The Survey of Water and Ammonia in the Galactic Center (SWAG): Molecular Cloud Evolution in the Central Molecular Zone [GA]

The Survey of Water and Ammonia in the Galactic Center (SWAG) covers the Central Molecular Zone (CMZ) of the Milky Way at frequencies between 21.2 and 25.4 GHz obtained at the Australia Telescope Compact Array at $\sim 0.9$ pc spatial and $\sim 2.0$ km s$^{-1}$ spectral resolution. In this paper, we present data on the inner $\sim 250$ pc ($1.4^\circ$) between Sgr C and Sgr B2. We focus on the hyperfine structure of the metastable ammonia inversion lines (J,K) = (1,1) – (6,6) to derive column density, kinematics, opacity and kinetic gas temperature. In the CMZ molecular clouds, we find typical line widths of $8-16$ km s$^{-1}$ and extended regions of optically thick ($\tau > 1$) emission. Two components in kinetic temperature are detected at $25-50$ K and $60-100$ K, both being significantly hotter than dust temperatures throughout the CMZ. We discuss the physical state of the CMZ gas as traced by ammonia in the context of the orbital model by Kruijssen et al. (2015) that interprets the observed distribution as a stream of molecular clouds following an open eccentric orbit. This allows us to statistically investigate the time dependencies of gas temperature, column density and line width. We find heating rates between $\sim 50$ and $\sim 100$ K Myr$^{-1}$ along the stream orbit. No strong signs of time dependence are found for column density or line width. These quantities are likely dominated by cloud-to-cloud variations. Our results qualitatively match the predictions of the current model of tidal triggering of cloud collapse, orbital kinematics and the observation of an evolutionary sequence of increasing star formation activity with orbital phase.

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N. Krieger, J. Ott, H. Beuther, et. al.
Fri, 20 Oct 17

Comments: 68 pages, 44 figures, accepted for publication in ApJS; slightly compressed images due to file size limitations, high resolution available at this http URL

Repercussions of thermal atmospheric tides on the rotation of terrestrial planets in the habitable zone [EPA]

Semidiurnal atmospheric thermal tides are important for terrestrial exoplanets in the habitable zone of their host stars. With solid tides, they torque these planets, thus contributing to determine their rotation states as well as their climate. Given the complex dynamics of thermal tides, analytical models are essential to understand its dependence on the structure and rotation of planetary atmospheres and the tidal frequency. In this context, the state of the art model proposed in the 60’s by Lindzen and Chapman explains well the properties of thermal tides in the asymptotic regime of Earth-like rapid rotators but predicts a non-physical diverging tidal torque in the vicinity of the spin-orbit synchronization. In this work, we present a new model that addresses this issue by taking into account dissipative processes through a Newtonian cooling. First, we recover the tidal torque recently obtained with numerical simulations using General Circulation Models (GCM). Second, we show that the tidal response is very sensitive to the atmospheric structure, particularly to the stability with respect to convection. A strong stable stratification is able to annihilate the atmospheric tidal torque, leading to synchronization, while a convective atmosphere will be submitted to a strong torque, leading to a non-synchronized rotation state.

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P. Auclair-Desrotour, S. Mathis and J. Laskar
Fri, 20 Oct 17

Comments: 5 pages, 3 figures, SF2A conference

Sensitive CO(1-0) Survey in Pegasus-Pisces Reduces CO-Dark Gas Inventory by Factor of Two [GA]

We conducted high-sensitivity, high-velocity resolution CO(1-0) observations in a region containing a portion of the diffuse molecular cloud MBM 53 to determine whether weak CO emission was present. The results of our observations increase the amount of CO-detectable molecular gas in the region by a factor of two. The increased molecular mass for the cloud, if applicable to the molecular clouds in the entire Pegasus-Pisces region, decreases the dark molecular gas content from 58% of the total H2 mass to ~ 30%. If the results for MBM53 are applicable to other diffuse clouds, then the fraction of dark gas directly detectable via sensitive CO(1-0) observations in diffuse molecular clouds is similar to that predicted by models for Giant Molecular Clouds.

