243 research outputs found
The relation between metallicity, stellar mass and star formation in galaxies: an analysis of observational and model data
We study relations between stellar mass, star formation and gas-phase
metallicity in a sample of 177,071 unique emission line galaxies from the
SDSS-DR7, as well as in a sample of 43,767 star forming galaxies at z=0 from
the cosmological semi-analytic model L-GALAXIES. We demonstrate that
metallicity is dependent on star formation rate at fixed mass, but that the
trend is opposite for low and for high mass galaxies. Low-mass galaxies that
are actively forming stars are more metal-poor than quiescent low-mass
galaxies. High-mass galaxies, on the other hand, have lower gas-phase
metallicities if their star formation rates are small. Remarkably, the same
trends are found for our sample of model galaxies. We find that massive model
galaxies with low gas-phase metallicities have undergone a gas-rich merger in
the past, inducing a starburst which exhausted their cold gas reservoirs and
shut down star formation. This led to a gradual dilution in the gas-phase
metallicities of these systems via accretion of gas. These model galaxies have
lower-than-average gas-to-stellar mass ratios and higher-than-average central
black hole masses. We confirm that massive galaxies with low gas-phase
metallicities in our observational sample also have very massive black holes.
We propose that accretion may therefore play a significant role in regulating
the gas-phase metallicities of present-day massive galaxies.Comment: 18 pages, 18 figures, 2 tables. Submitted to MNRA
Neonatal programming by immunological challenge: effects on ovarian function in the adult rat
Neonatal exposure to an immunological challenge (lipopolysaccharide, LPS) increases the activity of hypothalamo-pituitary–adrenal axis and sensitises the GNRH pulse generator to the inhibitory influence of stress in adult rats. We investigated the effects of neonatal exposure to LPS on various reproductive parameters during puberty and into adulthood in female rats. LPS (50 μg/kg, i.p.) or saline was administered on postnatal days 3 and 5. Vaginal opening was recorded, and oestrous cyclicity was monitored immediately post puberty and again at 8–9 weeks of age. At 10 weeks of age, the ovaries were removed and the number of follicles was counted, together with the thickness of the theca interna of the largest antral follicles. Ovarian sympathetic nerve activity was assessed immunohistochemically by measurement of the levels of ovarian low-affinity receptor of nerve growth factor (p75NGFR). In rats exposed to LPS in early life, there was a significant delay in puberty and disruption of oestrous cyclicity immediately post puberty, which persisted into adulthood. The follicle reserve was decreased, the thickness of the theca interna increased and the expression profile of ovarian p75NGFR increased in the neonatal LPS-treated animals. These data suggest that exposure to LPS during early neonatal life can have long-term dysfunctional effects on the female reproductive system, which might involve, at least in part, increased ovarian sympathetic nerve activity
Eliminating Error in the Chemical Abundance Scale for Extragalactic HII Regions
In an attempt to remove the systematic errors which have plagued the
calibration of the HII region abundance sequence, we have theoretically modeled
the extragalactic HII region sequence. We then used the theoretical spectra so
generated in a double blind experiment to recover the chemical abundances using
both the classical electron temperature + ionization correction factor
technique, and the technique which depends on the use of strong emission lines
(SELs) in the nebular spectrum to estimate the abundance of oxygen. We find a
number of systematic trends, and we provide correction formulae which should
remove systematic errors in the electron temperature + ionization correction
factor technique. We also provide a critical evaluation of the various
semi-empirical SEL techniques. Finally, we offer a scheme which should help to
eliminate systematic errors in the SEL-derived chemical abundance scale for
extragalactic HII regions.Comment: 24 pages, 9 Tables, 13 figures, accepted for publication in MNRAS.
Updated considering minor changes during the final edition process and some
few missing reference
Study of star-forming galaxies in SDSS up to redshift 0.4 II. Evolution from the fundamental parameters: mass, metallicity & SFR
To understand the formation and evolution of galaxies, it is important to
have a full comprehension of the role played by the metallicity, star formation
rate (SFR), morphology, and color. The interplay of these parameters at
different redshifts will substantially affect the evolution of galaxies and, as
a consequence, the evolution of them will provide important clues and
constraints on the galaxy evolution models. In this work we focus on the
evolution of the SFR, metallicity of the gas, and morphology of galaxies at low
redshift in search of signs of evolution. We use the S2N2 diagnostic diagram as
a tool to classify star--forming, composite, and AGN galaxies. We analyzed the
evolution of the three principal BPT diagrams, estimating the SFR and specific
SFR (SSFR) for our samples of galaxies, studying the luminosity and
mass-metallicity relations, and analyzing the morphology of our sample of
galaxies through the g-r color, concentration index, and SSFR. We found that
the S2N2 is a reliable diagram to classify star--forming, composite, and AGNs
galaxies. We demonstrate that the three principal BPT diagrams show an
evolution toward higher values of [OIII]5007/Hb due to a metallicity decrement.
