265 research outputs found
Discovering extremely compact and metal-poor, star-forming dwarf galaxies out to z ∼ 0.9 in the VIMOS Ultra-Deep Survey
We report the discovery of 31 low-luminosity (−14.5 ≳ M_(AB)(B) ≳ −18.8), extreme emission line galaxies (EELGs) at 0.2 ≲ z ≲ 0.9 identified by their unusually high rest-frame equivalent widths (100 ≤ EW[O iii] ≤ 1700 Å) as part of the VIMOS Ultra Deep Survey (VUDS). VIMOS optical spectra of unprecedented sensitivity (I_(AB) ~ 25 mag) along with multiwavelength photometry and HST imaging are used to investigate spectrophotometric properties of this unique sample and to explore, for the first time, the very low stellar mass end (M* ≲ 10^8M_⊙) of the luminosity-metallicity (LZR) and mass-metallicity (MZR) relations at z < 1. Characterized by their extreme compactness (R_50 < 1 kpc), low stellar mass and enhanced specific star formation rates (sSFR = SFR/M_* ~ 10^(-9)−10^(-7) yr^(-1)), the VUDS EELGs are blue dwarf galaxies likely experiencing the first stages of a vigorous galaxy-wide starburst. Using T_e-sensitive direct and strong-line methods, we find that VUDS EELGs are low-metallicity (7.5 ≲ 12 + log (O/H) ≲ 8.3) galaxies with high ionization conditions (log (q_(ion)) ≳ 8 cm s^(-1)), including at least three EELGs showing Heiiλ 4686 Å emission and four extremely metal-poor (≲10% solar) galaxies. The LZR and MZR followed by VUDS EELGs show relatively large scatter, being broadly consistent with the extrapolation toward low luminosity and mass from previous studies at similar redshift. However, we find evidence that galaxies with younger and more vigorous star formation – as characterized by their larger EWs, ionization and sSFR – tend to be more metal poor at a given stellar mass
A young stellar environment for the superluminous supernova PTF12dam
The progenitors of super luminous supernovae (SLSNe) are still a mystery.
Hydrogen-poor SLSN hosts are often highly star-forming dwarf galaxies and the
majority belongs to the class of extreme emission line galaxies hosting young
and highly star-forming stellar populations. Here we present a resolved
long-slit study of the host of the hydrogen-poor SLSN PTF12dam probing the kpc
environment of the SN site to determine the age of the progenitor. The galaxy
is a "tadpole" with uniform properties and the SN occurred in a star-forming
region in the head of the tadpole. The galaxy experienced a recent star-burst
superimposed on an underlying old stellar population. We measure a very young
stellar population at the SN site with an age of ~3 Myr and a metallicity of
12+log(O/H)=8.0 at the SN site but do not observe any WR features. The
progenitor of PTF12dam must have been a massive star of at least 60 M_solar and
one of the first stars exploding as a SN in this extremely young starburst.Comment: submitted to MNRAS letters. 5 pages, 3 figures, supplementary
material: 2 figures, 2 table
The environs of the H II region Gum 31
Aims. We analyze the distribution of the interstellar matter in the environs of the Hn region Gum 31, excited by the open cluster NGC 3324, located in the complex Carina region, with the aim of investigating the action of the massive stars on the surrounding neutral material.
Methods. We use neutral hydrogen 21-cm line data, radio continuum images at 0.843, 2.4 and 4.9 GHz, 12CO(1-0) observations, and IRAS and MSX infrared data.
