225 research outputs found
Images IV: Strong evolution of the oxygen abundance in gaseous phases of intermediate mass galaxies since z=0.8
Intermediate mass galaxies (logM(Msun)>10) at z~0.6 are the likeliest
progenitors of the present-day numerous population of spirals. There is growing
evidence that they have evolved rapidly since the last 6 to 8 Gyr ago, and
likely have formed a significant fraction of their stellar mass, often showing
perturbed morphologies and kinematics. We have gathered a representative sample
of 88 such galaxies and have provided robust estimates of their gas phase
metallicity. For doing so, we have used moderate spectral resolution
spectroscopy at VLT/FORS2 with unprecedented high S/N allowing to remove biases
coming from interstellar absorption lines and extinction to establish robust
values of R23=([OII]3727 + [OIII]4959,5007)/Hbeta. We definitively confirm that
the predominant population of z~0.6 starbursts and luminous IR galaxies (LIRGs)
are on average, two times less metal rich than the local galaxies at a given
stellar mass. We do find that the metal abundance of the gaseous phase of
galaxies is evolving linearly with time, from z=1 to z=0 and after comparing
with other studies, from z=3 to z=0. Combining our results with the reported
evolution of the Tully Fisher relation, we do find that such an evolution
requires that ~30% of the stellar mass of local galaxies have been formed
through an external supply of gas, thus excluding the close box model. Distant
starbursts & LIRGs have properties (metal abundance, star formation efficiency
& morphologies) similar to those of local LIRGs. Their underlying physics is
likely dominated by gas infall probably through merging or interactions. Our
study further supports the rapid evolution of z~0.4-1 galaxies. Gas exchanges
between galaxies is likely the main cause of this evolution.Comment: 21 pages, 12 figures, A&A, In pres
Chemical Abundances and Dust in Planetary Nebulae in the Galactic Bulge
We present mid-infrared Spitzer spectra of eleven planetary nebulae in the
Galactic Bulge. We derive argon, neon, sulfur, and oxygen abundances for them
using mainly infrared line fluxes combined with some optical line fluxes from
the literature. Due to the high extinction toward the Bulge, the infrared
spectra allow us to determine abundances for certain elements more accurately
that previously possible with optical data alone. Abundances of argon and
sulfur (and in most cases neon and oxygen) in planetary nebulae in the Bulge
give the abundances of the interstellar medium at the time their progenitor
stars formed; thus these abundances give information about the formation and
evolution of the Bulge. The abundances of Bulge planetary nebulae tend to be
slightly higher than those in the Disk on average, but they do not follow the
trend of the Disk planetary nebulae, thus confirming the difference between
Bulge and Disk evolution. Additionally, the Bulge planetary nebulae show
peculiar dust properties compared to the Disk nebulae. Oxygen-rich dust feature
(crystalline silicates) dominate the spectra of all of the Bulge planetary
nebulae; such features are more scarce in Disk nebulae. Additionally,
carbon-rich dust features (polycyclic aromatic hydrocarbons) appear in roughly
half of the Bulge planetary nebulae in our sample, which is interesting in
light of the fact that this dual chemistry is comparatively rare in the Milky
Way as a whole.Comment: 16 pages, 5 figures, accepted to Ap
The Gemini Deep Deep Survey. VII. The Redshift Evolution of the Mass-Metallicity Relation
We have investigated the mass-metallicity (M-Z) relation using galaxies at
0.4<z<1.0 from the Gemini Deep Deep Survey and Canada-France Redshift Survey.
Deep K and z' band photometry allowed us to measure stellar masses for 69
galaxies. From a subsample of 56 galaxies, for which metallicity of the
interstellar medium is also measured, we identified a strong correlation
between mass and metallicity, for the first time in the distant Universe. This
was possible because of the larger base line spanned by the sample in terms of
metallicity (a factor of 7) and mass (a factor of 400) than in previous works.
