In modern astrophysics, galaxies are described as complex ecosystems, whose fate is governed by the flow of gas from the large-scale cosmological environment, its conversion into stars and the ejection of part of the metal-enriched interstellar material.
These processes are known as the "galaxy baryon cycle" and the physics that regulates their efficiency still remains a critical problem in galaxy formation studies, particularly at early cosmic epochs, when star formation in galaxies is observed to occur at a much larger pace compared to the relatively quiet Milky Way galaxy at the present time.
The purpose of this research project is to advance our understanding of two key aspects of the problem:
(1) How do early galaxies acquire their gas content to fuel their star formation?
(2) When and how do galaxies get enriched with heavy elements and dust grains?
We plan to address these objectives by exploiting new and archival data acquired through the largest currently available telescope facilities on the ground and in space and running state-of-the-art hydro-dynamical simulations of galaxy growth at early cosmic times.
The proposed research stands on a unique combination of observational data and numerical tools.
From the observational point of view, we plan to exploit the current leading observational facilities to obtain a clear and definitive picture of the cold gas flows from the IGM onto massive galaxies at high redshift. This will provide one of the most critical missing pieces for a complete understanding of the "galaxy baryon cycle", adding precious elements to galaxy evolution models. More in details, our observational strategy consists in both making an extensive use of archival data and applying for time allocation on the most powerful telescopes (some observing proposals have been already submitted to the VLT and ALMA telescopes and are currently being evaluated).
This approach will enable us to achieve the following results:
-To map for the first time with extremely high sensitivity the cold streams of inflowing gas from the cosmic web. The extended distribution of molecular gas reported in Ginolfi et al. (2017) may be the tip of the iceberg of a much wider distribution of inflowing gas, that can be investigated with deeper ALMA observations.
-To detect signatures of Lyman alpha and optical nebular lines emission within the circumgalactic medium of Candels-5001. Lyman alpha observations, to carry out with VLT-MUSE, will enable us to detect the more diffuse and less metal-enriched component of the filaments. Detecting optical nebular lines (with VLT-SINFONI) is fundamental as they are the perfect tool to measure the mass content of the atomic phase within the circumgalactic medium and, more interestingly, the star formation occurring within the streams and their metallicity.
-To explore how common are inflowing streams around massive galaxies in over-densities and whether there is a redshift dependence or not. Models predict that below z~2 gravitational shock heating may halt streams at the interface with the dark matter halo and prevent further accretion (Sanchez-Almeida et al. 2014). We aim to investigate cold accretion through cosmic streams in redshift slots encompassing this transition phase. This goal will be addressed through a deep research of existing archival data, to be investigated with the most powerful techniques of data reduction and data analysis.
From the theoretical point of view, simulating the self-regulated assembly of galaxy ecosystems require advanced numerical techniques that are capable of following physical processes occurring on a vast range of spatial and temporal scales. From the few hundreds parsec scales of star forming giant molecular clouds to the few hundreds kilo-parsecs scales of the galaxy circumgalactic medium. And from the few tens of million year lifetimes of massive stars that explode as supernovae and enrich the interstellar medium with heavy elements od dust grains, to the billion years timescales of cosmological evolution from the dawn of the first stars to the redshift range of the target observations (see above). The proposed simulation will be carried out using the recently developed numerical code dustyGADGET (Graziani et al. 2017).
In details, we will make the first detailed predictions based on a simulation with fully integrated dust evolution, of the Hubble Ultra Deep Field, that is one of the first targets of the next generation JWST telescope. Previous attempts to predict the dust content of high-redshift galaxies have made the strong assumption that, independently of galaxy properties and redshift, a fixed fraction of gas-phase metals is condensed into dust grains. Theoretical and observational studies have convincingly demonstrated that this assumption is incorrect, particularly at the low-metallicities that are supposed to characterize galaxies in the reionization epoch (Remy-Ruyer et al. 2015; Mancini et al. 2015, 2016; Khakaleva & Gneidn 2016, Popping et al. 2017). The approach that we intend to follow is to use dustyGADGET to overcome this limitation, and simulate the dust content and spatial distribution of high-redshift galaxies in a self-consistent way. This is a key aspect to correctly estimate galaxy colours and to correctly assess fundamental physical properties, such as the star formation rate, stellar mass, molecular gas mass, from UV colours.
In addition, the use of ARGO, a well-studied zoom-in cosmological simulation of a massive star forming galaxy from z=100 down to z>2 (Feldman & Mayer 2014), to run dustyGADGET, will enable for the first time to understand the cycling of dust in the different phases of the ISM at different metallicities. This approach has never been attempted before, due to limitations of existing codes, where dust evolution is not modeled in a self-consistent way, with a few exceptions, which have either targeted the Milky Way Galaxy (McKinnon et al. 2016) or an isolated galaxy (Aoyama et al. 2017).