We found an evolution in the mass-metallicity relation of ~ 0.2 dex for the
redshift range 0.3 < z < 0.4 compared to our local one. From the analysis of
the evolution of the SFR and SSFR as a function of the stellar mass and
metallicity, we discovered a group of galaxies with higher SFR and SSFR at all
redshift samples, whose morphology is consistent with those of late-type
galaxies. Finally, the comparison of our local (0.04<z<0.1) with our higher
redshift sample (0.3<z<0.4), show that the metallicity, the SFR and morphology,
evolve toward lower values of metallicity, higher SFRs, and late--type
morphologies for the redshift range 0.3<z<0.4Comment: 16 pages, 15 figures. Accepted for publication in A&
The dependence of galaxy group star formation rates and metallicities on large scale environment
We construct a sample of 75,863 star forming galaxies with robust metallicity
and star formation rate measurements from the Sloan Digital Sky Survey Data
Release 7 (SDSS DR7), from which we select a clean sample of compact group (CG)
galaxies. The CGs are defined to be close configurations of at least 4 galaxies
that are otherwise apparently isolated. Our selection results in a sample of
112 spectroscopically identified compact group galaxies, which can be further
divided into groups that are either embedded within a larger structure, such as
a cluster or large group, or truly isolated systems. The compact groups then
serve as a probe into the influence of large scale environment on a galaxy's
evolution, while keeping the local density fixed at high values. We find that
the star formation rates (SFRs) of star forming galaxies in compact groups are
significantly different between isolated and embedded systems. Galaxies in
isolated systems show significantly enhanced SFR, relative to a control sample
matched in mass and redshift, a trend not seen in the embedded systems.
Galaxies in isolated systems exhibit a median SFR enhancement at fixed stellar
mass of +0.07 \pm 0.03 dex. These dependences on large scale environment are
small in magnitude relative to the apparent influence of local scale effects
found in previous studies, but the significance of the difference in SFRs
between our two samples constrains the effect of large scale environment to be
non-zero. We find no significant change in the gas-phase interstellar
metallicity for either the isolated or embedded compact group sample relative
to their controls. However, simulated samples that include artificial offsets
indicate that we are only sensitive to metallicity changes of log O/H >0.13 dex
(at 99% confidence), which is considerably larger than the typical metallicity
differences seen in previous environmental studies.Comment: Accepted for publication in MNRAS. 16 pages, 9 figure
Colon-Specific Drug Delivery Behavior of pH-Responsive PMAA/Perlite Composite
The preparation, characterization, and in vitro release of 5-aminosalicylic acid (5-ASA) from methacrylic acid (MAA)/perlite composites (APC) prepared via a sol–gel route are reported. The free-radical graft polymerization of methacrylic acid (MAA) onto perlite particles was studied experimentally. The grafting procedure consisted of surface activation with 3-(trimethoxysilyl) propyl methacrylate (TSPA), followed by free-radical graft polymerization of methacrylic acid (MAA) in ethyl acetate with 2,2′-azobisisobutyronitrile (AIBN) initiator. The composition of the composites hybrid materials was determined by FTIR spectroscopy. Equilibrium swelling studies were carried out in enzyme-free simulated gastric and intestinal fluids (SGF and SIF, respectively). The dried composites were immersed in a saturated solution of 5-ASA in water overnight and dried over a period of three days at room temperature and the in vitro release profiles were established separately in both (SGF, pH 1) and (SIF, pH 7.4). The 5-ASA concentration of the solution was measured using a UV-Vis spectrophotometer (205 nm) at different time intervals. The in vitro drug release test revealed that the release rate of 5-ASA in buffer solutions increased with the silica content in the composites; on the contrary, the increase of the content of 3-(trimethoxysilyl)propyl methacrylate (TSPA), a coupling agent, decreased the drug release rate
The continuous rise of bulges out of galactic disks
(abridged) This study revolves around dmB, a new distance- and
extinction-independent measure of the contribution by stellar populations older
than 9 Gyr to the mean r-band surface brightness of the bulge component in 135
late-type galaxies (LTGs) from the CALIFA survey, spanning a range of 2.6 dex
