187 research outputs found
HeI in the central Giant HII Region of NGC 5253. A 2D observational approach to collisional and radiative transfer effects
ABRIDGED: NGC5253 is an ideal laboratory for detailed studies of starburst
galaxies. We present for the first time in a starburst galaxy a 2D study of the
spatial behavior of collisional and radiative transfer effects in He^+. The HeI
lines are analysed based on data obtained with FLAMES and GMOS. Collisional
effects are negligible for transitions in the singlet cascade while relatively
important for those in the triplet cascade. In particular, they can contribute
up to 20% of the flux in the HeIl7065 line. Radiative transfer effects are
important over an extended and circular area of 30pc in diameter centered at
the Super Star Clusters. HeI abundance, y^+, has been mapped using extinction
corrected fluxes of six HeI lines, realistic assumptions for T_e, n_e, and the
stellar absorption equivalent width as well as the most recent emissivities. We
found a mean of 10^3 y^+ ~80.3 over the mapped area. The relation between the
excitation and the total helium abundance, y_tot, is consistent with no
abundance gradient. Uncertainties in the derivation of He abundances are
dominated by the adopted assumptions. We illustrated the difficulty of
detecting a putative He enrichment due to the presence of Wolf-Rayet stars in
the main GHIIR. Data are marginally consistent with an excess in the N/He ratio
in the N enriched area of the order of both, the atmospheric N/He ratios in WR
stars and the uncertainties estimated for the N/He ratios.Comment: Accepted in Astronomy and Astrophysics; the emissivities presented in
the Corrigendum, Porter et al. 2013, arXiv:1303.5115, have been include
Studying nearby galactic planetary nebulae with MUSE: on the physical properties of IC 418
Galaxie
The extinction and dust-to-gas structure of the planetary nebula NGC 7009 observed with MUSE
The large field and wavelength range of MUSE is well suited to mapping
Galactic planetary nebulae (PN). The bright PN NGC 7009 was observed with MUSE
on the VLT during the Science Verification of the instrument in seeing of 0.6".
Emission line maps in hydrogen Balmer and Paschen lines were formed from
analysis of the MUSE cubes. The measured electron temperature and density from
the MUSE cube were employed to predict the theoretical hydrogen line ratios and
map the extinction distribution across the nebula. After correction for the
interstellar extinction to NGC 7009, the internal dust-to-gas ratio (A_V/N_H)
has been mapped for the first time in a PN. The extinction map of NGC 7009 has
considerable structure, broadly corresponding to the morphological features of
the nebula. A large-scale feature in the extinction map, consisting of a crest
and trough, occurs at the rim of the inner shell. The nature of this feature
was investigated and instrumental and physical causes considered; no convincing
mechanisms were identified to produce this feature, other than mass loss
variations in the earlier asymptotic giant branch phase. The dust-to-gas ratio
A_V/N_H increases from 0.7 times the interstellar value to >5 times from the
centre towards the periphery of the ionized nebula. The integrated A_V/N_H is
about 2 times the mean ISM value. It is demonstrated that extinction mapping
with MUSE provides a powerful tool for studying the distribution of PN internal
dust and the dust-to-gas ratio. (Abridged.)Comment: 10 pages, 7 figures. Accepted by A&
LINER-like Extended Nebulae in ULIRGs: Shocks Generated by Merger Induced Flows
In this work we studied the two-dimensional ionization structure of the
circumnuclear and extranuclear regions in a sample of six low-z Ultraluminous
Infrared Galaxies using Integral Field Spectroscopy. The ionization conditions
in the extranuclear regions of these galaxies (~5-15 kpc) are typical of LINERs
as obtained from the Veilleux-Osterbrock line ratio diagnostic diagrams. The
range of observed line ratios is best explained by the presence of fast shocks
with velocities of 150 to 500 km s^{-1}, while the ionization by an AGN or
nuclear starburst is in general less likely. The comparison of the
two-dimensional ionization level and velocity dispersion in the extranuclear
regions of these galaxies shows a positive correlation, further supporting the
idea that shocks are indeed the main cause of ionization.
The origin of these shocks is also investigated. Despite the likely presence
of superwinds in the circumnuclear regions of these systems, no evidence for
signatures of superwinds such as double velocity components are found in the
extended extranuclear regions. We consider a more likely explanation for the
presence of shocks, the existence of tidally induced large scale gas flows
caused by the merging process itself, as evidenced by the observed velocity
fields characterized by peak-to-peak velocities of 400 km s^{-1}, and velocity
dispersions of up to 200 km s^{-1}.Comment: 21 pages, 5 figures, accepted for publication in The Astrophysical
Journa
p3d: a general data-reduction tool for fiber-fed integral-field spectrographs
The reduction of integral-field spectrograph (IFS) data is demanding work.
