On the influence of physical galaxy properties on Lyman-alpha escape in star-forming galaxies

[abridged] Among the different observational techniques used to select high-redshift galaxies, the hydrogen recombination line Lyman-alpha (Lya) is of particular interest as it gives access to the measurement of cosmological quantities such as the star formation rate of distant galaxy populations. However, the interpretation of this line and the calibration of such observables is still subject to serious uncertainties. Therefore, it important to understand under what conditions the Lya line can be used as a reliable star formation diagnostic tool. We use a sample of 24 Lya emitters at z ~ 0.3 with an optical spectroscopic follow-up to calculate the Lya escape fraction and its dependency upon different physical properties. We also examine the reliability of Lya as a star formation rate indicator. We combine these observations with a compilation of Lya emitters selected at z = 0 - 0.3 to assemble a larger sample. The Lya escape fraction depends clearly on the dust extinction following the relation fesc(Lya) = C(Lya) x 10^(-0.4 E(B-V) k(Lya)), but with a shallower slope than previously reported, with k(Lya) ~ 6.67 and C(Lya) = 0.22. However, the correlation does not follow the expected curve for a simple dust attenuation. We explore the various mechanisms than lead to fesc(Lya) values above the continuum extinction curve, i.e. to an enhancement of the Lya output. We also observe that the strength of Lya and the escape fraction appear unrelated to the galaxy metallicity. Regarding the reliability of Lya as a star formation rate (SFR) indicator, we show that the deviation of SFR(Lya) from the true SFR (as traced by the UV continuum) is a function of the observed SFR(UV), which can be seen as the decrease of fesc(Lya) with increasing UV luminosity. Moreover, we observe a redshift-dependence of this relationship revealing the underlying evolution of fesc(Lya) with redshift.Comment: 17 pages. Accepted for publicatio

Empirical Estimate of Lyman-alpha Escape Fraction in a Statistical Sample of Lyman-alpha Emitters

The Lyman-alpha (Lya) recombination line is a fundamental tool for galaxy evolution studies and modern observational cosmology. However, subsequent interpretations are still prone to a number of uncertainties. Besides numerical efforts, empirical data are urgently needed for a better understanding of Lya escape process. We empirically estimate the Lyman-alpha escape fraction fesc(Lya) in a statistically significant sample of z ~ 0 - 0.3 galaxies in order to calibrate high-redshift Lyman-alpha observations. An optical spectroscopic follow-up of a sub-sample of 24 Lyman-alpha emitters (LAEs) detected by GALEX at z ~ 0.2-0.3, combined with a UV-optical sample of local starbursts, both with matched apertures, allow us to quantify the dust extinction through Balmer lines, and to estimate the Lyman-alpha escape fraction from the Halpha flux corrected for extinction in the framework of the recombination theory. The global escape fraction of Lyman-alpha radiation spans nearly the entire range of values, from 0.5 to 100 %, and fesc(Lya) clearly decreases with increasing nebular dust extinction E(B-V). Several objects show fesc(Lya) greater than fesc(continuum) which may be an observational evidence for clumpy ISM geometry or for an aspherical ISM. Selection biases and aperture size effects may still prevail between z ~ 0.2-0.3 LAEs and local starbursts, which may explain the difference observed for fesc(Lya).Comment: 4 pages, 2 figures, accepted for publication in Astronomy and Astrophysic

On the contribution of ULXs to stellar feedback: an intermediate mass black hole candidate and the population of ULXs in the low-metallicity starburst galaxy ESO 338-4

X-ray radiation from accreting compact objects is an important part of stellar feedback. The metal-poor galaxy ESO 338-4 has experienced vigorous starburst during the last 40 Myr and contains some of the most massive super star clusters in the nearby Universe. Given its starburst age and its star-formation rate, ESO 338-4 is one of the most efficient nearby manufactures of neutron stars and black holes, hence providing an excellent laboratory for feedback studies. We compared X-ray images and spectra obtained by XMM-Newton and Chandra telescopes with integral field spectroscopic VLT MUSE observations in the optical to constrain the nature of strong X-ray emitters. X-ray observations uncover three ultraluminous X-ray sources (ULXs) in ESO 338-4. The brightest among them, ESO 338~X-1, has X-ray luminosity in excess of 10^{40} erg/s. We speculate that ESO 338-4 is powered by accretion on an intermediate-mass (~300Msun) black hole. We show that X-ray radiation from ULXs and hot superbubbles strongly contributes to HeII ionization and general stellar feedback in this template starburst galaxy.Comment: A&A, in pres

