Research

OVERVIEW

I have been working mainly in the field of Theoretical and Computational Astrophysics for more than a decade focusing on the formation and evolution of galaxies, groups and clusters of galaxies, and their interplay with the central supermassive black hole (SMBH). I am particularly keen to study the baryonic component and multiphase gaseous halos, which are key to current Astrophysics and Cosmology. I tackle the astrophysical problems from two major angles. On the one hand, I develop and carry out high-resolution (HR) 3D magnetohydrodynamic (MHD) simulations complemented by analytic models. The exponential advancement of the computational science has allowed us to study with unprecedented realism the baryon astrophysics. I devote myself to finding new physical models and principles which can explain the observed phenomena on a fundamental level and provide testable predictions. On the other hand, I am deeply involved in observational collaborations: I use simulations as controlled astrophysical experiments to accurately interpret the multiwavelength observations provided by the last/next-generation telescopes (as Chandra, XMM, Athena, ALMA, HST, Magellan, MUSE, Mustang-2). My research methods are supported by a strong knowledge of (astro)physics and computational science, allowing me to independently develop and apply diverse physical modules in state-of-the-art massively-parallel AMR (adaptive mesh refinement) codes.

The gaseous halos of cosmic structures can be considered as giant extended atmospheres shaped by thermo/hydrodynamical processes – in analogy to Earth weather. During the gravitational collapse in the potential well of hierarchically forming dark matter structures, the primordial gas is shock heated up to 10^7 K forming the plasma halo filling galaxies (10s kpc), groups (100s kpc), and clusters (a few Mpc; aka intracluster, intragroup, circumgalactic/intrahalo medium – ICM, IGrM, CGM). Deep multiwavelength observations have revealed the ubiquitous presence of condensing warm ionized gas (UV/optical), neutral gas (IR/21cm), and cold molecular gas (radio) out of the hot halos. The top-down multiphase condensation cascade has opened up the gate for the understanding of long-standing problems, since the condensed phase is both the by-product of the cooling process and the main fuel for SMBH accretion.

I currently lead the "BlackHoleWeather" program, which aims to unveil and unify the major processes involved in feeding and feedback of SMBHs over 10 orders of magnitude in space and time (see the Nature Astronomy review, Gaspari et al. 2020). The main areas tackled both from a numerical/theoretical and observational perspective are as follows (see also the right Figure).

1. Macro Feeding: Multiphase Condensation Cascade – Raining

Radiative emission induces the plasma to cool in the X-ray band via Bremsstrahlung, with most of the emission contained within the core (<0.1 r_vir). A major thrust of my recent investigations has been applied to unveil the multiphase condensation cascade. In the first two works of the series (Gaspari et al. 2012a, 2013a) – which many consider the pioneering work in the field – we discovered a new mechanism known as Chaotic Cold Accretion (CCA; also called ‘rain’ or precipitation). In a turbulent and heated halo, extended warm (T∼10000 K) filaments condense out of the hot plasma (due to line recombination) tracing the turbulent eddies (Gaspari et al. 2015) – the main governing criterion being t_cool/t_eddy∼1. The structure is formed by layers within layers, with the ionized (UV-optical) skin covering the inner neutral filaments (IR-21cm; Gaspari et al. 2017). The density peaks condense into cold molecular clouds (<50 K; radio) via nonlinear thermal instability, and assemble in giant associations supported by non-thermal pressure. Beside the tight cospatiality and thermodynamics between the phases, consistent with deep X-ray, optical, and radio data, the multiphase structures inherit as ensemble the parent plasma kinematics. This allows to easily trace plasma turbulence with the Halpha+[NII] or CO line-of-sight velocity dispersion of integrated spectra (Gaspari et al. 2018).

2. Micro Feeding: Chaotic Cold Accretion – CCA

Within r<100 pc, the potential is dominated by the SMBH. In a monophase plasma halo, the accretion flow proceeds in a low-amplitude regime (Bondi or ADAF), which is further suppressed by any turbulent or rotating motion (Gaspari et al. 2015, 2017). However, observed ICM/IGrM atmospheres are multiphase and far from being hydrostatic. To test the impact of each physics deviating from the classic Bondi theory (e.g., multi-temperature cooling, heating, turbulence, and rotation), we carried out – for the first time – zoomed-in 3D AMR simulations covering the galactic scale (50 kpc) down to the sub-pc scale, with a dynamical range of ~10^6 (Gaspari 2016 for a review). During the CCA phase (velocity dispersion > rotational velocity), the condensed clouds, filaments, and clumpy torus continuously interact in the nuclear region via inelastic collisions, progressively cancelling angular momentum and boosting the inflow rate toward the Schwarzschild region up to 100× the hot Bondi rate. The process can be simplified as quasi-spherical viscous accretion driven by the clump collisional viscosity (with mean free path ~100 pc). The flow is chaotic, with rapid variability described by flicker noise (Gaspari et al. 2017), commonly found in AGN light curves. Unlike hot flows, the CCA rapid variability and boosting are key for the SMBH to quickly react to the state of the entire host galaxy. CCA provides a natural explanation for the ubiquitous presence of clumpy high-velocity absorbers and emitters in AGN spectral studies, as the narrow and broad line region. CCA has been corroborated in the past years by multiwavelength observations (e.g., Voit et al. 2014, David et al. 2014, Werner et al. 2014, Tremblay et al. 2016) and is the subject of a growing number of observational campaigns.

