arXiv subjects

Laura Greggio

Publications and source records attributed to Laura Greggio.

At least 19 recordsLinked to original sources

arXiv2024

Recovered SN Ia rate from simulated LSST images

The Legacy Survey of Space and Time (LSST) will revolutionize Time Domain Astronomy by detecting millions of transients. In particular, it is expected to increment the number of type Ia supernovae (SNIa) of a factor of 100 compared to existing samples up to z~1.2. Such a high number of events will dramatically reduce statistical uncertainties in the analysis of SNIa properties and rates. However, the impact of all other sources of uncertainty on the measurement must still be evaluated. The comprehension and reduction of such uncertainties will be fundamental both for cosmology and stellar evolution studies, as measuring the SNIa rate can put constraints on the evolutionary scenarios of different SNIa progenitors. We use simulated data from the DESC Data Challenge 2 (DC2) and LSST Data Preview 0 (DP0) to measure the SNIa rate on a 15 deg2 region of the Wide-Fast-Deep area. We select a sample of SN candidates detected on difference images, associate them to the host galaxy, and retrieve their photometric redshifts (z-phot). Then, we test different light curves classification methods, with and without redshift priors. We discuss how the distribution in redshift measured for the SN candidates changes according to the selected host galaxy and redshift estimate. We measure the SNIa rate analyzing the impact of uncertainties due to z-phot, host galaxy association and classification on the distribution in redshift of the starting sample. We found a 17% average lost fraction of SNIa with respect to the simulated sample. As 10% of the bias is due to the uncertainty on the z-phot alone (which also affects classification when used as a prior), it results to be the major source of uncertainty. We discuss possible reduction of the errors in the measurement of the SNIa rate, including synergies with other surveys, which may help using the rate to discriminate different progenitor models.

astro-ph.CO↗

arXiv2023

Ultra diffuse galaxies in the Hydra I cluster from the LEWIS Project: Phase-Space distribution and globular cluster richness

Although ultra diffuse galaxies (UDGs) are found in large numbers in clusters of galaxies, the role of the cluster environment in shaping their low surface brightness and large sizes is still uncertain. Here we examine a sample of UDGs in the Hydra I cluster (D = 51 Mpc) with new radial velocities obtained as part of the LEWIS (Looking into the faintest with MUSE) project using VLT/MUSE data. Using a phase-space, or infall diagnostic, diagram we compare the UDGs to other known galaxies in the Hydra I cluster and to UDGs in other clusters. The UDGs, along with the bulk of regular Hydra I galaxies, have low relative velocities and are located near the cluster core, and thus consistent with very early infall into the cluster. Combining with literature data, we do not find the expected trend of GC-rich UDGs associated with earlier infall times. This result suggests that quenching mechanisms other than cluster infall should be further considered, e.g. quenching by strong feedback or in cosmic sheets and filaments. Tidal stripping of GCs in the cluster environment also warrants further modelling.

astro-ph.GA↗

arXiv2023

Looking into the faintEst WIth MUSE (LEWIS): on the nature of ultra-diffuse galaxies in the Hydra-I cluster.I. Project description and preliminary results

Looking into the faintEst WIth MUSE (LEWIS) is an ESO large observing programme aimed at obtaining the first homogeneous integral-field spectroscopic survey of 30 extremely low-surface brightness (LSB) galaxies in the Hydra I cluster of galaxies, with MUSE at ESO-VLT. The majority of LSB galaxies in the sample (22 in total) are ultra-diffuse galaxies (UDGs). The distribution of systemic velocities Vsys ranges between 2317 km/s and 5198 km/s and is centred on the mean velocity of Hydra I (Vsys = 3683 $\pm$ 46 km/s). Considering the mean velocity and the velocity dispersion of the cluster, 17 out of 20 targets are confirmed cluster members. To assess the quality of the data and demonstrate the feasibility of the science goals, we report the preliminary results obtained for one of the sample galaxies, UDG11. For this target, we derived the stellar kinematics, including the 2-dimensional maps of line-of-sight velocity and velocity dispersion, constrained age and metallicity, and studied the globular cluster (GC) population hosted by the UDG. Results are compared with the available measurements for UDGs and dwarf galaxies in literature. By fitting the stacked spectrum inside one effective radius, we find that UDG11 has a velocity dispersion $\sigma = 20 \pm 8$ km/s, it is old ($10\pm1$ Gyr), metal-poor ([M/H]=-1.17$\pm$0.11 dex) and has a total dynamical mass-to-light ratio M$/L_V\sim 14$, comparable to those observed for classical dwarf galaxies. The spatially resolved stellar kinematics maps suggest that UDG11 does not show a significant velocity gradient along either major or minor photometric axes. We find two GCs kinematically associated with UDG11. The estimated total number of GCs in UDG11, corrected for the spectroscopic completeness limit, is $N_{GC}= 5.9^{+2.2}_ {-1.8}$, which corresponds to a GC specific frequency of $S_N = 8.4^{+3.2}_{-2.7}$.