and 3 dex in total and bulge stellar mass (M*T~10^(8.9-11.5) M_solar and
M*B~10^(8.3-11.3) M_solar, respectively). The main insight from this study is
that LTG bulges form a continuous sequence of increasing dmB with increasing
M*T, M*B, stellar mass surface density S* and mass-weighted age and
metallicity: high-dmB bulges are the oldest, densest and most massive ones, and
vice versa. Furthermore, we find that the bulge-to-disk age and metallicity
contrast, as well as the bulge-to-disk mass ratio increase with M*T, raising
from, respectively, ~0 Gyr, 0 dex and 0.25 to ~3 Gyr, ~0.3 dex and 0.67 across
the mass range covered by our sample. Whereas gas excitation in lower-mass
bulges is invariably dominated by star formation (SF), LINER- and
Seyfert-specific emission-line ratios were exclusively documented in high-mass,
high-S* bulges. The continuity both in the properties of LTG bulges themselves
and in their age and metallicity contrast to their parent disks suggests that
these components evolve alongside in a concurrent process that leads to a
continuum of physical and evolutionary characteristics. Our results are
consistent with a picture where bulge growth in LTGs is driven by a
superposition of quick-early and slow-secular processes, the relative
importance of which increases with M*T. These processes, which presumably
combine in situ SF in the bulge and inward migration of material from the disk,
are expected to lead to a non-homologous radial growth of S* and a trend for an
increasing Sersic index with increasing galaxy mass.Comment: 24 pages, accepted for publication in A&
Flat-spectrum symmetric objects with ~1 kpc sizes I. The candidates
In order to understand the origin and evolution of radio galaxies, searches
for the youngest such sources have been conducted. Compact-medium symmetric
objects (CSO-MSOs) are thought to be the earliest stages of radio sources, with
possible ages of <10^3 yrs for CSOs (<1 kpc in size) and 10^4-10^5 yrs for MSOs
(1-15 kpc). From a literature selection in heterogeneous surveys, we have
established a sample of 37 confirmed CSOs. In addition, we only found three
confirmed flat-spectrum MSOs in the literature.
The typical CSO resides on a z<0.5 galaxy, has a flat radio spectrum
(a_thin<0.5; S_v proportional to v^-a), is <0.3 kpc in size, has an arm length
ratio <2, and well-aligned (theta<20 deg) opposite lobes with a flux density
ratio <10. In order to populate the 0.3-1 kpc size range (large CSOs) and also
in order to find more flat-spectrum MSOs, we have built a sample of 157 radio
sources with a_{1.40}^{4.85}<0.5 that were resolved with the VLA-A 8.4 GHz. As
first results, we have 'rediscovered' nine of the known CSO/MSOs while
identifying two new ~14 kpc MSOs and two candidate CSO/MSOs (which only lack
redshifts for final classification). We were able to reject 61 of the remaining
144 objects from literature information alone. In the series of papers that
starts with this one we plan to classify the remaining 83 CSO/MSO candidates
(thanks to radio and optical observations) as well as characterize the physical
properties of the (likely) many 0.3-15 kpc flat-spectrum CSO/MSOs to be found.Comment: 19 pages, 14 figures, 6 tables (note that Table 2, in landscape
format, has a separate file); accepted by MNRA
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A high-resolution map of human evolutionary constraint using 29 mammals.
The comparison of related genomes has emerged as a powerful lens for genome interpretation. Here we report the sequencing and comparative analysis of 29 eutherian genomes. We confirm that at least 5.5% of the human genome has undergone purifying selection, and locate constrained elements covering ∼4.2% of the genome. We use evolutionary signatures and comparisons with experimental data sets to suggest candidate functions for ∼60% of constrained bases. These elements reveal a small number of new coding exons, candidate stop codon readthrough events and over 10,000 regions of overlapping synonymous constraint within protein-coding exons. We find 220 candidate RNA structural families, and nearly a million elements overlapping potential promoter, enhancer and insulator regions. We report specific amino acid residues that have undergone positive selection, 280,000 non-coding elements exapted from mobile elements and more than 1,000 primate- and human-accelerated elements. Overlap with disease-associated variants indicates that our findings will be relevant for studies of human biology, health and disease
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