Cosmology and high redshift galaxies

The Diverse Progenitors of Supernovae of Type Ia

The nature of the progenitors of SNIa is still an unsettled question. While there is general consensus that the explosions originate from
the ignition of nuclear fuel under degenerate conditions in a White Dwarf (WD) member of a close binary system, the evolutionary paths leading to the event are controversial. Two competing scenarios are considered, one in which the donor star is a White Dwarf (double degenerate), and one in which it is a living star (single degenerate). In addition, explosion can occur when the WD mass has reached the Chandrasekhar limit, or when a sufficiently massive Helium layer has accumulated on top of the WD and detonates (sub-Chandra explosions). In the recent years more and more evidence for diversity of the spectrophotometric properties of SNIa’s has accumulated. Peculiar events, like Superchandra explosions, subluminous events, or the intriguing class of Type Iax Supernovae have supplemented the class of normal SNIa events in a non negligible proportion. The diversity of spectrophotometric propeties of SN of type Ia is a challenge for their use as distance indicators in cosmological applications. The aim of the thesis is to provide
a comprehensive overview of this diversity by collecting data from the literature, with special attention to the more exceptional events. Existing models for the SNIa explosions will provide a framework to develop associations between evolutionary paths and SNIa subtype. These will be used to compute the expected diversiy of events in galaxies with different star formation histories, to be tested against observational data. The thesis will finally include the discussion of the future prospects for this kind of analysis using the vast database provided by the LSST survey.

TeV astronomy, the problem of cosmic opacity at TeV photon energies, and the cosmic IR background

A. Franceschini

Observations of very high energy (TeV) photons are now routinely possible with Cherenkov telescopes. The team proposing the thesis collaborates with colleagues of the Physics Department involved in the MAGIC consortium. The thesis will concern the important question of the cosmological opacity of the universe to TeV photons due to their interaction with those of the cosmic optical and IR backgrounds (photon-photon collisions). The topic will not only address all the physical problems related with this phenomenon, but also will exploit TeV observations of BLAZARs over a wide redshift range to constrain the intensity of the extragalactic radiations and compare them with direct determinations and their model interpretation.

The Herschel space mission

A. Franceschini , G. Rodighiero

The Herschel Space Observatory has been one of the most important ESA mission of the decade  and a unique opportunity for European astronomers. The thesis advisor, co-investigator of the mission, is involved in a wide effort of exploiting Herschel for the exploration of the distant universe. The thesis will consider various aspects of such cosmological investigations, including model work of IR galaxy evolution, interpretation of Herschel data and even data from archival data, according to the preferences of the perspective student.

COSMOS, SWIRE, and Spitzer cosmological surveys

A.Franceschini, G. Rodighiero
The team proposing the thesis is actively involved in wide international collaborations dedicated to the exploitation of the most advanced instrumentation for cosmological investigations in the darkest sky regions. The forefront of these is the COSMOS project over a 2 sq. deg. field, including HST ACS data; SWIRE is a wide-area (49 sq. deg.) with Spitzer IR data. The thesis will consider the effects of environment on the formation and evolution of cosmic structures (galaxies and AGNs in particular).

H-ATLAS: co-evolution of Black Hole accretion rate and Star-Formation in galaxies across cosmic times
G.  Rodighiero, A. Franceschini

The self-regulation of host galaxies and their central supermassive black holes (BHs) is often invoked to explain the local scaling relations between BH mass and galaxy properties. In the last years it has also been shown that this correlates mainly with velocity dispersion, a relation that can be hardly explained without feedbak effects. As a consequence, determining the relative roles of the various processes that drive the coevolution of BHs and galaxies has emerged as a key goal of current astrophysics research.The emerging observational framework supports the idea that mergers and/or secular processes should play a major role in fuelling both star formation and SMBH growth in a large majority of galaxies displaying moderate nuclear activity.
The student will be involved in an extensive exploitation of the photometric and spectroscopic H-ATLAS data-set (the largest far-Infrared Herschel survey). In particular he/she will perform an SED fitting to a complete mass selected sample of star-forming galaxies in the low-redshift Universe. The adopted code (Magphys) performs an energetic balance between UV absorbed and IR re-emitted radiation, and it provides the total star formation rate (SFR) and the intrinsic AGN luminosity (i.e. proportional to the black hole accretion rate) of galaxies.  In the thesis the candidate will then integrate these two physical quantities to trace the evolution of black hole masses and the stellar mass of their hosts. This project forsees a collaboration with the theoretical group of the Southampton University (in particular prof F. Shankar), where the student is supposed to spend a period to implement a phenomenological and semi-analytical model to interpret the data (in particular to understand the actual roles of host stellar mass/velocity dispersion and BH mass)  and put them into a cosmological context.

