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In my current PhD work I have initially focused on scanning for [C II] in z ∼ 7 rest-frame UV selected galaxies in the Epoch of Reionisation (the first Billion years of cosmic time). Galaxies that form in this epoch are some of the first to form in the Universe and are therefore very important for our understanding of galaxy formation and evolution. This work has led to the spectroscopic confirmation of the galaxy A1703-zD1, at z=6.827 (Molyneux+22).
Galaxies first form in the so-called Epoch of Reionisation (the first billion years of cosmic time: z > 6.5), and understanding the physical properties of these primordial systems is fundamental to understanding galaxy evolution, as they represent the initial building blocks of the modern day galaxy population. We currently have a very limited understanding of the physical properties of these very high-redshift galaxies and a major challenge in studying these faint sources is that very few of these galaxies at z > 6.5 have spectroscopic redshift confirmations. The strategy for studying these objects in sub-millimetre wavelengths is to therefore first efficiently scan for the bright [C II]158 line, to obtain a precision measurement of the redshift. This measurement then opens the door to further physical characterisation, by follow-up observations of other fine structure lines such as [O III]88 to study the highly ionised gas in these systems, as well as taking deeper observations of [C II] to understand their size, structure and dynamics.
Illustration to show the key stages in the history of the Universe. Credit: NAOJ
In keeping with this step-wise process during my PhD, I have initially focused on scanning for [C II] in z ∼ 7 rest-frame UV selected galaxies, often known as Lyman Break Galaxies, in the Epoch of Reionisation. The target galaxies have been studied using the NOEMA interferometer, the first time it has been successfully used in studying "normal" galaxies at such high redshifts.
My work has led to the spectroscopic confirmation of the brightest known gravitationally lensed Lyman Break Galaxy in the Epoch of Reionisation, A1703-zD1, at a redshift of z=6.8269 +/- 0.0004. This source was selected behind the strong lensing cluster Abell 1703, with an intrinsic UV luminosity ∼ L∗z=7 luminosity and a very blue Spitzer/IRAC [3.6]–[4.5] colour, implying high equivalent width line emission of [O III]+Hβ. [C II] is reliably detected at 6.1sigma co-spatial with the rest-frame UV counterpart, showing similar spatial extent. Correcting for the lensing magnification, the [C ii] luminosity in A1703-zD1 is broadly consistent with the local L[CII] – SFR relation. We find a clear velocity gradient of 103 ± 22 km/s across the source which possibly indicates rotation or an ongoing merger.
Over the last few years ALMA has demonstrated its role as a “redshift machine” in the Epoch of Reionisation by confirming galaxies out to redshift z = 9. In this work we have demonstrated the ability of NOEMA to search for [C II] in ‘normal’ star-forming galaxies at z > 6, complementing ALMA by observing Epoch of Reionisation galaxies in the Northern Hemisphere, with [C II] as a reliable spectroscopic tracer of these distant systems. With the launch of JWST this capability will be particularly useful for rare, lensed sources and intrinsically luminous objects that will be discovered far outside the limited JWST survey area using the next generation of large area surveys, such as the Euclid mission and the Rubin observatory.
Top Panel: The top left panel shows the NOEMA cube collapsed over the frequency range 242.76-242.86GHz. The top right panel shows zoom-in HST imaging (grey-scale image) smoothed with a Gaussian filter equivalent to the psf size, while overlaid are the 2, 3, 4, and 5 sigma contours of the NOEMA narrowband (red contours). The filled ellipse in the bottom right corner indicates the beam size (1.57 × 1.31 arcsec).
Bottom Panel: The bottom panel shows the exact same as the top panel, but using the tapered datacube (tapering described in Section 2.1). The scale of the images are identical as well as the frequency range used to create the narrowband. Here, we also have a 6 sigma contour plotted in the right panel and the filled ellipse in the bottom right corner indicates the tapered beam size (2.9 x 1.47 arcsec).
Bottom Panel: The bottom panel shows the exact same as the top panel, but using the tapered datacube (tapering described in Section 2.1). The scale of the images are identical as well as the frequency range used to create the narrowband. Here, we also have a 6 sigma contour plotted in the right panel and the filled ellipse in the bottom right corner indicates the tapered beam size (2.9 x 1.47 arcsec).
The spectrum of A1703-zD1 extracted by summing the flux over the source from all pixels with S/N>2, using the tapered imaging. The red line shows the best fit Gaussian line profile. The grey shaded region gives the measured rms throughout the spectrum.
The [C II] line luminosity as a function of the star-formation rate, derived from the ultraviolet luminosity. The locally observed De Looze et al. 2014 relation is indicated by the dotted green line with the errors shown by the green shaded region. Sources with 3 sigma upper limits are shown by downward triangles. Our three targets are presented as red points. The black arrows in the top left indicate the magnification uncertainty for A1703-zD1. Changes in magnification affect both L[CII] and UV SFR, such that A1703-zD1 moves along the local relation. We use a compilation of z ≈ 6 – 7 sources for comparison (compiled from Matthee et al. 2019), shown in blue. In orange we highlight four lensed sources from the literature (Knudsen et al. 2016; Bradač et al. 2017; Fujimoto et al. 2021) for which three show an apparent offset from the local L[CII] – SFR relation, one of which shows a very clear offset (MS0451-H).
Main Panel: HST imaging overlaid with contours from the untapered datacube corresponding to 2,3 and 4 sigma [C II] emission collapsed over channels redwards (red contours) and bluewards (blue contours) of the measured line centre, showing evidence for a velocity gradient. The observations are spatially resolved, as shown by the filled ellipse indicating the beam size(1.57 × 1.31 arcsec).
Inset Panel: First moment map of the detected [C II] from A1703-zD1 (showing pixels above 2 sigma). We measure a velocity difference over the galaxy of 103 ±22 km s−1. We show the green arrow to illustrate the stretching of the arc in the image plane with respect to the source-planeimage of this source.
Inset Panel: First moment map of the detected [C II] from A1703-zD1 (showing pixels above 2 sigma). We measure a velocity difference over the galaxy of 103 ±22 km s−1. We show the green arrow to illustrate the stretching of the arc in the image plane with respect to the source-planeimage of this source.
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