216 research outputs found
The shallow boreholes at The AltotiBerina near fault Observatory (TABOO; northern Apennines of Italy)
Abstract. As part of an interdisciplinary research project, funded by the European Research Council and addressing the mechanics of weak faults, we drilled three 200–250 m-deep boreholes and installed an array of seismometers. The array augments TABOO (The AltotiBerina near fault ObservatOry), a scientific infrastructure managed by the Italian National Institute of Geophysics and Volcanology. The observatory, which consists of a geophysical network equipped with multi-sensor stations, is located in the northern Apennines (Italy) and monitors a large and active low-angle normal fault. The drilling operations started at the end of 2011 and were completed by July 2012. We instrumented the boreholes with three-component short-period (2 Hz) passive instruments at different depths. The seismometers are now fully operational and collecting waveforms characterised by a very high signal to noise ratio that is ideal for studying microearthquakes. The resulting increase in the detection capability of the seismic network will allow for a broader range of transients to be identified
An Ancient Metal-Poor Population in M32, and Halo Satellite Accretion in M31, Identified by RR Lyrae Stars
We present time-series photometry of two fields near M32 using archival
observations from ACS/WFC onboard HST. One field is centered about 2 arcmin
from M32 while the other is located 15 arcmin to the southeast of M31. We
identify a total of 1139 RR Lyrae variables of which 821 are ab-type and 318
are c-type. In the field near M32, we find a radial gradient in the density of
RR Lyraes relative to the center of M32. This gradient is consistent with the
surface brightness profile of M32 suggesting that a significant number of the
RR Lyraes in this region belong to M32. This provides further confirmation that
M32 contains an ancient stellar population formed around the same time as the
oldest population in M31 and the Milky Way. The RR Lyrae stars in M32 exhibit a
mean metal abundance of [Fe/H] ~ -1.42 +/- 0.02, which is ~15 times lower than
the metal abundance of the overall M32 stellar population. Moreover, the
abundance of RR Lyrae stars normalized to the luminosity of M32 in the field
analyzed further indicates that the ancient metal-poor population in M32
represents only a very minor component of this galaxy, consistent with the 1%
to 4.5% in mass inferred from the CMD analysis of Monachesi et al. In the other
field, we find unprecedented evidence for two populations of RR Lyraes in M31
as shown by two distinct sequences among the ab-type variables in the Bailey
Diagram. When interpreted in terms of metal abundance, one population exhibits
a peak at [Fe/H] ~ -1.3 and the other is at [Fe/H] ~ -1.9. One possible
interpretation of this result is that the more metal-rich population represents
the dominant M31 halo, while the metal-poorer group could be a disrupted dwarf
satellite galaxy orbiting M31. If true, this represents a further indication
that the formation of the M31 spheroid has been significantly influenced by the
merger and accretion of dwarf galaxy satellites. [abridged]Comment: 22 pages, 17 figures, accepted for publication in the MNRA
The origin of the Milky Way's halo age distribution
© 2018. The American Astronomical Society.. We present an analysis of the radial age gradients for the stellar halos of five Milky Way (MW) mass-sized systems simulated as part of the Aquarius Project. The halos show a diversity of age trends, reflecting their different assembly histories. Four of the simulated halos possess clear negative age gradients, ranging from approximately -7 to -19 Myr kpc-1, shallower than those determined by recent observational studies of the Milky Way's stellar halo. However, when restricting the analysis to the accreted component alone, all of the stellar halos exhibit a steeper negative age gradient with values ranging from -8 to -32 Myr kpc-1, closer to those observed in the Galaxy. Two of the accretion-dominated simulated halos show a large concentration of old stars in the center, in agreement with the Ancient Chronographic Sphere reported observationally. The stellar halo that best reproduces the current observed characteristics of the age distributions of the Galaxy is that formed principally by the accretion of small satellite galaxies. Our findings suggest that the hierarchical clustering scenario can reproduce the MW's halo age distribution if the stellar halo was assembled from accretion and the disruption of satellite galaxies with dynamical masses less than ∼109.5M, and a minimal in situ contribution
The Deepest Hubble Space Telescope Color-Magnitude Diagram of M32:Evidence for Intermediate-age Populations
