The Ly&alpha emission line is a highly efficient tool by which to identify and study evolving galaxies at the highest redshifts, down to the faintest levels. However, in order to use Ly&alpha as a physical probe of galaxies, it becomes vital to understand how the Ly&alpha output depends on the evolution of the intrinsic properties of galaxies as a function of redshift.
Local Ly&alpha emitters - I am currently leading several projects aiming at characterizing the Lyα escape physics and at providing a suitable local testbed for Lyα evolutionary studies.
First results based on a sample of z=0.3 Lyα emitting galaxies show that the geometry of the dust distribution around star-forming regions plays a crucial role in determining the Ly&alpha output. We found that the interstellar dust is likely distributed in clumps, with an associated covering fraction that is responsible for the Ly&alpha visibility. We also found that an AGN is present in about 17% of the galaxies, a much higher fraction than what is inferred for high-z samples of Ly&alpha emitters (more details in Scarlata et al. 2009a).
We are now extending the study of this sample to the mid-IR regime, probing directly the dust emission. The study at the longer wavelengths will allow to quantifying the total activity of these galaxies, including the intrinsic level of star formation and the contribution from -possibly hidden- AGN.
In Cycle 18, we have been awarded 25 HST orbits to obtain high reslution UV spectra of the z=0.3 Ly&alpha emitters. The COS spectra will provide the shape of the Ly&alpha profile, a fundamental diagnostic of the physical conditions of the ISM, since it encodes crucial information about the kinematics, column density, and dust content of the neutral gas (Scarlat et al. in prep.).
Extended Ly&alpha blobs - Searches for high z Ly&alpha emitters discover also rare, powerful and spatially extended Ly&alpha nebulae, often referred to as "blobs". The Ly&alpha emitting region in Ly&alpha blobs can reach up to hundreds of kpc, with Ly&alpha luminosities greater than 1010 solar luminosity. Apart from their extended size and impressive luminosities, what makes Ly&alpha blobs peculiar compared to normal star forming Ly&alpha emitters is the lack of a powerful UV source able to explain the gas ionisation.
Various mechanisms have been proposed to explain the observed Ly&alpha emission, including photoionisation by either powerful AGN or starbursts with geometrical effects masking the UV, shocks produced by supernova driven winds, and cooling radiation from gas falling into a dark matter halo (i.e., "cold accretion"). In Scarlata et al. (2009b) we have recently presented a detailed analysis of a Ly&alpha nebula identified in an overdense filamentary structure of Ly&alpha emitters at z=2.38. Using ground based and Sptitzer imaging, together with optical and mid-IR spectroscopy of the nebula, we have shown that this object might represent a candidate of the so-called “cold mode” of gas accretion.
ZEST - Galaxy structure (often quantified with the morphology) is a basic observable of galaxies since it is found to be related to more fundamental properties, such as stellar mass, gas content and environment. Characterizing galaxy morphology, especially in current large-area surveys, has to be done in a quantitative and not subjective manner, to allow for comparative studies both at different redshifts and among different surveys.
In Scarlata et al. (2007a) we developed ZEST, a new algorithm to quantify galaxy structure, based on a principal component analysis (PCA) of five non-parametric diagnostics (Gini, M20, Asymmetry, Concentration and elongation). The PCA reduces the redundancy of information present in these parameters, allowing a complete description of galaxies’ structure by means of only the first three principal components. ZEST has so far been the basis of more than 30 papers, studying the various connections between galaxy structure, stellar content, and environmental density, and their evolution with redshift.