astro-ph.GA↗

arXiv2022

Rubin Observatory LSST Transients and Variable Stars Roadmap

The Vera C. Rubin Legacy Survey of Space and Time holds the potential to revolutionize time domain astrophysics, reaching completely unexplored areas of the Universe and mapping variability time scales from minutes to a decade. To prepare to maximize the potential of the Rubin LSST data for the exploration of the transient and variable Universe, one of the four pillars of Rubin LSST science, the Transient and Variable Stars Science Collaboration, one of the eight Rubin LSST Science Collaborations, has identified research areas of interest and requirements, and paths to enable them. While our roadmap is ever-evolving, this document represents a snapshot of our plans and preparatory work in the final years and months leading up to the survey's first light.

astro-ph.IM↗

arXiv2022

Galaxy populations in the Hydra I cluster from the VEGAS survey II. The ultra-diffuse galaxy population

In this work, we extend the catalog of low-surface brightness (LSB) galaxies, including Ultra-Diffuse Galaxy (UDG) candidates, within $\approx 0.4R_{vir}$ of the Hydra I cluster of galaxies, based on deep images from the VST Early-type GAlaxy Survey (VEGAS). The new galaxies are found by applying an automatic detection tool and carrying out additional visual inspections of $g$ and $r$ band images. This led to the detection of 11 UDGs and 8 more LSB galaxies. For all of them, the cluster membership has been assessed using the color-magnitude relation derived for early-type giant and dwarf galaxies in Hydra I. The UDGs and new LSB galaxies found in Hydra I span a wide range of central surface brightness ($22.7 \lesssim \mu_{0,g} \lesssim 26.5$ mag/arcsec$^2$), effective radius ($0.6 \lesssim R_e \lesssim 4.0$ kpc) and color ($0.4 \leq g-r \leq 0.9$ mag), and have stellar masses in the range $\sim 5\times 10^6 - 2\times 10^8$M$_{\odot}$. The 2D projected distribution of both galaxy types is similar to the spatial distribution of dwarf galaxies, with over-densities in the cluster core and north of the cluster centre. They have similar color distribution and comparable stellar masses to the red dwarf galaxies. Based on photometric selection, we identify a total of 9 globular cluster candidates associated to the UDGs and 4 to the LSB galaxies, with the highest number of candidates in an individual UDG being three. We find that there are no relevant differences between dwarfs, LSB galaxies and UDGs: the structural parameters (that is surface brightness, size, colors, n-index) and GCs content of the three classes have similar properties and trends. This finding is consistent with UDGs being the extreme LSB tail of the size-luminosity distribution of dwarfs in this environment.

astro-ph.GA↗

arXiv2021

Formation of an ultra-diffuse galaxy in the stellar filaments of NGC3314A: caught in act?

The VEGAS imaging survey of the Hydra I cluster reveals an extended network of stellar filaments to the south-west of the spiral galaxy NGC3314A. Within these filaments, at a projected distance of ~40 kpc from the galaxy, we discover an ultra-diffuse galaxy (UDG) with a central surface brightness of $\mu_{0,g}\sim26$ mag arcsec$^{-2}$ and effective radius $R_e\sim3.8$ kpc. This UDG, named UDG32, is one of the faintest and most diffuse low-surface brightness galaxies in the Hydra~I cluster. Based on the available data, we cannot exclude that this object is just seen in projection on top of the stellar filaments, thus being instead a foreground or background UDG in the cluster. However, the clear spatial coincidence of UDG32 with the stellar filaments of NGC3314A suggests that it might have formed from the material in the filaments, becoming a detached, gravitationally bound system. In this scenario, the origin of UDG32 depends on the nature of the stellar filaments in NGC3314A, which is still unknown. They could result from ram-pressure stripping or have a tidal origin. In this letter, we focus on the comparison of the observed properties of the stellar filaments and UDG32, and speculate about their possible origin. The relatively red color ($g-r=0.54 \pm 0.14$~mag) of the UDG, similar to that of the disk in NGC3314A, combined with an age older than 1Gyr, and the possible presence of a few compact stellar systems, all point towards a tidal formation scenario inferred for the UDG32.

