The role of neutral hydrogen in the life of galaxies and AGN: A spectral stacking analysis

Katinka Gereb

Research output: ThesisThesis fully internal (DIV)

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Neutral hydrogen (HI) gas is thought to play an important role in the evolution of galaxies, as HI provides the primordial gas from which stars eventually form. Furthermore, HI is thought to be partly responsible for the triggering of one of the most energetic phenomena in the Universe, by feeding supermassive black holes, i.e. active galactic nuclei (AGN) in the centres of galaxies. In this thesis we investigate the HI properties of hundreds of galaxies to get a better understanding of the role of HI in the evolution of galaxies over the past 1.5 billion year. We also look at the gas properties of accreting supermassive black holes to investigate the interplay between AGN activity and the surrounding gas.

Using data taken with the Westerbork Synthesis Radio Telescope (WSRT), we carry out a spectral stacking analysis of HI. Stacking is the process of co-adding the spectra of individual galaxies, with the goal of recovering the average HI signal in the stacked sample. This method is particularly useful to enhance the sensitivity of individually non-detected sources, not only in the nearby Universe, but also at large distances where the flux emitted by individual sources falls below the sensitivity limit of current radio telescopes.

We find that the presence of HI is well correlated with the star formation (SF) history. In galaxies where cold gas (HI) is present, conditions are favourable for (residual) SF to be seen. Furthermore, galaxies with more gas have a higher star formation rate.

Young AGN are rich in unsettled HI gas, supporting the hypothesis that nuclear activity in radio galaxies is triggered through feeding of cold gas. Furthermore, the detection of HI in re-activated AGN suggests a link between neutral hydrogen gas and rejuvenation of the nuclear activity. If gas accretion is a periodic event in radio galaxies, we find indications that this effect will leave an imprint both on the AGN and star formation history.
Translated title of the contributionDe rol van neutraal waterstof in de levensloop van sterrenstelsels en AGN: Een analyse met gestapelde spectra
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • University of Groningen
  • Morganti, Raffaella, Supervisor
  • Oosterloo, Thomas, Co-supervisor
Award date26-Sep-2014
Place of Publication[S.l.]
Print ISBNs978-90-367-7268-6
Electronic ISBNs978-90-367-7267-9
Publication statusPublished - 2014

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