engleski

Studies of neutral hydrogen gas in distant galaxies using the spectral stacking technique

Neutral hydrogen gas (𝐻𝐼) is the building block of galaxies as it is a primary fuel for star formation. Direct observations of the 𝐻𝐼 21 cm emission line are restricted by the limited sensitivities of radio telescopes. To overcome this issue, we employ the relatively new technique of spectral stacking to push the redshift limits of radio observations further than otherwise possible. This thesis presents an 𝐻𝐼 spectral stacking analysis of galaxies within the Galaxy and Mass Assembly (GAMA) 9h field and over the redshift range 0.039<𝑧<0.134. 𝐻𝐼 data was collected with the Parkes radio telescope in New South Wales, Australia. Galaxy positions and spectroscopic redshifts are provided by the GAMA survey. By co-adding the spectra of 7049 galaxies extracted from the Parkes data cube, we obtain a strong integrated signal-to-noise ratio of S/N=11.9. The rms noise behaves in a Gaussian manner, with slight deviation caused by the residual radio frequency interference and continuum source emission in the data. We measure average 𝐻𝐼 properties by integrating stacked spectra over ±300 kms−1 from rest frame, the range equivalent to the maximum expected width of the 𝐻𝐼 profile. The resulting average flux density, average 𝐻𝐼 mass and average 𝐻𝐼 mass-to-light ratio are, respectively: 〈𝑆〉=(1.07±0.09)mJy, 〈𝑀𝐻𝐼〉=(2.97±0.38)×109ℎ−2𝑀⊙ and ⟨𝑀𝐻𝐼/𝐿𝑟⟩=(1.58±0.39)𝑀⊙𝐿⊙−1. We test two different strategies in calculating the cosmic 𝐻𝐼 mass density (Ω𝐻𝐼). To account for source confusion effect, we apply the confusion factor 𝒞=8.6, the estimate of the average number of galaxies within one beam width of the Parkes telescope (15.5 arcmin) with redshift ±0.002 (±300 kms−1). Derived values over the observed redshift range are Ω𝐻𝐼=(1.51±0.38)×10−4ℎ−1 and Ω𝐻𝐼=(0.51±0.07)×10−4ℎ−1. The confusion correction is likely too extreme and so our results are a factor of ~3 smaller than other literature values and predictions. In reality, it is unlikely that there is significant evolution of Ω𝐻𝐼 over the past 𝑧~1.67 Gyr.

neutral hydrogen gas, distant galaxies

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