Sirio Belli

Graduate Student, Caltech

sirio [at] astro [dot] caltech [dot] edu

Caltech MC 249-17
1200 E California Blvd
Pasadena, CA 91125
Office: Cahill 255



I am a graduate student in astrophysics at Caltech, working with Prof. Richard Ellis. I study high-redshift galaxies and try to understand the evolution of their physical properties, in particular their mass, size, and chemical composition. Most of my research is based on observations carried out at the Keck telescopes in Hawaii and at the beautiful Hale telescope at Palomar observatory in California.

Velocity Dispersions of Quiescent Galaxies at z > 1

The stellar velocity dispersion is one of the most fundamental properties of galaxies, since it traces the gravitational potential of the dark matter halo and is changed very little by galaxy merging. By taking very deep rest-frame optical spectra using Keck LRIS, we obtained velocity dispersion measurements for the largest sample of quiescent galaxies at z > 1. We could then calculate the dynamical masses of these galaxies, and compare them to the stellar masses. We found that the stellar mass - dynamical mass relation at z>1 is in very good agreement with the local one. Also, we were able to follow galaxies through cosmic time, by comparing objects with identical velocity dispersion at different redshifts. We demonstrated that quiescent galaxies undergo a significant growth in size and stellar mass over 0 < z < 1.6, even though they form very little stars. This evolution is consistent with the theoretical predictions of galaxy minor merging, and could explain the puzzling small sizes measured for quiescent galaxies at high redshift. (2014ApJ...783..117B)


Stellar versus dynamical masses for quiescent galaxies at z>1 (red points) and z=0 (grayscale map).


Metallicity residuals from the local fundamental metallicity relation for high-redshift gravitational arcs, as a function of stellar mass.

Gas Metallicity of Star-Forming Galaxies at High Redshift

Local star-forming galaxies show a strong correlation between stellar mass, star formation rate, and gas metallicity, which has been called the "fundamental metallicity relation". At fixed mass, galaxies with high star formation rate tend to have lower metallicity, a behavior that could be caused by the balance of gas inflows and outflows. If the physical processes behind the fundamental metallicity relation are universal, we expect galaxies to follow the same correlation even at high redshift. Testing this requires very challenging metallicity measurements for faint objects. We used near-infrared spectroscopy to study a sample of 1.5 < z < 3 galaxies that are gravitationally lensed by foreground clusters. The lensing magnification allows us to reach very faint objects and correspondingly very low star formation rates, a region of the parameter space never probed before. Even though there are still large uncertainties on the metallicity measurements, our study suggests that high-redshift galaxies follow the same fundamental metallicity relation that is observed in the local universe. (2013ApJ...772..141B)