The role of bars in disk galaxy evolution

  • Thomas Melvin

Student thesis: Doctoral Thesis


Galactic bars play in an important role in the evolution of disk galaxies. Bars also act as a tracer for dynamically cool, `mature' disk galaxies. Therefore, the
observational study of bars, especially at higher redshifts, provides an invaluable insight into how the disk galaxy population has evolved as the Universe has aged.
In this thesis, I present research that explores the redshift evolution of the bar fraction since z = 1, and the first look at the fraction of disk galaxies hosting bars out to z = 2. My research combines optical and infrared observations, taken by the Advanced Camera for Surveys and Wide Field Camera 3 instruments on board the Hubble Space Telescope, with visual morphological classifications made by citizen scientists as part of the Galaxy Zoo project.
After a careful analysis of the visual morphologies provided and the possible effects of a range of potential observational biases, I measure the redshift evolution of the bar fraction in the COSMOS field. For a volume limited sample (0.4 ≤ z ≤ 1.0; log(M/M) ≥10) of 2,380 disk galaxies, of which 317 are barred (fbar ∼13%), I find that the bar fraction has increased from 11±2% at z = 1, to 22±5% at z = 0.4. Furthermore, when splitting this sample into three stellar mass bins, I find that, whilst an increasing bar fraction with decreasing redshift is observed for all stellar masses, it is the most massive disk galaxies that drive the evolution observed.
Building on this work, I present the first observations of the bar fractions beyond z = 1. Using visual morphological classifications from Galaxy Zoo: CANDELS, a sample of 876 disk galaxies, with 123 being barred disks (fbar ∼ 14%) are identified across the redshift range 0.5 ≤ z ≤ 2.0. Selecting a sub-sample within the same region of the evolving galaxy luminosity function (brighter than L), we find that the bar fraction across the redshift range 0.5 ≤ z ≤ 2 (fbar = 10.7+6:3-3:5% after correcting for incompleteness) does not significantly evolve.
I present a recipe that describes how to explore the effects observational biases have on visual morphological classifications, before working through this recipe in detail in order to debias the galaxies in the Galaxy Zoo: Hubble catalogue. Of the 46,703 galaxies found within the redshift range 0.3 ≤ z ≤ 1.0, 24% (15,855) can be debiased, with a further 33% (15,338) having lower and upper limits placed on their original pfeatures vote-fractions. I use the newly debiased classifications to re-explore the redshift evolving bar fraction, finding that the trend is greater within these galaxies, increasing from fbar = 4±1% at z = 1, to fbar = 24±2% at z = 0.3.
In my exploration of barred disk galaxies in the COSMOS field, up to z = 1, I
identified a sample of 98 quiescent disk galaxies whose bar fraction is considerably higher (fbar ∼ 45±5%) than the overall disk sample. To investigate this observation further, I explore the evolving demographics of the red sequence since z = 1. Using rest-frame colours from the UltraVISTA catalogue (McCracken et al., 2012), I find that, when using debiased pfeatures classifications, the fraction of all disk galaxies that are part of the red sequence decreases from 26±2% at z = 1, to 11±2% at z ∼0. Additionally, I also find that the fraction of all barred disk galaxies that are part of the red sequence increases from 0% at z = 1, to 22±4% at z ∼ 0. I conclude that these results show a maturing disk galaxy population as the universe ages.
Date of AwardNov 2016
Original languageEnglish
Awarding Institution
  • University of Portsmouth
SupervisorKaren Masters (Supervisor) & Bob Nichol (Supervisor)

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