Projects

Probing neutral gas in high-redshift galaxies with IR lines.

The [CII] 158 &mu m fine - structure transition is the brightest emission line of the Galaxy and has already been detected in z > 1 starburst galaxies. Being the principal coolant of the cold neutral medium, this line is an ideal tracer of cold gas at high redshifts. We study the ALMA detectability of gas rich galaxies at z ~ 2 with high resolution hydrodynamical simulations, including radiative transfer of ionizing radiation and a subgrid prescription to compute the molecular fraction and the [CII] 158 &mu m emissivity. Future work will include additional IR lines and the effects of radiative transfer. (Collaborators: Xavier Prochaska, Art Wolfe, Daniel Ceverino, Avishi Dekel)

A test of the ALMA visibility for one of our models.

Blazar redshifts

Blazars, with their high intensity gamma-ray emission, are among the highest energy sources in the Universe. Blazars are active-galactic nuclei (AGN) powered by an accreting super-massive black hole that features two jets perpendicular to the plane of an accretion disk, with one jet pointing towards the observer. While properties at longer wavelengths (X-rays, UV, optical, and radio) have been extensively studied, the advent of new gamma-ray telescopes, such as Fermi and VERITAS, has opened for the first time the exploration of the Very High Energy (VHE) part of the blazar spectrum and the associated emission mechanisms. Despite the increasing number of known VHE sources, a strong limitation for studies of VHE blazars arises from a very practical reason: the lack of precise distances for these sources. These AGNs are all of the BL Lac type that, by definition, display weak or no lines. We are trying to improve our ability to establish blazar redshifts via optical monitoring and spectroscopic follow-up when targets enter a dim phase. We are also exploring the possibility to constarin redshift using UV or mm observations. (Collaborators: Amy Furniss, Xavier Prochaska, David Williams, Miroslava Dessauges-Zavadsky)

SLUG

SLUG is a new code to ``Stochastically Light Up Galaxies''. SLUG populates star clusters by randomly drawing stars from an initial mass function (IMF) and then following their time evolution with stellar models and an observationally-motivated prescription for cluster disruption. For a choice of star formation history, metallicity, and IMF, SLUG outputs synthetic photometry for clusters and field stars with a proper treatment of stochastic star formation. SLUG generates realistic distributions of star clusters, demonstrating the range of properties that result from finite sampling of an IMF and a random distribution of ages. The simulated data sets provide a quantitative means to address open problems in studies of star formation in galaxies and clusters, such as a test for IMF variations that are suggested by the systematic deficiency in the H-&alpha/UV ratio in outer disks or in dwarf galaxies. Check out our SLUG webpage. (Collaborators: Robert da Silva, Mark Krumholz and Frank Bigiel)

Damped Lyman-&alpha Systems in simulations

Before the advent of large arrays like SKA, the only way to study neutral atomic gas at high redshift is through quasar absorption line systems. However, this technique provides only statistical information on the neutral component of the IGM. Therefore, simulations become an important tool to understand where absorption line systems originate. Until recently, simulations could not reproduce all the observed properties of the IGM. Using the state-of-the-art AMR simulations from Ceverino et al. (2010) we are exploring if these high resolution calculations which naturally reproduce a multiphase medium can reproduced the observed statistics in the absorption line studies. Furthermore, we are investigating whether cold flows provide enough covering fraction and high velocities required to reproduced the observed DLA/LLS cross section and kinematics. (Collaborators: Daniel Ceverino, Avishi Dekel, Xavier Prochaska, Daniel Kasen)

Neutral hydrogen map of a massive galaxy at z ~2.3.

Imaging of high redshift Damped Lyman-&alpha Systems

DLAs are the highest HI column density absorption systems seen in QSO spectra and therefore they are a repository of neutral gas at high redshifts and the primary gas reservoir for star formation at high z. The study of these objects is critical to understand the evolution of the star formation rate (SFR) with redshift. Even if DLAs are the logical candidates for the precursors of present-day galaxies, the crucial missing piece in our understanding of DLA evolution is direct imaging of the absorbing galaxies at high redshifts (z>2) to obtain their typical spatial extents, luminosities, SFRs, morphologies and impact parameters. This paucity of information comes as a consequence of absorbers lying very close on the sky to the background QSOs, which are typically brighter by 8 magnitudes or more. However, when there are two galaxies foreground to a distant quasar, each exhibiting a damped Ly&alpha profile, the higher redshift galaxy acts as a sharp and severe blocking filter for the background quasar allowing the detection of emission from the lower redshift galaxy. (Collaborators: John M. O'Meara and Xavier Prochaska)

The star formation rate in the ALFALFA survey

Starting from a complete sample or radio selected galaxies, the optical identifications of the radio data was performed using SDSS. Through this investigation we understood that HI inhabits galaxies that are structurally similar to ordinary late-type galaxies and their HI content can be predicted from their optical luminosity. Typically, low surface brightness galaxies have low optical luminosity and contain small quantities of neutral hydrogen; in addition, very low surface brightness, massive Malin1 type galaxies are comfortably rare objects. After that, optical candidates were observed through narrow band filters, hunting for their H&alpha emission from gas ionized by massive stars, thus providing a complete survey of the current massive star formation from a radio selected sample. We study the efficiency of transformation of primordial gas (HI) into stars at the present cosmological epoch, in the various galaxy environments; in addition we quantify the total star formation rate in the local volume and we study dependencies of the star formation rate from parameters such as luminosity. (Collaborators: Giuseppe Gavazzi and Silvia Fabello)

The role of HI and H₂ in the star formation process and the molecular hydrogen deficiency in HI poor galaxies.