Many repetitive operations are required in order to convert raw data into,
typically a large number of, spectra. This effort can be markedly simplified
through the use of a tool or pipeline, which is designed to complete many of
the repetitive operations without human interaction. Here we present our
semi-automatic data-reduction tool p3d that is designed to be used with
fiber-fed IFSs. Important components of p3d include a novel algorithm for
automatic finding and tracing of spectra on the detector, and two methods of
optimal spectrum extraction in addition to standard aperture extraction. p3d
also provides tools to combine several images, perform wavelength calibration
and flat field data. p3d is at the moment configured for four IFSs. In order to
evaluate its performance we have tested the different components of the tool.
For these tests we used both simulated and observational data. We demonstrate
that for three of the IFSs a correction for so-called cross-talk due to
overlapping spectra on the detector is required. Without such a correction
spectra will be inaccurate, in particular if there is a significant intensity
gradient across the object. Our tests showed that p3d is able to produce
accurate results. p3d is a highly general and freely available tool. It is
easily extended to include improved algorithms, new visualization tools and
support for additional instruments. The program code can be downloaded from the
p3d-project web site http://p3d.sourceforge.netComment: 18 pages, 15 figures, 3 tables, accepted for publication in A&
Integral Field Spectroscopy based H\alpha\ sizes of local Luminous and Ultraluminous Infrared Galaxies. A Direct Comparison with high-z Massive Star Forming Galaxies
Aims. We study the analogy between local U/LIRGs and high-z massive SFGs by
comparing basic H{\alpha} structural characteristics, such as size, and
luminosity (and SFR) surface density, in an homogeneous way (i.e. same tracer
and size definition, similar physical scales). Methods. We use Integral Field
Spectroscopy based H{\alpha} emission maps for a representative sample of 54
local U/LIRGs (66 galaxies). From this initial sample we select 26 objects with
H{\alpha} luminosities (L(H{\alpha})) similar to those of massive (i.e. M\ast
\sim 10^10 M\odot or larger) SFGs at z \sim 2, and observed on similar physical
scales. Results. The sizes of the H{\alpha} emitting region in the sample of
local U/LIRGs span a large range, with r1/2(H{\alpha}) from 0.2 to 7 kpc.
However, about 2/3 of local U/LIRGs with Lir > 10^11.4 L\odot have compact
H{\alpha} emission (i.e. r1/2 < 2 kpc). The comparison sample of local U/LIRGs
also shows a higher fraction (59%) of objects with compact H{\alpha} emission
than the high-z sample (25%). This gives further support to the idea that for
this luminosity range the size of the star forming region is a distinctive
factor between local and distant galaxies of similar SF rates. However, when
using H{\alpha} as a tracer for both local and high-z samples, the differences
are smaller than the ones recently reported using a variety of other tracers.
Despite of the higher fraction of galaxies with compact H{\alpha} emission, a
sizable group (\sim 1/3) of local U/LIRGs are large (i.e. r1/2 > 2 kpc). These
are systems showing pre-coalescence merger activity and they are
indistinguishable from the massive high-z SFGs galaxies in terms of their
H{\alpha} sizes, and luminosity and SFR surface densities.Comment: Accepted for publication in A&A. (!5 pages, 7 figures, 2 tables
The ionized gas in the central region of NGC 5253: 2D mapping of the physical and chemical properties
ABRIDGED: NGC5253 was previously studied by our group with the aim to
elucidate in detail the starburst interaction processes. Some open issues
regarding the 2D structure of the main properties of the ionized gas remain to
be addressed. Using IFS data obtained with FLAMES, we derived 2D maps for
different tracers of electron density (n_e), electron temperature (T_e) and
ionization degree. The maps for n_e as traced by several line ratios are
compatible with a 3D stratified view of the nebula with the highest n_e in the
innermost layers and a decrease of n_e outwards. To our knowledge, this is the
first time that a T_e map based on [SII] lines for an extragalactic object is
presented. The joint interpretation of our two T_e maps is consistent with a
T_e structure in 3D with higher temperatures close to the main ionizing source
surrounded by a colder and more diffuse component. The highest ionization
degree is found at the peak of emission for the gas with relatively high
ionization in the main GHIIR and lower ionization degree delineating the more
extended diffuse component. Abundances for O, Ne and Ar are constant over the
mapped area within <0.1 dex. The mean 12+log(O/H) is 8.26 while the relative
abundances of log(N/O), log(Ne/O) and log(Ar/O) were \sim-1.32, -0.65 and
-2.33, respectively. There are two locations with enhanced N/O. The first
(log(N/O)\sim-0.95) is associated to two super star clusters. The second
(log(N/O)\sim-1.17), reported here for the first time, is associated to two
moderately massive (2-4x10^4 M_sun) and relatively old (\sim10 Myr) clusters. A
comparison of the N/O map with those produced by strong line methods supports
the use of N2O2 over N2S2 in the search for chemical inhomogeneities within a
galaxy. The results on the localized nitrogen enhancement were used to compile
and discuss the factors that affect the complex relationship between Wolf-Rayet
stars and N/O excess.Comment: 16 pages, 14 figures, accepted for publication in A&
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