No missing baryons in massive spirals!

The relation between galaxies and their dark matter haloes is central in understanding galaxy formation. Out of the gas associated to the haloes, only a small fraction was believed to be efficiently converted into stars (at most ~20% for Milky Way-like discs). This inefficiency is typically parametrised with the stellar-to-halo mass ratio f* = M* / Mh, also called star-formation efficiency. We determine f* for a large sample of nearby disc galaxies from the SPARC database, which goes from tiny dwarf irregulars to massive spirals. We analyse their HI rotation curves and we decompose them fitting for the dark matter halo mass. We find the following surprising results: 1) the most massive spirals are the most efficient star factories in the Universe; 2) when also their cold and hot gas are considered, the most massive spirals have virtually no missing baryons.
We provide here all the rotation curve decompositions to the 175 SPARC galaxies.

Mass & shape of the Milky Way's dark halo

The 2nd Data Release of the Gaia space mission provided data of unprecedented quality to study the dynamics of our Galaxy. In particular we now know with excellent accuracy how almost all the known ~150 globular cluster (GCs) are orbiting around in the halo and disc of the Milky Way. We model their dynamics with sophisticated distribution function models, this allows us to simultaneously recover both their phase space distribution and the mass and shape of the dark matter halo of our Galaxy.

Action-based distribution functions

Most of my work involves dynamical models for galaxies expressed as analytic distribution functions (DF) of the action integrals. With simple double power-laws in the actions one can reproduce systems with a wide variety of density profiles (see Figure, adapted from Posti et al. 2015).
Allowing the DF to depend on three actions any model can be trivially set rotating or flattened by velocity anisotropy by controlling the parametrised rotation and dispersion profiles. Moreover, the fact that the actions are adiabatic invariant makes these models uniquely suited to self-consistently represent a galaxy made up by multiple components.

Finding Milky Way halo stars

In the golden era of Galactic Surveys (Gaia, RAVE, APOGEE, LAMOST, WEAVE) finding halo stars is crucial for studies of the formation history of the Milky Way. Using a multi-component action-based dynamical model for the Galaxy one can identify unambiguously halo- from disc-stars given their orbit. Given the Gaia DR1 dataset of stars in the Solar Neighbourhood, complemented with radial velocities from RAVE, we identify ~1000 halo stars in action space (see Figure, from Posti et al. 2017). With this sample we study the stellar velocity ellipsoid, which we find to be mildly triaxial and aligned with the spherical coordinates, suggesting that the total Gravitational potential is nearly spherical.

Fall relation: observational determination

Stellar mass (M*) and specific angular momentum (j*) are two fundamental, independent quantities subject to physical conservation laws. We determine observationally their correlation, the Fall relation, in a sample of local spiral galaxies (drawn from the SPARC sample), from dwarf irregulars to massive spirals. j* is accurately determined by reconstructing its radial profile for all galaxies. We find the Fall relation to be a single, unbroken power-law over 4.5 orders of magnitude in stellar mass (see Figure, from Posti et al. 2018b). This finding has a great impact in constraining galaxy formation models.

Stellar-to-halo specific angular momentum

Spirals and ellipticals follow the Fall relation: their specific angular momentum j* correlates with their stellar mass M* as a power-law of slope 2/3 (similarly to dark matter haloes) and at fixed M* spirals have 5x the j* of ellipticals.
We use this and the fact that the log M* - log Mh is non-linear, to derive the stellar-to-halo specific angular momentum relation. Also this turns out to be non-linear, with a similar behaviour to the stellar-to-halo mass relation (see Figure, from Posti et al. 2018a). We, then, find a strong correlation between fj = j* / jh and f* = M* / Mh, which can be understood with a simple inside-out cooling formation model.

Galaxy-halo growth

Galaxies are observed to adhere to similar scaling laws both at z=0 and at high-redshift. Hence galaxy evolution, which makes galaxies bigger and more massive with time, does not disrupt such tight correlations. This can be understood in terms of dark matter haloes driving the evolution of galaxies (see Figure, from Posti et al. 2014).
In fact, halo size- and velocity dispersion-growth is remarkably similar to that observed for galaxies and if one indeed assumes that a stellar-to-halo mass and size relation is in place at all redshift, then all the observed evolution (up to z~2) can be driven by the haloes.

Instabilities in hot astrophysical plasmas

The question of whether gas clouds can condense in an hot astrophysical plasma is very important, for instance to understand if cold gas can feed star-formation in the disc of a spiral galaxy or at the centre of galaxy clusters. We study the conditions in which linear (thermal-)perturbations can grow exponentially in realistic atmospheres of galaxies and galaxy clusters: we find unstable modes can always be found, but the interplay between thermal conduction, magnetic field and rotation is crucial to determine whether condensation is possible (see Figure, from Nipoti & Posti 2014).

Additional resources

Check out my Research Gate profile. I'm also on the Math Genealogy Project!


Complete Curriculum Vitae (in .pdf)


See my Github repositories!


Link to ADS publication list