Results. Adopting a distance of 3 kpc for the Hn region and the ionizing cluster, we derived an electron density of 33± 3 cm-3 and an ionized mass of (3.3 ±1.1) × 103 M⊙ based on the radio continuum data at 4.9 GHz. The Hl 21-cm line images revealed an Hl shell surrounding the Hn region. The Hl structure is 10.0 ±1.7 pc in radius, has a neutral mass of 1500 ±500 M⊙, and is expanding at 11 km s-1. The associated molecular gas amounts to (1.1 ±0.5) × 105 M⊙, being its volume density of about 350 cm-3. This molecular shell could represent the remains of the cloud where the young open cluster NGC 3324 was born or could have originated by the shock front associated with the Hn region. The difference between the ambient density and the electron density of the Hn region suggests that the Hn region is expanding. The distributions of the ionized and molecular material, along with that of the emission in the MSX band A, suggest that a photodissociation region has developed at the interface between the ionized and molecular gas. The copious UV photon flux from the early type stars in NGC 3324 keeps the Hn region ionized. The characteristics of a relatively large number of the IRAS, MSX, and 2MASS point sources projected onto the molecular envelope are compatible with protostellar candidates, showing the presence of active star forming regions. Very probably, the expansion of the Hn region has triggered stellar formation in the molecular shell.Instituto Argentino de RadioastronomíaInstituto de Astrofísica de La PlataFacultad de Ciencias Astronómicas y Geofísica
Using photoionization models to derive carbon and oxygen gas-phase abundances in the rest UV
We present a new method to derive oxygen and carbon abundances using the ultraviolet (UV) lines emitted by the gas phase ionized by massive stars. The method is based on the comparison of the nebular emission-line ratios with those predicted by a large grid of photoionization models. Given the large dispersion in the O/H–C/O plane, our method first fixes C/O using ratios of appropriate emission lines and, in a second step, calculates O/H and the ionization parameter from carbon lines in the UV. We find abundances totally consistent with those provided by the direct method when we apply this method to a sample of objects with an empirical determination of the electron temperature using optical emission lines. The proposed methodology appears as a powerful tool for systematic studies of nebular abundances in star-forming galaxies at high redshift.EPM acknowledges support from the Spanish MICINN through grants AYA2010-21887-C04-01 and AYA2013-47742-C4-1-P and the Junta de Andalucía for grant EXC/2011 FQM-7058. RA acknowledges the support from the ERC Advanced Grant 695671 ‘QUENCH’ and the FP7 SPACE project ‘ASTRODEEP’ (Ref. No.: 312725), supported by the European Commission
The star formation history and metal content of the "Green Peas". New detailed GTC-OSIRIS spectrophotometry of three galaxies
We present deep broad-band imaging and long-slit spectroscopy of three
compact, low-mass starburst galaxies at redshift z\sim0.2-0.3, also referred to
as Green Peas (GP). We measure physical properties of the ionized gas and
derive abundances for several species with high precision. We find that the
three GPs display relatively low extinction, low oxygen abundances, and
remarkably high N/O ratios We also report on the detection of clear signatures
of Wolf-Rayet (WR) stars in these galaxies. We carry out a pilot spectral
synthesis study using a combination of both population and evolutionary
synthesis models. Their outputs are in qualitative agreement, strongly
suggesting a formation history dominated by starbursts. In agreement with the
presence of WR stars, these models show that these GPs currently undergo a
major starburst producing between ~4% and ~20% of their stellar mass. However,
as models imply, they are old galaxies having had formed most of their stellar
mass several Gyr ago. The presence of old stars has been spectroscopically
verified in one of the galaxies by the detection of Mg I 5167, 5173 absorption
line. Additionally, we perform a surface photometry study based on HST data,
that indicates that the three galaxies posses an exponential low-surface
brightness envelope. If due to stellar emission, the latter is structurally
compatible to the evolved hosts of luminous BCD/HII galaxies, suggesting that
GPs are identifiable with major episodes in the assembly history of local BCDs.
These conclusions highlight the importance of these objects as laboratories for
studying galaxy evolution at late cosmic epochs.Comment: 30 pages, 9 figures and 7 tables. Accepted for publication in Ap
An extreme [OIII] emitter at : a low metallicity Lyman continuum source
[Abridged] We investigate the physical properties of a Lyman continuum
emitter candidate at with photometric coverage from to MIPS
24m band and VIMOS/VLT and MOSFIRE/Keck spectroscopy. Investigation of the
UV spectrum confirms a direct spectroscopic detection of the Lyman continuum
emission with . Non-zero Ly flux at the systemic redshift and
high Lyman- escape fraction suggest a low HI column density. The weak C
and Si low-ionization absorption lines are also consistent with a low covering
fraction along the line of sight. The
[OIII] equivalent width is one of the
largest reported for a galaxy at
(, rest-frame) and the NIR spectrum shows that this is mainly due to an
extremely strong [OIII] emission. The large observed [OIII]/[OII] ratio ()
and high ionization parameter are consistent with prediction from
photoionization models in case of a density-bounded nebula scenario.
