36 research outputs found
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&
High resolution spectroscopy of the BCD galaxy Haro 15: II. Chemodynamics
We present a detailed study of the physical properties of the nebular
material in four star-forming knots of the blue compact dwarf galaxy Haro 15.
Using long-slit and echelle spectroscopy obtained at Las Campanas Observatory,
we study the physical conditions (electron density and temperatures), ionic and
total chemical abundances of several atoms, reddening and ionization structure,
for the global flux and for the different kinematical components. The latter
was derived by comparing the oxygen and sulphur ionic ratios to their
corresponding observed emission line ratios (the and ' plots) in
different regions of the galaxy. Applying the direct method or empirical
relationships for abundance determination, we perform a comparative analysis
between these regions. The similarities found in the ionization structure of
the different kinematical components implies that the effective temperatures of
the ionizing radiation fields are very similar in spite of some small
differences in the ionization state of the different elements. Therefore the
different gaseous kinematical components identified in each star forming knot
are probably ionized by the same star cluster. However, the difference in the
ionizing structure of the two knots with knot A showing a lower effective
temperature than knot B, suggests a different evolutionary stage for them
consistent with the presence of an older and more evolved stellar population in
the first.Comment: 21 pages, 6 figures, 8 tables, accepted by MNRA
Integral field spectroscopy of nitrogen overabundant blue compact dwarf galaxies
We study the spatial distribution of the physical properties and of oxygen
and nitrogen abundances in three Blue Compact Dwarf Galaxiess (HS 0128+2832, HS
0837+4717 and Mrk 930) with a reported excess of N/O in order to investigate
the nature of this excess and, particularly, if it is associated with
Wolf-Rayet (WR) stars We have observed these BCDs by using PMAS integral field
spectroscopy in the optical spectral range (3700 - 6900 {\AA}), mapping their
physical-chemical properties, using both the direct method and appropriate
strong-line methods. We make a statistical analysis of the resulting
distributions and we compare them with the integrated properties of the
galaxies. Our results indicate that outer parts of the three galaxies are
placed on the "AGN-zone" of the [NII]/H{\alpha} vs. [OIII]/H{\beta} diagnostic
diagram most likely due to a high N/O combined with the excitation structure in
these regions. From the statistical analysis, it is assumed that a certain
property can be considered as spatially homogeneous (or uniform) if a normal
gaussian function fits its distribution in several regions of the galaxy.
Moreover, a disagreement between the integrated properties and the mean values
of the distribution usually appears when a gaussian does not fit the
corresponding distribution. We find that for Mrk 930, the uniformity is found
for all parameters, except for electron density and reddening. The rotation
curve together with the H{\alpha} map and UV images, reveal a perturbed
morphology and possible interacting processes. The N/O is found to be constant
in the three studied objects at spatial scales of the order of several kpc so
we conclude that the number of WR stars estimated from spectroscopy is not
sufficient to pollute the ISM and to produce the observed N/O excess in these
objectsComment: 17 pages, 14 figures, accepted for publication in Astronomy &
Astrophysic
A 2D multiwavelength study of the ionized gas and stellar population in the Giant HII Region NGC 588
We present an analysis of NGC588 based on IFS data with PMAS, together with Spitzer images at 8 mi and 24 mi. The extinction distribution in the optical shows complex structure, with maxima correlating in position with those of the emission at 24 mi and 8 mi. The Ha luminosity absorbed by the dust within the GHIIR reproduces the structure observed in the 24 mi image, supporting the use of this band as a tracer of recent star formation. A velocity difference of ~50 km/s was measured between the areas of high and low surface brightness, which would be expected if NGC588 were an evolved GHIIR. Line ratios used in the BPT diagnostic diagrams show a larger range of variation in the low surface brightness areas. The ranges are ~0.5 