Browsing by Author "Maddox, Natasha"
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Item Comparison of Hi and optical redshifts of galaxies - The impact of redshift uncertainties on spectral line stacking(Oxford University Press, 2013) Maddox, Natasha; Hess, Kelley M.; Blyth, S.-L.; Jarvis, MattAccurate optical redshifts will be critical for spectral co-adding techniques used to extract detections from below the noise level in ongoing and upcoming surveys for Hi, which will extend our current understanding of gas reservoirs in galaxies to lower column densities and higher redshifts. We have used existing, high quality optical and radio data from the SDSS and ALFALFA surveys to investigate the relationship be- tween redshifts derived from optical spectroscopy and neutral hydrogen (Hi) spectral line observations.We find that the two redshift measurements agree well, with a negli- gible systematic offset and a small distribution width. Employing simple simulations, we determine how the width of an ideal stacked Hi profile depends on these redshift offsets, as well as larger redshift errors more appropriate for high redshift galaxy sur- veys. The width of the stacked profile is dominated by the width distribution of the input individual profiles when the redshift errors are less than the median width of the input profiles, and only when the redshift errors become large, 150 kms−1, do they significantly affect the width of the stacked profile. This redshift accuracy can be achieved with moderate resolution optical spectra. We provide guidelines for the number of spectra required for stacking to reach a specified mass sensitivity, given tele- scope and survey parameters, which will be useful for planning optical spectroscopy observing campaigns to supplement the radio data.Item The star formation rates of QSOs(Royal Astronomical Society, 2022) Symeonidis, Myrto; Maddox, Natasha; Jarvis, Matthew J.We examine the far-IR properties of a sample of 5391 optically selected QSOs in the 0.5 < z < 2.65 redshift range down to log [νLν,2500(erg/s)] > 44.7, using SPIRE data from Herschel-ATLAS. We split the sample in a grid of 74 luminosity-redshift bins and compute the average optical–infrared spectral energy distribution (SED) in each bin. By normalising an intrinsic AGN template to the AGN optical power (at 5100˚A) we decompose the total infrared emission (LIR; 8—1000µm) into an AGN (LIR,AGN) and star-forming component (LIR,SF). We find that the AGN contribution to LIR increases as a function of AGN power which manifests as a reduction of the ‘far-IR bump’ in the average QSO SEDs. We note that LIR,SF does not correlate with AGN power; the mean star formation rates (SFRs) of AGN host galaxies are a function of redshift only and they range from ∼6 M⊙/yr at z ∼ 0 to a plateau of . 200 M⊙/yr at z ∼ 2.6.