Neutral gas properties of Lyman continuum emitting galaxies: column densities and covering fractions from UV absorption lines [GA]

The processes allowing the escape of ionizing photons from galaxies into the intergalactic medium are poorly known. To understand how Lyman continuum (LyC) photons escape galaxies, we constrain the HI covering fractions and column densities using ultraviolet HI and metal absorption lines of 18 star-forming galaxies which have Lyman series observations. Nine of these galaxies are confirmed LyC emitters. We fit the stellar continuum, dust attenuation, metal, and HI properties to consistently determine the UV attenuation, as well as the column densities and covering factors of neutral hydrogen and metals. We use synthetic interstellar absorption lines to explore the systematics of our measurements. Then we apply our method to the observed UV spectra of low-redshift and z-2 galaxies. The observed HI lines are found to be saturated in all galaxies. An indirect approach using OI column densities and the observed O/H abundances yields HI column densities of 18.6 to 20 cm-2. These columns are too high to allow the escape of ionizing photons. We find that the known LyC leakers have HI covering fractions less than unity. Ionizing photons escape through optically thin holes/channels in a clumpy interstellar medium. Our simulations confirm that the HI covering fractions are accurately recovered. The SiII and HI covering fractions scale linearly, in agreement with observations from stacked Lyman break galaxy spectra at z-3. Thus, with an empirical correction, the SiII absorption lines can also be used to determine the HI coverage. Finally, we show that a consistent fitting of dust attenuation, continuum and absorption lines is required to properly infer the covering fraction of neutral gas and subsequently to infer the escape fraction of ionizing radiation. These measurements can estimate the LyC escape fraction, as we demonstrate in a companion paper.

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S. Gazagnes, J. Chisholm, D. Schaerer, et. al.
Tue, 20 Feb 18

Comments: We have incorporated the initial referee comments and will resubmit to A&A shortly. Comments are welcome

Sun-Sized Water Vapor Masers in Cepheus A [GA]

We present the first VLBI observations of a Galactic water maser (in Chepeus A) made with a very long baseline interferometric array involving the RadioAstron Earth-orbiting satellite station as one of its elements. We detected two distinct components at -16.9 and 0.6 km/s with a fringe spacing of 66 microarcseconds. In total power, the 0.6 km/s component appears to be a single Gaussian component of strength 580 Jy and width of 0.7 km/s. Single-telescope monitoring showed that its lifetime was only 8~months. The absence of a Zeeman pattern implies the longitudinal magnetic field component is weaker than 120 mG. The space-Earth cross power spectrum shows two unresolved components smaller than 15 microarcseconds, corresponding to a linear scale of 1.6 x 10^11 cm, about the diameter of the Sun, for a distance of 700 pc, separated by 0.54 km/s in velocity and by 160 +/-35 microarcseconds in angle. This is the smallest angular structure ever observed in a Galactic maser. The brightness temperatures are greater than 2 x 10^14K, and the line widths are 0.5 km/s. Most of the flux (about 87%) is contained in a halo of angular size of 400 +/- 150 microarcseconds. This structure is associated with the compact HII region HW3diii. We have probably picked up the most prominent peaks in the angular size range of our interferometer. We discuss three dynamical models: (1) Keplerian motion around a central object, (2) two chance overlapping clouds, and (3) vortices caused by flow around an obstacle (i.e., von Karman vortex street) with Strouhal number of about~0.3.

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A. Sobolev, J. Moran, M. Gray, et. al.
Tue, 20 Feb 18

Comments: 15 pages, 9 figures. Accepted for publication in ApJ, February 16, 2018

SEAGLE – I: A pipeline for simulating and modeling strong lenses from cosmological hydrodynamic simulations [CEA]

