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Register a new userThe Lyman Alpha Reference Sample IX: Revelations from deep surface photometry
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The Lyman Alpha Reference Sample (LARS) of 14 star-forming galaxies offers a wealth of insight into the workings of these local analogs to high-redshift star-forming galaxies. The sample has been well-studied in terms of LyA and other emission line properties, such as HI mass, gas kinematics, and morphology. We analyze deep surface photometry of the LARS sample in UBIK broadband imaging obtained at the Nordic Optical Telescope and the Canada-France-Hawaii Telescope, and juxtaposition their derived properties with a sample of local high-redshift galaxy analogs, namely, with blue compact galaxies (BCGs). We construct radial surface brightness and color profiles with both elliptical and isophotal integration, as well as RGB images, deep contours, color maps, a burst fraction estimate, and a radial mass-to-light ratio profile for each LARS galaxy. Standard morphological parameters like asymmetry, clumpiness, the Gini and M20 coefficients are [...] analyzed, as well as isophotal asymmetry profiles for each galaxy. [...] We compare the LARS to the properties of the BCG sample and highlight the differences. Several diagnostics indicate that the LARS galaxies have highly disturbed morphologies even at the level of the faintest isophotes [...]. The ground-based photometry [...] reveals previously unexplored isophotes [...]. The burst fraction estimate suggests a spatially more extended burst region in LARS than in the BCGs. [...] The galaxies in the LARS sample appear to be in earlier stages of a merger event compared to the BCGs. Standard morphological diagnostics like asymmetry, clumpiness, Gini and M20 coefficients cannot separate the two samples, although an isophotal asymmetry profile successfully captures the average difference in morphology. These morphological diagnostics do not show any correlation with the equivalent width or the escape fraction of Lyman Alpha. [abridged]
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We examine the dust geometry and Ly{alpha} scattering in the galaxies of the Lyman Alpha Reference Sample (LARS), a set of 14 nearby (0.02 < $z$ < 0.2) Ly{alpha} emitting and starbursting systems with Hubble Space Telescope Ly{alpha}, H{alpha}, and H{beta} imaging. We find that the global dust properties determined by line ratios are consistent with other studies, with some of the LARS galaxies exhibiting clumpy dust media while others of them show significantly lower Ly{alpha} emission compared to their Balmer decrement. With the LARS imaging, we present Ly{alpha}/H{alpha} and H{alpha}/H{beta} maps with spatial resolutions as low as $sim$ 40 pc, and use these data to show that in most galaxies, the dust geometry is best modeled by three distinct regions: a central core where dust acts as a screen, an annulus where dust is distributed in clumps, and an outer envelope where Ly{alpha} photons only scatter. We show that the dust that affects the escape of Ly{alpha} is more restricted to the galaxies central regions, while the larger Ly{alpha} halos are generated by scattering at large radii. We present an empirical modeling technique to quantify how much Ly{alpha} scatters in the halo, and find that this characteristic scattering distance correlates with the measured size of the Ly{alpha} halo. We note that there exists a slight anti-correlation between the scattering distance of Ly{alpha} and global dust properties.
We present new H I imaging and spectroscopy of the 14 UV-selected star-forming galaxies in the Lyman Alpha Reference Sample (LARS), aimed for a detailed study of the processes governing the production, propagation, and escape of Ly$alpha$ photons. New H I spectroscopy, obtained with the 100m Green Bank Telescope (GBT), robustly detects the H I spectral line in 11 of the 14 observed LARS galaxies (although the profiles of two of the galaxies are likely confused by other sources within the GBT beam); the three highest redshift galaxies are not detected at our current sensitivity limits. The GBT profiles are used to derive fundamental H I line properties of the LARS galaxies. We also present new pilot H I spectral line imaging of 5 of the LARS galaxies obtained with the Karl G. Jansky Very Large Array (VLA). This imaging localizes the H I gas and provides a measurement of the total H I mass in each galaxy. In one system, LARS 03 (UGC 8335 or Arp 238), VLA observations reveal an enormous tidal structure that extends over 160 kpc from the main interacting systems and that contains $>$10$^9$ M$_{odot}$ of H I. We compare various H I properties with global Ly$alpha$ quantities derived from HST measurements. The measurements of the Ly$alpha$ escape fraction are coupled with the new direct measurements of H I mass and significantly disturbed H I velocities. Our robustly detected sample reveals that both total H I mass and linewidth are tentatively correlated with key Ly$alpha$ tracers. Further, on global scales, these data support a complex coupling between Ly$alpha$ propagation and the H I properties of the surrounding medium.
We study young star-forming clumps on physical scales of 10-500 pc in the Lyman-Alpha Reference Sample (LARS), a collection of low-redshift (z = 0.03-0.2) UV-selected star-forming galaxies. In each of the 14 galaxies of the sample, we detect clumps for which we derive sizes and magnitudes in 5 UV-optical filters. The final sample includes $sim$1400 clumps, of which $sim$600 have magnitude uncertainties below 0.3 in all filters. The UV luminosity function for the total sample of clumps is described by a power-law with slope $alpha = -2.03^{+0.11}_{-0.13}$. Clumps in the LARS galaxies have on average $Sigma_{SFR}$ values higher than what observed in HII regions of local galaxies and comparable to typical SFR densities of clumps in z = 1-3 galaxies. We derive the clumpiness as the relative contribution from clumps to the UV emission of each galaxy, and study it as a function of galactic-scale properties, i.e. $Sigma_{SFR}$ and the ratio between rotational and dispersion velocities of the gas ($v_s/sigma_0$). We find that in galaxies with higher $Sigma_{SFR}$ or lower $v_s/sigma_0$, clumps dominate the UV emission of their host systems. All LARS galaxies with Ly$alpha$ escape fractions larger than 10% have more than 50% of the UV luminosity from clumps. We tested the robustness of these results against the effect of different physical resolutions. At low resolution, the measured clumpiness appears more elevated than if we could resolve clumps down to single clusters. This effect is small in the redshift range covered by LARS, thus our results are not driven by the physical resolution.
We report upon new results regarding the Lya output of galaxies, derived from the Lyman alpha Reference Sample (LARS), focusing on Hubble Space Telescope imaging. For 14 galaxies we present intensity images in Lya, Halpha, and UV, and maps of Halpha/Hbeta, Lya equivalent width (EW), and Lya/Halpha. We present Lya and UV light profiles and show they are well-fitted by Sersic profiles, but Lya profiles show indices systematically lower than those of the UV (n approx 1-2 instead of >~4). This reveals a general lack of the central concentration in Lya that is ubiquitous in the UV. Photometric growth curves increase more slowly for Lya than the FUV, showing that small apertures may underestimate the EW. For most galaxies, however, flux and EW curves flatten by radii ~10 kpc, suggesting that if placed at high-z, only a few of our galaxies would suffer from large flux losses. We compute global properties of the sample in large apertures, and show total luminosities to be independent of all other quantities. Normalized Lya throughput, however, shows significant correlations: escape is found to be higher in galaxies of lower star formation rate, dust content, mass, and several quantities that suggest harder ionizing continuum and lower metallicity. Eight galaxies could be selected as high-z Lya emitters, based upon their luminosity and EW. We discuss the results in the context of high-z Lya and UV samples. A few galaxies have EWs above 50 AA, and one shows f_escLya of 80%; such objects have not previously been reported at low-z.
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