| Abstract |
This thesis presents the activity demographics of an Infrared-selected sample of galaxies in the local Universe. Our goal is to investigate the nuclear activity and the star-forming activity in galaxy-wide and sub-galactic (kpc) physical scales.
First, we investigate how activity classification depends on the proportion of the host galaxy's light that is included in the nuclear spectral extraction aperture. We use both observed long-slit spectra and spectra from simulated elliptical apertures of different sizes for a set of 14 galaxies. We examine how the line ratios used in optical activity classification diagnostics, change and possibly influence the resulting classifications as a larger portion of starlight is gradually incorporated in the extracted aperture. We find that starlight subtraction can mitigate but not remove the effect of host galaxy contamination in the nuclear aperture. Furthermore, galaxies with extranuclear star formation can show higher [O III]λ5007/H/β ratios with increasing aperture, in contrast to the naive expectation that integrated light will only dilute the nuclear emission lines. Both fiber and long slit spectroscopy ascertain that galaxies will have a different placement in the diagnostic diagrams and on certain occasions a different classification as more starlight is included in the aperture of spectral extraction.
We present activity demographics and host-galaxy properties of IR-selected galax¬ies in the local Universe, using the representative Star Formation Reference Survey (SFRS; Ashby et al. 2011). Our classification scheme is based on a combination of the three standard optical emission-line diagrams (BPT) and IR-color diagnostics. We em¬ploy total IR colors to identify obscured active galactic nuclei (AGN), but furthermore we use also nuclear IR colors to account for host-galaxy light contamination. Using the well defined weights of the SFRS galaxies with respect to the parent PSCz sam¬ple, we derive the fractions of different activity types and their respective host-galaxy properties for the parent sample of IR-selected galaxies. We find that the PSCz com¬prises 71% Η II galaxies, 13% Seyferts, 3% Transition Objects, and 13% Low-Ionization Nuclear Emission-Line Regions (LINERs). For the SFRS sample we derive nuclear star-formation rates and gas-phase metallicities for the star-forming galaxies (SFGs = Η II), host-galaxy metallicities for all activity types with available long-slit spectroscopy, and measure abundance gradients for a subset of 12 face-on galaxies. The majority of SFGs show a narrow range of metallicities, close to solar, and flat metallicity profiles. We find that based on their host-galaxy and nuclear properties, the dominant ionizing source in the SFRS (PSCz) transition objects is star-forming activity. LINERs are found mostly in massive hosts with low L(60^m), low dust temperatures, and low LΗα surface densi¬ties, indicating older stellar populations as their main ionizing source rather than AGN activity.
We describe a sub-galactic main sequence (SGMS) relating star-formation rate sur¬face density (ΣSFR) and stellar mass density (Σ*) for distinct regions within star forming galaxies, including their nuclei. We use a sample of 246 nearby star-forming galaxies from the SFRS and demonstrate that the SGMS holds down to ~1 kpc scales with a slope of α = 0.91 and a dispersion of 0.31 dex, similar to the well-known main sequence (MS) measured for globally integrated star formation rates (SFRs) and stellar masses. The SGMS slope depends on galaxy morphology, with late-type galaxies (Sc—Irr) hav¬ing a slope α = 0.97, and early-type spirals (Sa—Sbc) having α = 0.81. The SGMS constructed from sub-regions of individual galaxies has on average the same character¬istics as the composite SGMS from all galaxies. The SGMS for galaxy nuclei shows a dispersion similar to that seen for other sub-regions. Our analysis shows that sampling a limited range of the SFR—M* space results in either sub-linearity or super-linearity of the SGMS slope. For nearly all galaxies, both the SFR and stellar mass peak in the nu¬cleus, indicating that circumnuclear clusters are among the most actively star-forming regions in the galaxy and the most massive. The nuclear SFR also correlates with total galaxy mass, forming a distinct sequence from the standard MS of star-formation. The nuclear main sequence will be useful for studying bulge growth and for characterizing feedback processes connecting AGN and star formation.
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