367 research outputs found
Ultra Long Period Cepheids: a primary standard candle out to the Hubble flow
The cosmological distance ladder crucially depends on classical Cepheids
(with P=3-80 days), which are primary distance indicators up to 33 Mpc. Within
this volume, very few SNe Ia have been calibrated through classical Cepheids,
with uncertainty related to the non-linearity and the metallicity dependence of
their period-luminosity (PL) relation. Although a general consensus on these
effects is still not achieved, classical Cepheids remain the most used primary
distance indicators. A possible extension of these standard candles to further
distances would be important. In this context, a very promising new tool is
represented by the ultra-long period (ULP) Cepheids (P \geq 80 days), recently
identified in star-forming galaxies. Only a small number of ULP Cepheids have
been discovered so far. Here we present and analyse the properties of an
updated sample of 37 ULP Cepheids observed in galaxies within a very large
metallicity range of 12+log(O/H) from ~7.2 to 9.2 dex. We find that their
location in the colour(V-I)-magnitude diagram as well as their Wesenheit (V-I)
index-period (WP) relation suggests that they are the counterparts at high
luminosity of the shorter-period (P \leq 80 days) classical Cepheids. However,
a complete pulsation and evolutionary theoretical scenario is needed to
properly interpret the true nature of these objects. We do not confirm the
flattening in the studied WP relation suggested by Bird et al. (2009). Using
the whole sample, we find that ULP Cepheids lie around a relation similar to
that of the LMC, although with a large spread (~0.4 mag).Comment: 8 pages, 4 figures, accepted for publication in Astrophysics & Space
Scienc
Nearby early-type galaxies with ionized gas. The UV emission from GALEX observations
We present GALEX far-ultraviolet (FUV, =1538 \AA) and
near-ultraviolet (NUV, =2316 \AA) surface photometry of 40
early-type galaxies (ETGs) selected from a wider sample of 65 nearby ETGs
showing emission lines in their optical spectra. We derive FUV and NUV surface
brightness profiles, (FUV-NUV) colour profiles and D integrated
magnitudes. We extend the photometric study to the optical {\it r} band from
SDSS imaging for 14 of these ETGs. In general, the (FUV-NUV) radial colour
profiles become redder with galactocentric distance in both rejuvenated ( Gyr) and old ETGs. Colour profiles of NGC 1533, NGC 2962, NGC 2974, NGC
3489, and IC 5063 show rings and/or arm-like structures, bluer than the body of
the galaxy, suggesting the presence of recent star formation. Although seven of
our ETGs show shell systems in their optical image, only NGC 7135 displays
shells in the UV bands. We characterize the UV and optical surface brightness
profiles, along the major axis, using a Sersic law. The Sersic law exponent,
, varies from 1 to 16 in the UV bands. S0 galaxies tend to have lower values
of (). The Sersic law exponent seems to be a watershed: ETGs
with tend to have [/Fe] greater than 0.15, implying a short
star-formation time scale. We find a significant correlation between the
FUVNUV colour and central velocity dispersions , with the UV colours
getting bluer at larger . This trend is likely driven by a combined
effect of `downsizing' and of the mass-metallicity relation.Comment: Accepted for publication in MNRAS, 33 pages, 7 figure
Impact of IFN lambda 3/4 single nucleotide polymorphisms on the cytomegalovirus reactivation in autologous stem cell transplant patients
