whole rather than localized in a metal-poor region of the galaxy.

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M. Thorp and E. Levesque
Wed, 26 Jul 17

Comments: 6 pages, 4 figures, to be submitted to AJ

The reverberation signatures of rotating disc winds in active galactic nuclei [GA]

The broad emission lines (BELs) in active galactic nuclei (AGN) respond to ionizing continuum variations. The time and velocity dependence of their response depends on the structure of the broad-line region: its geometry, kinematics and ionization state. Here, we predict the reverberation signatures of BELs formed in rotating accretion disc winds. We use a Monte Carlo radiative transfer and ionization code to predict velocity-delay maps for representative high- (C$~IV$) and low-ionization (H$\alpha$) emission lines in both high- and moderate-luminosity AGN. Self-shielding, multiple scattering and the ionization structure of the outflows are all self-consistently taken into account, while small-scale structure in the outflow is modelled in the micro-clumping approximation. Our main findings are: (1) The velocity-delay maps of smooth/micro-clumped outflows often contain significant negative responses. (2)~The reverberation signatures of disc wind models tend to be rotation dominated and can even resemble the classic “red-leads-blue” inflow signature. (3) Traditional “blue-leads-red” outflow signatures can usually only be observed in the long-delay limit. (4) Our models predict lag-luminosity relationships similar to those inferred from observations, but systematically underpredict the observed centroid delays. (5) The ratio between “virial product” and black hole mass predicted by our models depends on viewing angle. Our results imply that considerable care needs to be taken in interpreting data obtained by observational reverberation mapping campaigns. In particular, basic signatures such as “red-leads-blue”, “blue-leads-red” and “blue and red vary jointly” are not always reliable indicators of inflow, outflow or rotation. This may help to explain the perplexing diversity of such signatures seen in observational campaigns to date.

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S. Mangham, C. Knigge, J. Matthews, et. al.
Wed, 26 Jul 17

Comments: 15 pages, 17 figures, 2 tables. Accepted by MNRAS 20/7/2017