to 1.2 dex for [NII]/Ha, 0.7 to 1.7 dex for [SII]/Ha, and 0.3 to 0.5 dex for [OIII]/Hb. Ratios corresponding to large ionization parameter (U) are found between the peak of the emission in Hb and the main ionizing source decreasing radially outwards within the region. Differences between the integrated and local values of the U tracers can be as high as ~0.8 dex. [OII]/Hb and [OIII]/[OII] yield similar local values for U and consistent with those expected from the integrated spectrum of an HII region ionized by a single star. The ratio [SII]/Ha departs significantly from the range predicted by this scenario, indicating the complex ionization structure in GHIIRs. There is a significant scatter in derivations of Z using strong line tracers as a function of position, caused by variations in the degree of ionization. The scatter is smaller for N2O3 which points to this tracer as a better Z tracer than N2. The comparison between integrated and local line ratio values indicates that measurements of the line ratios of GHIIR in galaxies at distances >~25 Mpc may be dominated by the ionization conditions in their low surface brightness areas.AM-I, EP-M and JMV acknowledge partial funding through research projects AYA2007-67965-C03-02 from the Spanish PNAYA and CSD2006-00070 1st Science with GTC of the MICINN. MR is supported by a Marie Curie Intra European Fellowship within the 7th European Community Framework Programme. CK, as a Humboldt Fellow, acknowledges support from the Alexander von Humboldt Foundation, Germany
Piecing together the puzzle of NGC 5253: abundances, kinematics and WR stars
We present Gemini-S/GMOS-IFU optical spectroscopy of four regions near the
centre of the nearby (3.8 Mpc) dwarf starburst galaxy NGC 5253. This galaxy is
famous for hosting a radio supernebula containing two deeply embedded massive
super star clusters, surrounded by a region of enhanced nitrogen abundance that
has been linked to the presence of WR stars. We detected 11 distinct sources of
red WR bump (CIV) emission over a 20" (~350 pc) area, each consistent with the
presence of ~1 WCE-type star. WC stars are not found coincident with the
supernebula, although WN stars have previously been detected here. We performed
a multi-component decomposition of the H\alpha\ line across all four fields and
mapped the kinematics of the narrow and broad (FWHM = 100-250 km/s) components.
These maps paint a picture of localised gas flows, as part of multiple
overlapping bubbles and filaments driven by the star clusters throughout the
starburst. We confirm the presence of a strong H\alpha\ velocity gradient over
~4.5" (~80 pc) coincident with the region of N/O enhancement, and high gas
density known from previous study, and interpret this as an accelerating
ionized gas outflow from the supernebula clusters. We measure the ionized gas
abundances in a number of regions in the outer IFU positions and combine these
with measurements from the literature to assess the radial abundance
distribution. We find that the O/H and N/H profiles are consistent with being
flat. Only the central 50 pc exhibits the well-known N/O enhancement, and we
propose that the unusually high densities/pressures in the supernebula region
have acted to impede the escape of metal-enriched hot winds from the star
clusters and allow them to mix with the cooler phases, thus allowing these
freshly processed chemicals to be seen in the optical.Comment: 16 pages, accepted to A&
A photoionization model of the spatial distribution of the optical and mid-IR properties in NGC595
We present a set of photoionization models that reproduce simultaneously the
observed optical and mid-infrared spatial distribution of the HII region NGC595
in the disk of M33 using the code CLOUDY. Both optical (PMAS-Integral Field
Spectroscopy) and mid-infrared (8 mi and 24 mi bands from Spitzer) data provide
enough spatial resolution to model in a novel approach the inner structure of
the HII region. We define a set of elliptical annular regions around the
central ionizing cluster with an uniformity in their observed properties and
consider each annulus as an independent thin shell structure. For the first
time our models fit the relative surface brightness profiles in both the
optical (Halpha, [OII], [OIII]) and the mid-infrared emissions (8 mi and 24
mi), under the assumption of a uniform metallicity (12+log(O/H) = 8.45; Esteban
et al. 2009) and an age for the stellar cluster of 4.5 Myr (Malumuth et al.