Furthermore, the
is
comparable to recent measurements reported at , in the reionization
epoch. We also investigate the possibility of an AGN contribution to explain
the ionizing emission but most of the AGN identification diagnostics suggest
that stellar emission dominates instead. This source is currently the first
high- example of a Lyman continuum emitter exhibiting indirect and direct
evidences of a Lyman continuum leakage and having physical properties
consistent with theoretical expectation from Lyman continuum emission from a
density-bounded nebula.Comment: 14 pages, 11 figures, accepted for publication in A&A. Minor
modifications, Figure 2 updated, Figure 9 adde
Search for Blue Compact Dwarf Galaxies During Quiescence
Blue Compact Dwarf (BCD) galaxies are metal poor systems going through a
major starburst that cannot last for long. We have identified galaxies which
may be BCDs during quiescence (QBCD), i.e., before the characteristic starburst
sets in or when it has faded away. These QBCD galaxies are assumed to be like
the BCD host galaxies. The SDSS/DR6 database provides ~21500 QBCD candidates.
We also select from SDSS/DR6 a complete sample of BCD galaxies to serve as
reference. The properties of these two galaxy sets have been computed and
compared. The QBCD candidates are thirty times more abundant than the BCDs,
with their luminosity functions being very similar except for the scaling
factor, and the expected luminosity dimming associated with the end of the
starburst. QBCDs are redder than BCDs, and they have larger HII region based
oxygen abundance. QBCDs also have lower surface brightness. The BCD candidates
turn out to be the QBCD candidates with the largest specific star formation
rate (actually, with the largest H_alpha equivalent width). One out of each
three dwarf galaxies in the local universe may be a QBCD. The properties of the
selected BCDs and QBCDs are consistent with a single sequence in galactic
evolution, with the quiescent phase lasting thirty times longer than the
starburst phase. The resulting time-averaged star formation rate is low enough
to allow this cadence of BCD -- QBCD phases during the Hubble time.Comment: Accepted for publication in ApJ. 17 pages. 13 Fig
Extremely Metal-Poor Galaxies: The HI Content
Extremely metal-poor (XMP) galaxies are chemically, and possibly dynamically,
primordial objects in the local Universe. Our objective is to characterize the
HI content of the XMP galaxies as a class, using as a reference the list of 140
known local XMPs compiled by Morales-Luis et al. (2011). We have observed 29
XMPs, which had not been observed before at 21 cm, using the Effelsberg radio
telescope. This information was complemented with HI data published in
literature for a further 53 XMPs. In addition, optical data from the literature
provided morphologies, stellar masses, star-formation rates and metallicities.
Effelsberg HI integrated flux densities are between 1 and 15 Jy km/s, while
line widths are between 20 and 120 km/s. HI integrated flux densities and line
widths from literature are in the range 0.1 - 200 Jy km/s and 15 - 150 km/s,
respectively. Of the 10 new Effelsberg detections, two sources show an
asymmetric double-horn profile, while the remaining sources show either
asymmetric (7 sources) or symmetric (1 source) single-peak 21 cm line profiles.
An asymmetry in the HI line profile is systematically accompanied by an
asymmetry in the optical morphology. Typically, the g-band stellar
mass-to-light ratios are ~0.1, whereas the HI gas mass-to-light ratios may be
up to 2 orders of magnitude larger. Moreover, HI gas-to-stellar mass ratios
fall typically between 10 and 20, denoting that XMPs are extremely gas-rich. We
find an anti-correlation between the HI gas mass-to-light ratio and the
luminosity, whereby fainter XMPs are more gas-rich than brighter XMPs,
suggesting that brighter sources have converted a larger fraction of their HI
gas into stars. The dynamical masses inferred from the HI line widths imply
that the stellar mass does not exceed 5% of the dynamical mass, while the
\ion{H}{i} mass constitutes between 20 and 60% of the dynamical mass.
(abridged)Comment: 30 pages, accepted for A&
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