Research :: PhD Topics for 2017 :: Galaxies

Topics under the thematic line The assembly history of galaxies resolved in space and timeavailable for the 2017 Call (7 PhD Topics):

  • 2017/329 - Investigating Structure Formation around Massive Galaxies through a Radio-Infrared Synergy
  • 2017/330 - The First Radio Galaxies in the Universe
  • 2017/331 - Objective criteria for the selection of the most distant radio galaxies
  • 2017/332c - Testing the impact of kappa-distributed electron energies in quasar nebulae
  • 2017/333 – Diffuse ionized gas and Lyman continuum photon escape in spiral galaxies
  • 2017/334 – Spatial distribution of α-elements in CALIFA galaxies
  • 2017/335 – The importance of star-formation, black-hole accretion and other physical mechanisms into the Far-IR – radio correlation

For further details, please see the listing below for abstracts and advisors. Prospective candidates are welcome to contact directly the proposers of the topics for inquiries and further details.

2017/329 - Investigating Structure Formation around Massive Galaxies through a Radio-Infrared Synergy

Advisors: José Afonso (IA U.Lisboa & FCUL), Israel Matute (IA U.Lisboa), Hugo Messias (Joint ALMA Observatory)

One of the greatest challenges facing observational cosmology is understanding the formation of large scale structure in the Universe. Hierarchical models for structure formation developed over the last few years, achieving the high degree of predictive success that they do, are however still unconstrained, in particular in helping to understand how the light (galaxies) traces the underlying (dark) matter and how this relation evolves over time. We will address this problem by performing a systematic study of the evolution of the densest regions of the Universe, as traced by the most massive galaxies and their environments, improving our understanding of how the most massive regions of the Universe form and evolve. This will only be possible by using data from a deep mid-infrared wide-field survey, the extended Spitzer Extragalactic Representative Volume Survey (SERVS), now including over 2700 hours of deep mid-infrared observations and capable of finally overcoming long-standing observational limitations.

University: Lisboa

Type: This topic may correspond to a mixed fellowship (up to 1 year abroad).

2017/330 - The First Radio Galaxies in the Universe

Advisors: José Afonso (IA U.Lisboa & FCUL), Israel Matute (IA U.Lisboa), Hugo Messias (Joint ALMA Observatory)

Recent observations of the highest redshift quasars and radio galaxies pinpoint the early growth of supermassive black holes (SMBH) that trigger the formation of active galactic nuclei (AGNs) at redshifts greater than 7. It is anticipated that radio emission can be detected from such early AGN, although its characteristics are still quite undetermined. The importance of such detection, however, is extremely high. It will: (a) provide us with a lighthouse that reveals the physics of the first accretion episodes to the first SMBHs in the Universe; (b) allow the direct study of the neutral gas throughout the Epoch of Reionisation itself with the next generation of radio telescopes, through the observation and study of the HI 21cm forest against such early AGN; (c) allow us to trace the early growth of Large Scale Structure in the Universe. After decades of laborious work, trying to understand the deepest radio observations, the conditions are now finally right to develop a project that can make us understand where are the “first radio galaxies” and how to find them with upcoming radio telescopes.

University: Lisboa

Type: This topic may correspond to a mixed fellowship (up to 1 year abroad).

2017/331 - Objective criteria for the selection of the most distant radio galaxies

Advisors: José Afonso (IA U.Lisboa & FCUL), Israel Matute (IA U.Lisboa), Hugo Messias (Joint ALMA Observatory)

Studies of the high redshift Universe have relied on a number of methods to identify increasingly distant and, consequently, younger galaxies. Radio selection has been reborn as one of the most promising methods, as powerful radio AGN can currently be detected at radio frequencies at essentially any redshift. Using the next generation of deep radio surveys, these searches are now being pushed to new sensitivity levels, to find radio AGN at the highest distances and at the earliest formation stages, presumably well within the Epoch of Reionization. Using multiwavelength data (x-rays, optical, infrared and millimetre, besides the radio itself), the faint radio population has started to be characterized, contributing not only to a better understanding of the radio sky, but, even more interestingly, selecting elusive, more extreme sources that can only be understood with the upcoming full capabilities of the Atacama Large Millimetre Array. These studies are exciting, but can only currently be performed over very small areas. Extending such studies to virtually the whole sky is a prime objective of the next generation of radio surveys, and this project aims to play a pivotal role in such expansion.

