585 research outputs found
Using Lyman-alpha to detect galaxies that leak Lyman continuum
We propose to infer ionising continuum leaking properties of galaxies by
looking at their Lyman-alpha line profiles. We carry out Lyman-alpha radiation
transfer calculations in two models of HII regions which are porous to ionising
continuum escape: 1) the so-called "density bounded" media, in which massive
stars produce enough ionising photons to keep the surrounding interstellar
medium transparent to the ionising continuum, i.e almost totally ionised, and
2) "riddled ionisation-bounded" media, surrounded by neutral interstellar
medium, but with holes, i.e. with a covering factor lower than unity. The
Lyman-alpha spectra emergent from these configurations have distinctive
features: 1) a "classical" asymmetric redshifted profile in the first case, but
with a small shift of the maximum of the profile compare to the systemic
redshift (Vpeak < 150 km/s); 2) a main peak at the systemic redshift in the
second case (Vpeak = 0 km/s), with, as a consequence, a non-zero Lyman-alpha
flux bluewards the systemic redshift. Assuming that in a galaxy leaking
ionising photons, the Lyman-alpha component emerging from the leaking star
cluster(s) dominates the total Lyman-alpha spectrum, the Lyman-alpha shape may
be used as a pre-selection tool to detect Lyman continuum (LyC) leaking
galaxies, in objects with well determined systemic redshift, and high spectral
resolution Lyman-alpha spectra (R >= 4000). The examination of a sample of 10
local starbursts with high resolution HST-COS Lyman-alpha spectra and known in
the literature as LyC leakers or leaking candidates, corroborates our
predictions. Observations of Lyman-alpha profiles at high resolution should
show definite signatures revealing the escape of Lyman continuum photons from
star-forming galaxies.Comment: A&A in pres
Grid of Lya radiation transfer models for the interpretation of distant galaxies
Lya is a key diagnostic for numerous observations of distant star-forming
galaxies. It's interpretation requires, however, detailed radiation transfer
models. We provide an extensive grid of 3D radiation transfer models simulating
the Lya and UV continuum radiation transfer in the interstellar medium of
star-forming galaxies. We have improved our Monte Carlo MCLya code, and have
used it to compute a grid of 6240 radiation transfer models for homogeneous
spherical shells containing HI and dust surrounding a central source. The
simulations cover a wide range of parameter space. We present the detailed
predictions from our models including in particular the Lya escape fraction
fesc, the continuum attenuation, and detailed Lya line profiles. The Lya escape
fraction is shown to depend strongly on dust content, but also on other
parameters (HI column density and radial velocity). The predicted line profiles
show a great diversity of morphologies ranging from broad absorption lines to
emission lines with complex features. The results from our simulations are
distributed in electronic format. Our models should be of use for the
interpretation of observations from distant galaxies, for other simulations,
and should also serve as an important base for comparison for future, more
refined, radiation transfer models.Comment: Accepted for publication in Astronomy & Astrophysics. Results from
simulations available at http://obswww.unige.ch/sf
First consistent Lyα profile and UV spectral modeling of z ~ 3 LGBs with a 3D radiative transfer code
We developped a 3D Monte Carlo Lyα radiation transfer code to understand the diversity of Lyα line profiles observed in star forming galaxies and related objects (Verhamme et al. 2006). Our code allows for prescribed arbitrary hydrogen density, ionisation, temperature structures, and dust distributions, and arbitrary velocity fields and UV photon sources.Here we present results from the first modelling of the Lyα line and of the UV spectrum with our code of a sample of z ~ 3 Lyman break galaxies observed by Steidel and collaborators (Pettini et al. 2002) and taken from the FORS Deep Field (Tapken et al. 2006). A simple model of an expanding neutral shell surrounding a starburst region can reproduce the whole variety of spectra ranging from double-peaked profiles to asymetric emission lines, P-Cygni profiles or broad absorption. The main determining parameters are the outflow velocity and the dust content. Other parameters such as the hydrogen column density, the intrinsic Lyα emission and hence SFR, and the intrinsic Lyα line widths can be determined consistently taking all radiation transfer effects into accoun