astro-ph.GA↗

arXiv2021

The VST Early-type GAlaxy Survey: Exploring the Outskirts and Intra-cluster Regions of Galaxies in the Low-surface-brightness Regime

The VST Early-type GAlaxy Survey (VEGAS) is a deep, multi-band (u, g, r, i) imaging survey, carried out with the 2.6-metre VLT Survey Telescope (VST) at ESO's Paranal Observatory in Chile. VEGAS combines the wide (1-square-degree) OmegaCAM imager and long integration times, together with a specially designed observing strategy. It has proven to be a gold mine for studies of features at very low surface brightness, down to levels of mu_g~27-30 magnitudes arcsec^(-2), over 5-8 magnitudes fainter than the dark sky at Paranal. In this article we highlight the main science results obtained with VEGAS observations of galaxies across different environments, from dense clusters of galaxies to unexplored poor groups and in the field.

astro-ph.GA↗

arXiv2020

On the delay times of merging double neutron stars

The merging rate of double neutron stars (DNS) has a great impact on many astrophysical issues, including the interpretation of gravitational waves signals, of the short Gamma Ray Bursts (GRBs), and of the chemical properties of stars in galaxies. Such rate depends on the distribution of the delay times (DDT) of the merging events. In this paper we derive a theoretical DDT of merging DNS following from the characteristics of the clock controlling their evolution. We show that the shape of the DDT is governed by a few key parameters, primarily the lower limit and the slope of the distribution of the separation of the DNS systems at birth. With a parametric approach we investigate on the observational constraints on the DDT from the cosmic rate of short GRBs and the Europium to Iron ratio in Milky Way stars, taken as tracer of the products of the explosion. We find that the local rate of DNS merging requires that about 1 percent of neutron stars progenitors live in binary systems which end their evolution as merging DNS within a Hubble time. The redshift distribution of short GRBs does not yet provide a strong constraint on the shape of the DDT, although the best fitting models have a shallow DDT. The chemical pattern in Milky Way stars requires an additional source of Europium besides the products from merging DNS, which weakens the related requirement on the DDT. At present both constraints can be matched with the same DDT for merging DNS.

astro-ph.HE↗

arXiv2019

Correlation of the rate of Type Ia supernovae with the parent galaxy properties: Light and shadows

The identification of the progenitors of Type Ia Supernovae (SNIa) is extremely important in several astrophysical contexts, ranging from stellar evolution in close binary systems to evaluating cosmological parameters. Determining the distribution of the delay times (DTD) of SNIa progenitors can shed light on their nature. In this paper we investigate on the diagnostic capabilities on the DTD of the correlation between the SNIa rate and the parent galaxy properties by examining its systematics with the various parameters at play: simple stellar population models, the adopted description for the star formation history in galaxies, and the way in which the masses of the galaxies are evaluated. We compute models for the correlations of the SNIa rate with the parent galaxy color and specific star formation rate for a variety of input ingredients, and for a few astrophysically motivated DTD laws. The models are compared to the results of three independent observational surveys. We find that the scaling of the SNIa rate with the properties of the parent galaxy is sensitive to all input ingredients mentioned above. This is a severe limitation on the possibility to discriminate alternative DTDs. In addition, current surveys show some discrepancies for the rate measured in the reddest and bluest galaxies, likely due to limited statistics and inhomogeneity of the observations. For galaxies with intermediate colors the rates are in agreement, leading to a robust determination of the productivity of SNIa from stellar populations of $\simeq$ 0.8 events per 1000 \msun. Large stastistics of SNIa events along with accurate measurements of the star formation history in the galaxies are required to derive firm constraints on the DTD. LSST will achieve these results by providing the homogeneous, unbiased and vast database on both SNIa and galaxies.

