Dust and gas in local galaxies in the equatorial H-ATLAS fields

One of the main challenges for extragalactic astronomy is to understand how the baryonic components of galaxies evolve from simple clouds of unenriched atomic gas into complex systems consisting of stars, dust, heavy elements and the different gas phases we observe today. This transformation is driven by the ongoing star formation in galaxies, yet there are many other poorly understood interactions between the different constituents that strongly affect the evolution. The physical properties of the galaxy population have been observed to change over time. For example, the stellar mass is built up monotonically, and the star formation rate within galaxies has changed drastically over the past ∼ 10 billion years. The main challenge in galaxy evolution is to procure a more detailed understanding of the various physical and chemical processes responsible for the observed changes in the physical properties of galaxies. Galaxies evolve over cosmic time, and thus much too slow to observe any changes directly. In order to study how the physical properties of a galaxy change, they need to be compared to the physical properties of galaxies at different evolutionary stages. There are two approaches to achieve this. The first one is to study the average change in the galaxy properties with redshift. Many studies (see next sections) have used this approach to study the redshift-evolution of various galaxy properties and these have dramatically changed our understanding of galaxy evolution. However, because of the difficulties of observing very distant objects, this method can only be used to study the evolution out to a given limiting redshift, which is determined by the used wavelength and telescope. Especially for atomic hydrogen (HI) gas, which has a hyperfine emission line at 21 cm, the current generation of telescopes strongly restricts the observations to the relatively nearby Universe. And since the HI gas is a key component in galaxy evolution, it is hard to get a detailed understanding of galaxy evolution beyond the local Universe. One way to extract information for HI beyond the most nearby sources is to use a ‘stacking’ analysis technique. Stacking is the process of combining many low signal-to-noise observations of different individual objects in order to retrieve a high-significance statistical detection (e.g. Delhaize et al., 2013). This technique enables studies of the changes in average galaxy properties out to larger distances (and thus larger lookback times). Part III of this thesis describes the ‘HI-stacking’ analysis of dust-selected sources. The second approach to study galaxy evolution is to investigate the differences in galaxy properties between galaxies at different evolutionary stages, rather than between galaxies at different times. In this context, the evolutionary stage of a galaxy can be defined by its gas fraction, i.e. by how much gas has been converted into stars. So as galaxies evolve, they move from high to low gas fractions and the changes in the physical galaxy properties are studied as a function of gas fraction rather than as a function of time. The rate at which galaxies evolve is determined by their star formation rate, which is in turn dependent on the galaxy’s halo mass and environment. Galaxies span a wide range of halo masses and environments, and hence a correspondingly large range in star formation rates. By the current epoch, some galaxies have converted most of their gas into stars, whereas others still mainly consist of gas. Galaxies in the local Universe span a range of different evolutionary stages due to the differences in their star formation histories. Depending on how the sources are selected, a sample can consist of more evolved or more unevolved sources. In Part I of this thesis, we present a local HI-selected sample and compare to a local stellar mass selected and local dust-selected sample in order to study the changes in galaxy properties over as much of the evolutionary track as possible. Galaxy evolution entails much more than the formation of stars from the available gas. As stars evolve, they synthesise metals (i.e. all elements except hydrogen and helium) in their cores, and subsequently expel them into the interstellar medium (ISM) at the end of the stars’ lives. About half of these metals are locked up in dust grains (Whittet, 1992). This dust absorbs about 30 to 50% of the optical light emitted by galaxies (e.g. Driver et al., 2016; Viaene et al., 2016) and enshrouds some of the most interesting environments in these galaxies. It is therefore difficult to develop a thorough understanding of galaxy evolution without also understanding how dust affects the observations. In Part I of this thesis we will put additional focus on the dust content of galaxies selected by their HI, dust and stellar content. In Part II, we determine the metal content of the same galaxies and study how dust is formed and destroyed by comparing dust and chemical evolution models (that predict the build-up of dust, gas and metals) with observed galaxy properties. This thesis describes three distinct, but closely related, research projects I conducted during the course of my PhD, all dealing with cosmic dust and HI gas in the context of galaxy evolution. This introductory chapter briefly summarises the current theoretical and observational framework of galaxy evolution, with a focus on cosmic dust and HI gas. Chapter 2 explains how we have selected the HIGH sample (HI-selected Galaxies in H-ATLAS) and dealt with observational issues. Chapter 3 describes the pipeline that was developed to perform the photometry and the SED fitting code that was used to determine the galaxy properties. Chapter 4 details the scaling relations between the different galaxy properties and how the galaxy properties of HIGH compare to a stellar selected and dust-selected sample. Chapter 5 explains how metallicities have been derived using fibre optical spectroscopy. In Chapter 6, we study dust sources and sinks by comparing models of the build-up of dust, gas and metals with the observed properties of galaxies. Chapter 7 presents the HI-stacking methods and preliminary results. Finally, in Chapter 8 we summarise our conclusions and describe potential future work

Do bulges stop stars forming?