In this paper we introduce the SEAGLE (i.e. Simulating EAGLE LEnses) program, that approaches the study of galaxy formation through strong gravitational lensing, using a suite of high-resolution hydrodynamic simulations, Evolution and Assembly of GaLaxies and their Environments (EAGLE) project. We introduce the simulation and analysis pipeline and present the first set of results from our analysis of early-type galaxies. We identify and extract an ensemble of simulated lens galaxies and use the GLAMER ray-tracing lensing code to create mock lenses similar to those observed in the SLACS and SL2S surveys, using a range of source parameters and galaxy orientations, including observational effects such as the Point-Spread-Function (PSF), pixelization and noise levels, representative of single-orbit observations with the Hubble Space Telescope (HST) using the ACS-F814W filter. We subsequently model these mock lenses using the code LENSED, treating them in the same way as observed lenses. We also estimate the mass model parameters directly from the projected surface mass density of the simulated galaxy, using an identical mass model family. We perform a three-way comparison of all the measured quantities with real lenses. We find the average total density slope of EAGLE lenses, $t=2.26\; (0.25\; \rm{rms})$ to be higher than SL2S, $t=2.16$ or SLACS, $t=2.08$. We find a very strong correlation between the external shear ($\gamma$) and the complex ellipticity ($\epsilon$), with $\gamma \sim \epsilon/4$. This correlation indicates a degeneracy in the lens mass modeling. We also see a dispersion between lens modeling and direct fitting results, indicating systematical biases.

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S. Mukherjee, L. Koopmans, R. Metcalf, et. al.
Tue, 20 Feb 18

Comments: Submitted to MNRAS

A study of the HI and optical properties of Low Surface Brightness galaxies: spirals, dwarfs and irregulars [GA]

We present a study of the HI and optical properties of nearby ($z$ $\le$ 0.1) Low Surface Brightness galaxies (LSBGs). We started with a literature sample of $\sim$900 LSBGs and divided them into three morphological classes: spirals, irregulars and dwarfs. Of these, we could use $\sim$490 LSBGs to study their HI and stellar masses, colours and colour magnitude diagrams, and local environment, compare them with normal, High Surface Brightness (HSB) galaxies and determine the differences between the three morphological classes. We found that LSB and HSB galaxies span a similar range in HI and stellar masses, and have a similar $M_{\rm HI}$/$M_{\star}$–$M_{\star}$ relationship. Among the LSBGs, as expected, the spirals have the highest average HI and stellar masses, both of about 10$^{9.8}$ $M_\odot$. The LSGBs’ ($g$–$r$) integrated colour is nearly constant as function of HI mass for all classes. In the colour magnitude diagram, the spirals are spread over the red and blue regions whereas the irregulars and dwarfs are confined to the blue region. The spirals also exhibit a steeper slope in the $M_{\rm HI}$/$M_{\star}$–$M_{\star}$ plane. Within their local environment we confirmed that LSBGs are more isolated than HSB galaxies, and LSB spirals more isolated than irregulars and dwarfs. Kolmogorov-Smirnov statistical tests on the HI mass, stellar mass and number of neighbours indicates that the spirals are a statistically different population from the dwarfs and irregulars. This suggests that the spirals may have different formation and HI evolution than the dwarfs and irregulars.

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M. Honey, W. Driel, M. Das, et. al.
Tue, 20 Feb 18

Comments: 37 pages, 9 figures, Accepted for publication in MNRAS

Orbits in elementary, power-law galaxy bars: 1. Occurence and role of single loops [GA]

Orbits in galaxy bars are generally complex, but simple closed loop orbits play an important role in our conceptual understanding of bars. Such orbits are found in some well-studied potentials, provide a simple model of the bar in themselves, and may generate complex orbit families. The precessing, power ellipse (p-ellipse) orbit approximation provides accurate analytic orbit orbits in symmetric galaxy potentials. It remains useful for finding and fitting simple loop orbits in the frame of a rotating bar with bar-like and symmetric power-law potentials. Second order perturbation theory yields two or fewer simple loop solutions in these potentials. Numerical integrations in the parameter space neighborhood of perturbation solutions reveal zero or one ac- tual loops in a range of such potentials with rising rotation curves. These loops are embedded in a small parameter region of similar, but librating orbits, which have a subharmonic frequency superimposed on the basic loop. These loops and their librat- ing companions support annular bars. Solid bars can be produced in more complex potentials, as shown by an example with power-law indices varying with radius. The power-law potentials can be viewed as the elementary constituents of more complex potentials. Numerical integrations also reveal interesting classes of orbits with multiple loops. In two-dimensional, self-gravitating bars, with power-law potentials, single loop orbits are very rare. This result suggests that gas bars or oval distortions are unlikely to be long-lived, and that complex orbits or three-dimensional structure must support self-gravitating stellar bars.

