A [CII] and [NII] study of the interstellar medium in z = 4.5 submillimetre galaxies

Desmet, Pascale and Wardlow, Julie (2024) A [CII] and [NII] study of the interstellar medium in z = 4.5 submillimetre galaxies. PhD thesis, Lancaster University.

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Abstract

Submillimetre galaxies (SMGs) are an enigmatic population, playing host to some of the highest star formation rates (SFRs) seen in the Universe. They are known to be an important population at their peak around z ∼ 2 but have a tail out to redshift > 5 where they are more poorly studied. Far-infrared (IR) emission lines are regularly used to study galaxies such as SMGs, with [CII] 158 μm often being used due to its brightness which enables observations at high redshifts. This has lead to its increased use across a range of galaxy types for studies of the interstellar medium (ISM) of galaxies at high redshift. To investigate the properties of high redshift SMGs, and assess the use of [CII] as a tracer of gas and star formation requires additional complimentary data. The [NII] 205 μm line is the ideal candidate to enable more meaning to be deduced from the [CII] line, as it allows the amount of [CII] originating from different regions of the ISM to be determined. In this thesis we present new ALMA observations of 12 SMGs targeting the [CII] 158 μm and [NII] 205 μm emission lines. The data were taken as follow up to untargeted detections from previous studies of SMGs. We write a custom data reduction process for these observations, using CASA to tailor different data products to the varied science goals presented here. We first process the data to optimise recovered flux, implementing a custom region finding algorithm to create extraction regions that account for the disturbed (i.e. non-elliptical) morphologies that are present in some of the SMGs. We detect [CII] and [NII] in six of the targeted galaxies, and one additional source that is within the field of view of the observations, bringing the total to seven. We use these seven galaxies to investigate the [CII] deficit, a phenomenon whereby galaxies with high IR luminosities have a lower [CII] to far-IR luminosity ratio than expected, based on lower IR luminosity galaxies. There is a moderate deficit in 3–4 of our sources (depending on which relation the deficit is measured from), consistent with other observations of high redshift SMGs. We then assess the amount of [CII] that originates from the ionised ISM vs photodissociation regions (PDRs) using the ratio of the [CII] and [NII] luminosities. The [CII] emission in the z ∼ 4.5 SMGs is mostly originating from PDRs (70–95% from PDRs) in our galaxies. We find a correlation (Spearman’s rank correlation coefficient = 0.64) between the amount of [CII] originating from the ionised ISM and the strength of the [CII] deficit. We use this correlation to calibrate a correction to the observed [CII] luminosity using the [NII] luminosity. This corrects for the effect of the deficit on calculated SFR from the [CII] line. The equation to perform the correction is as follows, [CII]C = B([CII]O2/[N II])a where [CII]C is the corrected [CII] luminosity, [CII]O is the observed [CII] luminosity and [NII] is the observed [NII] 205 μm luminosity. We measure log(B) = 1.43 ± 0.16 and a = −0.701 ± 0.13, and log(B) = 1.49 ± 0.17 and a = −0.719 ± 0.14 for two different calibrations of [CII]-SFR. We next use data that has been reprocessed to higher resolution and perform a study of the gas and dust in the same seven sources. We measure [CII] and Band 7 continuum sizes that are consistent with studies of SMGs at lower redshifts, and find that on average the size of the [CII] emission is 1.47 ± 0.13 times larger than the Band 7 continuum, and the [NII] is 1.38 ± 0.27 times larger than Band 7 continuum, supporting theories of the star formation being more compact than the gas in SMGs. We measure values of gas mass fraction and depletion timescales that are consistent with lower redshift SMGs. A kinematic analysis of the [CII] finds four rotating gas disks in our sources. We conclude that the prevalence of rotating gas disks suggests that the sources are unlikely to be late stage mergers. We provide evidence that z ∼ 4.5 SMGs are consistent with being the progenitors of z ~ 2 massive elliptical galaxies, through their number densities, mass-size relationships and gas depletion timescales.