Lyman-alpha spectral properties of five newly discovered Lyman continuum emitters
We have recently reported the discovery of five low redshift Lyman continuum
(LyC) emitters (LCEs, hereafter) with absolute escape fractions fesc(LyC)
ranging from 6 to 13%, higher than previously found, and which more than
doubles the number of low redshift LCEs.We use these observations to test
theoretical predictions about a link between the characteristics of the
Lyman-alpha (Lya) line from galaxies and the escape of ionising photons. We
analyse the Lya spectra of eight LCEs of the local Universe observed with the
Cosmic Origins Spectrograph onboard the Hubble Space Telescope (our five
leakers and three galaxies from the litterature), and compare their strengths
and shapes to the theoretical criteria and comparison samples of local
galaxies: the Lyman Alpha Reference Survey, Lyman Break Analogs, Green Peas,
and the high-redshift strong LyC leaker Ion2. Our LCEs are found to be strong
Lya emitters, with high equivalent widths, EW(Lya)> 70 {\AA}, and large Lya
escape fractions, fesc(Lya) > 20%. The Lya profiles are all double-peaked with
a small peak separation, in agreement with our theoretical expectations. They
also have no underlying absorption at the Lya position. All these
characteristics are very different from the Lya properties of typical
star-forming galaxies of the local Universe. A subset of the comparison samples
(2-3 Green Pea galaxies) share these extreme values, indicating that they could
also be leaking. We also find a strong correlation between the star formation
rate surface density and the escape fraction of ionising photons, indicating
that the compactness of star-forming regions plays a role in shaping low column
density paths in the interstellar medium of LCEs. The Lya properties of LCEs
are peculiar: Lya can be used as a reliable tracer of LyC escape from galaxies,
in complement to other indirect diagnostics proposed in the literature.Comment: 11 pages, 10 figures, accepted for publication in A&
MUSE-inspired view of the quasar Q2059-360, its Lyman alpha blob, and its neighborhood
The radio-quiet quasar Q2059-360 at redshift is known to be close to
a small Lyman blob (LAB) and to be absorbed by a proximate damped
Ly (PDLA) system.
Here, we present the Multi Unit Spectroscopic Explorer (MUSE) integral field
spectroscopy follow-up of this quasi-stellar object (QSO). Our primary goal is
to characterize this LAB in detail by mapping it both spatially and spectrally
using the Ly line, and by looking for high-ionization lines to
constrain the emission mechanism.
Combining the high sensitivity of the MUSE integral field spectrograph
mounted on the Yepun telescope at ESO-VLT with the natural coronagraph provided
by the PDLA, we map the LAB down to the QSO position, after robust subtraction
of QSO light in the spectral domain.
In addition to confirming earlier results for the small bright component of
the LAB, we unveil a faint filamentary emission protruding to the south over
about 80 pkpc (physical kpc); this results in a total size of about 120 pkpc.
We derive the velocity field of the LAB (assuming no transfer effects) and map
the Ly line width. Upper limits are set to the flux of the N V , C IV , He II , and C III] lines. We have discovered two probable Ly emitters at the
same redshift as the LAB and at projected distances of 265 kpc and 207 kpc from
the QSO; their Ly luminosities might well be enhanced by the QSO
radiation. We also find an emission line galaxy at near the line of
sight to the QSO.
