Paper Review: Measurement of rotation in two galaxies in the Epoch of Reionization from ALMA-detected [CII] emission

In this blog post we are going to be discussing the paper by R Smit et al – 2017.

As the Universe cooled from the big bang, structures such as baryons eventually formed. Once the temperature of the Universe dropped below that of the ionization potential of Hydrogen, protons and electrons could combine to form neutral hydrogen (this is the origin of the CMB). The state of the universe at this point, was neutral, until the epoch of reionization began. This reionization was sparked from high energy sources in the early universe, such as quasars and the very first generation of stars and galaxies.

This carries certain implications on observation in the high redshift universe, and as the redshift increases beyond z~7, the photons that are typically emitted from these distant sources are scattered by the increasingly neutral IGM. This makes spectroscopic confirmation of these sources difficult.

The CII line, λ157.74μm, is often used as an alternative to Lyman alpha to trace star formation, is a ‘coolant’ for interstellar gases, and can also be used to probe the internal velocity structure of galaxies.

In this paper, they are looking at two galaxies at redshifts in the range of 6.6<z<6.9. First, they used photometry to select the galaxies and then used the spectra and the CII line to confirm theses galaxies and more accurately determine their redshifts.

They use data from ALMA and collapse the cube over a range of frequencies in order to get an image of each of the galaxies, as shown in Figure 1 in the paper. This is similar to what we have been doing with our ALMA data in project 1 where we have been collapsing our cube over many different velocity ranges to get a set of slices of the region around and including CR7. They used similar methods as us with random apertures to determine the local noise in the two images and find the signal to noise ratio of the two galaxies.

The main focus of the paper was to use the CII line to look at the velocity differences in the galaxies and measure their rotation. They find the CII emission to be spatially resolved and can use this to more closely probe the image. This can be achieved as a feature of the cubes of data that ALMA produces, in which a third axis of frequency in the image allows measurement of small changes in redshift. Below are the velocity maps they produced from this data cube.

emma_paper_1

They measured values of rotation of (numbers above in caption), which are further supported as evidence for a symmetric rotating disk by looking at results of hydrodynamic simulations of cooling gases which do indeed form regular disk galaxies.

A similar method has been carried for CR7, the galaxy we are currently working on and the velocity maps created by David Sobral are shown below. These are looking at the different clumps within the galaxy and you can see that it is safe to interpret that there is internal rotation within the clumps and also rotation of each the clump about the other clumps.

emma_paper_2

Another focus of the paper was to look at how these galaxies fit with the relation between CII luminosity and SFR assuming a Chabrier initial mass function. For local galaxies, the CII line luminosity has a strong correlation with star formation rate. We looked at the SFR for the different clumps of CR7 assuming the same IMF as in this paper.

We, as a group, found this paper interesting to read because of how closely it relates to the work we have been doing over the last few weeks. We also find it incredible the amount of information you can infer for galaxies that are so far away and being new to this field it is surprising how much you learn every day both through your own work and through reading about other people’s findings.

-Emma, Lauren, Laurence