The host institution for this project is involved (with co-Is and co-PIs) in two of the most ambitious projects in the pre-Square Kilometre Array era, surveys that will map the entire sky at 1.4GHz at microJansky levels. The Evolutionary Map of the Universe (EMU), to be performed with the Australia Square Kilometre Array Pathfinder, will cover the sky at declinations below 30deg, while the Westerbork Observations of the Deep APERTIF Northern-Sky (WODAN) will cover the northern regions. Both surveys should start producing data from 2017. Together, the EMU and WODAN surveys will produce a unique dataset that, together with other multiwavelength data being obtained or soon to be obtained, will be able to find the most extreme and unique radio galaxies, including the first-generation of powerful AGN in the Universe, in the Epoch of Reionization. This project proposes to (a) establish a set of objective criteria for the selection of very high redshift radio galaxies; (b) find and analyse candidates for very high redshift radio galaxies, including the preparation of follow-up observations of particularly interesting candidates with ALMA; (c) play an active role in the optimization of the next generation of ultra-deep whole-sky radio surveys, the EMU and WODAN projects, in order to explore more efficiently the highest redshift Universe.

University: Lisboa

Type: This topic may correspond to a mixed fellowship (up to 1 year abroad).

2017/332c - Testing the impact of kappa-distributed electron energies in quasar nebulae

Advisors: Andrew Humphrey (IA U.Porto)

A long-held assumption is that the electron energies in extrasolar warm ionized nebulae (HII regions, quasar-ionized nebulae, etc.) follow the Maxwell-Boltzmann (MB) distribution. However, direct measurements from a number of plasmas within our Solar system have revealed the presence non-equilibrium electron energy distributions, which have been characterised by the ‘kappa-distribution’ (KD). Armed with the MAPPINGS 1e photoionization code – the first to include kappa-distributed electron energies – an extensive exploration of photoionization model parameter space will be conducted, to ascertain the generalized impact of KD election energies on quasar / AGN line spectra, and how this impact depends on various model parameters (e.g., density, metallicity, kappa, ionization parameter, ionizing spectral energy distribution). Observational tests for the presence of KD in photoionized nebulae will be developed. Moreover, diagnostics of gas physical conditions with the least (or no) dependence on the value of kappa, or on the choice of KD of MB cases, will be developed. The possible impact on BPT diagrams (and similar), and on the spectral classification of emission line galaxies, will also be elucidated. In addition, the student will study feedback activity in high-z radio-loud AGN using integral field spectroscopic observations from MUSE, if time permits.

University: Porto

Restrictions: This is a closed topic.

2017/333 – Diffuse ionized gas and Lyman continuum photon escape in spiral galaxies

Advisors: Polychronis Papaderos (IA U.Porto), Jean Michel Gomes (IA U.Porto)

Diffuse ionized gas (DIG) is an ubiquitous component in the disk and halo of spiral galaxies. The excitation mechanisms and ionization conditions of the DIG pose a long-standing enigma. The prevailing picture though is that the DIG originates from Lyman continuum (LyC) photons escaping from sites of ongoing star formation and their reprocessing into nebular emission on scales of ~1 kpc away from HII regions. The mechanisms facilitating escape and transport of LyC radiation are unclear, it is yet likely that a key role is played by injection of energy and momentum by stellar winds and SNe into a porous multi- phase interstellar medium. Various lines of evidence indicate that the diffuse, low-surface brightness DIG contributes ~20-50% of the total Hα emission in late-type galaxies, a fact that may introduce a substantial observational bias on estimates of star formation rates in high-redshift galaxies where DIG emission is barely detectable. This project aims at a detailed investigation of the physical properties of the DIG in a representative sample of nearby face-on spiral galaxies using narrow/broad-band imaging, spatially resolved integral field spectroscopy from the CALIFA survey (http://califa.caha.es) and advanced spectral synthesis models. A central question to be addressed concerns the relation between the fractional contribution of the DIG to the total Hα luminosity and the star formation history and structural properties of spiral galaxies.

University: Porto

2017/334 – Spatial distribution of α-elements in CALIFA galaxies

Advisors: Jean Michel Gomes (IA U.Porto), Polychronis Papaderos (IA U.Porto).

A long-standing puzzle in extragalactic research concerns the anomalous abundances of so-called α-elements (e.g., C, N, O, Ne, Si, S, Mg and Na) relative to iron (Fe) in early-type galaxies (ETGs). These elements are generally enhanced relative to Fe by an “enhanced-ratio” (perhaps “enhancement-ratio”?) [E/Fe] correlating with the stellar velocity dispersion (hence, the total stellar mass) of an ETG. The dominant physical mechanism responsible for this trend is still unknown yet fundamental to the understanding of the chemo-dynamical evolution of ETGs across their entire mass spectrum.

