Research :: PhD Topics for 2015 :: Galaxies

Topics under the thematic line The assembly history of galaxies resolved in space and timeavailable for the 2015 Call (8 PhD Topics):

  • 2015/309 – A large double-blind Lyα-Hα survey at z~2.2: what does Lyα tell us
  • 2015/310 – Unveiling the 3-D structure of distant super-clusters: the roles of environment, mass and galaxy properties at z~1
  • 2015/311 – The high redshift Universe with the largest emission line surveys: dawn of MOONS and EUCLID
  • 2015/312 – Investigating Structure Formation around Massive Galaxies through a Radio- Infrared Synergy
  • 2015/313 – Objective criteria for the selection of the most distant radio galaxies
  • 2015/314 – The First Radio Galaxies in the Universe
  • 2015/315 – Spatial distribution of α-elements in CALIFA galaxies
  • 2015/316 – The dust and molecular content of normal primeval galaxies

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.

2015/309 - A large double-blind Lyα-Hα survey at z~2.2: what does Lyα tell us?

Advisors: David Sobral (IA-U.Lisboa)

Currently, almost all studies rely on the Hydrogen Lyman-α (Lyα) emission line tosurvey, study and understand the distant Universe (z>2-3), as it is often the only feature available to spectroscopically confirm/study such galaxies. However, the escape fraction of Lyα (fescape) is highly uncertain at z>2, and much is unknown about what Lyα actually traces. How much are we missingby relying on it? How biased is our current view of the very high redshift, almost completely based on Lyα? Can we finally calibrate and understand Lyα and Lyα emitters?

In order to answer such questions, the student will conduct and work on very large (~5-10 degLyα surveys at z~2-3 (the likely peak of the star-formation history). This includes a perfectly matched Lyα-Hα survey at z=2.23 using custom-made narrow-band filters specifically designed forthis project (the first has already been delivered to the INT; PI: D. Sobral; deeper observations willbe conducted with CFHT in Hawaii). By measuring Lyα/Hα ratios for a sample of hundreds ofgalaxies at z=2.23, the student will robustly measure fescape and the Lyα/Hα ratio as a function ofmass, colour, environment and SFR and empirically calibrate Lyα for the first time, with veryimportant applications/consequences for z>2 studies. The student will also lead the follow-up (usinge.g. X-shooter and VIMOS on the VLT – some data already being taken) of many of the sources,resulting in the addition of accurate metallicity and dust extinction measurements (from a wealth ofemission-line ratios). This will be a unique and significant contribution towards unveiling the natureof Lyα emitters with a large, representative sample.

Furthermore, by conducting by far the largest survey (>2-4 orders of magnitude larger in volume than any other) for the most luminous Lyα emitters at z~2-3, the student will also detect >3000powerful Lyα emitters and >100 Lyα “blobs” (the largest ~contiguous objects found in the Universe,many times the size of a single galaxy), determine their Luminosity Function for the first time and measure their correlation function and evolution. This will provide the first robust sample that can be directly compared with the highest redshift samples, to directly test whether there is evolution inthe bright end of the Lyα luminosity function. This project will allow the student to observe on largetelescopes to obtain the data directly (~20 nights over the first years), but also to do follow-upstudies with e.g. VLT or ALMA to unveil and detail the nature of misterious Lyα emitters and blobs.

University: Lisboa

2015/310 - Unveiling the 3-D structure of distant super-clusters: the roles of environment, mass and galaxy properties at z~1

Advisors: David Sobral (IA-U.Lisboa), Jean Michel Gomes (IA-U.Porto), Polychronis Papaderos (IA-U.Porto)

Why were galaxies in the distant Universe so efficient at producing new stars? What were the roles of “nature” (stellar mass) and “nurture” (environment) in the past, how did they change with cosmic time and is there a connection between those and the declining star formation activity? Is our current view of how galaxies form and evolve correct? By probing a very wide range of environments (from fields to clusters of galaxies) and masses, we are now obtaining a much better picture of the roles and inter-dependences of mass and environment in the distant Universe. However, there are significant limitations in current studies, due to the small sample sizes, lack of multi-wavelenght data in cluster fields, projection effects, and the dilution/confusion of environments (e.g. filaments vs small groups). In order to obtain the sharp view that we need, overcoming current limitations (from the use of photometric redshifts), the student will start by using the VLT (with VIMOS [PI: D. Sobral], 40 hours of observations already conducted at the VLT, all in excellent conditions) to accurately map in 3-D a unique super-structure at z = 0.84 in the COSMOS field (10×13 Mpc). This massive, large structure contains 3 confirmed massive X-ray clusters/groups and shows a striking filamentary structure of star-forming galaxies. With >1000 galaxies residing in such structure, the student will measure accurate redshifts (from both emission and absorption lines) and make a detailed/accurate 3-D map of the complex structure, identifying filaments, fields, outskirts, small groups and the cluster cores. The student will obtain independent mass estimates from the absorption lines, and map SFRs down to even the least active galaxies, but also detect post-starburst galaxies (K+As) and map their fraction in the cluster, group, filament and field environments over the entire structure. The unique modelling capabilities developed at IA-CAUP, will be fully implemented to interpret and extract even more information from the high S/N, high quality spectra. The results of this project will be directly compared with a large field survey, recently approved by ESO (LEGA-C, 128 nights on the VLT, Co-I: D. Sobral), will reveal exactly where star formation activity is being enhanced/quenched, clearly disentangling the roles of mass and environment in the distant Universe in a robust way for the first time. The student will then fully explore the rich multi-wavelenght data-set to detail and expand the conclusions of the study (including with SED-modelling/fitting), particularly by investigating the morphologies (with Hubble Space Telescope imaging), but also to look at radio and far-infrared (Spitzer + Herschel) properties of galaxies residing in the various environments within the super-structure and in the general field environment.

