State of the gas in intense lensed starbursts
George, Richard David
The most intensely star-forming galaxies lie at z ∼ 2 and are thought to be the progenitors of the most massive galaxies today, yet study of this important population has been hampered by vast quantities of dust, making them almost invisible in the optical and ultraviolet (UV) regimes, and by the low sensitivity and angular resolution of many infrared (IR) facilities. Chapter 2 describes the use of the flux and angular extent boost provided by strong gravitational lensing in the detailed study of individual high-redshift dusty star-forming galaxies (DSFGs). The low number density of such systems has been overcome by recent wide area far-infrared (FIR)–mm surveys, and a sample of candidate systems which are bright enough to study with single-dish FIR telescopes are assembled from these surveys. The chapter further describes spectra of these galaxies obtained using the the Spectral and Photometric Imaging REceiver (SPIRE; Griffin et al. 2010) Fourier transform spectrometer (FTS) on board the Herschel Space Observatory (Pilbratt et al. 2010), exploiting the increased flux densities to search for FIR atomic and ionic spectral lines: important coolants of warm gas surrounding star-formation regions. Chapter 3 describes the first “blind” redshift obtained using Herschel, via the detection of [C ii] 158 μm in one of our spectra. Confirmation of this redshift was provided by detection of CO lines with the Combined Array for Research in Millimeter-wave Astronomy (CARMA) and the Plateau de Bure Interferometer (PdBI), and along with multi-wavelength photometric follow-up, allowed a characterisation of the galaxy, indicating both a merger-driven starburst and an active galactic nucleus (AGN) within the system. Chapter 4 describes the first detection of a massive outflow of molecular gas at high-redshift. Stacking five repeat spectra of the Cosmic Eyelash, one of the best-studied strongly lensed DSFGs, one of the massive star-forming clumps is shown to drive this outflow, albeit likely at a velocity lower than that required to become unbound from the hosting gravitational potential well. Chapter 5 describes line measurements and spectral energy distribution (SED) fitting from the full set of spectra and Herschel PACS mini scan maps. The spectra are stacked to search for faint lines, and compared to a detailed interstellar medium (ISM) model to determine average physical properties of the star-forming gas. Photodissociation region (PDR) properties are found to be similar to those derived using other models, however a cosmic ray ionisation rate of 103 times that of the Milky Way, expected in galaxies of this type cannot reproduce the observed line ratios, in particular the low [O i] 63 μm flux. Chapter 6 finally describes the conclusions drawn from the work presented in this thesis and how these data and analysis add to our knowledge and interpretation of high-redshift DSFGs.
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