Three main scenarios have been proposed for these discrepancies: a) a varying star-formation rate efficiency in massive ETGs, b) a non-universality of the stellar initial mass function (IMF) in the sense of a “top-heavy” IMF and c) selective loss of elements due to galactic winds. All these scenarios attempt reproducing the observed [E/Fe] ratios as essentially the result of chemical enrichment by Type II and Type Ia Supernovae, each acting on different timescales, and with a relative frequency closely linked to the galaxy star formation history.

Studies of stellar populations in galaxies have dramatically advanced in the last decade. Instead of using a few hand-picked Lick indices, fluxes and integral colours to constrain the star formation- and chemical enrichment history of galaxies, modern spectral synthesis codes and computing facilities now permit a detailed modelling of the full optical spectrum of a galaxy in a pixel-by-pixel approach. These modelling tools and the availability of high-quality data sets (e.g., 2dF, 6dF, SDSS and GAMA surveys) offer a promising avenue for a better understanding on how galaxies form and evolve through time. However, all spectral synthesis studies carried out over the past decade on the basis of these single-fibre spectroscopic surveys lack the necessary spatial coverage and resolution to study the radial trends in galaxies (cf e.g. Gomes et al. 2016b&c).

Only recently spatially-resolved data from Integral Field Spectroscopy (IFS) has become available, permitting the study of radial abundance patterns of α-elements in galaxies with unprecedented detail. An innovative aspect of this PhD research project is the use of IFS data for ~600 local Hubble-type galaxies from The Calar Alto Legacy Integral Field spectroscopy Area survey (CALIFA – http://califa.caha.es) to determine the 2D α-element distribution in a spatially resolved pattern. This observational input will be combined with the derived Star-Formation Histories and structural properties of ETGs from CALIFA with the goal of developing new evolutionary diagnostics for ETGs and shedding light into the origin of the α-element enhancement in these systems.

University: Porto

2017/335 – The importance of star-formation, black-hole accretion and other physical mechanisms into the Far-IR – radio correlation

Advisors: Cirino Pappalardo (IA U.Lisboa), Israel Matute (IA U.Lisboa), Jose Afonso (IA U.Lisboa)

One of the most challenging astrophysical questions is to determine when and how stars form throught the history of the Universe. One of the best indicators of this so-called star formation rate is the far infrared (FIR) emission of galaxies. The interstellar dust present in galaxies is the responsible for the absorption of the UV energy coming from the radiation field of young (and massive) stars that is later re- emitted thermally in the FIR. The infered number of massive stars is then a good proxy of the current star-formation rate of the galaxies. The contribution to this emission from the older stellar population present in the galaxy is still a matter of debate and, depending on their contributing fraction, could strongly affect our star-formation estimates. In this context, we observe in galaxies a strong connection between the radio and the FIR emission. Although radio emission can be related to the current star formation, the apparent direct connection between them is far from obvious due to the contribution (as in the case of the FIR) of additional physical processes not directly linked to star-formation. In the radio, electrons accelerated to relativistic energies in jets coming from the accretion of matter into a super massive blackhole (SMBH) in Active Galactic Nuclei (AGN), can in many cases overshine the emission from young stellar populations. Despite all these different emission mechanisms, the relation is surprisingly tight and universal for galaxies over several orders or magnitude in mass and luminosity. Nowadays, a complete theoretical understanding for the tightness of this correlation is still missing. An invaluable insight into the physics of this correlation could be gain by considering a statistical large sample of galaxies were we can precisely measure both the radio continuum and FIR properties. With modern radio continuum surveys and with the large FIR database produced by the Herschel Space Observatory, such samples can be built and explored.

The goal of this project is to investigate the influence of the average dust mass and dust temperature into the FIR-radio correlation as well as the significance of the AGN contribution. With this objective in mind, the student will build a large sample of galaxies with high quality data both in the radio and at FIR wavelengths. We will investigate the scatter in the FIR-radio correlation as a function of different dust mass bins, and as a function of redshift. The multi-wavelength coverage available for a large fraction of the sources will allow us to investigate in detail the physics related to the different dust heating mechanisms. This study will shed light on which conditions the FIR-Radio correlations is still reliable. The data sample and results obtained during the thesis will be crucial for future studies of star-forming galaxies. In particular the proposed analysis will be important to understand the faint radio population that will be uncovered by the future radio continuum survey, such as the SKA pathfinder and precursor radio continuum surveys from ASKAP (Norris et al. 2011), LOFAR (Rottgering et al. 2011), MeerKAT (Jarvis 2012), and the Jansky Very Large Array (JVLA).

University: Lisboa