A complete and coherent understanding of the physical processes that regulate the birth of stars has not yet been achieved, nor a unanimous consent was reached on the effects of the environment on the star formation in galaxies member of rich clusters. We focus on the local and global Schmidt law and we investigate how cluster galaxies have their star formation activity perturbed. We collect multifrequency imaging for a sample of spiral galaxies, member of the Virgo cluster and of the local field; we compute the surface density profiles for the young and for the bulk of the stellar components, for the molecular and for the atomic gas. Our analysis shows that the bulk of the star formation correlates with the molecular gas, but the atomic gas is important or even crucial in supporting the star formation activity in the outer part of the disks. Moreover, we show that cluster members which suffer from a moderate HI removal have their molecular component and their SFR quenched, while highly perturbed galaxies show an additional truncation in their star forming disks. Our results are consistent with a model in which the atomic hydrogen is the fundamental fuel for the star formation, either directly or indirectly through the molecular phase; therefore galaxies whose HI reservoirs have been depleted suffer from starvation or even from truncation of their star formation activity. For the first time we provide a direct evidence of molecular gas deficiency in cluster galaxies. (Collaborators: Mark Krumholz and Giuseppe Gavazzi)

SFR radial profiles normalized to the stellar continuum for normal galaxies (continuous lines) and galaxies with reduced HI (dashed lines). The three panels are for increasing HI loss.

H&alpha follow-up for the ALFALFA survey

ALFALFA (Giovanelli et al. 2005) is a survey currently underway at Arecibo designed to map approximately one fifth of the sky in the HI line, out to a distance of 250 Mpc. The local extragalactic sky visible to Arecibo is rich, containing the central longitudes of the Supergalactic Plane in and around the Virgo cluster (Binggeli et al. 1985), the main ridge of the Pisces-Perseus Supercluster, and the extensive filaments connecting A1367, Coma and Hercules. Data taking for ALFALFA was initiated in February 2005, and completion of the full survey is projected to require 5-6 years. Extragalactic HI sources with M(HI) ~ 10⁷ solar masses will be detectable throughout most of the Local Supercluster, including the Virgo cluster, thus allowing a robust determination of the faint end of the HIMF. The survey will provide a census of HI in the surveyed sky area, making it especially useful in synergy with other wide area surveys such as SDSS, 2MASS, GALEX, ASTROF, HERSCHEL etc. I took part at two observational runs with professor Gavazzi, member of the ALFALFA collaboration, to continue the H&alpha imaging observations of ALFALFA detections in the Virgo region and its immediate surroundings. (Collaborators: Giuseppe Gavazzi and Silvia Fabello)

The strip covered by ALFALFA observations and the H&alpha imaging follow-up.

Low-frequency measurements of the CMB spectrum

*Top panels: uncertainties grater than 0.02 K do not allow the detection of the chemical potential. Bottom panels: uncertainties of 0.02 K and 0.01 K allow the detection of &mu .*

The measurements between 60-600 GHz of the FIRAS instrument on board of COBE show that the spectrum of the cosmic background radiation (CMB) is well described with a black body of temperature T=2.725 +/- 0.001 K. However, energy injections in the radiation field could have upset the equilibrium between matter and radiation and distorted the CMB spectrum: in particular, between z=10⁷ and z= 10⁵, the spectrum could have assumed a Bose-Einstein distribution, characterised beside the temperature by the chemical potential &mu . The analysis of FIRAS data gives the possibility to detect only an upper limit for &mu, being its value very small and appreciable only at frequencies lower than &nu&le 1 GHz. The aim of my work was to study what sensibility is required to detect &mu with low frequencies measurements in the interval between 400 MHz and 3 GHz. From a simulation of the spectra, it appears clear that to detect &mu with a significance of 1&sigma and 2&sigma the uncertainty on the low frequency measurements must be no grater than 0.02 K and 0.01 K, respectively. This involves a problem: at low frequencies the sources of errors are various and they make difficult, or maybe impossible, to obtain a CMB detection with uncertainties lower than 100 mK. We also showed that it would be profitable to measure the CMB at different frequencies using a unique instrument to reduce the dispersion of the measurements presently available in the literature. (Collaborators: Mario Zannoni)