Shedding light on the lives of star forming galaxies

This thesis examines two separate aspects related to the evolution of star forming galaxies using spectroscopic and photometric measurements taken from large area sky surveys. The first part of the thesis focuses on the local evolution of star formation rate density, and the second part explores the universality of the stellar initial mass function (IMF). The first part of this thesis is motivated by the published measurements of the low z H luminosity (LH) functions,, that show a large dispersion in the local number density of sources (0.5 1 Mpc-3dex-1), and correspondingly in the SFR density. The possible causes for these discrepancies include limited volume sampling, biases arising from survey sample selection, and different methods of correcting for dust obscuration and AGN contamination. The Galaxy And Mass Assembly (GAMA) survey and Sloan Digital Sky Survey (SDSS) provide deep spectroscopic observations over a wide sky area enabling detection of a large sample of star-forming galaxies spanning 0.001 SFR H (solar masses per year) 100 with which to robustly measure the evolution of the SFR density in the lowz universe. The large number of high SFR galaxies present in our sample allows an improved measurement of the bright end of the luminosity function, indicating that the decrease in at bright luminosities is best described by a Saunders functional form rather than the traditional Schechter function. This result is consistent with other published luminosity functions in the FIR and radio. For GAMA and SDSS we find the rband apparent magnitude limit, combined with the subsequent requirement for H detection leads to an incompleteness due to missing bright H sources with faint rband magnitudes. To correct for this incompleteness, we use the lowest redshift LHabsolute (rband) magnitude and LHstellar mass distributions as a reference to model the higherz bivariate luminosity functions, thereby approximating the contribution from the missing optically faint star forming galaxies to the local star formation rate and stellar mass densities. The second part of this thesis is motivated by the recent extragalactic studies indicating variations in the stellar IMF. The IMF describes the distribution in stellar masses produced from a burst of star formation. For more than fifty years, the implicit assumption underpinning most areas of research involving the IMF has been that it is universal, regardless of time and environment. We measure the highmass IMF slope for a sample of lowtomoderate redshift galaxies from the GAMA survey. The large range in luminosities and galaxy masses of the sample permits the exploration of underlying IMF dependencies. A strong IMFstar formation rate dependency is discovered, which shows that highly star forming galaxies form proportionally more massive stars (they have IMFs with flatter powerlaw slopes) than galaxies with low star formation rates. This has a significant impact on a wide variety of galaxy evolution studies, all of which rely on assumptions about the slope of the IMF. Our result is supported by, and provides an explanation for, the results of numerous recent explorations suggesting a variation of or evolution in the IMF

Shedding light on the lives of star forming galaxies

This thesis examines two separate aspects related to the evolution of star forming galaxies using spectroscopic and photometric measurements taken from large area sky surveys. The first part of the thesis focuses on the local evolution of star formation rate density, and the second part explores the universality of the stellar initial mass function (IMF). The first part of this thesis is motivated by the published measurements of the low z H luminosity (LH) functions,, that show a large dispersion in the local number density of sources (0.5 1 Mpc-3dex-1), and correspondingly in the SFR density. The possible causes for these discrepancies include limited volume sampling, biases arising from survey sample selection, and different methods of correcting for dust obscuration and AGN contamination. The Galaxy And Mass Assembly (GAMA) survey and Sloan Digital Sky Survey (SDSS) provide deep spectroscopic observations over a wide sky area enabling detection of a large sample of star-forming galaxies spanning 0.001 SFR H (solar masses per year) 100 with which to robustly measure the evolution of the SFR density in the lowz universe. The large number of high SFR galaxies present in our sample allows an improved measurement of the bright end of the luminosity function, indicating that the decrease in at bright luminosities is best described by a Saunders functional form rather than the traditional Schechter function. This result is consistent with other published luminosity functions in the FIR and radio. For GAMA and SDSS we find the rband apparent magnitude limit, combined with the subsequent requirement for H detection leads to an incompleteness due to missing bright H sources with faint rband magnitudes. To correct for this incompleteness, we use the lowest redshift LHabsolute (rband) magnitude and LHstellar mass distributions as a reference to model the higherz bivariate luminosity functions, thereby approximating the contribution from the missing optically faint star forming galaxies to the local star formation rate and stellar mass densities. The second part of this thesis is motivated by the recent extragalactic studies indicating variations in the stellar IMF. The IMF describes the distribution in stellar masses produced from a burst of star formation. For more than fifty years, the implicit assumption underpinning most areas of research involving the IMF has been that it is universal, regardless of time and environment. We measure the highmass IMF slope for a sample of lowtomoderate redshift galaxies from the GAMA survey. The large range in luminosities and galaxy masses of the sample permits the exploration of underlying IMF dependencies. A strong IMFstar formation rate dependency is discovered, which shows that highly star forming galaxies form proportionally more massive stars (they have IMFs with flatter powerlaw slopes) than galaxies with low star formation rates. This has a significant impact on a wide variety of galaxy evolution studies, all of which rely on assumptions about the slope of the IMF. Our result is supported by, and provides an explanation for, the results of numerous recent explorations suggesting a variation of or evolution in the IMF