astro-ph.HE↗

arXiv2019

A new delay time distribution for merging neutron stars tested against Galactic and cosmic data

The merging of two neutron stars (MNS) is thought to be the source of short gamma-ray bursts (SGRB) and gravitational wave transients, as well as the main production site of r-process elements like Eu. We have derived a new delay time distribution (DTD) for MNS systems from theoretical considerations and we have tested it against (i) the SGRB redshift distribution and (ii) the Galactic evolution of Eu and Fe, in particular the [Eu/Fe] vs [Fe/H] relation. For comparison, we also tested other DTDs, as proposed in the literature. To address the first item, we have convolved the DTD with the cosmic star formation rate, while for the second we have employed a detailed chemical evolution model of the Milky Way. We have also varied the DTD of Type Ia SNe (the main Fe producers), the contribution to Eu production from core-collapse SNe, as well as explored the effect of a dependence on the metallicity of the occurrence probability of MNS. Our main results can be summarized as follows: (i) the SGRB redshift distribution can be fitted using DTDs for MNS that produce average timescales of 300-500 Myr; (ii) if the MNS are the sole producers of the Galactic Eu and the occurrence probability of MNS is constant the Eu production timescale must be on the order of <30 Myr; (iii) allowing for the Eu production in core-collapse SNe, or adopting a metallicity-dependent occurrence probability, allow us to reproduce both observational constraints, but many uncertainties are still present in both assumptions.

astro-ph.HE↗

arXiv2018

The Stellar Halo of the Spiral Galaxy NGC 1560

We report on the detection of a stellar halo around NGC 1560, a 10^9 Mo spiral galaxy member of the Maffei group. We obtained deep images in the r and i bands using the 10.4m Gran Telescopio Canarias in a field centered at ~ 3.7 arcmin (projected distance of 3.5 kpc) from the center of this galaxy. The luminosity function and the CMD show a clear excess of stars with respect to the expected foreground level at magnitudes fainter than the RGB tip at the distance of NGC 1560. The color of the halo stars implies a metallicity of Z ~ Zo/50, while their counts correspond to a stellar mass of ~ 10^7 Mo in the sampled region. Assuming a power law profile for the surface mass density of the halo, our data suggest a total stellar mass of 10^8 Mo for the halo of NGC 1560.

astro-ph.GA↗

arXiv2017

A VST and VISTA study of globular clusters in NGC253

Aims. We analyze the properties of the sources in the NGC253 to define an up to date catalog of GC candidates in the galaxy. Methods. Our analysis is based on the science verification data of two ESO survey telescopes, VST and VISTA. Using ugri photometry from VST and JKs from VISTA, GC candidates were selected using the morpho-photometric and color properties of spectroscopically confirmed GCs available in the literature. The strength of the results was verified against available archival HST/ACS data from the GHOSTS survey. Results. The adopted GC selection leads to the definition of a sample of ~350 GC candidates. At visual inspection, we find that 82 objects match all the requirements for selecting GC candidates and 155 are flagged as uncertain GC candidate; 110 are unlikely GCs, most likely background galaxies. Furthermore, our analysis shows that four of the previously spectroscopically confirmed GCs, i.e., ~20% of the total spectroscopic sample, are more likely either background galaxies or high-velocity Milky Way stars. The radial density profile of the selected best candidates shows the typically observed r1/4-law radial profile. The analysis of the color distributions reveals only marginal evidence of the presence of color bimodality, which is normally observed in galaxies of similar luminosity. The GC luminosity function does not show the typical symmetry, mainly because of the lack of bright GCs. Part of the bright GCs missing might be at very large galactocentric distances or along the line of sight of the galaxy dusty disk. Conclusions. Using ugriJKs photometry we purged the list of GCs with spectroscopic membership and photometric GC candidates in NGC 253. Our results show that the use of either spectroscopic or photometric data only does not generally ensure a contaminant-free sample and a combination of both spectroscopy and photometry is preferred.

astro-ph.GA↗

arXiv2017

Type Ia supernovae with and without blueshifted narrow Na I D lines - how different is their structure?