This correlation is much stronger and tighter than the luminosity-metallicity,
confirming that stellar mass is a more meaningful physical parameter than
luminosity. We find clear evidence for temporal evolution in the M-Z relation
in the sense that at a given mass, a galaxy at z=0.7 tends to have lower
metallicity than a local galaxy of similar mass. We use the z=0.1 SDSS M-Z
relation, and a small sample of z=2.3 Lyman break galaxies with known mass and
metallicity, to propose an empirical redshift-dependent M-Z relation, according
to which the stellar mass and metallicity in small galaxies evolve for a longer
time than in massive galaxies. This relation predicts that the generally metal
poor damped Lyman-alpha galaxies have stellar masses of the order of 10^8.8
M_sun (with a dispersion of 0.7 dex) all the way from z=0.2 to z=4. The
observed redshift evolution of the M-Z relation can be reproduced remarkably
well by a simple closed-box model where the key assumption is an e-folding time
for star formation which is higher or, in other words, a period of star
formation that lasts longer in less massive galaxies than in more massive
galaxies. Such a picture supports the downsizing scenario for galaxy formation.Comment: ApJ in pres
Sulfur, Chlorine, and Argon Abundances in Planetary Nebulae. IV: Synthesis and the Sulfur Anomaly
We have compiled a large sample of O, Ne, S, Cl, and Ar abundances which have
been determined for 85 galactic planetary nebulae in a consistent and
homogeneous manner using spectra extending from 3600-9600 Angstroms. Sulfur
abundances have been computed using the near IR lines of [S III] 9069,9532
along with [S III] temperatures. We find average values, expressed
logarithmically with a standard deviation, of log(S/O)=-1.91(+/-.24),
log(Cl/O)=-3.52(+/-.16), and log(Ar/O)=-2.29(+/-.18), numbers consistent with
previous studies of both planetary nebulae and H II regions. We also find a
strong correlation between [O III] and [S III] temperatures among planetary
nebulae. In analyzing abundances of Ne, S, Cl, and Ar with respect to O, we
find a tight correlation for Ne-O, and loose correlations for Cl-O and Ar-O.
All three trends appear to be colinear with observed correlations for H II
regions. S and O also show a correlation but there is a definite offset from
the behavior exhibited by H II regions and stars. We suggest that this S
anomaly is most easily explained by the existence of S^+3, whose abundance must
be inferred indirectly when only optical spectra are available, in amounts in
excess of what is predicted by model-derived ionization correction factors.
Finally for the disk PNe, abundances of O, Ne, S, Cl, and Ar all show gradients
when plotted against galactocentric distance. The slopes are statistically
indistinguishable from one another, a result which is consistent with the
notion that the cosmic abundances of these elements evolve in lockstep.Comment: 43 pages, including 11 figures. Accepted for publication in the
Astronomical Journal. See also astro-ph/0106213 for Northern sample results,
astro-ph/0109161 and astro-ph/0108336 for the data and abundance information
for the Southern sample, and astro-ph/020954
The iron abundance of the Magellanic Bridge
High-resolution HST ultra-violet spectra for five B-type stars in the
Magellanic Bridge and in the Large and Small Magellanic Clouds have been
analysed to estimate their iron abundances. Those for the Clouds are lower than
estimates obtained from late-type stars or the optical lines in B-type stars by
approximately 0.5 dex. This may be due to systematic errors possibly arising
from non-LTE effects or from errors in the atomic data as similar low Fe
abundances having previously been reported from the analysis of the
ultra-violet spectra of Galactic early-type stars. The iron abundance estimates
for all three Bridge targets appear to be significantly lower than those found
for the SMC and LMC by approximately -0.5 dex and -0.8 dex respectively and
these differential results should not be affected by any systematic errors
present in the absolute abundance estimates. These differential iron abundance
estimates are consistent with the underabundances for C, N, O, Mg and Si of
approximately -1.1 dex relative to our Galaxy previously found in our Bridge
targets. The implications of these very low metal abundances for the Magellanic
Bridge are discussed in terms of metal deficient material being stripped from
the SMC.Comment: Accepted for publication in MNRA
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