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C. Struck
Tue, 20 Feb 18

Comments: 16 pos., 8 figs., MNRAS accepted

CHANG-ES XI: Circular Polarization in the Cores of Nearby Galaxies [GA]

We detect 5 galaxies in the CHANG-ES (Continuum Halos in Nearby Galaxies — an EVLA Survey) sample that show circular polarization (CP) at L-band in our high resolution data sets. Two of the galaxies (NGC~4388 and NGC~4845) show strong Stokes $V/I\,\equiv\,m_C\,\sim\,2$\%, two (NGC~660 and NGC~3628) have values of $m_C\sim \,0.3$\%, and NGC~3079 is a marginal detection at $m_C\sim \,0.2$\%. The two strongest $m_C$ galaxies also have the most luminous X-ray cores and the strongest internal absorption in X-rays. We have expanded on our previous Faraday conversion interpretation and analysis and provide analytical expressions for the expected $V$ signal for a general case in which the cosmic ray electron energy spectral index can take on any value. We provide examples as to how such expressions could be used to estimate magnetic field strengths and the lower energy cutoff for CR electrons. Four out of our detections are {\it resolved}, showing unique structures, including a {\it jet} in NGC~4388 and a CP `conversion disk’ in NGC~4845. The conversion disk is inclined to the galactic disk but is perpendicular to a possible outflow direction. Such CP structures have never before been seen in any galaxy to our knowledge. None of the galaxy cores show linear polarization at L-band. Thus CP may provide a unique probe of physical conditions deep into radio AGNs.

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J. Irwin, R. Henriksen, M. Wezgowiec, et. al.
Tue, 20 Feb 18

Comments: 30 pages, 4 figures, accepted to MNRAS

The Influence of Galaxy Environment on the Stellar Initial Mass Function of Early-Type Galaxies [GA]

In this paper we investigate whether the stellar initial mass function of early-type galaxies depends on their host environment. To this purpose, we have selected a sample of early-type galaxies from the SPIDER catalogue, characterized their environment through the group catalogue of Wang et al. and used their optical SDSS spectra to constrain the IMF slope, through the analysis of IMF-sensitive spectral indices. To reach a high enough signal-to-noise ratio, we have stacked spectra in velocity dispersion ($\sigma_0$) bins, on top of separating the sample by galaxy hierarchy and host halo mass, as proxies for galaxy environment. In order to constrain the IMF, we have compared observed line strengths to predictions of MIUSCAT/EMILES synthetic stellar population models, with varying age, metallicity, and “bimodal” (low-mass tapered) IMF slope ($\rm \Gamma_b$). Consistent with previous studies, we find that $\rm \Gamma_b$ increases with $\sigma_0$, becoming bottom-heavy (i.e. an excess of low-mass stars with respect to the Milky-Way-like IMF) at high $\sigma_0$. We find that this result is robust against the set of isochrones used in the stellar population models, as well as the way the effect of elemental abundance ratios is taken into account. We thus conclude that it is possible to use currently state-of-the-art stellar population models and intermediate resolution spectra to consistently probe IMF variations. For the first time, we show that there is no dependence of $\Gamma_b$ on environment or galaxy hierarchy, as measured within the $3″$ SDSS fibre, thus leaving the IMF as an intrinsic galaxy property, possibly set already at high redshift.

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G. Rosani, A. Pasquali, F. Barbera, et. al.
Tue, 20 Feb 18

Comments: N/A