23 research outputs found
Extended Tidal Structure In Two Lyman Alpha-Emitting Starburst Galaxies
We present new VLA C-configuration HI imaging of the Lyman Alpha-emitting
starburst galaxies Tol 1924-416 and IRAS 08339+6517. The effective resolution
probes neutral gas structures larger than 4.7 kpc in Tol 1924-416, and larger
than 8.1 kpc in IRAS 08339+6517. Both systems are revealed to be tidally
interacting: Tol 1924-416 with ESO 338-IG04B (6.6 arcminutes = 72 kpc minimum
separation), and IRAS 08339+6517 with 2MASX J08380769+6508579 (2.4 arcminutes =
56 kpc minimum separation). The HI emission is extended in these systems, with
tidal tails and debris between the target galaxies and their companions. Since
Lyman Alpha emission has been detected from both of these primary systems,
these observations suggest that the geometry of the ISM is one of the factors
affecting the escape fraction of Lyman Alpha emission from starburst
environments. Furthermore, these observations argue for the importance of
interactions in triggering massive star formation events.Comment: ApJ, in press; 11 pages, 2 color figure
Magellan/MMIRS near-infrared multi-object spectroscopy of nebular emission from star forming galaxies at 2<z<3
To investigate the ingredients, which allow star-forming galaxies to present
Lyalpha line in emission, we studied the kinematics and gas phase metallicity
(Z) of the interstellar medium. We used multi-object NIR spectroscopy with
Magellan/MMIRS to study nebular emission from z=2-3 star-forming galaxies
discovered in 3 MUSYC fields. We detected emission lines from four active
galactic nuclei and 13 high-z star-forming galaxies, including Halpha lines
down to a flux of 4.E-17 erg/sec/cm^2. This yielded 7 new redshifts. The most
common emission line detected is [OIII]5007, which is sensitive to Z. We were
able to measure Z for 2 galaxies and to set upper(lower) limits for another
2(2). The Z values are consistent with 0.3<Z/Zsun<1.2. Comparing the Lyalpha
central wavelength with the systemic redshift, we find
Delta_v(Lyalpha-[OIII])=70-270 km/sec. High-redshift star-forming galaxies,
Lyalpha emitting (LAE) galaxies, and Halpha emitters appear to be located in
the low mass, high star-formation rate (SFR) region of the SFR versus stellar
mass diagram, confirming that they are experiencing burst episodes of star
formation, which are building up their stellar mass. Their Zs are consistent
with the relation found for z<2.2 galaxies in the Z versus stellar mass plane.
The measured Delta_v(Lyalpha-[OIII]) values imply that outflows of material,
driven by star formation, could be present in the z=2-3 LAEs of our sample.
Comparing with the literature, we note that galaxies with lower Z than ours are
also characterized by similar Delta_v(Lyalpha-[OIII]) velocity offsets. Strong
[OIII] is detected in many Lyalpha emitters. Therefore, we propose the
Lyalpha/[OIII] flux ratio as a tool for the study of high-z galaxies; while
influenced by Z, ionization, and Lyalpha radiative transfer in the ISM, it may
be possible to calibrate this ratio to primarily trace one of these effects.Comment: 22 pages, 13 figures, 6 table
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 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
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
First detection of Lyman continuum escape from a local starburst galaxy
The dominating reionization source in the young universe has yet to be
identified. Possible candidates include metal poor starburst dwarf galaxies of
which the Blue Compact Galaxy Haro 11 may represent a local counterpart. Using
the Far Ultraviolet Spectroscopic Explorer (FUSE) we obtained spectra of Haro
11 to search for leaking ionizing radiation. A weak signal shortwards of the
Lyman break is identified as Lyman continuum (LyC) emission escaping from the
ongoing starburst. From profile fitting to weak metal lines we derive column
densities of the low ionization species. Adopting a metallicity typical of the
H II regions of Haro 11, the corresponding H I column density is optically
thick in the LyC. Therefore most of the LyC photons must escape through
transparent holes in the interstellar medium. Using spectral evolutionary
models we constrain the escape fraction of the produced LyC photons to between
4 and 10%, assuming a normal Salpeter IMF. We argue that in a hierarchical
galaxy formation scenario, this allows for a substantial contribution to cosmic
reionization by starburst dwarf galaxies at high redshifts.Comment: Accepted for publication in Astronomy and Astrophysic
The First Deep ACS Lyman alpha Images of Local Starburst Galaxies
We report the first results from a deep Lya imaging program of local
starburst galaxies with the Advanced Camera for Surveys (ACS) of the Hubble
Space Telescope. The two observed galaxies ESO 350-IG038 and SBS 0335-052 have
luminosities similar to those of the Magellanic Clouds but differ in their
chemical composition. ESO 350-IG038 has an oxygen abundance of 1/8 solar,
whereas SBS 0335-052 is known to have one of the lowest abundances among blue
galaxies (~1/30). The ACS imaging reveals a complex Lya morphology, with
sometimes strong offsets between the emission of Lya and the location of
stellar light, ionized gas traced by Halpha, and the neutral gas. Overall, more
Lya photons escape from the more metal- and dust-rich galaxy ESO 350-IG038. The
absence of clear SBS 0335-052 Lya emission over all observed knots, whatever
their dust content or/and color indices, contradicts model expectations of a
lower escape fraction from dust-rich gas due to destruction of Lya photons by
dust grains. Rather, the results are in qualitative agreement with models
suggesting the kinematic properties of the gas as the dominant Lya escape
regulator. If the properties of the two observed galaxies are representative
for starburst galaxies in general, Lya will be difficult to interpret as a
star-formation indicator, in particular if based on Lya imaging at low spatial
resolution.Comment: 6 pages, 2 figures (1 in color) New version of Fig
Nuclear spirals as feeding channels to the Supermassive Black Hole: the case of the galaxy NGC 6951
We report the discovery of gas streaming motions along nuclear spiral arms
towards the LINER nucleus of the galaxy NGC 6951. The observations, obtained
using the GMOS integral field spectrograph on the Gemini North telescope,
yielded maps of the flux distributions and gas kinematics in the Halpha,
[NII]6584 and [SII]6717,31 emission lines of the inner 7x5 arcsec^2 of the
galaxy. This region includes a circumnuclear star-forming ring with radius
500pc, a nuclear spiral inside the ring and the LINER nucleus. The kinematics
of the ionized gas is dominated by rotation, but subtraction of a kinematic
model of a rotating exponential disk reveals deviations from circular rotation
within the nuclear ring which can be attributed to (1) streaming motions along
the nuclear spiral arms and (2) a bipolar outflow which seems to be associated
to a nuclear jet. On the basis of the observed streaming velocities and
geometry of the spiral arms we estimate a mass inflow rate of ionized gas of
3x10^(-4) Msun/yr, which is of the order of the accretion rate necessary to
power the LINER nucleus of NGC 6951. Similar streaming motions towards the
nucleus of another galaxy with LINER nucleus -- NGC 1097 -- have been reported
by our group in a previous paper. Taken together, these results support a
scenario in which nuclear spirals are channels through which matter is
transferred from galactic scales to the nuclear region to feed the supermassive
black hole.Comment: 25 pages, 6 eps figures, accepted for publication in Ap