In studies on intermediate- and high-resolution spectra of Type Ia supernovae (SNe Ia), some objects exhibit narrow Na-I D absorptions often blueshifted with respect to the rest wavelength within the host galaxy. The absence of these in other SNe Ia may reflect that the explosions have different progenitors: blueshifted Na-I D features might be explained by the outflows of 'single-degenerate' systems (binaries of a white dwarf with a non-degenerate companion). In this work, we search for systematic differences among SNe Ia for which the Na-I D characteristics have been clearly established in previous studies. We perform an analysis of the chemical abundances in the outer ejecta of 13 'spectroscopically normal' SNe Ia (five of which show blueshifted Na lines), modelling time series of photospheric spectra with a radiative-transfer code. We find only moderate differences between 'blueshifted-Na', 'redshifted-Na' and 'no-Na' SNe Ia, so that we can neither conclusively confirm a 'one-scenario' nor a 'two-scenario' theory for normal SNe Ia. Yet, some of the trends we see should be further studied using larger observed samples: Models for blueshifted-Na SNe tend to show higher photospheric velocities than no-Na SNe, corresponding to a higher opacity of the envelope. Consistently, blueshifted-Na SNe show hints of a somewhat larger iron-group content in the outer layers with respect to the no-Na subsample (and also to the redshifted-Na subsample). This agrees with earlier work where it was found that the light curves of no-Na SNe - often appearing in elliptical galaxies - are narrower, i.e. decline more rapidly.

astro-ph.SR↗

arXiv2015

Studying stellar halos with future facilities

Stellar halos around galaxies retain fundamental evidence of the processes which lead to their build up. Sophisticated models of galaxy formation in a cosmological context yield quantitative predictions about various observable characteristics, including the amount of substructure, the slope of radial mass profiles and three dimensional shapes, and the properties of the stellar populations in the halos. The comparison of such models with the observations provides constraints on the general picture of galaxy formation in the hierarchical Universe, as well as on the physical processes taking place in the halos formation. With the current observing facilities, stellar halos can be effectively probed only for a limited number of nearby galaxies. In this paper we illustrate the progress that we expect in this field with the future ground based large aperture telescopes (E-ELT) and with space based facilities as JWST.

astro-ph.GA↗

arXiv2014

A panoramic VISTA of the stellar halo of NGC 253

Outskirts of large galaxies contain important information about the galaxy formation and assembly process, and resolved star count studies can probe the extremely low surface brightness of the outer halos. We use images obtained with the VISTA telescope to construct spatially resolved J vs Z-J colour-magnitude diagrams (CMDs) of NGC 253, a nearly edge-on disk galaxy in the Sculptor group. The very deep photometry, down to J ~ 23.5, and the wide area covered allows us to trace the red giant branch (RGB) and asymptotic giant branch (AGB) stars that belong to the outer disk and the halo of NGC 253, out to 50 kpc along the galaxy minor axis. We confirm the existence of an extra planar stellar component of the disk, with a very prominent southern shelf and a symmetrical feature on the north side. The only additional visible sub-structure is an overdensity in the north-west part of the halo at about 28 kpc from the plane and extending over ~ 20 kpc parallel with the disk of the galaxy. From the stellar count profile along the major axis we measure the transition from the disk to the halo at a radial distance of about 25 kpc, where a clear break appears in the number density profile. The isodensity contours show that the inner halo is a flattened structure that blends with a more extended, diffuse, rounder outer halo. Such external structure can be traced to the very edge of our image out to 50 kpc from the disk plane. The number density profile of the stars in the stellar halo follows a power law with index -1.6, as function of radius. The CMD shows a very homogeneous stellar population across the whole field; by comparison with theoretical isochrones we conclude that the RGB stars are ~ 8 Gyr old or more, while the AGB stars trace a population of about 2 x 10^8 Mo, formed from ~ 0.5 to a few Gyr ago. Surprisingly, part of this latter population appears scattered over a wide area.

astro-ph.GA↗

arXiv2013

Studying the metallicity gradient in Virgo Ellipticals with E-ELT photometry of resolved stars