Cytomegalovirus (CMV) infection represents one of the main cause mortality after Stem Cell Transplantation. Recently, a protective effect of the T allele of rs12979860 IL28B Single Nucleotide Polymorphisms (SNPs) against CMV infection in the allogenic stem cell transplantation was suggested. We investigate whether the rs12979860 IL28B SNP and the relative rs368234815 (IFNλ4) genotype may affect the incidence of active CMV infection in Autologous stem cell transplantation (Auto-SCT) setting. The study included 99 patients who underwent to Auto-SCT. IL28 and IFNΔ4 SNPs were correlated with CMV reactivation along with other clinical and treatment parameters. CMV reactivation by CMV DNAemia was evaluated once a week until day 100 from Auto-SCT. CMV reactivation was documented in 50% (TT-ΔG/ΔG), 35% (CC-TT/TT) and 29.2% (CT-TT/ΔG) of the patients respectively. No differences in CMV copies number were recorded at reactivation between different IL28/IFNλ4 genotypes. The analysis of patients older than 60 years showed a significantly higher incidence of active CMV infection in the TT-ΔG/ΔG (83%) population with respect to CC-TT/TT (21%) and CT-TT/ΔG (40%) patients. Our data suggest a negative role of TT-ΔG/ΔG genotype in the CMV reactivation in Auto-SCT. The exposure to rituximab and the pre-infusion presence of anti CMV IgG also significantly influenced CMV reactivation
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
I Zw 18 revisited with HST/ACS and Cepheids: New Distance and Age
We present new V and I-band HST/ACS photometry of I Zw 18, the most
metal-poor blue compact dwarf (BCD) galaxy in the nearby universe. It has been
argued in the past that I Zw 18 is a very young system that started forming
stars only 1 Gyr) red
giant branch (RGB) stars may also exist. Our new data, once combined with
archival HST/ACS data, provide a deep and uncontaminated optical
color-magnitude diagram (CMD) that now strongly indicates an RGB. The RGB tip
(TRGB) magnitude yields a distance modulus (m-M)_0 = 31.30 +/- 0.17, i.e., D =
18.2 +/- 1.5 Mpc. The time-series nature of our observations allows us to also
detect and characterize for the first time three classical Cepheids in I~Zw~18.
The time-averaged Cepheid and magnitudes are compared to the VI
reddening-free Wesenheit relation predicted from new non-linear pulsation
models specifically calculated at the metallicity of I Zw 18. For the one
bona-fide classical Cepheid with a period of 8.63 days this implies a distance
modulus (m-M)_0 = 31.42 +/- 0.26. The other two Cepheids have unusually long
periods (125.0 and 129.8 d) but are consistent with this distance. The coherent
picture that emerges is that I Zw 18 is older and farther away than previously
believed. This rules out the possibility that I Zw 18 is a truly primordial
galaxy formed recently (z < 0.1) in the local universe.Comment: 12 pages, 3 figures, submitted to ApJ
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
The lenticular galaxy NGC3607: stellar population, metallicity and ionised gas
In this work we derive clues to the formation scenario and ionisation source
of the lenticular galaxy NGC 3607 by means of metallicity gradients, stellar
population and emission lines properties. We work with long-slit spectroscopy
from which we (i) study the radial distribution of the equivalent widths of
conspicuous metallic absorption features, (ii) infer on the star-formation
history (with a stellar population synthesis algorithm), and (iii) investigate
the ionisation source responsible for a few strong emission lines. Negative
radial gradients are observed for most of the absorption features of NGC 3607.
