• Laurea in Physics, University of Bologna
• PhD in Astronomy, University of Bologna

Black Hole Feedback

Alessandro Ballone's thesis

Research interest:
My current interests involve the evolution of gas in galaxies and clusters of galaxies. The range of astrophysical issues that relate to this problem is remarkable: complicated hydrodynamics, dark matter in galaxies, galaxy formation and evolution, chemical enrichment of ISM and ICM, AGN and star formation feedback, cosmological evolution of galaxy clusters, physics of dust in hot plasmas, cosmic magnetic fields, and many others.
With the lauch of the new generation of X-Ray telescopes (Chandra and XMM) the understanding of this problem was expected to perform a quantum leap. Ironically enough, the first data from these observatories have instead confounded and astonished the X-Ray community. The failure to detect the cooling gas is deteriorating the astronomers faith in the traditional concept of "cooling flow", expected to explain the X-Ray brightest objects. Our understanding of the astrophysics of hot gas in galaxies and clusters is in a moment of recession! Actually, this is real fun. As Albert Einstein said, "in the middle of difficulty lies opportunity".

In recent years I worked on different topics concerning the origin and evolution of hot ISM/ICM and its relevance to galaxy evolution. My primary tool to attack these problems are (multidimensional) numerical simulations, at all scales from cluster of galaxies down to dwarf galaxies.
Few selected topics are listed below.

Metal enrichment: we combined gasdynamics and chemical evolution to succesfully explain the observed abundance profiles in the ISM of giant ellipticals (Es). A reasonable combination of (early) SNII, SNIa and stellar winds produces a central abundance of ~2 solar at the center, and a steep decline at large radii. A necessary ingredient is the chemical dilution by an inflowing external gas which accretes on the galaxy. Unfortunately, we found that the SN rate which explain the metals inside Es fails to explain the metal content of clusters. This is still an unresolved problem.

Rotating galaxies: almost all Es slowly rotate. The standard cooling flow models predict very flat X-Ray isophotes within r_eff, and gas cooling in a large disk. Neither effect is observed. We showed that this can be solved if a significant level of turbulence is present, as indicated by H_alpha observations. Turbulent diffusion transports away the angular momentum from the central regions, without appreciably reducing the abundance gradients.

Entropy evolution of groups and clusters: the aberrant behaviour of small groups in the L_x-T and other relations can be explained if something heats the ICM and raise its (central) entropy. We compared two extreme scenarios: internal and external heating. We find external (pre)heating unsatisfactory. Internal heating by SNe (and AGN?) agree better with observations, but require a high heating efficiency.

Heated cooling flows: as soon as the (cluster) cooling flow model was proposed (in the 70's), several astronomers proposed that some form of heating was actually balancing the radiative cooling. The lack of detectable cooling in current X-Ray spectra has strongly revived the hunt for a reasonable heating model. A central AGN is the usual suspect. The sharp vision of Chandra allows for the first time to study in detail the interaction between the AGN and the ISM. We have shown that randomly oriented X-Ray cavities, surrounded by bright cold rims, are a natural result of central heating. However, we also found that heated flows do not agree with observations, especially at galactic scale. The absence of cooling flows implied by recent observations is not explained yet.
Here is a hydro simulation of the effect of central heating on the ISM of an elliptical galaxy. Temperature evolution (darker regions are hotter). Click here
Here is shown the effect of a jet on the ISM. Click here

Starbursts and galactic winds: In galaxies with high star formation rate the energy released by the supernova explosions can push the interstellar medium away and generate a galactic wind. The physics of this gas flows is rather complicated and not well understood. This is a shame, given the crucial role of winds (and in general the star formation feedback) on the evolution of galaxies. Several years ago we started a project dedicated to the investigation of galactic winds, especially in dwarf galaxies, where the effect of the SNe explosions is more dramatic. Recently, we have completed a series of 3D simulations of the effect of ram pressure stripping from an external intergalactic medium and central starbursts on the ISM of disky, rotating dwarf galaxies.
Here is an animation of a galactic winds in a rotating dwarf galaxy. Gas density evolution. Click here

Dust in early-type galaxies: The amount and the origin of dust in elliptical galaxies is controversial. Early IRAS observations returned a very conplicated situation. The far-IR emission does not correlate with any galactic parameter (Lb, Lx, etc.). The easiest explanation is that the dust is acquired through random accretion events (mergers). We started a project to investigate the origin and evolution of dust in ellipticals using ISO observations and developing new theoretical models which take into account the injection of dust by the stellar winds, grain sputtering, heating by stellar radiation and electron collisions, the possible enhanced cooling by dusty gas, etc. The emission of internally produced distributed dust is usually just below the ISO detection limit. On the other hand, ISO detects many early type galaxies in the far-IR. Only in a few galaxies the FIR emssion is extended, most are consistent with point sources at ISO resolution. Is this consistent with the merging scenario? Also, we find dust masses in excess by a factor of 10 or more with respect to those inferred from the IRAS data, a result of the higher ISO sensitivity to emission from cold dust.


    Collaborators     My publications on ADS        

Myriam, click here!