265 research outputs found
Chemical abundances and properties of the ionized gas in NGC 1705
We obtained [O III] narrow-band imaging and multi-slit MXU spectroscopy of
the blue compact dwarf (BCD) galaxy NGC 1705 with FORS2@VLT to derive chemical
abundances of PNe and H II regions and, more in general, to characterize the
properties of the ionized gas. The auroral [O III]\lambda4363 line was detected
in all but one of the eleven analyzed regions, allowing for a direct estimate
of their electron temperature. The only object for which the [O III]\lambda4363
line was not detected is a possible low-ionization PN, the only one detected in
our data. For all the other regions, we derived the abundances of Nitrogen,
Oxygen, Neon, Sulfur and Argon out to ~ 1 kpc from the galaxy center. We detect
for the first time in NGC 1705 a negative radial gradient in the oxygen
metallicity of -0.24 \pm 0.08 dex kpc^{-1}. The element abundances are all
consistent with values reported in the literature for other samples of dwarf
irregular and blue compact dwarf galaxies. However, the average (central)
oxygen abundance, 12 + log(O/H)=7.96 \pm 0.04, is ~0.26 dex lower than previous
literature estimates for NGC 1705 based on the [O III]\lambda4363 line. From
classical emission-line diagnostic diagrams, we exclude a major contribution
from shock excitation. On the other hand, the radial behavior of the emission
line ratios is consistent with the progressive dilution of radiation with
increasing distance from the center of NGC 1705. This suggests that the
strongest starburst located within the central 150 pc is responsible for
the ionization of the gas out to at least 1 kpc. The gradual dilution of
the radiation with increasing distance from the center reflects the gradual and
continuous transition from the highly ionized H II regions in the proximity of
the major starburst into the diffuse ionized gas.Comment: Accepted for publication on A
PNe and H II regions in the starburst irregular galaxy NGC 4449 from LBT MODS data
We present deep 350010000 spectra of H II regions and planetary
nebulae (PNe) in the starburst irregular galaxy NGC 4449, acquired with the
Multi Object Double Spectrograph at the Large Binocular Telescope. Using the
"direct" method, we derived the abundance of He, N, O, Ne, Ar, and S in six H
II regions and in four PNe in NGC 4449. This is the first case of PNe studied
in a starburst irregular outside the Local Group. Our H II region and PN sample
extends over a galacto-centric distance range of 2 kpc and spans
0.2 dex in oxygen abundance, with average values of and for H II regions and PNe, respectively. PNe and H
II regions exhibit similar oxygen abundances in the galacto-centric distance
range of overlap, while PNe appear more than 1 dex enhanced in nitrogen
with respect to H II regions. The latter result is the natural consequence of N
being mostly synthesized in intermediate-mass stars and brought to the stellar
surface during dredge-up episodes. On the other hand, the similarity in O
abundance between H II regions and PNe suggests that NGC 4449' s interstellar
medium has been poorly enriched in elements since the progenitors of
the PNe were formed. Finally, our data reveal the presence of a negative oxygen
gradient for both H II regions and PNe, whilst nitrogen does not exhibit any
significant radial trend. We ascribe the (unexpected) nitrogen behaviour as due
to local N enrichment by the conspicuous Wolf-Rayet population in NGC 4449.Comment: Accepted for publication on Ap
Star Formation Histories of the LEGUS Dwarf Galaxies (I): recent History of NGC1705, NGC4449 and Holmberg II
We use HST observations from the Legacy Extragalactic UV Survey to
reconstruct the recent star formation histories (SFHs) of three actively
star-forming dwarf galaxies, NGC4449, Holmberg II and NGC1705, from their UV
color-magnitude diagrams (CMDs). We apply a CMD fitting technique using two
independent sets of stellar isochrones, PARSEC-COLIBRI and MIST, to assess the
uncertainties related to stellar evolution modelling. Irrespective of the
adopted stellar models, all the three dwarfs are found to have had almost
constant star formation rates (SFRs) in the last 100-200 Myr, with modest
enhancements (a factor of 2) above the 100 Myr-averaged-SFR. Significant
differences among the three dwarfs are found in the overall SFR, the timing of
the most recent peak and the SFRarea. The Initial Mass Function (IMF) of
NGC1705 and Holmberg II is consistent with a Salpeter slope down to 5
M, whereas it is slightly flatter, s, in NGC4449. The SFHs
derived with the two different sets of stellar models are consistent with each
other, except for some quantitative details, attributable to their input
assumptions. They also share the drawback that all synthetic diagrams predict a
clear separation in color between upper main sequence and helium burning stars,
which is not apparent in the data. Since differential reddening, significant in
NGC4449, or unresolved binaries don't appear to be sufficient to fill the gap,
