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The stellar Initial Mass Function
in red and dead massive galaxies
The stellar Initial Mass Function
in red and dead massive galaxies
Historically, the stellar Initial Mass Function (IMF) has always been assumed to be universal and equal to that measured in the solar neighborhood.
Questioning over thirty years of well-established results, I have, among others, challenged this universality (Spiniello et al. 2012). I have used state-of-the-art stellar-population models and the observed strength of various IMF-sensitive absorption-line features, as well as the combination of gravitational lensing and dynamical analysis to demonstrate that the IMF slope is bottom-heavy (dominated by dwarfs stars) for more massive galaxies.
I have defined and characterized a non-degenerate set of optical line indices (based on stellar absorption lines) that allowed me to establish a linear trend of a steepening IMF slope with increasing stellar velocity dispersion in red and dead massive galaxies (Spiniello et al. 2014).
I have also showed that the non-Universality of the IMF is a model-independent result, robust against the choice of the stellar population model (Spiniello et al. 2015). Finally, together with my collaborators, we have been the first able to constrain not only the slope but also the low-mass cut-off of the IMF (Barnabè et al. 2013, Spiniello et al. 2015).
More recently, thanks to Integral-Field Data, I have also shown that the IMF variation is not a global result, but rather a local one since the low-mass IMF slope varies also within a single (massive) galaxy.
For instance, based on Multi-Unit Spectroscopic Explorer (MUSE) data, I found clear evidence for a radial variation at the low-mass end of the IMF in the central regions of the giant early-type galaxy NGC 4486 (M87, Sarzi et al. 2018). Such IMF variation corresponds to over a factor 2 increase in stellar mass-to-light ratio (M/L) compared to the case of a Milky Way IMF, also consistent with independent constraints on M/L radial variations in M87 from dynamical models.
We also looked into the abundance of sodium in M87, which turned up to be super-Solar over the entire radial range of our MUSE observations and to exhibit a considerable negative gradient. These findings suggest an additional role of metallicity in boosting the Na-yields in the central, metal-rich regions of M87 during its early and brief star formation history.
WHAT DOES (NOT) DRIVE THE IMF VARIATION?
WHAT DOES (NOT) DRIVE THE IMF VARIATION?
Thanks to high signal-to-noise integral field MUSE spectra on the bright central galaxy of the Hydra I cluster, NGC 3311, in the recent letter Barbosa, Spiniello, Arnaboldi et al. 2021 we unambiguously invalidate the previously observed direct correlation between the IMF slope and the local stellar velocity dispersion.
This relation is only a spatial coincidence between the region with the largest stellar velocity dispersion and the region where the oldest in situ population is found and dominates the light.
We confirm that the stellar content in the innermost region of NGC 3311 follows a bottom-heavy IMF with the tightest correlations found between stellar age and the IMF and between the galactocentric radius and the IMF.
The variation of the IMF at its low-mass end is not due to kinematical, dynamical, or global properties in NGC 3311. We speculate instead that the IMF might be dwarf-dominated in the "red nuggets" that formed through a very short and intense star formation episode at high redshifts (z > 2) when the Universe was denser and richer in gas, and which then ended up being the central cores of today's giant ellipticals.
Figure: IMF slope versus stellar velocity dispersion. Blue circles indicate data points for NGC 3311, colored according to the galactocentric distance. For other works, solid lines refer to local variations within galaxies (with a symbol indicating the center of the objects) and dashed lines refer to global variation across galaxies.
For more details on the fitting methods and on the other stellar populations parameters of NGC3311, check out the companion paper:
STRONG GRAVITATIONAL LENSING
STRONG GRAVITATIONAL LENSING
XLENS: THE X-SHOOTER LENS SURVEY
XLENS: THE X-SHOOTER LENS SURVEY
I have designed and carried out the X-Shooter Lens Survey (XLENS, Spiniello et al. 2011, Spiniello et al. 2015) which aims at study the interplay of dark matter and stellar content in the inner regions of massive early-type galaxies and simultaneously constraining the low-mass end of the IMF slope by combining strong gravitational lensing, dynamical models, and spectroscopic stellar population analysis. SL is by far the most accurate mass-measurement technique available for the central regions of ETGs, providing a one-shot, purely gravity-dependent measurement of the total (luminous + dark) projected mass enclosed by the lensed images. When lensing is combined with stellar dynamics and stellar population analysis it also allow to fully separate luminous from dark matter and measure the fraction dark matter enclosed within a certain aperture as well as the stellar and dynamical mass-to-light ratios. Finally, adding also spectroscopic stellar population studies, the XLENS project is able to disentangle the stellar and dark matter content of galaxies and, for the first time, directly constrain the normalization, shape (x), and cut-off mass (Mlow) of the low-mass end of the IMF and correlate these results with other galaxy properties such as galaxy mass, size, stellar density, and/or stellar velocity dispersion (Barnabé et al. 2013, Spiniello et al. 2015).
Gotta chatch'em all
Gotta chatch'em all
Strong gravitational lenses are valuable systems that can provide unique insights into a large number of open issue in cosmology and extragalactic astrophysics. For instance, strong galaxy-galaxy lensing is by far the most accurate mass-measurement technique available for the central regions of massive galaxies, providing a one-shot, purely gravity-dependent measurement of the mass enclosed by the lensed images. Thus, when combined with dynamical analysis, lensing provides excellent means of investigating dark matter in galaxies (Barnabé, Spiniello & Koopmans 2015).