This LAB shares the same general characteristics as the 17 others surrounding
radio-quiet QSOs presented previously. However, there are indications that it
may be centered on the PDLA galaxy rather than on the QSO.Comment: Accepted for publication in Astronomy & Astrophysics; 16 pages, 19
figure
Detection of high Lyman continuum leakage from four low-redshift compact star-forming galaxies
Following our first detection reported in Izotov et al. (2016), we present
the detection of Lyman continuum (LyC) radiation of four other compact
star-forming galaxies observed with the Cosmic Origins Spectrograph (COS)
onboard the Hubble Space Telescope (HST). These galaxies, at redshifts of
z~0.3, are characterized by high emission-line flux ratios [OIII]5007/[OII]3727
> 5. The escape fractions of the LyC radiation fesc(LyC) in these galaxies are
in the range of ~6%-13%, the highest values found so far in low-redshift
star-forming galaxies. Narrow double-peaked Lyalpha emission lines are detected
in the spectra of all four galaxies, compatible with predictions for Lyman
continuum leakers. We find escape fractions of Lyalpha, fesc(Lyalpha) ~20%-40%,
among the highest known for Lyalpha emitters (LAEs). Surface brightness
profiles produced from the COS acquisition images reveal bright star-forming
regions in the center and exponential discs in the outskirts with disc scale
lengths alpha in the range ~0.6-1.4 kpc. Our galaxies are characterized by low
metallicity, ~1/8-1/5 solar, low stellar mass ~(0.2 - 4)e9 Msun, high star
formation rates SFR~14-36 Msun/yr, and high SFR densities Sigma~2-35
Msun/yr/kpc^2. These properties are comparable to those of high-redshift
star-forming galaxies. Finally, our observations, combined with our first
detection reported in Izotov et al. (2016), reveal that a selection for compact
star-forming galaxies showing high [OIII]5007/[OII]3727 ratios appears to pick
up very efficiently sources with escaping Lyman continuum radiation: all five
of our selected galaxies are LyC leakers.Comment: 21 pages, 14 figures, accepted for publication in MNRAS; corrected
Lyalpha escape fraction
Intense CIII] 1907,1909 emission from a strong Lyman continuum emitting galaxy
We have obtained the first complete ultraviolet (UV) spectrum of a strong
Lyman continuum(LyC) emitter at low redshift -- the compact, low-metallicity,
star-forming galaxy J1154+2443 -- with a Lyman continuum escape fraction of 46%
discovered recently. The Space Telescope Imaging Spectrograph spectrum shows
strong Lya and CIII] 1909 emission, as well as OIII] 1666. Our observations
show that strong LyC emitters can have UV emission lines with a high equivalent
width (e.g. EW(CIII]) rest-frame), although their equivalent
widths should be reduced due to the loss of ionizing photons. The intrinsic
ionizing photon production efficiency of J1154+2443 is high, erg Hz, comparable to that of other recently discovered
LyC emitters. Combining our measurements and earlier
determinations from the literature, we find a trend of increasing with increasing CIII] 1909 equivalent width, which can be understood by
a combination of decreasing stellar population age and metallicity. Simple
ionization and density-bounded photoionization models can explain the main
observational features including the UV spectrum of J1154+2443.Comment: 5 pages, 4 figures. Accepted for publication in A&A Letter
Do galaxies that leak ionizing photons have extreme outflows?
To reionize the early universe, high-energy photons must escape the galaxies
that produce them. It has been suggested that stellar feedback drives galactic
outflows out of star-forming regions, creating low density channels through
which ionizing photons escape into the inter-galactic medium. We compare the
galactic outflow properties of confirmed Lyman continuum (LyC) leaking galaxies
to a control sample of nearby star-forming galaxies to explore whether the
outflows from leakers are extreme as compared to the control sample. We use
data from the Cosmic Origins Spectrograph on the Hubble Space Telescope to
measure the equivalent widths and velocities of Si II and Si III absorption
lines, tracing neutral and ionized galactic outflows. We find that the Si II
and Si III equivalent widths of the LyC leakers reside on the low-end of the
trend established by the control sample. The leakers' velocities are not
statistically different than the control sample, but their absorption line
profiles have a different asymmetry: their central velocities are closer to
their maximum velocities. The outflow kinematics and equivalent widths are
consistent with the scaling relations between outflow properties and host
galaxy properties -- most notably metallicity -- defined by the control sample.
