141 research outputs found
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&
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
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
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
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
Chemical abundances of damped Lyman alpha systems in the XQ-100 survey
The XQ-100 survey has provided high signal-noise spectra of 100 redshift
3-4.5 quasars with the X-Shooter spectrograph. The metal abundances for 13
elements in the 41 damped Lyman alpha systems (DLAs) identified in the XQ-100
sample are presented, and an investigation into abundances of a variety of DLA
classes is conducted. The XQ-100 DLA sample contains five DLAs within 5000 km/s
of their host quasar (proximate DLAs; PDLAs) as well as three sightlines which
contain two DLAs within 10,000 km/s of each other along the same line-of-sight
(multiple DLAs; MDLAs). Combined with previous observations in the literature,
we demonstrate that PDLAs with logN(HI)<21.0 show lower [S/H] and [Fe/H]
(relative to intervening systems with similar redshift and N(HI)), whilst
higher [S/H] and [Si/H] are seen in PDLAs with logN(HI)>21.0. These abundance
discrepancies are independent of their line-of-sight velocity separation from
the host quasar, and the velocity width of the metal lines (v90). Contrary to
previous studies, MDLAs show no difference in [alpha/Fe] relative to single
DLAs matched in metallicity and redshift. In addition, we present follow-up
UVES data of J0034+1639, a sightline containing three DLAs, including a
metal-poor DLA with [Fe/H]=-2.82 (the third lowest [Fe/H] in DLAs identified to
date) at z=4.25. Lastly we study the dust-corrected [Zn/Fe], emphasizing that
near-IR coverage of X-Shooter provides unprecedented access to MgII, CaII and
TiII lines (at redshifts 3-4) to provide additional evidence for subsolar
[Zn/Fe] ratio in DLAs.Comment: Accepted to MNRAS. 19 pages plus Appendix material (102 pages total
Extremely strong CIV 1550 nebular emission in the extremely low-metallicity star-forming galaxy J2229+2725
Using Hubble Space Telescope (HST)/Cosmic Origins Spectrograph (COS)
observations of one of the most metal-poor dwarf star-forming galaxies (SFG) in
the local Universe, J2229+2725, we have discovered an extremely strong nebular
CIV 1549, 1551 emission-line doublet, with an equivalent width of 43A, several
times higher than the value observed so far in low-redshift SFGs. Together with
other extreme characteristics obtained from optical spectroscopy (oxygen
abundance 12+log(O/H)=7.085+/-0.031, ratio O32 = I([OIII]5007)/I([OII]3727) ~
53, and equivalent width of the Hbeta emission line EW(Hbeta) = 577A), this
galaxy greatly increases the range of physical properties for dwarf SFGs at low
redshift and is a likely analogue of the high-redshift dwarf SFGs responsible
for the reionization of the Universe. We find the ionizing radiation in
J2229+2725 to be stellar in origin and the high EW(CIV 1549,1551) to be due to
both extreme ionization conditions and a high carbon abundance, with a
corresponding log C/O = -0.38, that is ~ 0.4 dex higher than the average value
for nearby low-metallicity SFGs.Comment: 6 pages, 4 figures, accepted for publikation in Monthly Notices of
the Royal Astronomical Society Letter
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