Metal-THINGS: On the metallicity and ionization of ULX sources in NGC 925

We present an analysis of the optical properties of three Ultra Luminous X-ray (ULX) sources identified in NGC 925. We use Integral field unit data from the George Mitchel spectrograph in the context of the Metal-THINGS survey. The optical properties for ULX-1 and ULX-3 are presented, while the spaxel associated with ULX-2 had a low S/N, which prevented its analysis. We also report the kinematics and dimensions of the optical nebula associated with each ULX using ancillary data from the PUMA Fabry-Perot spectrograph. A BPT analysis demonstrates that most spaxels in NGC 925 are dominated by star-forming regions, including those associated with ULX-1 and ULX-3. Using the resolved gas-phase metallicities, a negative metallicity gradient is found, consistent with previous results for spiral galaxies, while the ionization parameter tends to increase radially throughout the galaxy. Interestingly, ULX-1 shows a very low gas metallicity for its galactocentric distance, identified by two independent methods, while exhibiting a typical ionization. We find that such low gas metallicity is best explained in the context of the high-mass X-ray binary population, where the low-metallicity environment favours active Roche lobe overflows that can drive much higher accretion rates. An alternative scenario invoking accretion of a low-mass galaxy is not supported by the data in this region. Finally, ULX-3 shows both a high metallicity and ionization parameter, which is consistent with the progenitor being a highly-accreting neutron star within an evolved stellar population region.Comment: Accepted by Ap

Galaxy and mass assembly (GAMA): A deeper view of the mass, metallicity and SFR relationships

A full appreciation of the role played by gasmetallicity (Z), star formation rate (SFR) and stellar mass (M*) is fundamental to understanding how galaxies form and evolve. The connections between these three parameters at different redshifts significantl

Galaxy and mass assembly: Resolving the role of environment in galaxy evolution

We present observations of 18 galaxies from the Galaxy And Mass Assembly (GAMA) survey made with the SPIRAL optical integral field unit (IFU) on the Anglo-Australian Telescope. The galaxies are selected to have a narrow range in stellar mass (6 × 109

Galaxy and Mass Assembly (GAMA): active galactic nuclei in pairs of galaxies

There exist conflicting observations on whether or not the environment of broad- and narrowline active galatic nuclei (AGN) differ and this consequently questions the validity of the AGN unification model. The high spectroscopic completeness of the Galaxy and Mass Assembly (GAMA) survey makes it ideal for a comprehensive analysis of the close environment of galaxies. To exploit this, and conduct a comparative analysis of the environment of broad- and narrow-line AGN within GAMA, we use a double-Gaussian emission line fitting method to model the more complex line profiles associated with broad-line AGN. We select 209 type 1 (i.e. unobscured), 464 type 1.5–1.9 (partially obscured), and 281 type 2 (obscured) AGN within the GAMA II data base. Comparing the fractions of these with neighbouring galaxies out to a pair separation of 350 kpc h−1 and Δz < 0.012 shows no difference between AGN of different type, except at separations less than 20 kpc h−1 where our observations suggest an excess of type 2 AGN in close pairs. We analyse the properties of the galaxies neighbouring our AGN and find no significant differences in colour or the star formation activity of these galaxies. Further to this, we find that Σ5 is also consistent between broad- and narrow-line AGN. We conclude that the observations presented here are consistent with AGN unification

Galaxy And Mass Assembly (GAMA): Linking star formation histories and stellar mass growth

WWe present evidence for stochastic star formation histories in low-mass (M* <1010M⊙) galaxies from observations within the Galaxy And Mass Assembly (GAMA) survey. For ̃73 000 galaxies between 0.05 < z < 0.32, we calculate star formation rate

Galaxy And Mass Assembly (GAMA): stellar mass estimates

This paper describes the first catalogue of photometrically derived stellar mass estimates for intermediate-redshift (z < 0.65; median z= 0.2) galaxies in the Galaxy And Mass Assembly (GAMA) spectroscopic redshift survey. These masses, as well as the full set of ancillary stellar population parameters, will be made public as part of GAMA data release 2. Although the GAMA database does include near-infrared (NIR) photometry, we show that the quality of our stellar population synthesis fits is significantly poorer when these NIR data are included. Further, for a large fraction of galaxies, the stellar population parameters inferred from the optical-plus-NIR photometry are formally inconsistent with those inferred from the optical data alone. This may indicate problems in our stellar population library, or NIR data issues, or both; these issues will be addressed for future versions of the catalogue. For now, we have chosen to base our stellar mass estimates on optical photometry only. In light of our decision to ignore the available NIR data, we examine how well stellar mass can be constrained based on optical data alone. We use generic properties of stellar population synthesis models to demonstrate that restframe colour alone is in principle a very good estimator of stellar mass-to-light ratio, M*/Li. Further, we use the observed relation between restframe (g−i) and M*/Li for real GAMA galaxies to argue that, modulo uncertainties in the stellar evolution models themselves, (g−i) colour can in practice be used to estimate M*/Li to an accuracy of ≲0.1 dex (1σ). This ‘empirically calibrated' (g−i)-M*/Li relation offers a simple and transparent means for estimating galaxies' stellar masses based on minimal data, and so provides a solid basis for other surveys to compare their results to z≲0.4 measurements from GAM