In this paper, we use the Herschel Reference Survey to make a direct test of the hypothesis that the growth of a stellar bulge leads to a reduction in the star-formation efficiency of a galaxy (or conversely a growth in the gas-depletion timescale) as a result of the stabilisation of the gaseous disk by the gravitational field of the bulge. We find a strong correlation between star-formation efficiency and specific star-formation rate in galaxies without prominent bulges and in galaxies of the same morphological type, showing that there must be some other process besides the growth of a bulge that reduces the star-formation efficiency in galaxies. However, we also find that galaxies with more prominent bulges (Hubble types E to Sab) do have significantly lower star-formation efficiencies than galaxies with later morphological types, which is at least consistent with the hypothesis that the growth of a bulge leads to the reduction in the star-formation efficiency. The answer to the question in the title is therefore, yes and no: bulges may reduce the star-formation efficiency in galaxies but there must also be some other process at work. We also find that there is a significant but small difference in the star-formation efficiencies of galaxies with and without bars, in the sense that galaxies with bars have slightly higher star-formation efficiencies.Comment: Accepted for publication in MNRA

The Galaxy end sequence

A common assumption is that galaxies fall in two distinct regions of a plot of specific star formation rate (SSFR) versus galaxy stellar mass: a star-forming galaxy main sequence (GMS) and a separate region of ‘passive’ or ‘red and dead galaxies’. Starting from a volume-limited sample of nearby galaxies designed to contain most of the stellar mass in this volume, and thus representing the end-point of ≃12 billion years of galaxy evolution, we investigate the distribution of galaxies in this diagram today. We show that galaxies follow a strongly curved extended GMS with a steep negative slope at high galaxy stellar masses. There is a gradual change in the morphologies of the galaxies along this distribution, but there is no clear break between early-type and late-type galaxies. Examining the other evidence that there are two distinct populations, we argue that the ‘red sequence’ is the result of the colours of galaxies changing very little below a critical value of the SSFR, rather than implying a distinct population of galaxies. Herschel observations, which show at least half of early-type galaxies contain a cool interstellar medium, also imply continuity between early-type and late-type galaxies. This picture of a unitary population of galaxies requires more gradual evolutionary processes than the rapid quenching process needed to explain two distinct populations. We challenge theorists to predict quantitatively the properties of this ‘Galaxy End Sequence’

FDR4ATMOS (Task A): Improving SCIAMACHY Level 1 and add calibrated lunar data

The project FDR4ATMOS (Fundamental Data Records in the domain of satellite Atmospheric Composition) has been initiated by the European Space Agency (ESA). Task A of the project covers the improvement of the SCIAMACHY Level 1b degradation correction, with the aim to remove ozone trends from the SCIAMACHY Level 2 data set that were introduced during the development of baseline version 9 (both data sets not released). We will also, for the first time, add calibrated lunar data to Level 1, covering the whole spectral range of SCIAMACHY and the full mission time. The SCIAMACHY processing chain for better Ozone total column data: After the full re-processing of the SCIAMACHY mission with the updated processor versions, the validation showed that the total Ozone column drifted downward by nearly 2% over the mission lifetime. This drift is likely caused by changes in the degradation correction in the Level 1 processor, that led to subtle changes in the spectral structures. These are misinterpreted as an atmospheric signature. We updated the Level 0-1 processor accordingly and a full mission re-processing was done. As a major improvement we additionally incorporated calibrated lunar data in the SCIAMACHY Level 1b product. In the new Level 1b product we will provide the individual scans of the moon as well as disk integrated and calibrated lunar irradiance and reflectance. The instrument performed regular lunar observations building up a unique 10 year data set of lunar spectra from the UV to the SWIR with moderately high spectral resolution. SCIAMACHY scanned the full lunar disk and over the ten year mission time made 1123 observations of the moon. Most satellites can only observe the moon under very specific geometries due to instrument-viewing and orbit restrictions. SCIAMACHY, however, with a two mirror pointing system was much less constrained and was able to observe the moon under an extreme large variation of geometries (especially during dedicated lunar observation campaigns), allowing it thus potentially to tie different satellites and geometry observations together. During the individual lunar observations, SCIAMACHY only saw a small slice of the Moon and scanned over the moon in order to obtain data for the full disk. We combined the individual calibrated scans, correcting for scan speed and the fact the Moon does not fill the entire slit length. The calculation of distance-normalized lunar reflectances did not require an external solar spectrum, but used solar measurements of SCIAMACHY itself. This version of Level 1 will also be the first one that replaces the ENVISAT byte stream format with the netCDF format that is aligned with the product format of other atmospheric sensors like the Sentinels The paper will present the improvements of the Level 1 product, the results of the quality control and validation

