The Luminous Polycyclic Aromatic Hydrocarbon Emission Features: Applications to High Redshift Galaxies and Active Galactic Nuclei
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The co-evolution of star-formation and supermassive black hole (SMBH) accretion in galaxies is one of the key problems in galaxy formation theory. Understanding the formation of galaxies, and their subsequent evolution, will be coupled to intensive study of the evolution of SMBHs. This thesis focuses on studying diagnostics of star-formation and SMBH accretion to develop tools to study this co-evolution. Chapter 2 consists of using mid-infrared (mid-IR) spectroscopy from the Spitzer Infrared Spectrograph (IRS) to study the nature of star-formation and SMBH accretion. The mid-IR spectra cover wavelengths 5-38µm, spanning the polycyclic aromatic hydrocarbon (PAH) features and important atomic diagnostic lines. We divide our sample into a subsample of galaxies with Spitzer IRAC colors indicative of warm dust heated by an AGN (IRAGN) and those galaxies whose colors indicate star-formation processes (non-IRAGN). In both the IRAGN and star-forming samples, the luminosity in the PAH features correlates strongly with [Ne II]λ12.8µm emission line, from which we conclude that the PAH luminosity directly traces the instantaneous star-formation rate (SFR) in both the IRAGN and star-forming galaxies. There is no measurable difference between the PAH luminosity ratios of L11:3/L7:7 and L6:2/L7:7 for the IRAGN and non-IRAGN, suggesting that AGN do not significantly excite or destroy PAH molecules on galaxy-wide scales. In chapter 3, I calibrate the PAH luminosity as a SFR indicator. We provide a new robust SFR calibration using the luminosity emitted from PAH molecules at 6.2µm, 7.7µm and 11.3µm. The PAH features emit strongly in the mid-IR mitigating dust extinction, containing on average 5 – 10% of the total IR luminosity in galaxies. We use mid-IR spectroscopy from the Spitzer/IRS, and data covering other SFR indicators (Hα emission and rest-frame 24µm continuum emission). The PAH luminosity correlates linearly with the SFR as measured by the Hα luminosity (corrected for attenuation using the mono-chromatic rest-frame 24um emission), with a tight scatter of <0.15 dex. The scatter is comparable to that between SFRs derived from the Paα and dust-corrected Hα emission lines, implying the PAH features may be as accurate a SFR indicator as the Hydrogen recombination lines. Because the PAH features are so bright, our PAH SFR calibration enables an efficient way to measure SFRs in distant galaxies with JWST to SFRs as low as ~10 M⨀ yr^-1 to z <~ 2. We use Spitzer/IRS observations of PAH features in lensed star-forming galaxies at 1 < z < 3 to demonstrate the utility of the PAHs to derive SFRs as accurate as those available from Paα. Chapter 4 is the application of the PAH SFRs for galaxies with AGN to demonstrate the reliability for studies of the co-evolution of star-formation and SMBH accretion. We present a study of the contribution from star-formation in galaxies of varying AGN activity (from pure star-forming galaxies to quasars) as a function of total IR luminosity using a sample of 220 galaxies. We use mid-IR spectroscopy from the Spitzer/IRS and photometry from the MIPS 24µm, 70µm and 160µm bands with partial coverage of the sample with the Herschel 160µm band for the quasars. The contribution from star-formation to the total IR luminosity implied by the PAH emission decreases with increasing IR luminosity. We find a similar result to previous studies for the correlation between SFR, i.e. PAH luminosity, and AGN luminosity for quasars of LSF ∞ L0:67±0:10/AGN and LSF ∞ L0:55±0:15/AGN for the 11.3µm PAH feature only (which has been shown to be the most reliable PAH feature in the vicinity of AGN). This may indicate the PAH luminosity remains a reliable tracer of the SFR for galaxies with strong AGN contributions (i.e. quasars), as we did not subtract off the AGN component before measuring the SFR from the PAH luminosity.
Shipley, Heath (2015). The Luminous Polycyclic Aromatic Hydrocarbon Emission Features: Applications to High Redshift Galaxies and Active Galactic Nuclei. Doctoral dissertation, Texas A & M University. Available electronically from
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