Additionally, we use the Ly\alpha\ profiles to show that the Si II equivalent
width scales with the Ly\alpha\ peak velocity separation. We determine that the
low equivalent widths of the leakers are likely driven by low metallicities and
low H I column densities, consistent with a density-bounded ionization region,
although we cannot rule out significant variations in covering fraction. While
we do not find that the LyC leakers have extreme outflow velocities, the low
maximum-to-central velocity ratios demonstrate the importance of the
acceleration and density profiles for LyC and Ly\alpha\ escape. [abridged]Comment: 17 pages, 8 Figures. Accepted for publication in Astronomy &
Astrophysic
Accurately predicting the escape fraction of ionizing photons using restframe ultraviolet absorption lines
The fraction of ionizing photons that escape high-redshift galaxies
sensitively determines whether galaxies reionized the early universe. However,
this escape fraction cannot be measured from high-redshift galaxies because the
opacity of the intergalactic medium is large at high redshifts. Without methods
to indirectly measure the escape fraction of high-redshift galaxies, it is
unlikely that we will know what reionized the universe. Here, we analyze the
far-ultraviolet (UV) H I (Lyman series) and low-ionization metal absorption
lines of nine low-redshift, confirmed Lyman continuum emitting galaxies. We use
the H I covering fractions, column densities, and dust attenuations measured in
a companion paper to predict the escape fraction of ionizing photons. We find
good agreement between the predicted and observed Lyman continuum escape
fractions (within ) using both the H I and ISM absorption lines. The
ionizing photons escape through holes in the H I, but we show that dust
attenuation reduces the fraction of photons that escape galaxies. This means
that the average high-redshift galaxy likely emits more ionizing photons than
low-redshift galaxies. Two other indirect methods accurately predict the escape
fractions: the Ly escape fraction and the optical [O III]/[O II] flux
ratio. We use these indirect methods to predict the escape fraction of a sample
of 21 galaxies with rest-frame UV spectra but without Lyman continuum
observations. Many of these galaxies have low escape fractions (\%), but 11 have escape fractions \%. The methods presented here will
measure the escape fractions of high-redshift galaxies, enabling future
telescopes to determine whether star-forming galaxies reionized the early
universe.Comment: Accepted for publication in A&A. 12 pages, 5 figure
J1154+2443: a low-redshift compact star-forming galaxy with a 46 per cent leakage of Lyman continuum photons
We report the detection of the Lyman continuum (LyC) radiation of the compact
star-forming galaxy (SFG) J1154+2443 observed with the Cosmic Origins
Spectrograph (COS) onboard the Hubble Space Telescope. This galaxy, at a
redshift of z=0.3690, is characterized by a high emission-line flux ratio
O32=[OIII]5007/[OII]3727=11.5. The escape fraction of the LyC radiation
fesc(LyC) in this galaxy is 46 per cent, the highest value found so far in
low-redshift SFGs and one of the highest values found in galaxies at any
redshift. The narrow double-peaked Lya emission line is detected in the
spectrum of J1154+2443 with a separation between the peaks Vsep of 199 km/s,
one of the lowest known for Lya-emitting galaxies, implying a high fesc(Lya).
Comparing the extinction-corrected Lya/Hb flux ratio with the case B value we
find fesc(Lya) = 98 per cent. Our observations, combined with previous
detections in the literature, reveal an increase of O32 with increasing
fesc(LyC). We also find a tight anticorrelation between fesc(LyC) and Vsep. The
surface brightness profile derived from the COS acquisition image reveals a
bright star-forming region in the centre and an exponential disc in the
outskirts with a disc scale length alpha=1.09 kpc. J1154+2443, compared to
other known low-redshift LyC leakers, is characterized by the lowest
metallicity, 12+logO/H=7.65+/-0.01, the lowest stellar mass M*=10^8.20 Msun, a
similar star formation rate SFR=18.9 Msun/yr and a high specific SFR of
1.2x10^-7 yr^-1.Comment: 16 pages, 8 figures, accepted for publication in MNRAS. arXiv admin
note: text overlap with arXiv:1605.0516
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