The causes of the red sequence, the blue cloud, the green valley, and the green mountain

The galaxies found in optical surveys fall in two distinct regions of a diagram of optical colour versus absolute magnitude: the red sequence and the blue cloud with the green valley in between. We show that the galaxies found in a submillimetre survey have almost the opposite distribution in this diagram, forming a `green mountain'. We show that these distinctive distributions follow naturally from a single, continuous, curved Galaxy Sequence in a diagram of specific star-formation rate versus stellar mass without there being the need for a separate star-forming galaxy Main Sequence and region of passive galaxies. The cause of the red sequence and the blue cloud is the geometric mapping between stellar mass/specific star-formation rate and absolute magnitude/colour, which distorts a continuous Galaxy Sequence in the diagram of intrinsic properties into a bimodal distribution in the diagram of observed properties. The cause of the green mountain is Malmquist bias in the submillimetre waveband, with submillimetre surveys tending to select galaxies on the curve of the Galaxy Sequence, which have the highest ratios of submillimetre-to-optical luminosity. This effect, working in reverse, causes galaxies on the curve of the Galaxy Sequence to be underrepresented in optical samples, deepening the green valley. The green valley is therefore not evidence (1) for there being two distinct populations of galaxies, (2) for galaxies in this region evolving more quickly than galaxies in the blue cloud and the red sequence, (c) for rapid quenching processes in the galaxy population

The causes of the red sequence, the blue cloud, the green valley, and the green mountain

The galaxies found in optical surveys fall in two distinct regions of a diagram of optical colour versus absolute magnitude: the red sequence and the blue cloud, with the green valley in between. We show that the galaxies found in a submillimetre survey have almost the opposite distribution in this diagram, forming a \u27green mountain\u27. We show that these distinctive distributions follow naturally from a single, continuous, curved Galaxy Sequence in a diagram of specific star formation rate versus stellar mass, without there being the need for a separate star-forming galaxy main sequence and region of passive galaxies. The cause of the red sequence and the blue cloud is the geometric mapping between stellar mass/specific star formation rate and absolute magnitude/colour, which distorts a continuous Galaxy Sequence in the diagram of intrinsic properties into a bimodal distribution in the diagram of observed properties. The cause of the green mountain isMalmquist bias in the submillimetre waveband, with submillimetre surveys tending to select galaxies on the curve of the Galaxy Sequence, which have the highest ratios of submillimetre-to-optical luminosity. This effect, working in reverse, causes galaxies on the curve of the Galaxy Sequence to be underrepresented in optical samples, deepening the green valley. The green valley is therefore not evidence (1) for there being two distinct populations of galaxies, (2) for galaxies in this region evolving more quickly than galaxies in the blue cloud and the red sequence, and (3) for rapid-quenching processes in the galaxy population

Reproducing the Universe: a comparison between the EAGLE simulations and the nearby DustPedia galaxy sample

We compare the spectral energy distributions (SEDs) and inferred physical properties for simulated and observed galaxies at low redshift. We exploit UV-submillimetre mock fluxes of ~7000 z=0 galaxies from the EAGLE suite of cosmological simulations, derived using the radiative transfer code SKIRT. We compare these to ~800 observed galaxies in the UV-submillimetre range, from the DustPedia sample of nearby galaxies. To derive global properties, we apply the SED fitting code CIGALE consistently to both data sets, using the same set of ~80 million models. The results of this comparison reveal overall agreement between the simulations and observations, both in the SEDs and in the derived physical properties, with a number of discrepancies. The optical and far-infrared regimes, and the scaling relations based upon the global emission, diffuse dust and stellar mass, show high levels of agreement. However, the mid-infrared fluxes of the EAGLE galaxies are overestimated while the far-UV domain is not attenuated enough, compared to the observations. We attribute these discrepancies to a combination of galaxy population differences between the samples, and limitations in the subgrid treatment of star-forming regions in the EAGLE-SKIRT post-processing recipe. Our findings show the importance of detailed radiative transfer calculations and consistent comparison, and provide suggestions for improved numerical models.Comment: 17 pages, 14 figures, accepted for publication in MNRA