We present the deepest optical color-magnitude diagram (CMD) to date of the local elliptical galaxy M32. We have obtained F435W and F555W photometries based on Hubble Space Telescope (HST) Advanced Camera for Surveys/High-Resolution Channel images for a region 110 '' from the center of M32 (F1) and a background field (F2) about 320 '' away from M32 center. Due to the high resolution of our Nyquist-sampled images, the small photometric errors, and the depth of our data (the CMD of M32 goes as deep as F435W similar to 28.5 at 50% completeness level), we obtain the most detailed resolved photometric study of M32 yet. Deconvolution of HST images proves to be superior than other standard methods to derive stellar photometry on extremely crowded HST images, as its photometric errors are similar to 2x smaller than other methods tried. The location of the strong red clump in the CMD suggests a mean age between 8 and 10 Gyr for [Fe/H] = -0.2 dex in M32. We detect for the first time a red giant branch bump and an asymptotic giant branch (AGB) bump in M32 which, together with the red clump, allow us to constrain the age and metallicity of the dominant population in this region of M32. These features indicate that the mean age of M32's population at similar to 2' from its center is between 5 and 10 Gyr. We see evidence of an intermediate-age population in M32 mainly due to the presence of AGB stars rising to M-F555W similar to -2.0. Our detection of a blue component of stars (blue plume) may indicate for the first time the presence of a young stellar population, with ages of the order of 0.5 Gyr, in our M32 field. However, it is likely that the brighter stars of this blue plume belong to the disk of M31 rather than to M32. The fainter stars populating the blue plume indicate the presence of stars not younger than 1 Gyr and/or BSSs in M32. The CMD of M32 displays a wide color distribution of red giant branch stars indicating an intrinsic spread in metallicity with a peak at [Fe/H] similar to -0.2. There is not a noticeable presence of blue horizontal branch stars, suggesting that an ancient population with [Fe/H] <-1.3 does not significantly contribute to the light or mass of M32 in our observed fields. M32's dominant population of 8-10 Gyr implies a formation redshift of 1 less than or similar to z(f) less than or similar to 2, precisely when observations of the specific star formation rates and models of "downsizing" imply galaxies of M32's mass ought to be forming their stars. Our CMD therefore provides a "ground truth" of downsizing scenarios at z = 0. Our background field data represent the deepest optical observations yet of the inner disk and bulge of M31. Its CMD exhibits a broad color spread of red giant stars indicative of its metallicity range with a peak at [Fe/H] similar to -0.4 dex, slightly more metal-poor than M32 in our fields. The observed blue plume consists of stars as young as 0.3 Gyr, in agreement with previous works on the disk of M31. The detection of bright AGB stars reveals the presence of intermediate-age population in M31, which is, however, less significant than that in M32 at our field's location
The Star Formation History of M32
We use deep HST ACS/HRC observations of a field within M32 (F1) and an M31
background field (F2) to determine the star formation history (SFH) of M32 from
its resolved stellar population. We find that 2-5Gyr old stars contribute
\som40%+/- 17% of M32's mass, while 55%+/-21% of M32's mass comes from stars
older than 5 Gyr. The mass-weighted mean age and metallicity of M32 at F1 are
=6.8+/-1.5 Gyr and =-0.01+/-0.08 dex. The SFH additionally
indicates the presence of young (<2 Gyr old), metal-poor ([M/H]\sim-0.7) stars,
suggesting that blue straggler stars contribute ~2% of the mass at F1; the
remaining \sim3% of the mass is in young metal-rich stars. Line-strength
indices computed from the SFH imply a light-weighted mean age and metallicity
of 4.9 Gyr and [M/H] = -0.12 dex, and single-stellar-population-equivalent
parameters of 2.9+/-0.2 Gyr and [M/H]=0.02+/-0.01 dex at F1 (~2.7 re). This
contradicts spectroscopic studies that show a steep age gradient from M32's
center to 1re. The inferred SFH of the M31 background field F2 reveals that the
majority of its stars are old, with \sim95% of its mass already acquired 5-14
Gyr ago. It is composed of two dominant populations; \sim30%+/-7.5% of its mass
is in a 5-8 Gyr old population, and \sim65%+/-9% of the mass is in a 8-14 Gyr
old population. The mass-weighted mean age and metallicity of F2 are
=9.2+/-1.2 Gyr and =-0.10+/-0.10 dex, respectively. Our results
suggest that the inner disk and spheroid populations of M31 are
indistinguishable from those of the outer disk and spheroid. Assuming the mean
age of M31's disk at F2 (\sim1 disk scale length) to be 5-9 Gyr, our results
agree with an inside-out disk formation scenario for M31's disk.Comment: Accepted to ApJ. 24 pages, 18 figures. A high-resolution version can
be downloaded from http://www.astro.rug.nl/~monachesi/monachesi-sfh.pd
