70,599 research outputs found
On the compact HII galaxy UM 408 as seen by GMOS-IFU: Physical conditions
We present Integral Field Unit GMOS-IFU data of the compact HII galaxy UM408,
obtained at Gemini South telescope, in order to derive the spatial distribution
of emission lines and line ratios, kinematics, plasma parameters, and oxygen
abundances as well the integrated properties over an area of 3"x4".4(~750x1100
pc) located in the central part of the galaxy. The starburst in this area is
resolved into two giant regions of ~375 and 250pc diameter, respectively. The
ages of these two regions, estimated using Hb equivalent widths, suggest that
they are coeval events of ~5Myr with stellar masses of ~10^4M_o. We have also
used [OIII]/Hb and [SII]/Ha ratio maps to explore the excitation mechanisms in
this galaxy. The Ha emission line was used to measure the radial velocity and
velocity dispersion. We derived an integrated oxygen abundance of
12+log(O/H)=7.87 summing over all spaxels in our field of view. An average
value of 12+log(O/H)=7.77 and a difference of D(O/H)=0.47 between the minimum
and maximum values (7.58+-0.06-8.05+-0.04) were found, considering all data
points where the oxygen abundance was measured. The spatial distribution of
oxygen abundance does not show any significant gradient across the galaxy. On
the other hand, the bulk of data points are lying in a region of +-2sigma
dispersion (with sigma=0.1 dex) around the average value, confirming that this
compact HII galaxy as other previously studied dwarf irregular galaxies is
chemically homogeneous. Therefore, the new metals processed and injected by the
current star formation episode are possibly not observed and reside in the hot
gas phase, whereas the metals from previous events are well mixed and
homogeneously distributed through the whole extent of the galaxy.Comment: 30 pages, 10 figures. AJ accepte
Advances in small lasers
M.T.H was supported by an Australian Research council Future Fellowship research grant for this work. M.C.G. is grateful to the Scottish Funding Council (via SUPA) for financial support.Small lasers have dimensions or modes sizes close to or smaller than the wavelength of emitted light. In recent years there has been significant progress towards reducing the size and improving the characteristics of these devices. This work has been led primarily by the innovative use of new materials and cavity designs. This Review summarizes some of the latest developments, particularly in metallic and plasmonic lasers, improvements in small dielectric lasers, and the emerging area of small bio-compatible or bio-derived lasers. We examine the different approaches employed to reduce size and how they result in significant differences in the final device, particularly between metal- and dielectric-cavity lasers. We also present potential applications for the various forms of small lasers, and indicate where further developments are required.PostprintPeer reviewe
Random Walks Along the Streets and Canals in Compact Cities: Spectral analysis, Dynamical Modularity, Information, and Statistical Mechanics
Different models of random walks on the dual graphs of compact urban
structures are considered. Analysis of access times between streets helps to
detect the city modularity. The statistical mechanics approach to the ensembles
of lazy random walkers is developed. The complexity of city modularity can be
measured by an information-like parameter which plays the role of an individual
fingerprint of {\it Genius loci}.
Global structural properties of a city can be characterized by the
thermodynamical parameters calculated in the random walks problem.Comment: 44 pages, 22 figures, 2 table
Detecting filaments in the ultra-high energy cosmic ray distribution
We propose and test new statistical tools to study the distribution of cosmic
rays based on the use of the Minimal Spanning Tree. The method described is
particularly sensitive to filamentary structures, as those expected to arise
from strong sources of charged cosmic rays which get deflected by intervening
magnetic fields. We also test the method with data available from the AGASA and
SUGAR surface detector arrays.Comment: minor changes, matching the published version in Astroparticle
Physic
The M81 Group Dwarf Irregular Galaxy DDO 165. II. Connecting Recent Star Formation with ISM Structures and Kinematics
We compare the stellar populations and complex neutral gas dynamics of the
M81 group dIrr galaxy DDO 165 using data from the HST and the VLA. Paper I
identified two kinematically distinct HI components, multiple localized high
velocity gas features, and eight HI holes and shells (the largest of which
spans ~2.2x1.1 kpc). Using the spatial and temporal information from the
stellar populations in DDO 165, we compare the patterns of star formation over
the past 500 Myr with the HI dynamics. We extract localized star formation
histories within 6 of the 8 HI holes identified in Paper I, as well as 23 other
regions that sample a range of stellar densities and neutral gas properties.
From population synthesis modeling, we derive the energy outputs (from stellar
winds and supernovae) of the stellar populations within these regions over the
last 100 Myr, and compare with refined estimates of the energies required to
create the HI holes. In all cases, we find that "feedback" is energetically
capable of creating the observed structures in the ISM. Numerous regions with
significant energy inputs from feedback lack coherent HI structures but show
prominent localized high velocity gas features; this feedback signature is a
natural product of temporally and spatially distributed star formation. In DDO
165, the extended period of heightened star formation activity (lasting more
than 1 Gyr) is energetically capable of creating the observed holes and high
velocity gas features in the neutral ISM.Comment: The Astrophysical Journal, in press. Full-resolution version
available on request from the first autho
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