1996). Our models also reproduce the observed uniformity of the R23 parameter
and the increase of the [OII]/[OIII] ratio due to the decrease of the
ionization parameter. The variation of the Halpha profile is explained in terms
of the differences of the occupied volume (the product of filling factor and
total volume of the shell) in a matter-bounded geometry, which also allows to
reproduce the observed pattern of the extinction. The 8 mi/24 mi ratio is low
(ranging between 0.04 and 0.4) because it is dominated by the surviving of
small dust grains in the HII region, while the PAHs emit more weakly because
they cannot be formed in these thin HII gas shells. The ratio is also well
fitted in our models by assuming a dust-to-gas ratio in each annulus compatible
with the integrated estimate for the whole HII region after the 70 mi, and 160
mi Spitzer observations.Comment: Accepted for publication in MNRAS, 9 pages, 17 figure
First survey of Wolf-Rayet star populations over the full extension of nearby galaxies observed with CALIFA
The search of extragalactic regions with conspicuous presence of Wolf-Rayet
(WR) stars outside the Local Group is challenging task due to the difficulties
in detecting their faint spectral features. In this exploratory work, we
develop a methodology to perform an automated search of WR signatures through a
pixel-by-pixel analysis of integral field spectroscopy (IFS) data belonging to
the Calar Alto Legacy Integral Field Area survey, CALIFA. This technique
allowed us to build the first catalogue of Wolf-Rayet rich regions with
spatially-resolved information, allowing to study the properties of these
complexes in a 2D context. The detection technique is based on the
identification of the blue WR bump (around He II 4686 {\AA}, mainly associated
to nitrogen-rich WR stars, WN) and the red WR bump (around C IV 5808 {\AA} and
associated to carbon-rich WR stars, WC) using a pixel-by-pixel analysis. We
identified 44 WR-rich regions with blue bumps distributed in 25 galaxies of a
total of 558. The red WR bump was identified only in 5 of those regions. We
found that the majority of the galaxies hosting WR populations in our sample
are involved in some kind of interaction process. Half of the host galaxies
share some properties with gamma-ray burst (GRB) hosts where WR stars, as
potential candidates to being the progenitors of GRBs, are found. We also
compared the WR properties derived from the CALIFA data with stellar population
synthesis models, and confirm that simple star models are generally not able to
reproduce the observations. We conclude that other effects, such as the binary
star channel (which could extend the WR phase up to 10 Myr), fast rotation or
other physical processes that causes the loss of observed Lyman continuum
photons, are very likely affecting the derived WR properties, and hence should
be considered when modelling the evolution of massive stars.Comment: 33 pages, accepted for publication in A&
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
The baryonic content and Tully-Fisher relation at z~0.6
[abr.] Using the multi-integral-field spectrograph GIRAFFE at VLT, we
previsouly derived the stellar-mass Tully-Fisher Relation (smTFR) at z~0.6, and
found that the distant relation is systematically offset by roughly a factor of
two toward lower masses. We extend the study of the evolution of the TFR by
establishing the first distant baryonic TFR. To derive gas masses in distant
galaxies, we estimate a gas radius and invert the Schmidt-Kennicutt law between
star formation rate and gas surface densities. We find that gas extends farther
out than the UV light from young stars, a median of ~30%. We present the first
baryonic TFR (bTFR) ever established at intermediate redshift and show that,
within an uncertainty of +/-0.08 dex, the zeropoint of the bTFR does not appear
to evolve between z~0.6 and z=0. The absence of evolution in the bTFR over the
past 6 Gyr implies that no external gas accretion is required for distant
rotating disks to sustain star formation until z=0 and convert most of their
gas into stars. Finally, we confirm that the larger scatter found in the
distant smTFR, and hence in the bTFR, is caused entirely by major mergers. This
scatter results from a transfer of energy from bulk motions in the progenitors,
to random motions in the remnants, generated by shocks during the merging.
Shocks occurring during these events naturally explain the large extent of
ionized gas found out to the UV radius in z~0.6 galaxies. All the results
presented in this paper support the ``spiral rebuilding scenario'' of Hammer
and collaborators, i.e., that a large fraction of local spiral disks have been
reprocessed during major mergers in the past 8 Gyr.Comment: Accepted for publication in A&A, v3 addressing comments from the
refere