Paper Review: A bright lensed galaxy at z = 5:4 with strong Ly-α emission

This paper presents the findings of McGreer et al. regarding a galaxy discovered by his team in May 2012. The subject galaxy is known as J1414+5446 throughout the paper, and lies at redshift z=5.424 (7.469×1010 light years away). It is one of the brightest galaxies at this redshift and beyond, as well as being a strong Ly-α emitter. However, when first discovered, the research team classified it as a quasar, using what is known as the Likelihood method; essentially, bodies are compared to known quasars and non-quasars, and assigned an individual likelihood of being a quasar. For this scenario, since there are no known quasars at this distance, the team had to use simulations instead.

Ly- α emission is produced when a hydrogen electron goes to the first energy level from the second energy level, and is thought to be an indicator of a prolonged outburst of star formation, but very little is known about LAEs (Ly- α emitters)

McGreer and his team were able to obtain HST (Hubble Space Telescope) images of the galaxy, which show it has four different components, or clumps (here labelled A, B, C, D). They found that Clump A is responsible for the elevated levels of Ly-α radiation being detected. The paper shows that there is also a foreground object, and that Clump B could belong to this object instead of J1414+5446. This then leads to an uncertainty in whether Clump C is in the foreground or at high redshift, since it doesn’t have strong Ly-α emission.

A HST image showing the clumpy nature of J1414+5446.

In Section 4 of the paper, the authors discuss the physical properties of J1414+5446, summarising it in the table below. Since the team used several different telescopes to observe the galaxy, the images they obtain allow lots of missing pieces of information to be filled in, plus some uncertainty to be constrained. The second galaxy in front makes interpreting anything from the Large Binocular Telescope (LBT) fairly complicated, but the HST images put constraints on the UV radiation coming from Clump A (the Ly-α emitter); this allowed the team to adjust and correct for any emission coming from the foreground object. They observed a Ly-α emission of 11.5 x10-16 erg s-1 cm-2, which is very high. Many of the features of this emission point to there being an outflow – an expanding shell composed of hydrogen atoms.

Table showing the physical properties of J1414+5446.

J1414+5446 is unlikely to be an AGN (Active Galactic Nucleus), since no emission line of NV was detected (although the team state that the possibility of it being a AGN cannot be ruled out). However, they did manage to pick up an NIV emission line. Obtaining a reading for CIV was not easy since this had a strong presence in the sky in this area anyway, so distinguishing what is coming from the galaxy and what isn’t is tricky, but they managed to get a “questionable CIV detection”. They also found slight detections of SiII.

Using simulations again, the team found that the best fit model was where J1414+5446 had a young component of 5 x106 years with an SFR (star formation rate) of 100Mꙩ yr-1 and a mass of 6×108Mꙩ, as well as an older component with an age of 1 x109 years, a SFR of 400Mꙩ yr-1, and a mass of 3×1011Mꙩ. The values for the SFR are highly uncertain, so ought to be taken with a pinch of salt.

All images here are taken from the paper, which can be found here.

This paper contains a lot of useful information about high redshift Ly-α emitters, and since CR7 falls into this category, much of the information is relevant. We are also finding a similar chemical composition, and so we can link the two galaxies fairly well. As mentioned above, J1414+5446 is clumpy in nature, with two to four clumps. CR7 has three clumps, so again, comparisons can be drawn with relative ease.