SHARDS: The bending of the star-forming galaxies Main Sequence: What is the major physical process driving the evolution of massive galaxies?

G. Rodighiero, C. Mancini, A. Franceschini, A. Renzini

At z<1.5 the stellar mass/Star-Formation Rate (SFR) correlation of star-forming galaxies (i.e., the so-called Main Sequence, MS) is not linear at all masses, but it bends at the high-mass end, due to a population of sources with reduced SFR at such high stellar mass. The fact that such objects host large bulges has recently suggested that the internal formation of the bulges was the late event that induced massive galaxies to quench.
However, in a recent work (Mancini et al. 2017, in prep.) an accurate bulge/disc decomposition performed in 7 optical/near-IR HST bands for 16 galaxies below the main sequence at 0.5<z<1 showed that while the bulges hosted in these systems are virtually all maximally old, with ages approaching the age of the Universe at the time of the observation, disks are very young (0.3-1.5 Gyr). If confirmed, these results would suggest that the bending of the MS is likely due to rejuvenation events, caused by the interaction of passive massive galaxies with smaller gas-rich systems.
During the thesis, the candidate will perform a bulge/disk decomposition (with Galfit and similar tools) on the 24 contiguous medium-band maps from the SHARDS programme (Perez-Gonzalez et al 2013), covering the whole optical wavelength range, between 5000 and 9500 A. He/she will then apply a detailed SED fitting procedure to the SED of the sources, deriving with improved spectral resolution (R~50)  the age of the bulge and disc  components. The analysis will be also extended to a larger sample, including all the main sequence galaxies at 0.5<z<1 in the GOODS-N field, and to higher-redshift galaxies in the ongoing SHARDS programs in the HST Frontier Fields.

SHARDS: a comprehensive study of Cosmic Star Formation up to redshift z~2
G. Rodighiero, L. Rodriguez-Munoz, C. Mancini, A. Franceschini

In the last decades, the cosmic star formation (SF) history of the Universe has been thoroughly studied to  better understand galaxy formation and evolution. In this context, emission lines (ELs; i.e., H-alpha) reveal as some of the most direct SF tracers.  We propose to carry out a comprehensive study of emission line galaxies (ELGs) up to redshift z~2 exploiting the spectro-photometric Survey for High-z Absorption Red and Dead Sources (SHARDS, Pérez-González et al. 2013), an ESO/GTC Large Program. SHARDS covers GOODS-N field (~130 arcmin^2) and provides deep photometry in 25 medium-band filters (reaching magnitude ~27, at 3 sigma) throughout the wavelength range 5000-9500 A.  The power of using narrow/medium-band filters for the detection of ELGs (i.e., star-forming) has already been proved in previous studies. In this project, [OII]3727A emitters will be detected in the redshift range between ~0.3 and ~2.4. With this sample, the student will compute the evolution of the EL luminosity functions, and their contribution to the cosmic SF rate density.
The candidate will be further involved in the ongoing SHARDS observations in the HST Frontier Fields and for this part of the project, he/she is expected to spend a period at the UCM (Madrid; Prof. Pérez-González) to work on GTC observations and related data reduction and analysis. The UV to far-IR photometric datasets and spectroscopic data available on GOODS-N will allow the student to perform a high quality characterization of these star-forming galaxies using a SED-fitting methodology, and a panoramic perspective on the SF activity throughout cosmological timescales.