12,506 research outputs found
The oxygen abundance in the IFU era
Spatially-resolved information of gas-phase emission provided by integral
field units (IFUs) are allowing us to perform a new generation of emission-line
surveys, based on large samples of HII regions and full two-dimensional
coverage. Here we present two highlights of our current studies employing this
technique: 1) A statistical approach to the abundance gradients of spiral
galaxies, which indicates an -universal- radial gradient for oxygen abundance;
and 2) The discovery of a new scaling relation of HII regions in spiral
galaxies, the "local" mass-metallicity relation of star-forming galaxies.Comment: 6 pages, to appear in Highlights of Spanish Astrophysics VII,
Proceedings of the X Scientific Meeting of the Spanish Astronomical Society
held on July 9-13, 2012, in Valencia, Spai
Inner and outer star forming regions over the disks of spiral galaxies. I. Sample characterization
Context. The knowledge of abundance distributions is central to understanding
the formation and evolution of galaxies. Most of the relations employed for the
derivation of gas abundances have so far been derived from observations of
outer disk HII regions, despite the known differences between inner and outer
regions. Aims. Using integral field spectroscopy (IFS) observations we aim to
perform a systematic study and comparison of two inner and outer HII regions
samples. The spatial resolution of the IFS, the number of objects and the
homogeneity and coherence of the observations allow a complete characterization
of the main observational properties and differences of the regions. Methods.
We analyzed a sample of 725 inner HII regions and a sample of 671 outer HII
regions, all of them detected and extracted from the observations of a sample
of 263 nearby, isolated, spiral galaxies observed by the CALIFA survey.
Results. We find that inner HII regions show smaller equivalent widths, greater
extinction and luminosities, along with greater values of
[NII]{\lambda}6583/H{\alpha} and [OII]{\lambda}3727/[OIII]{\lambda}5007
emission-line ratios, indicating higher metallicites and lower ionization
parameters. Inner regions have also redder colors and higher photometric and
ionizing masses, although Mion/Mphot is slighty higher for the outer regions.
Conclusions. This work shows important observational differences between inner
and outer HII regions in star forming galaxies not previously studied in
detail. These differences indicate that inner regions have more evolved stellar
populations and are in a later evolution state with respect to outer regions,
which goes in line with the inside-out galaxy formation paradigm.Comment: 16 page
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Listeria monocytogenes cell-to-cell spread in epithelia is heterogeneous and dominated by rare pioneer bacteria.
Listeria monocytogenes hijacks host actin to promote its intracellular motility and intercellular spread. While L. monocytogenes virulence hinges on cell-to-cell spread, little is known about the dynamics of bacterial spread in epithelia at a population level. Here, we use live microscopy and statistical modeling to demonstrate that L. monocytogenes cell-to-cell spread proceeds anisotropically in an epithelial monolayer in culture. We show that boundaries of infection foci are irregular and dominated by rare pioneer bacteria that spread farther than the rest. We extend our quantitative model for bacterial spread to show that heterogeneous spreading behavior can improve the chances of creating a persistent L. monocytogenes infection in an actively extruding epithelium. Thus, our results indicate that L. monocytogenes cell-to-cell spread is heterogeneous, and that rare pioneer bacteria determine the frontier of infection foci and may promote bacterial infection persistence in dynamic epithelia. Editorial note:This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter)
PPAK Wide-field Integral Field Spectroscopy of NGC 628: II. Emission line abundance analysis
In this second paper of the series, we present the 2-dimensional (2D)
emission line abundance analysis of NGC 628, the largest object within the PPAK
Integral Field Spectroscopy (IFS) Nearby Galaxies Survey: PINGS. We introduce
the methodology applied to the 2D IFS data in order to extract and deal with
large spectral samples, from which a 2D abundance analysis can be later
performed. We obtain the most complete and reliable abundance gradient of the
galaxy up-to-date, by using the largest number of spectroscopic points sampled
in the galaxy, and by comparing the statistical significance of different
strong-line metallicity indicators. We find features not previously reported
for this galaxy that imply a multi-modality of the abundance gradient
consistent with a nearly flat-distribution in the innermost regions of the
galaxy, a steep negative gradient along the disc and a shallow gradient or
nearly-constant metallicity beyond the optical edge of the galaxy. The N/O
ratio seems to follow the same radial behaviour. We demonstrate that the
observed dispersion in metallicity shows no systematic dependence with the
spatial position, signal-to-noise or ionization conditions, implying that the
scatter in abundance for a given radius is reflecting a true spatial physical
variation of the oxygen content. Furthermore, by exploiting the 2D IFS data, we
were able to construct the 2D metallicity structure of the galaxy, detecting
regions of metal enhancement, and showing that they vary depending on the
choice of the metallicity estimator. The analysis of axisymmetric variations in
the disc of NGC 628 suggest that the physical conditions and the star formation
history of different-symmetric regions of the galaxy have evolved in a
different manner.Comment: Accepted for publication in MNRAS, 40 pages, 22 figures, online data:
http://www.ast.cam.ac.uk/ioa/research/ping
SUSIG: an on-line signature database, associated protocols and benchmark results
We present a new online signature database (SUSIG). The database consists of two parts that are collected using different pressure-sensitive tablets ( one with and the other without an LCD display). A total of 100 people contributed to each part, resulting in a database of more than 3,000 genuine signatures and 2,000 skilled forgeries. The genuine signatures in the database are real signatures of the contributors. In collecting skilled forgeries, forgers were shown the signing process on the monitor and were given a chance to practice. Furthermore, for a subset of the forgeries ( highly skilled forgeries), this animation was mapped onto the LCD screen of the tablet so that the forgers could trace over the mapped signature. Forgers in this group were also informed of how close they were to the reference signature, so that they could improve their forgery quality. We describe the signature acquisition process and several verification protocols for this database. We also report the performance of a state-of-the-art signature verification system using the associated protocols. The results show that the highly skilled forgery set is significantly more difficult compared to the skilled forgery set, providing researchers with challenging forgeries. The database is available through http://icproxy.sabanciuniv.edu:215
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