University: Lisboa (2 years) & Porto (2 years)

2015/311 - The high redshift Universe with the largest emission line surveys: dawn of MOONS and EUCLID

Advisors: David Sobral (IA-U.Lisboa), Bahram Mobasher (UC Riverside)

What are the physical drivers of galaxy formation and evolution? How much (and why) did galaxies like our own change across cosmic time (e.g. metallicities, stellar mass, gas fractions)? When, how and through which physical mechanisms are galaxies “quenched”? In order to make progress, the student will explore our unique, large >10 deg^2 narrow-band surveys that have yielded 1000s of galaxies (similar to the Milky Way), selected in the same way across the last 12 billion years (with WIRCam/CFHT and WFCAM/UKIRT) with Hα,Hβ+[OIII] and [OII]. These are ideal samples to study the metallicities, dust extinction, clustering and evolution of “typical” star-forming galaxies all the way to z~5. By using observations that have already been taken (e.g. MOSFIRE and DEIMOS/Keck, FMOS/Subaru, and VIMOS/VLT), the student will be able to gain unprecedentedly detailed information on a large sample of galaxies. These will not only improve our understanding of the number counts, luminosity function and evolution of all major emission lines, which is of upmost importance for planning dark energy experiments such as EUCLID and WFIRST, but will also prepare the first samples that will be explored with MOONS (part of the data were already used to develop the MOONS science case). The student will be able to answer many key questions: 1) What are the typical halo masses for which different star-forming galaxies reside in? 2) How strong is their evolution in metallicity and amount of dust across cosmic time? 3) How biased is our understanding of star formation at high-z based on UV surveys only? Answers to these questions using our well selected samples will provide some of the strongest tests/constraints to the most sophisticated models of galaxy formation and evolution (e.g. EAGLE, Illustris). The results will also be fully compared and contrasted to the best surveys at z~0. By selection, all of the targets have known emission line fluxes and are selected in the same way, thus being extremely competitive against any other method of selection and follow-up. Another unique aspect of this project is that there are significant over-densities in the very large samples of line emitters, and thus the student will be able to confirm and characterise the high redshift structures, derive accurate metallicities, measure the mass-metallicity relation and identify AGN for a sample of hundreds of emission-line selected galaxies and investigate if the environment plays a role in the evolution of these galaxies. Several detailed follow-up studies are foreseen, particularly with X-SHOOTER, MOSFIRE and other instruments. The student will also work on preparing the first targets for MOONS and collaborate in the production of a fully working pipeline for the instrument, and thus will be in an ideal position to explore the new instrument when MOONS becomes available by ~ 2018-2019, but also to be in a leading position when EUCLID starts observing in early 2020.

University: Lisboa

Note: This topic requires a mixed fellowship with up to one year in the UC Riverside.

2015/312 - Investigating Structure Formation around Massive Galaxies through a Radio- Infrared Synergy

Advisors: José Afonso (IA-U.Lisboa)

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 Spitzer Extragalactic Representative Volume Survey (SERVS), which has just finished the data processing for a 1400 hour-long observational programme capable of finally overcoming long-standing observational limitations.

University: Lisboa

2015/313 - Objective criteria for the selection of the most distant radio galaxies

Advisors: José Afonso (IA-U.Lisboa)

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 by 2015. 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

2015/314 - The First Radio Galaxies in the Universe

Advisors: José Afonso (IA-U.Lisboa)

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

2015/315 - 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 is 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 toFe by an “enhanced-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 resolution to study the radial trends in galaxies.

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) 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

2015/316 - The dust and molecular content of normal primeval galaxies

Advisors: Hugo Messias (IA-U.Lisboa), José Afonso (IA-U.Lisboa)

In these recent years, we have witnessed a significant increase in the numbers of galaxies observed at an epoch when the Universe was extremely young, less than one tenth of its current age. This was possible due to the advent of deep space- and ground-based near-infrared (nIR) surveys. However, the characterisation of these galaxies faces large uncertainties given the fact that, at those distances, the nIR analysis targets the rest-frame UV alone, where only the hottest stars emit and dust obscuration is critical. In order to finally understand the true nature of these primeval sources, long-wavelength (mm-to-cm) observations are required. State-of-the-art facilities working in this spectral range now provide continuum and spectral information all at once, enabling the study of dust and gas contents, and dynamics in galaxies. This project focuses on thousands of normal galaxies found in the early Universe, addressing dust particle formation and growth, and chemical enrichment.

University: Lisboa