GASEOUS HALOS

Star-forming galaxies are surrounded by a multiphase, low-density gas component built by a combination of material accreting onto their disc from the intergalactic medium and gas expelled from their disc by supernova feedback. Studying this disc-halo interplay is important to understand how galaxies replenish their ‘fuel’ for star formation. In my research I have shown how the Milky Way is able to sustain its star formation by harvesting gas from its hot circumgalactic medium via the galactic fountain mechanism. This analysis was based on the modelling of the available emission and absorption-line data. We have recently extended this study to nearby disc galaxies from the HALOGAS HI Survey.

Galactic fountain in the Milky Way
Galactic fountain in the Milky Way

Scheme of the galactic fountain framework: fountain clouds are ejected from the disc by stellar feedback and interact with the galactic hot corona, triggering its condensation and subsequent accretion. This process can be observed in emission (cold gas in the front and in condensing knots) and ionised absorption lines (in the warm-hot wakes).

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Galactic fountain: data vs models
Galactic fountain: data vs models

Comparison between a representative HI channel map (v=-45 km/s) of the Milky Way and those predicted by our models (bottom panels). The models are, from the left: a thin disc model, a disc model plus a slow rotating and inflowing layer of extraplanar HI, a dynamical model of the galactic fountain without condensation of coronal gas, and the fountain model with condensation. The latter is in better agreement with the data (see regions A and B).

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Extraplanar HI and galactic fountain
Extraplanar HI and galactic fountain

Comparison between the mass of extraplanar HI in late-type galaxies (from the HALOGAS survey) and that predicted by a simple model of supernova feedback-powered galactic fountain. The model matches the data remarkably well.

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Galactic fountain in the Milky Way
Galactic fountain in the Milky Way

Scheme of the galactic fountain framework: fountain clouds are ejected from the disc by stellar feedback and interact with the galactic hot corona, triggering its condensation and subsequent accretion. This process can be observed in emission (cold gas in the front and in condensing knots) and ionised absorption lines (in the warm-hot wakes).

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Extraplanar gas: NGC891 vs Gasoline
Extraplanar gas: NGC891 vs Gasoline

Comparison between the optical and HI morphology of the edge-on galaxy NGC 891 (on the left) and those of a similar system simulated with Gasoline. Stellar feedback produces a faint layer of extraplanar HI (in shades of blue).

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Dark and luminous bars in APOSTLE
Dark and luminous bars in APOSTLE

Dark matter halos can be significantly prolate at their centre, producing a bar-line morphology in the stellar component and strong non-circular motions in the gas. This is an example from the APOSTLE hydrodynamical simulations.

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Environmental HI stripping in EAGLE
Environmental HI stripping in EAGLE

Stellar and HI morphology for satellite galaxies in the EAGLE simulations. The top row shows satellites affected by ram-pressure stripping processes, the middle row shows tidally stripped satellites, the bottom row shows satellite-satellite flybys. The magnitude of the three environmental effects increases from left to right.

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Extraplanar gas: NGC891 vs Gasoline
Extraplanar gas: NGC891 vs Gasoline

Comparison between the optical and HI morphology of the edge-on galaxy NGC 891 (on the left) and those of a similar system simulated with Gasoline. Stellar feedback produces a faint layer of extraplanar HI (in shades of blue).

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NUMERICAL SIMULATIONS

​Numerical simulations are powerful tools to study the physics of gas in galaxies. In my work I have used state-of-the art hydrodynamical simulations, both in a cosmological (EAGLE, IllustrisTNGAPOSTLE) and in an isolated framework to study different aspects associated to the galactic HI physics, like how stellar feedback affects the kinematics and morphology of HI discs, how the HI kinematics of dwarf galaxies is influenced by the shape of their dark matter halo, and the mode by which the environment regulates the HI content of galaxies. All these studies were accompanied by a detailed comparison with the observations.

SCALING RELATIONS

Galaxies obey to simple scaling laws that link their luminous mass to their rotational speed, specific angular momentum, dark matter and black hole mass. These empirical scaling laws are particularly informative of the co-evolution between galaxies, their host dark matter halos and the material accreting from the intergalactic space. I have participated in several studies aimed at the characterisation and theoretical understanding of these scaling laws and their evolution with cosmic time.

The coevolution between BHs, galaxies and DM halos
The coevolution between BHs, galaxies and DM halos

Relations between supermassive BH masses, dark matter halo masses and stellar fractions for a sample of 55 nearby galaxies. Circles show ETGs, squares show LTGs hosting classical bulges, stars show LTGs hosting pseudo bulges. Markers are color-coded by Hubble type. The dashed lines show our fiducial galaxy evolution model at z=0 (blue: star forming; red: quiescent).

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Tully-Fisher relation and UDGs
Tully-Fisher relation and UDGs

HI kinematics of ultra-diffuse galaxies (UDGs) suggest that these systems are out of the baryonic Tully-Fisher relation! What is the origin of these galaxies?

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Fall relation at z=1
Fall relation at z=1

Stellar mass - specific angular momentum relation (or "Fall relation") for a sample of regularly rotating, main sequence galaxy at z=1. The data indicate that the Fall relation has not evolved from z=1 to z=0 (~8 Gyr).

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The coevolution between BHs, galaxies and DM halos
The coevolution between BHs, galaxies and DM halos

Relations between supermassive BH masses, dark matter halo masses and stellar fractions for a sample of 55 nearby galaxies. Circles show ETGs, squares show LTGs hosting classical bulges, stars show LTGs hosting pseudo bulges. Markers are color-coded by Hubble type. The dashed lines show our fiducial galaxy evolution model at z=0 (blue: star forming; red: quiescent).

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STELLAR & AGN FEEDBACK

Feedback from supernovae and/or AGN is thought to have a key role in quenching star formation in galaxies either by injecting energy into the circumgalactic medium, delaying its cooling and subsequent accretion onto the disc, or by producing large-scale outflows which can empty a galaxy of its gas reservoir. I study these processes using optical and near-IR integral field spectroscopic data, which allow me to characterise the dynamics and energetics of the diffuse gas in and around galaxies

IMAGE PROCESSING

​Photometric and astrometric measurements on images from single and multi-conjugated adaptive optics suffer from the variability of the PSF in time and across the field of view. In collaboration with Prof. E. Tolstoy and Dr. D. Massari I am developing SuperStar, a new software for photo/astro-metric measurements in stellar fields. The software processes the image while iteratively building a grid of numerical PSFs using as an input the image itself. Also, I collaborate with groups working on a-priori characterisation of the PSF via PSF-reconstruction techniques.

Flowing chart of the SuperStar code
Flowing chart of the SuperStar code

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SuperStar vs DAOPHOT
SuperStar vs DAOPHOT

Comparing the performance of SuperStar and DAOPHOT on an artificial image (see text). Left panels: zoomed view on a corner of the synthetic image (on top), DAOPHOT residuals (middle) and SuperStar residuals (bottom). Right panels: photo and astro-metric precision as a function of the input stellar magnitude for the entire image.

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The case of M4
The case of M4

Photo/astro-metric precision achieved using 24 exposures of a bright star towards the center of the globular cluster M4. Measurements using PSF-reconstruction techniques are a factor ~5-10 more precise.

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Flowing chart of the SuperStar code
Flowing chart of the SuperStar code

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