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E. Donate and L. Magnani
Fri, 20 Oct 17

Comments: N/A

The impact of dark energy on galaxy formation. What does the future of our Universe hold? [CEA]

We investigate the effect of the accelerated expansion of the Universe due to a cosmological constant, $\Lambda$, on the cosmic star formation rate. We utilise hydrodynamical simulations from the EAGLE suite, comparing a $\Lambda$CDM Universe to an Einstein-de Sitter model with $\Lambda=0$. Despite the differences in the rate of growth of structure, we find that dark energy, at its observed value, has negligible impact on star formation in the Universe. We study these effects beyond the present day by allowing the simulations to run forward into the future ($t>13.8$ Gyr). We show that the impact of $\Lambda$ becomes significant only when the Universe has already produced most of its stellar mass, only decreasing the total co-moving density of stars ever formed by ${\approx}15\%$. We develop a simple analytic model for the cosmic star formation rate that captures the suppression due to a cosmological constant. The main reason for the similarity between the models is that feedback from accreting black holes dramatically reduces the cosmic star formation at late times. Interestingly, simulations without feedback from accreting black holes predict an upturn in the cosmic star formation rate for $t>15$ Gyr due to the rejuvenation of massive ($ > 10^{11} \mathrm{M}_{\odot}$) galaxies. We briefly discuss the implication of the weak dependence of the cosmic star formation on $\Lambda$ in the context of the anthropic principle.

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J. Salcido, R. Bower, L. Barnes, et. al.
Fri, 20 Oct 17

Comments: Submitted to MNRAS. 16 pages, 11 figures. The EAGLE simulation data is available at this http URL

Spectral Energy Distribution Variations of Nearby Seyfert Galaxies During AGN Watch Monitoring Programs [GA]

We present and analyse quasi-simultaneous multi-epoch spectral energy distributions (SEDs) of seven reverberation-mapped AGNs for which accurate black hole mass measurements and suitable archival data are available from the ‘AGN Watch’ monitoring programs. We explore the potential of optical-UV and X-ray data, obtained within 2 days, to provide more accurate SED-based measurements of individual AGN and quantify the impact of source variability on key measurements typically used to characterise the black hole accretion process plus on bolometric correction factors at 5100 {\AA}, 1350 {\AA} and for the 2-10 keV X-ray band, respectively. The largest SED changes occur on long timescales (>1 year). For our small sample, the 1 micron to 10 keV integrated accretion luminosity typically changes by 10% on short time-scales (over 20 days), by ~30% over a year, but can change by 100% or more for individual AGN. The EUV gap is the most uncertain part of the intrinsic SED, introducing a ~25% uncertainty in the accretion-induced luminosity, relative to the model independent interpolation method that we adopt. That aside, our analysis shows that the uncertainty in the accretion-induced luminosity, the Eddington luminosity ratio and the bolometric correction factors can be reduced (by a factor of two or more) by use of SEDs built from data obtained within 20 days. However, mass accretion rate and mass accretion efficiency are mostly limited by the unknown EUV emission and the unknown details of the central engine and our aspect angle.

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E. Kilerci-Eser and M. Vestergaard
Fri, 20 Oct 17

Comments: Accepted for publication in MNRAS, 44 pages, 16 figures

Fast and accurate Voronoi density gridding from Lagrangian hydrodynamics data [IMA]

Voronoi grids have been successfully used to represent density structures of gas in astronomical hydrodynamics simulations. While some codes are explicitly built around using a Voronoi grid, others, such as Smoothed Particle Hydrodynamics (SPH), use particle-based representations and can benefit from constructing a Voronoi grid for post-processing their output. So far, calculating the density of each Voronoi cell from SPH data has been done numerically, which is both slow and potentially inaccurate. This paper proposes an alternative analytic method, which is fast and accurate. We derive an expression for the integral of a cubic spline kernel over the volume of a Voronoi cell and link it to the density of the cell. Mass conservation is ensured rigorously by the procedure. The method can be applied more broadly to integrate a spherically symmetric polynomial function over the volume of a random polyhedron.

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M. Petkova, G. Laibe and I. Bonnell
Fri, 20 Oct 17

Comments: 26 pages, 6 figures. For a sample implementation of the described algorithm, see this https URL

A technique of removing large-scale variations in astronomical observations [IMA]

In many astrophysical systems, smoothly-varying large-scale variations coexist with small-scale fluctuations. For example, a large-scale velocity or density gradient can exist in molecular clouds that exhibit small-scale turbulence. In redshifted 21cm observations, we also have two types of signals – the Galactic foreground emissions that change smoothly and the redshifted 21cm signals that change fast in frequency space. Sometimes the large-scale variations make it difficult to extract information on small-scale fluctuations. We propose a simple technique to remove smoothly varying large-scale variations. Our technique relies on multi-point structure functions and can obtain the magnitudes of small-scale fluctuations. It can also help us to filter out large-scale variations and retrieve small-scale maps. We discuss applications of our technique to astrophysical observations.

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J. Cho
Fri, 20 Oct 17

Comments: 9 pages, 7 figures, submitted