Strong gravitational lensing is also a very effective and successful way to investigate the distant universe, thanks to the source light magnification. Lensed quasars (QSOs), especially quadruples can work as crucial cosmological test providing firm constraints on the Hubble constant and other cosmological parameters (H0LiCOW) via Time-Delay Cosmography (TDCOSMO).
Unfortunately, strong gravitational lenses are an intrinsically rare class of objects, as they require close alignment of a background source (typically at redshifts zs ≈ 2 or beyond) with galaxies (zl ≈ 0.5-0.7) acting as deflectors or lenses. It is for this reason that with my research, I set out to find and spectroscopically confirm as many as possible previously undiscovered gravitational lenses as possible from wide-sky photometrical surveys (Li et al. 2020, Agnello&Spiniello 2019, Spiniello et al. 2019b, Spiniello et al. 2019a, Sergeyev, Spiniello et al. 2018).
KiDS-SQuaD : The KiDS Strongly lensed QUAsar Detection project
KiDS-SQuaD : The KiDS Strongly lensed QUAsar Detection project
With the KiDS Strongly lensed QUAsar Detection project, or KiDS-SQuaD, we set out to find as many previously undiscovered gravitational lenses as possible in the Kilo Degree Survey (KiDS), taking advantage of the high quality imaging provided by the survey (Spiniello et al. 2018, Khramtsov et al. 2019).
To reach our aims, we apply different methods (colour cuts, morphological criteria, machine learning techniques) to the KiDS data within the current footprint (DR3 and DR4) in order to increase the completeness of our search, which we can estimate by testing our methods blindly on already discovered lenses. Our final goal is to increase the number of known lensed quasars and build up a statistically relevant number of lensing systems, spanning a wide range of parameters, such as the mass of the deflector, the redshift and nature of the source, and the lensing geometrical configuration.
Some of the best candidates or confirmed lenses we found so far:
The haloes of central galaxies in clusters and the Intracluster light with discrete tracers.
The haloes of central galaxies in clusters and the Intracluster light with discrete tracers.
The Fornax Cluster VLT Spectroscopic Survey (FVSS)
The Fornax Cluster VLT Spectroscopic Survey (FVSS)
The Cold-Dark Matter scenario predicts that, as the Universe cooled, clumps of dark matter (DM) began to condense, and within them gas began to collapse, forming the first galaxies which then experienced structure growth via the so-called "merging tree". During this growth, various physical processes (e.g tidal interactions, ram pressure stripping, gas accretion) contributed to shape galaxies and the DM surrounding them. These processes are expected to have left their imprints in the outskirts of galaxies and beyond, out to the intracluster regions. Here, dynamical times are longer and galaxy formation mechanisms leave signatures of gravitational interactions, like shells or tidal tales, in the kinematics of their components or in the chemical composition of the stars.
The Fornax cluster is the most massive galaxy structure after the Virgo cluster within 20 Mpc and it is an ideal target to study the effect of the environment on the structure and assembly of galaxies of any type, from the massive central giant early-type systems to the dwarf galaxies. Despite its regular appearance, it has been found that the assembly of Fornax is still ongoing. Although its core seems in an evolved phase and most of the bright cluster members are early-type galaxies, the presence of stellar and GC tidal streams (FDS) have revealed that there are still signs of active galaxy interactions in the region inside 200 kpc, which mirrors the large scale activity.
To kinematically map the complexity of the cluster core out to at least 200 kpc using discrete kinematical tracers (Globular Clusters, Planetary Nebuale and Ultra Compact Dwarfs) and ultimately connect the large scale kinematics down to the scale of dwarf galaxies, we have started a multi-instrument observational effort called the Fornax Cluster VLT Spectroscopic Survey (FVSS). As a part of this effort we have acquired integral field spectroscopy of dwarfs galaxies, a counter dispersed imaging run with FORS2 to detect and measure radial velocities of planetary nebulae (PNe) and multi-object spectroscopy of globular clusters (GCs) and Ultra Compact Dwarfs (UCDs) candidates with VIMOS/VLT.
From the line of sight velocity distribution of PNe and GCs, we identify a "transition radius" (~10') mapping the transition from the halo of the central galaxy into the cluster potential (right panel below).
From the 2D spatial distribution of PNe velocities we identified two streams of PNe connecting the central bright galaxies with other galaxies in the group (left panel below), that might indicate gravitational interaction that, however, did not happen in a very recent epoch. One of these "bridges" was already identified from deep photometry.
VEXAS
the VISTA EXTENSION to auxiliary surveys
VEXAS
the VISTA EXTENSION to auxiliary surveys
The aim of the VISTA EXtension to Auxiliary Surveys (VEXAS), is to provide spatial coverage that is as uniform as possible in the multi-wavelength sky in order to provide the astronomical community with reference magnitudes and colours for various scientific uses: object classification (e.g. quasars, galaxies, and stars; high-z galaxies, white dwarfs); photometric redshifts of large galaxy samples; searches of exotic objects (e.g. extremely red objects and lensed quasars).
The VEXAS collection, comprising nine cross-matched multi-wavelength photometric catalogues where each object has a match in at least two surveys, is currently the widest and deepest public optical-to-IR photometric and spectroscopic database in the southern hemisphere.
The survey, its scientific goals, the single tables and few expamples of scientific topics in which VEXAS can be of precious help are presented in the Data Release I (Spiniello & Agnello, 2019) .
The Data Release II, presenting a machine-learning based classification of source in the Southern Hemisphere with optical and infrared magnitudes is presented in Khramtsov, Spiniello, Agnello & Sergeyev, 2021
The VEXAS tables are publicly available from the ESO Phase 3 Science Portal, or from the Catalog Facility, as well as on the google drive and the github page of the VEXAS team.
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