we suggest this calls for a revision of both sets of stellar evolutionary
tracks.Comment: 22 pages, 17 figures, accepted for publication on Ap
Chemical abundances and radial velocities in the extremely metal-poor galaxy DDO 68
We present chemical abundances and radial velocities of six HII regions in
the extremely metal-poor star-forming dwarf galaxy DDO 68. They are derived
from deep spectra in the wavelength range 3500 - 10,000 {\AA}, acquired with
the Multi Object Double Spectrograph (MODS) at the Large Binocular Telescope
(LBT). In the three regions where the [O III]4363 {\AA} line was
detected, we inferred the abundance of He, N, O, Ne, Ar, and S through the
"direct" method. We also derived the oxygen abundances of all the six regions
adopting indirect method calibrations. We confirm that DDO 68 is an extremely
metal-poor galaxy, and a strong outlier in the luminosity - metallicity
relation defined by star-forming galaxies. With the direct-method we find
indeed an oxygen abundance of 12+log(O/H)=7.140.07 in the northernmost
region of the galaxy and, although with large uncertainties, an even lower
12+log(O/H)=6.960.09 in the "tail". This is, at face value, the most
metal-poor direct abundance detection of any galaxy known. We derive a radial
oxygen gradient of -0.060.03 dex/kpc (or -0.30 dex ) with the
direct method, and a steeper gradient of -0.120.03 dex/kpc (or -0.59 dex
) from the indirect method. For the -element to oxygen
ratios we obtain values in agreement with those found in other metal-poor
star-forming dwarfs. For nitrogen, instead, we infer much higher values,
leading to log(N/O), at variance with the suggested existence of a
tight plateau at in extremely metal poor dwarfs. The derived helium mass
fraction ranges from Y=0.2400.005 to Y=0.250.02, compatible with
standard big bang nucleosynthesis. Finally, we measured HII region radial
velocities in the range 479522 km/s from the tail to the head of the
"comet", consistent with the rotation derived in the HI.Comment: Accepted for publication in MNRA
Emission lines in early-type galaxies: active nuclei or stars?
We selected 27244 nearby, red, giant early-type galaxies (RGEs) from the
Sloan Digital Sky Survey (SDSS). In a large fraction (53%) of their spectra the
[O III] emission line is detected, with an equivalent width (EW) distribution
strongly clustered around ~0.75 A. The vast majority of those RGEs for which it
is possible to derive emission line ratios (amounting to about half of the
sample) show values characteristic of LINERs. The close connection between
emission lines and stellar continuum points to stellar processes as the most
likely source of the bulk of the ionizing photons in RGEs, rather than active
nuclei. In particular, the observed EW and optical line ratios are consistent
with the predictions of models in which the photoionization comes from to hot
evolved stars. Shocks driven by supernovae or stellar ejecta might also
contribute to the ionization budget. A minority, ~4%, of the galaxies show
emission lines with an equivalent that is width a factor of ~2 greater than the
sample median. Only among them are Seyfert-like spectra found. Furthermore, 40%
of this subgroup have a radio counterpart, compared to ~6% of the rest of the
sample. These characteristics argue in favor of an AGN origin for their
emission lines. Emission lines diagnostic diagrams do not reveal a distinction
between the AGN subset and the other members of the sample, and consequently
they are not a useful tool for establishing the dominant source of the ionizing
photons, which is better predicted by the EW of the emission lines.Comment: Accepted for publication in A&
HST resolves stars in a tiny body falling on the dwarf galaxy DDO 68
We present new Hubble Space Telescope (HST) imaging of a stream-like system
associated with the dwarf galaxy DDO 68, located in the Lynx-Cancer Void at a
distance of D12.65 Mpc from us. The stream, previously identified in deep
Large Binocular Telescope images as a diffuse low surface brightness structure,
is resolved into individual stars in the F606W (broad V) and F814W (I)
images acquired with the Wide Field Camera 3. The resulting V, I
color-magnitude diagram (CMD) of the resolved stars is dominated by old
(age1-2 Gyr) red giant branch (RGB) stars. From the observed RGB tip,
we conclude that the stream is at the same distance as DDO 68, confirming the
physical association with it. A synthetic CMD analysis indicates that the large
majority of the star formation activity in the stream occurred at epochs
earlier than 1 Gyr ago, and that the star formation at epochs more recent
than 500 Myr ago is compatible with zero. The total stellar mass of the
stream is , about 1/100 of that of DDO~68. This is a
striking example of hierarchical merging in action at the dwarf galaxy scales.Comment: ApJ in pres
Anthropogenic sinkholes of the city of Naples, Italy: an update
