A [CII] and [NII] study of the interstellar medium in z = 4.5 submillimetre galaxies
Creators: name: Desmet, Pascale
Publisher: name: Lancaster University
Year: 2024
Subjects: galaxies
galaxies: evolution
galaxies: high-redshift
ISM: general
submillimeter: galaxies
submillimeter: ISM
Submillimeter astronomy
SUBMILLIMETER-SELECTED GALAXIES

description: 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 &gt; 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]<sub>C</sub> = B([CII]O<sup>2</sup>/[N
 II])<sup>a</sup>
 
 where [CII]<sub>C</sub> is the corrected [CII] luminosity,
 [CII]<sup>O</sup> 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.