Research :: PhD Topics for 2014 :: Galaxies

Topics under the thematic line The assembly history of galaxies resolved in space and timeavailable for the 2014 Call (8 PhD Topics):

  • 2014/301 – A large double-blind Lyα-Hα survey at z~2.2: what does Lyα really tell us?
  • 2014/302 – The KMOS/VLT revolution: rotation curves, metallicities, dust extinction and galaxy formation and evolution with hundreds of galaxies at 0.8 < z < 2.23 vs z~0
  • 2014/303 – Unveiling the 3-D structure of a remarkable distant super-cluster: the roles of environment, mass and galaxy properties at z~1
  • 2014/304 - Objective criteria for the selection of the most distant radio galaxies
  • 2014/305 – The First Radio Galaxies in the Universe
  • 2014/306 - Investigating Structure Formation around Massive Galaxies through a Radio-Infrared Synergy
  • 2014/307 – The nature and formation history of pseudo-bulges in galaxies
  • 2014/308 – Probing Galaxy Evolution and Feedback Using Giant Gaseous Structures Associated with Powerful Radio-Loud Galaxies at High Redshift

For further details, please see the listing below for abstracts and advisors.

2014/301 – A large double-blind Lyα-Hα survey at z~2.2: what does Lyα really tell us?

Advisors: David Sobral (CAAUL) and Huub Rottgering (Leiden)

An extremely large number of studies rely on the Hydrogen Lyman-α (Lyα) emission line to survey, 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 missing by 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 deg2 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 for this project (the first has already been delivered to the INT; PI: D. Sobral). By measuring Lyα/Hα ratios for a sample of hundreds of galaxies at z=2.23, the student will robustly measure fescape and the Lyα/Hα ratio as a function of mass, colour, environment and SFR and empirically calibrate Lyα for the first time, with very important applications/consequences for z>2 studies. The student will also lead the follow-up (using e.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 of emission-line ratios). This will be a significant contribution towards unveiling the nature of 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 >3000 powerful 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 in the bright end of the Lyα luminosity function. This project will allow the student to observe on large telescopes to obtain the data directly (~20 nights over the first years), but also to do follow-up studies with e.g. VLT or ALMA to unveil and detail the nature of misterious Lyα emitters and blobs.

Notes: This topic requires a mixed fellowship (up to 1 year in Leiden).

2014/302 – The KMOS/VLT revolution: rotation curves, metallicities, dust extinction and galaxy formation and evolution with hundreds of galaxies at 0.8 < z < 2.23 vs z~0

Advisors: David Sobral (CAAUL), Mark Swinbank (Durham), Jean Michel Gomes(CAUP), Polychronis Papaderos (CAUP)

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, dynamics)? When, how and through which physical mechanisms are galaxies “quenched”? In order to make progress, the student will explore our unique, large ~10 deg2 narrow-band surveys that have yielded 1000s of galaxies (similar to the Milky Way), selected in the same way across the last 11 billion years (with WIRCam/CFHT and WFCAM/UKIRT). These are ideal samples to study the metallicities, dust extinction and rotation curves of “typical” star-forming galaxies and how these have evolved from the peak of the star-formation history (z~2.5) till today. By using KMOS (observations already started and thus all data is guaranteed both as PI and as part of GTO collaboration), the student will be able to gain unprecedentedly detailed information on a large sample of galaxies. KMOS, with its 24 Integral Field Units (IFUs) allows to target up to 24 galaxies at the same time, obtaining an image and a near-infrared spectrum for each pixel. This is a unique opportunity to map the distribution and intensity of star formation, dynamics and metallicity on ~4 kpc scales and address (and to fully interpret them): (i) What is the fraction of primitive disks, spheroids and mergers? (ii) Is the distribution of star formation at high-z more centrally concentrated than comparably luminous/turbulent galaxies at z~0? and iii) Are chemical abundance gradients weaker or stronger than local spiral galaxies and do those change with time? 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. By selection, all of the targets have known Hα fluxes and all are sufficiently bright so their resolved properties can be recovered and the survey efficiency will be >95% (GTO observations already yielded samples of ~400 galaxies). The results will be fully compared and contrasted to the best local IFU surveys (including artificially redshifting a variety of local galaxies and fully addressing biases and systematics), and will be interpreted using our unique 3D modelling capabilities developed at CAUP. Another unique aspect of this project is that there are significant over-densities in the very large samples of Hα emitters, and thus, with KMOS, the student will be able to confirm and characterise the high redshift structures, derive accurate metallicities, measure the mass-metallicity relation, obtain Balmer decrement extinctions and identify AGN for a sample of hundreds of Hα-selected galaxies and investigate if the environment plays a role in setting these galaxy properties.