3. Micro Feedback: Multiphase AGN Outflows, Entrainment, Radiation

Within 100 Schwarzschild radii, the binding energy of the infalling clouds is converted into mechanical energy in the form of ultrafast outflows (>10000 km/s), or collimated jets (for large black bole spin values), hence triggering the active galactic nucleus (AGN) feedback phase. This has been investigated in-depth with a suite of general-relativistic (GR), radiative MHD simulations (Sadowski & Gaspari 2017) complemented by global HD runs (Gaspari & Sadowski 2017). Notably, regardless of the plasma microphysics (e.g., electron-ion temperature), the micro (horizon-scale) mechanical efficiency remains 3±1%. Semi-analytic models show that the inner ultrafast outflow progressively interacts with the ambient medium (via shocks and Kelvin-Helmholtz instabilities), loading mass along the path and decreasing its velocity down to 500 km/s (while conserving energy), thus creating neutral and molecular macro outflows. Predicted values are well consistent with the exponentially growing data of multiwavelength (ALMA, IRAM, XMM) AGN outflows, showing a tightly correlated kinematics between all the major gas phases, with mass outflow rates and velocities ranging ∼0.1--100 M⊙/yr and 10000--100 km/s, from the ionized to molecular outflows respectively (Tombesi et al. 2013).

4. Macro Feedback: Plasma Astrophysics – Turbulence, Heating, Conduction

Gaspari et al. (2009, 2011a,b, 2012a,b) showed how the macro feedback returns the plasma halo to a high entropy state reaching global, but not local, thermal equilibrium. As the anisotropic outflow reaches r∼10 kpc, the ambient pressure balances its ram pressure, inflating under-dense bubbles. The X-ray cavities store part of the feedback energy as enthalpy and release it during buoyancy while being disrupted by the turbulent backflow. The cocoon shock contributes to restore the lost entropy via irreversible heating, later weakening into sound waves. The macro imprint of kinetic feedback can be traced with the metals injected by the galaxy stellar evolution, which are advected up to 100 kpc radii. The large-scale bulk motions ultimately decay into subsonic turbulent motions (∼200 km/s) which mix and partially heat the hot halo at the end of the eddy Kolmogorov cascade. The Fourier power spectrum (PS) is key to constrain the plasma astrophysics: the PS amplitude of ICM density fluctuations (driven by gravity and sound waves) is linearly related to the level of turbulent motions (or Mach number), as discovered by Gaspari & Churazov (2013) with electron-ion plasma simulations. Thermal conduction washes out the filamentary structures, steepening the density (but not velocity) PS slope. The novel PS estimate of transport properties in the bulk of the ICM (Gaspari et al. 2013, 2014a) and up to r_vir (Khatri & Gaspari 2016) shows that real hot halos host subsonic turbulence and strongly suppressed conduction (<1% of Spitzer value). The suppression is corroborated by the survival of ram-pressure stripped IGrM in several clusters studied by our team, as A2142 (Eckert et al. 2014, 2017) and Hydra-A (De Grandi et al. 2016). The fluctuation PS method is nowadays widely employed: the brightest 33 Chandra clusters have been mapped finding Mach_3d∼0.3 (Hofmann et al. 2016a).

5. Global Self-Regulation: Evolution of Galaxies, Groups, and Clusters

The macro AGN heating diminishes the cooling rate, preventing condensation and quenching feeding – similar to a thermostat – for several 100 cycles during the 10 Gyr evolution (Gaspari et al. 2011a,b, 2012a,b). Global self-regulation means Pout∼Lx (Gaspari & Sadowski 2017) with X-ray luminosities in the range 10^41--10^45 erg/s (galactic to cluster halos; Anderson et al. 2016). Unopposed radiative cooling would lead instead to a catastrophic cooling flow due to the loss of pressure, with dramatic cooling/star formation rates up to 1000 M⊙/yr and peaked density cores – all ruled out by observations (e.g., Molendi et al. 2016). A breakthrough result is that CCA regulating the kinetic AGN feedback leads to quenching cooling rates by 20 fold with no entropy profile inversion (i.e., gentle heating), solving the long-standing cooling flow problem in massive galaxies, groups, and clusters (Gaspari et al. 2013b for a review). The CCA-kinetic regulation naturally explains the 2 orders of magnitude suppression of the soft X-ray emission (Gaspari 2015), the X-ray scaling relations as Lx--Tx (Gaspari et al. 2014b), and the faster duty cycle in lower mass, cooler halos (Gaspari et al. 2012b). Bondi-thermal models instead overheat most cool cores, inconsistently with their common presence over the past 10 Gyr (see our Chandra-SPT study in McDonald et al. 2017). Cosmological subgrid simulations probe the Mpc and high redshift regime. In Lau et al. (2017) and Biffi et al. (2016), we find hot halos to be continuously perturbed by turbulent motions via AGN feedback and cosmic accretion. The same cosmological runs clarified the evolution of plasma SZ profiles (Planelles et al. 2017), the scaling relations (Truong et al. 2016), and the metal enrichment history (Biffi et al. 2017).

In a thorough investigation, we showed how the macro properties of X-ray halos are tightly correlated with the masses of SMBHs in clusters, groups, and smaller galaxies (Gaspari et al. 2019), even better than the stellar counterparts (see the bottom two Figures). This shows how crucial is to understand and link the micro and macro properties of feeding and feedback in a unified theory, which is the long-term goal of the "BlackHoleWeather" program that I am leading.