In recent years, the study of anthropogenic sinkholes in densely urbanized areas has attracted the attention of both researchers and land management entities. The city of Naples (Italy) has been frequently affected by processes generating such landforms in the last decades: for this reason, an update of the sinkhole inventory and a preliminary susceptibility estimation are proposed in this work. Starting from previous data, not modified since 2010, a total of 270 new events occurred in the period February 2010–June 2021 were collected through the examination of online newspapers, local daily reports, council chronicle news and field surveys. The final consistence of the updated inventory is of 458 events occurred between 1880 and 2021, distributed through time with an increasing trend in frequency. Spatial analysis of sinkholes indicates a concentration in the central sector of the city, corresponding to its ancient and historic centre, crossed by a dense network of underground tunnels and cavities. Cavity-roof collapse is confirmed as one of the potential genetic types, along with processes related to rainfall events and service lines damage. A clear correlation between monthly rainfall and the number of triggered sinkholes was identified. Finally, a preliminary sinkhole susceptibility assessment, carried out by Frequency Ratio method, confirms the central sector of city as that most susceptible to sinkholes and emphasizes the predisposing role of service lines, mostly in the outermost areas of the city
LBT/MODS spectroscopy of globular clusters in the irregular galaxy NGC 4449
We present intermediate-resolution (R1000) spectra in the
3500-10,000 A range of 14 globular clusters in the magellanic irregular
galaxy NGC 4449 acquired with the Multi Object Double Spectrograph on the Large
Binocular Telescope. We derived Lick indices in the optical and the
CaII-triplet index in the near-infrared in order to infer the clusters' stellar
population properties. The inferred cluster ages are typically older than
9 Gyr, although ages are derived with large uncertainties. The clusters
exhibit intermediate metallicities, in the range
[Fe/H], and typically sub-solar []
ratios, with a peak at . These properties suggest that i) during the
first few Gyrs NGC 4449 formed stars slowly and inefficiently, with galactic
winds having possibly contributed to the expulsion of the -elements,
and ii) globular clusters in NGC 4449 formed relatively "late", from a medium
already enriched in the products of type Ia supernovae. The majority of
clusters appear also under-abundant in CN compared to Milky Way halo globular
clusters, perhaps because of the lack of a conspicuous N-enriched,
second-generation of stars like that observed in Galactic globular clusters.
Using the cluster velocities, we infer the dynamical mass of NGC 4449 inside
2.88 kpc to be M(2.88 kpc)=. We
also report the serendipitous discovery of a planetary nebula within one of the
targeted clusters, a rather rare event.Comment: Accepted for publication in MNRAS; corrected typo in author lis
Multiscenario flood hazard assessment using probabilistic runoff hydrograph estimation and 2D hydrodynamic modelling
In this paper, we aim to define a procedure of flood hazard assessment applicable to large river basins in which flood events can be induced/sustained by the full basin area or by fractions of the total area as functions of the extent of the triggering precipitation event. The proposed procedure is based on a combined approach accounting for (1) the reconstruction of intensity–duration–frequency curves expressing the magnitude in terms of intensity for multiple return periods; (2) the application of the soil conservation service method for runoff estimation from a selected rainfall scenario considering some characteristics of the basin (i.e. soil type, land use/treatment, surface condition, and antecedent moisture conditions); (3) 2D hydrodynamic modelling conducted by the HEC-RAS model using runoff hydrographs as hydrological input data; (4) the reconstruction of flood hazard maps by overlaying multiple inundation maps depicting flood extent for different return periods. To account for the variability in the extent of the triggering precipitation event and the resulting input hydrograph, multiple contributing areas are considered. The procedure is tested at the archaeological site of Sybaris in southern Italy, which is periodically involved in flood events of variable magnitude. The obtained results highlight that the variable extent of the floodable area is strongly conditioned by the extent of the contributing area and return period, as expected. The archaeological site is always involved in the simulated flooding process, except for the smallest contributing area for which only a 300-year event involves this part of the site. Our findings may be useful for developing and supporting flood risk management plans in the area. The developed procedure might be easily exported and tested in other fluvial contexts in which evaluations of multiple flood hazard scenarios, due to the basin geometry and extent, are needed
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