Automated control procedures and first results from the temporary seismic monitoring of the 2012 Emilia sequence
After moderate to strong earthquakes in Italy or in the surrounding areas, the Istituto Nazionale di Geofisica e Vulcanologia (INGV; National Institute for Geophysics and Volcanology) activates a temporary seismic network infrastructure. This is devoted to integration with the Italian National Seismic Network (RSN) [Delladio 2011] in the epicentral area, thus improving the localization of the aftershocks distribution after a mainshock. This infrastructure is composed of a stand-alone, locally recording part (Re.Mo.) [Moretti et al. 2010] and a real-time telemetered part (Re.Mo.Tel.) [Abruzzese et al. 2011a, 2011b] that can stream data to the acquisition centers in Rome and Grottaminarda. After the May 20, 2012, Ml 5.9 earthquake in the Emilia region (northern Italy), the temporary network was deployed in the epicentral area; in particular, 10 telemetered and 12 stand-alone stations were installed [Moretti et al. 2012, this volume]. Using the dedicated connection between the acquisition center in Rome and the Ancona acquisition sub-center [Cattaneo et al. 2011], the signals of the real-time telemetered stations were acquired also in this sub-center. These were used for preliminary quality control, by adopting the standard procedures in use here (see next paragraph, and Monachesi et al. [2011]). The main purpose of the present study is a first report on this quality check, which should be taken into account for the correct use of these dat
The Panchromatic Hubble Andromeda Treasury XI: The Spatially-Resolved Recent Star Formation History of M31
We measure the recent star formation history (SFH) across M31 using optical
images taken with the \texit{Hubble Space Telescope} as part of the
Panchromatic Hubble Andromeda Treasury (PHAT). We fit the color-magnitude
diagrams in ~9000 regions that are ~100 pc 100 pc in projected size,
covering a 0.5 square degree area (~380 kpc, deprojected) in the NE
quadrant of M31. We show that the SFHs vary significantly on these small
spatial scales but that there are also coherent galaxy-wide fluctuations in the
SFH back to ~500 Myr, most notably in M31's 10-kpc star-forming ring. We find
that the 10-kpc ring is at least 400 Myr old, showing ongoing star formation
over the past ~500 Myr. This indicates the presence of molecular gas in the
ring over at least 2 dynamical times at this radius. We also find that the
ring's position is constant throughout this time, and is stationary at the
level of 1 km/s, although there is evidence for broadening of the ring due to
diffusion of stars into the disk. Based on existing models of M31's ring
features, the lack of evolution in the ring's position makes a purely
collisional ring origin highly unlikely. We find that the global SFR has been
fairly constant over the last ~500 Myr, though it does show a small increase at
50 Myr that is 1.3 times the average SFR over the past 100 Myr. During the last
~500 Myr, ~60% of all SF occurs in the 10-kpc ring. Finally, we find that in
the past 100 Myr, the average SFR over the PHAT survey area is
M yr with an average deprojected intensity of M yr kpc, which yields a total SFR of ~0.7
M yr when extrapolated to the entire area of M31's disk. This
SFR is consistent with measurements from broadband estimates. [abridged]Comment: 23 pages, 17 figures, 2 tables, accepted for publication in Ap
PHAT Stellar Cluster Survey I. Year 1 Catalog and Integrated Photometry
The Panchromatic Hubble Andromeda Treasury (PHAT) survey is an on-going
Hubble Space Telescope (HST) multi-cycle program to obtain high spatial
resolution imaging of one-third of the M31 disk at ultraviolet through
near-infrared wavelengths. In this paper, we present the first installment of
the PHAT stellar cluster catalog. When completed, the PHAT cluster catalog will
be among the largest and most comprehensive surveys of resolved star clusters
in any galaxy. The exquisite spatial resolution achieved with HST has allowed
us to identify hundreds of new clusters that were previously inaccessible with
existing ground-based surveys. We identify 601 clusters in the Year 1 sample,
representing more than a factor of four increase over previous catalogs within
the current survey area (390 arcmin^2). This work presents results derived from
the first \sim25% of the survey data; we estimate that the final sample will
include \sim2500 clusters. For the Year 1 objects, we present a catalog with
positions, radii, and six-band integrated photometry. Along with a general
characterization of the cluster luminosities and colors, we discuss the cluster
luminosity function, the cluster size distributions, and highlight a number of
individually interesting clusters found in the Year 1 search.Comment: 26 pages, 22 figures, Accepted by Ap
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