The escape of Lyman photons from a young starburst: the case of Haro 11

(Abridged) Lyman-alpha (Lya) is a dominant probe of the galaxy population at high-z. However, interpretation of data drawn from Lya alone hinges on the Lya escape fraction which, due to the complex radiative transport, may vary greatly. Here we map the Lya emission from local starburst Haro 11, a Lya emitter and the only known candidate for low-z Lyman continuum emission (LyC). To aid in the interpretation we perform a detailed multi-wavelength analysis and model the stellar population, dust distribution, ionising photon budget, and star-cluster population. We use archival X-ray observations to further constrain properties of the starburst and estimate the HI column density. The Lya morphology is found to be strongly decoupled from stellar and nebular (H-alpha) morphologies. General surface photometry finds only very slight correlation between Lya and H-halpha, E(B-V), and stellar age. Only around the central Lya-bright cluster do we find the Lya/Ha ratio at values predicted by recombination theory. The total Lya escape fraction is found to be just 3%. We compute that ~90% of the Lya photons that escape do so after undergoing multiple resonance scattering events, masking their point of origin. This leads to a largely symmetric distribution and, by increasing the distance that photons must travel to escape, decreases the escape probability significantly. While dust must ultimately be responsible for the destruction of Lya, it plays little role in governing the observed morphology, which is regulated more by ISM kinematics and geometry. We find tentative evidence for local Lya equivalent width in the immediate vicinity of star-clusters being a function of cluster age, consistent with hydrodynamic studies. We estimate the ionising photon production and further constrain the escape fraction at 900 AA to <~9% .Comment: In press for MNRAS. 18 pages, 9 figures. Version with full resolution images to be found at http://www.astro.su.se/~matthew/english/papers/hayes_lya_haro11_hires.pd

Full chirality transfer in the synthesis of hindered tertiary boronic esters under in situ lithiation–borylation conditions

Using non-cryogenic lithiation–borylation, sterically hindered tertiary neopentyl glycol boronic esters can be prepared from secondary benzylic carbamates with full chirality transfer.</p

A copper-catalyzed asymmetric oxime propargylation enables the synthesis of the gliovirin tetrahydro-1,2-oxazine core

The bicyclic tetrahydro-1,2-oxazine subunit of gliovirin is synthesized through a diastereoselective copper-catalyzed cyclization of an N-hydroxyamino ester. Oxidative elaboration to the fully functionalized bicycle was achieved through a series of mild transformations. Central to this approach was the development of the first catalytic, enantioselective propargylation of an oxime to furnish a key N-hydroyxamino ester intermediate

Continuum subtracting Lyman-alpha images: Low redshift studies using the Solar Blind Channel of HST/ACS

[ABRIDGED] We are undertaking an imaging study of local star-forming galaxies in the Lyman-alpha (Lya) emission line using the Solar Blind Channel (SBC) of the ACS onboard the HST. Observations have been obtained in Lya and H-alpha (Ha) and 6 continuum filters between ~1500AA and the I-band. Previously (Hayes et al 2005) we demonstrated that the production of Lya line-only images in the SBC-only data-set is non-trivial and that supporting data is a requirement. We here develop various methods of continuum subtraction and assess their relative performance using a variety of spectral energy distributions (SED) as input. We conclude that simple assumptions about the behavior of the ultraviolet continuum consistently lead to results that are wildly erroneous, and determine that an SED fitting approach is essential. Moreover, a single component stellar or stellar+nebular spectrum is not always sufficient for realistic template SEDs and, in order to successfully recover the input observables, care must be taken to control contributions of nebular gas and underlying stellar populations. Independent measurements of the metallicity must first be obtained, but details of the initial mass function play only a small role. We identify the need to bin together pixels in our data to obtain signal-to-noise (S/N) of ~10 in each band before processing. At S/N=10 we can recover Lya fluxes accurate to ~30% for Lya lines with intrinsic equivalent width [W(Lya)] of 10AA. This accuracy improves to ~10% for W(Lya)=100AA. We describe the image processing applied to the observations presented in \"Ostlin et al. (2009) and the associated data-release. We also present simulations for an observing strategy for an alternative low-redshift SBC Lya imaging campaign using adjacent combinations of long-pass filters at slightly higher redshift.Comment: Accepted by the Astronomical Journa

The Lyman Alpha Reference Sample. VIII. Characterizing Lyman-Alpha Scattering in Nearby Galaxies

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.Comment: 32 pages, 51 figures, accepted to Ap

The Lyman alpha Reference Sample: Extended Lyman alpha Halos Produced at Low Dust Content

We report on new imaging observations of the Lyman alpha emission line (Lya), performed with the Hubble Space Telescope, that comprise the backbone of the Lyman alpha Reference Sample (LARS). We present images of 14 starburst galaxies at redshifts 0.028 < z < 0.18 in continuum-subtracted Lya, Halpha, and the far ultraviolet continuum. We show that Lya is emitted on scales that systematically exceed those of the massive stellar population and recombination nebulae: as measured by the Petrosian 20 percent radius, RP20, Lya radii are larger than those of Halpha by factors ranging from 1 to 3.6, with an average of 2.4. The average ratio of Lya-to-FUV radii is 2.9. This suggests that much of the Lya light is pushed to large radii by resonance scattering. Defining the "Relative Petrosian Extension" of Lya compared to Halpha, \xi_ext = RP20_Lya / RP20_Ha, we find \xi_ext to be uncorrelated with total Lya luminosity. However \xi_ext is strongly correlated with quantities that scale with dust content, in the sense that a low dust abundance is a necessary requirement (although not the only one) in order to spread Lya photons throughout the interstellar medium and drive a large extended Lya halo.Comment: Published in ApJ Letters ~~ 6 pages using emulateapj, 4 figures ~~ Higher-resolution, larger, nicer jpeg versions of Figures 1 and 2 can be found here: http://xayes.org/pub/press_lars.htm