129 research outputs found
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
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 < < 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 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
The Lyman Alpha Reference Sample: III. Properties of the Neutral ISM from GBT and VLA Observations
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 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 M of H I. We compare various H I
properties with global Ly quantities derived from HST measurements. The
measurements of the Ly 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 tracers. Further, on global scales,
these data support a complex coupling between Ly propagation and the H
I properties of the surrounding medium.Comment: Preprint form, 16 figures, accepted in Ap
- âŠ