Optical and near infrared properties of massive galaxies

  • Tim D. Higgs

Student thesis: Doctoral Thesis


In this thesis, we present a comparison of the evolution of the massive galaxies in the 7.8Gyr since redshift z=1 to the evolution predicted from galaxy formation models.

Observing the most massive galaxies in the Universe at high redshift is challenging due to their red colours, owing to both their intrinsically red Spectral Energy Distributions (SEDs) and their redshift. In Chapter 1, We produce a method using catalogue-level datato produce matched aperture photometry for the SDSS and UKIDSS surveys in order to extend the wavelength coverage of a sample of galaxies in order to improve the precision with which models can be fitted to photometric data for these high redshift galaxies.

Our matched photometry has consistent colours with those of the full processing of SDSS+UKIDSS images performed by the GAMA survey, and produces magnitudes within ∼0.1 magnitudes of the GAMA photometry for all galaxies. This is reducedto within 0.04 magnitudes when all blended sources are excluded. We compute stellar masses by fitting a Maraston et al. (2009) LRG model to both our derived photometry and that of the GAMA processing, and find that our photometry’s best fit stellar masses are within ∼0.2 dex of that which comes from the GAMA photometry, demonstrating that the method is consistent with that of a full processing, and that it is possible to quickly compute matched photometry for large area surveys of complimentary wavelength coverage.This is of vital importance for upcoming surveys eg. DES, VISTA, EUCLID etc.

Fitting Stellar Population Models to galaxy photometry is a widely used technique in order to convert from observables (colours, magnitudes) to physical properties (mass,absolute magnitude, age). In spite of their widespread use, the optical and Near Infrared(NIR) properties of stellar population models are still subject to debate. Two of the most commonly used models are those of (Maraston, 2005) (M05) and (Bruzual & Charlot,2003) (BC03), which can differ greatly in the NIR due to the M05 models’ inclusion of the TP-AGB phase, which was neglected for BC03 models. We explore the ability of these models to reproduce measured optical+NIR properties of galaxies in Chapter 3.

We produce matched optical+NIR photometry for the sub sample of the galaxies surveyed by Zibetti et al. (2013) (Z13) which lie within the UKIDSS imaging area in an attempt to reproduce the findings of Z13, who conclude that their optical and NIR spectroscopy is better fit by models from Bruzual & Charlot (2003) than similar models from Maraston et al (2005). We compare the observed optical+NIR Spectral Energy Distributions(SEDs) to those of BC03 and M05 models, as well as the approximate Z13 NIR fluxes. Z13 found that M05 models fitted to the optical data and extrapolated into the NIR displayed excess flux in the NIR relative to the data, and BC03 models are better at reproducing the data. However, we show that our data is consistent with both sets of models, and on average brighter in the NIR than that of Z13. We also compare the strength of spectral features in the optical to rest frame optical and optical-NIR colours, and show that our set of Composite Stellar Population (CSP) models agree well with data, with a preference for the M05 models, showing the validity of using these models on massive galaxies.

A measurement of the Stellar Mass Function (SMF) of galaxies is a powerful tool in detecting evolution of the galaxy population. With a statistically complete sample of a galaxy population down to a given stellar mass, it is possible to calculate a statistically complete SMF down to this mass. Comparison of the shape of this SMF to that of a similar sample over a different redshift interval allows the evolution of galaxies over this redshift interval to be calculated, in order to determine whether these galaxies are forming stars, merging or simply passively evolving.

For this purpose, in 4 compute matched SDSS+UKIDSS photometry for the AAomega UKIDSS SDSS (AUS) survey. This is a 145.416 deg2 area survey of Luminous Red Galaxies (LRGs) from redshift z∼0.5 to z∼1 located within Stripe 82. We fit this photometryto a Maraston et al. (2009) Luminous Red Galaxy (LRG) template to give stellar masses, and scale masses according to the magnitude difference between the matched photometry and the SDSS model photometry in order to produce “total” stellar masses.We produce a volume-weighted SMF for the survey, and find that our SMF is consistent with the Maraston et al. (2013) SMF from the BOSS survey, meaning that the most massive galaxies in the universe are evolving passively from z=1 to the present day, which is a challenge to hierarchical models of galaxy formation.

Comparison of observed SMFs to those produced by galaxy formation models is a method of testing the ability of the models to reproduce the evolution displayed by the real galaxy population. This is therefore a test of the physics included within the models,with the level of agreement between the simulation and the real galaxy SMF being indicative of whether the modelling has incorporated all the processes in action in the real universe. In order to test the ability of the state of the art semi analytical models of Henriques et al. (2013) (H13 hereafter), we compare SMFs of the simulated galaxies to those of the AUS and BOSS surveys in Chapter 5. The H13 galaxies were tailored via the application of both the AUS and BOSS colour and magnitude cuts, and SMFs calculated within light cones of the same area as the surveys in order to compare equal volumes.

Our findings extend the conclusions of Maraston et al. (2013), namely that the most massive galaxies in the simulations are not sufficiently massive to agree with the observed galaxy population at this redshift. By extending this analysis to redshift z∼1, we can confirm that the discrepancy is larger at higher redshift, with the difference between the most massive galaxies in the simulations and those observed being log(M/M⊙) ≃0.2at z≃0.6–0.7, whereas going beyond this to the range z≃0.7–1 the difference becomes log(M/M⊙) ≃0.25, as can be seen in Figure 5.6, which demonstrates that the simulations are failing to either form, or assemble, the mass quickly enough to reproduce the observations. Instead, the simulations continue to assemble mass through to low redshiftat a higher rate than is seen in the galaxy SMF. These discrepancies may indicate that the physics of the simulations is not fully accounting for the real processes in the Universe,and that we do not yet have a model capable of reproducing the galaxy population in the real universe. Clearly semi analytical galaxy simulations need to be modified in order to reproduce the observations, before being further challenged by upcoming spectroscopic surveys of galaxies at redshifts as high as z=2 eg. eBoss, DESI.
Date of AwardAug 2014
Original languageEnglish
Awarding Institution
  • University of Portsmouth
SupervisorBob Nichol (Supervisor), Claudia Maraston (Supervisor) & Will Percival (Supervisor)

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