The next generation of large aperture ground based telescopes will offer the opportunity to perform accurate stellar photometry in very crowded fields. This future capability will allow one to study in detail the stellar population in distant galaxies. In this paper we explore the effect of photometric errors on the stellar metallicity distribution derived from the color distribution of the Red Giant Branch stars in the central regions of galaxies at the distance of the Virgo cluster. We focus on the analysis of the Color-Magnitude Diagrams at different radii in a typical giant Elliptical galaxy obtained from synthetic data constructed to exemplify observations of the European Extremely Large Telescope. The simulations adopt the specifications of the first light high resolution imager MICADO and the expected performance of the Multi-Conjugate Adaptive Optics Module MAORY. We find that the foreseen photometric accuracy allows us to recover the shape of the metallicity distribution with a resolution $\lesssim 0.4$ dex in the inner regions ($μ_{\rm B}$ = 20.5 mag arcsec$^{-2}$) and $\simeq 0.2$ dex in regions with $μ_{\rm B}$ = 21.6 mag arcsec$^{-2}$, that corresponds to approximately half of the effective radius for a typical giant elliptical in Virgo. At the effective radius ($μ_{\rm B} \simeq 23$ mag arcsec$^{-2}$), the metallicity distribution is recovered with a resolution of $\simeq 0.1$ dex. It will thus be possible to study in detail the metallicity gradient of the stellar population over (almost) the whole extension of galaxies in Virgo. We also evaluate the impact of moderate degradations of the Point Spread Function from the assumed optimal conditions and find similar results, showing that this science case is robust.

astro-ph.IM↗

arXiv2013

Calibrating stellar population models with Magellanic Cloud star clusters

Stellar population models are commonly calculated using star clusters as calibrators for those evolutionary stages that depend on free parameters. However, discrepancies exist among different models, even if similar sets of calibration clusters are used. With the aim of understanding these discrepancies, and of improving the calibration procedure, we consider a set of 43 Magellanic Cloud (MC) clusters taking age and photometric information from the literature. We carefully assign ages to each cluster based on up-to-date determinations ensuring that these are as homogeneous as possible. To cope with statistical fluctuations, we stack the clusters in five age bins deriving for each of them integrated luminosities and colors. We find that clusters become abruptly red in optical and optical-IR colors as they age from ~0.6 to 1 Gyr, which we interpret as due to the development of a well-populated thermally pulsing asymptotic giant branch (TP-AGB). We argue that other studies missed this detection because of coarser age binnings. Maraston (2005; M05) and Girardi et al. (2010) models predict the presence of a populated TP-AGB at ~0.6 Gyr, with a correspondingly very red integrated color, at variance with the data; Bruzual & Charlot (2003) and Conroy et al. (2009) models run within the error bars at all ages. The discrepancy between the synthetic colors of M05 models and the average colors of MC clusters results from the now obsolete age scale adopted. Finally, our finding that the TP-AGB phase appears to develop between ~0.6-1 Gyr is dependent on the adopted age scale for the clusters and may have important implications for stellar evolution.

astro-ph.GA↗

arXiv2012

HST/ACS Photometry of Old Stars in NGC 1569: The Star Formation History of a Nearby Starburst

(abridged) We used HST/ACS to obtain deep V- and I-band images of NGC 1569, one of the closest and strongest starburst galaxies in the Universe. These data allowed us to study the underlying old stellar population, aimed at understanding NGC 1569's evolution over a full Hubble time. We focus on the less-crowded outer region of the galaxy, for which the color-magnitude diagram (CMD) shows predominantly a red giant branch (RGB) that reaches down to the red clump/horizontal branch feature (RC/HB). A simple stellar population analysis gives clear evidence for a more complicated star formation history (SFH) in the outer region. We derive the full SFH using a newly developed code, SFHMATRIX, which fits the CMD Hess diagram by solving a non-negative least squares problem. Our analysis shows that the relative brightnesses of the RGB tip and RC/HB, along with the curvature and color of the RGB, provide enough information to ameliorate the age-metallicity-extinction degeneracy. The distance/reddening combination that best fits the data is E(B-V) = 0.58 +/- 0.03 and D = 3.06 +/- 0.18 Mpc. Star formation began ~ 13 Gyr ago, and this accounts for the majority of the mass in the outer region. However, the initial burst was followed by a relatively low, but constant, rate of star formation until ~ 0.5-0.7 Gyr ago when there may have been a short, low intensity burst of star formation.

astro-ph.GA↗