Compared to the external parts, the central region has a deficiency of alpha
elements and higher metallicity, which implies different star-formation
histories in both regions. At least three star formation episodes are detected,
with ages within 1-13 Gyr. The dynamical mass and the gradient slope are
consistent with mergers being important contributors to the formation mechanism
of NGC 3607. Emission-line ratios indicate the presence of a LINER at the
centre of NGC 3607. Contribution of hot, old stars to the gas ionisation
outside the central region is detected. Evidence drawn from this work suggest
small mergers as important contributors to the formation of NGC 3607, a
scenario consistent with the star-formation episodes.Comment: 10 pages, 11 figs; accepted for publication in A&
Multi-Epoch HST Observations of IZw18: Characterization of Variable Stars at Ultra-Low Metallicities
Variable stars have been identified for the first time in the very metal-poor
Blue Compact dwarf galaxy IZw18, using deep multi-band (F606W,
F814W)time-series photometry obtained with the Advanced Camera for Surveys
(ACS) on board the Hubble Space Telescope (HST). We detected 34 candidate
variable stars in the galaxy. We classify three of them as Classical Cepheids,
with periods of 8.71, 125.0 and 130.3 days, respectively, and other two as long
period variables with periodicities longer than a hundred days. These are the
lowest metallicity Classical Cepheids known so far, thus providing the
opportunity to explore and fit models of stellar pulsation fo Classical
Cepheids at previously inaccessible metallicities. The period distribution of
the confirmed Cepheids is markedly different from what is seen in other nearby
galaxies, which is likely related to the star bursting nature of IZw18. By
applying to the 8.71 days Cepheid theoretical Wesenheit (V,I) relations based
on new pulsation models of Classical Cepheids specifically computed for the
extremely low metallicity of this galaxy (Z=0.0004, Y=0.24), we estimate the
distance modulus of IZw18 to be mu_0= 31.4pm0.2 D=19.0^{+1.8}_{-1.7}Mpc) for
canonical models of Classical Cepheids, and of 31.2pm0.2 mag
(D=17.4^{+1.6}_{-1.6}Mpc) using over luminous models. The theoretical modeling
of the star's light curves provides mu_0=31.4pm0.1 mag, D=19.0^{+0.9}_{-0.9}
Mpc, in good agreement with the results from the theoretical Wesenheit
relations. These pulsation distances bracket the distance of 18.2pm1.5Mpc
inferred by Aloisi et al. (2007) using the galaxy's Red Giant Branch Tip.Comment: 13 Pages, 6 Figures, accepted, Ap
The starburst phenomenon from the optical/near-IR perspective
The optical/near-IR stellar continuum carries unique information about the
stellar population in a galaxy, its mass function and star-formation history.
Star-forming regions display rich emission-line spectra from which we can
derive the dust and gas distribution, map velocity fields, metallicities and
young massive stars and locate shocks and stellar winds. All this information
is very useful in the dissection of the starburst phenomenon. We discuss a few
of the advantages and limitations of observations in the optical/near-IR region
and focus on some results. Special attention is given to the role of
interactions and mergers and observations of the relatively dust-free starburst
dwarfs. In the future we expect new and refined diagnostic tools to provide us
with more detailed information about the IMF, strength and duration of the
burst and its triggering mechanisms.Comment: 6 pages, 3 figures, to appear in "Starbursts: from 30 Doradus to
Lyman Break Galaxies" 2005, eds. R. de Grijs and R. M. Gonzalez Delgado
(Kluwer
The Importance of Nebular Continuum and Line Emission in Observations of Young Massive Star Clusters
In this spectroscopic study of infant massive star clusters, we find that
continuum emission from ionized gas rivals the stellar luminosity at optical
wavelengths. In addition, we find that nebular line emission is significant in
many commonly used broad-band HST filters including the F814W I-band, the F555W
V-band and the F435W B-band. Two young massive clusters (YMCs) in NGC 4449 were
targeted for spectroscopic observations after Reines et al. (2008a) discovered
an F814W I-band excess in their photometric study of radio-detected clusters in
the galaxy. The spectra were obtained with the Dual Imaging Spectrograph on the
3.5 m APO telescope. We supplement these data with HST and SDSS photometry. By
comparing our data to the Starburst99 and GALEV models, we find that nebular
continuum emission competes with the stellar light in our observations and that
the relative contribution is largest in the U- and I-bands, where the Balmer
and Paschen jumps are located. The spectra also exhibit strong line emission
including the [SIII] 9069,9532 lines in the HST F814W I-band. We find that the
combination of nebular continuum and line emission can account for the F814W
I-band excess found by Reines et al. (2008a). In an effort to provide a
benchmark for estimating the impact of ionized gas emission on photometric
observations of YMCs, we compute the relative contributions of the stellar
continuum, nebular continuum, and emission lines to the total flux of a 3
Myr-old cluster through various HST filter/instrument combinations, including
filters in the WFC3. We urge caution when comparing observations of YMCs to
evolutionary synthesis models since nebular emission can have a large impact on
magnitudes and colors of young (< 5 Myr) clusters, significantly affecting
inferred properties such as ages, masses and extinctions. (Abridged)Comment: 13 pages, 11 figures, Accepted for Publication in Ap
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