Studying Lyman-α-galaxies with strong gravitational lensing

November 01, 2018
Strong gravitational lensing is an extremely powerful tool to go beyond the current limits in angular resolution and to investigate the high-redshift, i.e. distant Universe. Scientists at MPA take advantage of this phenomenon to perform a detailed study of 17 Lyman-α-galaxies and present an analysis of the sizes and star formation rates of their reconstructed ultra-violet (UV) continuum emission.

Lyman-α-emitting (LAE) galaxies represent a unique probe of the young Universe, about 1 to 2 billion years after the Big Bang. Typical LAEs are characterised by high-ionisation and strong star formation with low metallicity (i.e. few elements heavier than hydrogen and in general a low mass. The Lyman-α emission is produced when electrons recombine with the ionized hydrogen atoms and the properties cited above, combined with low dust content, allow for the escape of a significant fraction of these photons. While this emission is thought to have had a crucial role in the reionisation of the young Universe, very little is known about the detailed structure of these galaxies and, most importantly, about the mechanism that leads to the production of these high-energy photons.

So far the study of these high-redshift objects has been limited to quantifying the properties of their strong optical lines. Alternatively, many efforts have been spent to identify local analogues, i.e. nearby galaxies presenting similar physical and morphological characteristics. Both these approaches, however, require significant investment in telescope time.

Another resource to study these galaxies lies in high-resolution imaging studies that so far have been very useful to reveal their structure. LAEs are found to be quite compact objects and there is no evidence that they change their size as they evolve. Moreover they are surrounded by a large Lyman-α-emitting halo which is on average 10 times more extended than the UV continuum emitting region. Also this halo does not show an evolution in size with redshift. However, such studies are currently limited by the angular resolution of the observations and struggle to reveal the detailed structure of these objects.

Strong gravitational lensing can be used to overcome these limits. The first statistically significant sample of LAEs at z~2.5 strongly lensed by early type galaxies at z~0.5, has recently been revealed by observations with the Hubble Space Telescope. Due to the lensing magnification by a factor of about 20, we can access and probe the detailed structure of these galaxies at scales around 100 pc (some 300 light-years).

We have studied the intrinsic properties of the UV-continuum emission of these LAEs and we found that they have a median star formation rate of 1.4 solar masses per year with peaks of up to 54 solar masses per year. (This is actually a lower limit as we could not correct for dust attenuation.) We have found these galaxies to be quite compact, with a median size of about 500 pc and a range of radii from 200 to 1800 pc – our Milky Way with about 60 000 pc is gigantic compared to these LAE. Interestingly, they show quite complex morphologies with several compact and diffuse components, while in some cases they appear to be interacting. In two cases (14 percent of 17?) the galaxies seem to have off-axis components that may be associated with mergers.

Most interestingly, our LAEs are found to be quite elliptical, with a mean axis-ratio of about 0.5. This morphology is consistent with disk-like structures of star-formation for three-quarters of our sample, which would rule out models where the Lyman-α-emission is only seen perpendicular to the disk to favour instead clumpy models. Our results are in agreement with the studies of non-lensed LAEs at similar redshifts, but are more robust given the improved angular resolution of our analysis and given that no stacking techniques are needed.

With 200 pc, our lower limit on the intrinsic size of these objects is a factor of two smaller than what is achieved in non-lensed LAEs studies. In general our analysis further promotes gravitational lensing as a powerful tool to analyse and resolve the detailed structure of high-redshift galaxies, allowing the study of their physical and morphological properties at a resolution otherwise only achievable with nearby targets.

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