Notes: This is a topic for a mixed fellowship (up to one year in Durham).

2014/303 – Unveiling the 3-D structure of a remarkable distant super-cluster: the roles of environment, mass and galaxy properties at z~1

Advisors: David Sobral (CAAUL), Jean Michel Gomes (CAUP), Polychronis Papaderos (CAUP)

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 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, coupled with a large field survey, 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.

Moreover, due to the fact that this super-structure is in the COSMOS field, the student will be able to fully explore the rich multi-wavelenght data-set to detail and expand the conclusions of the study (including with SED-modelling/fitting at IA-CAUP), 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.

2014/304 - Objective criteria for the selection of the most distant radio galaxies

Advisor: José Afonso (CAAUL)

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.

2014/305 – The First Radio Galaxies in the Universe

Advisor: José Afonso (CAAUL)

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 undeterminated. 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.

2014/306 - Investigating Structure Formation around Massive Galaxies through a Radio-Infrared Synergy

Advisor: José Afonso (CAAUL)

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.

2014/307 – The nature and formation history of pseudo-bulges in galaxies

Advisors: Polychronis Papaderos (CAUP) and Jean Michel Gomes (CAUP)

The featureless appearance of bulges in Hubble-type galaxies has for decades sustained the view that these high-surface brightness spheroidal components are largely “simple” in terms of their assembly history, formed on a short timescale early on, and having experienced little evolution over the past several Gyr. However, our early understanding of bulges as essentially scaled ellipticals has undergone a substantial revision over the last years. It is now recognized that central luminosity components that closely resemble classical bulges can also form over much longer timescales through disk instabilities and ensuing star forming activity at the centers of galaxies. The nature and formation history of these “pseudo-bulges” is enigmatic and of considerable relevance to our understanding of the structural and spectrophotometric evolution of galaxies in general.

This PhD project aims at a spatially resolved investigation of the star formation- and chemical enrichment history of pseudo-bulges. A unique aspect of its methodology is the combined application of surface photometry and spectral population synthesis to a large sample of galaxies from the Calar Alto Legacy Integral Field spectroscopy Area survey (http://califa.caha.es) with the goal of conclusively addressing the star formation history (SFH) and the chemical abundance patterns of pseudo-bulges. One of the central questions to be investigated is whether pseudo-bulges form in a quasi-continuous manner over several Gyrs of galactic evolution and their SFH can be parametrized through a simple functional form involving integral or structural properties of their host galaxy (e.g., total stellar mass; central surface brightness and exponential scale length of the underlying disk). Additionally, this project will include a comparative study of pseudo-bulges and classical bulges with the goal of the identification of new empirical discriminators between them and yield robust observational constraints to theoretical models of pseudo-bulge formation and evolution.

2014/308 – Probing Galaxy Evolution and Feedback Using Giant Gaseous Structures Associated with Powerful Radio-Loud Galaxies at High Redshift

Advisor: Andrew Humphrey (CAUP) and Patricio Lagos (CAUP)

Powerful radio-loud active galaxies at high redshifts (z>2) are often associated with ~100 kpc scale nebulae that emit strongly in the line of Lyman-alpha. Moreover, many also show associated extended Lyman-alpha absorbers that are at least as large. The global properties of these gaseous structures are, at present, poorly understood. This project aims for a substantially improved characterisation of the Lyman-alpha emitting and absorbing gases, making use of new observations from state of the art observing facilities.