Paper Review: Star formation at z = 2.481 in the lensed galaxy SDSS J1110+6459

This review looks at a paper and a letter about the observation of a lensed galaxy located at redshift z ~ 2 , and how gravitational lensing is used as a natural telescope to probe star-formation at resolutions not obtainable with today’s most advanced telescopes. Computer models are used to reconstruct an image of the lensed galaxy as it were to appear without lensing, making it possible to locate and measure star-forming ‘clumps’ within the galaxy.

According to Einstein’s Theory of General Relativity, gravity is the result of mass bending space-time around it, which can bend the path traveled by light. A far away galaxy located behind a nearer, more massive galaxy cluster will have its light bent around the cluster as it travels to us. This distorts the image of the distant galaxy, resulting in arcs that can be seen around the centre of cluster. The galaxy SGAS J111020.0+645950.8 (henceforth referred to as SGAS J1110) is lensed by the galaxy cluster SDSS J1110+6459 (henceforth referred to as SDSS J1110), resulting in an arc formed of three images shown in the picture below.

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SGAS J1110 is located at redshift z = 2.481 (3.232×1023 km away), SDSS J1110 is located at redshift z = 0.659 (8.585×1022 km away). The lensing effect results in a magnification of about thirty times for SGAS J1110. The three arcs A1, A2, A3 are lensed images of SGAS J1110. Redshifts are obtained from spectroscopy taken by the Gemini North telescope (for SDSS J1110) and the Multi Mirror Telescope (for SGAS J1110). Imaging for the lensed galaxy was obtained by the Hubble Space Telescope.

Spectroscopy of SGAS J1110 from the Blue Channel Spectrograph on the MMT reveals features that are common in the ultraviolet (UV) spectra of starburst galaxies, indicating a high rate of star formation. Surveys of galaxies at different redshifts show that star formation peaked at around z ~ 2 , a time referred to as the ‘cosmic noon’.

The lensed images of SGAS J1110 reveal clumps of star-forming regions. Clumps at high redshifts are typically unresolved with the best telescopes today; the Hubble Space Telescope can resolve structure within galaxies at ~ 530 pc at a redshift of z = 1 in the rest-frame optical wavelengths. The typical size of clumps in high redshift galaxies in Hubble Space Telescope (HST) imaging is reported to be ~ 1 kpc , a scale thought to be the critical size scale for star formation in the distant universe. Gravitational lensing allows us to overcome these resolution limits and probe scales less than 100 pc in bright, lensed galaxies such as SGAS J1110.

A model of the lensing effect caused by SDSS J1110 is computed using a software called LENSTOOL. The modelling is done iteratively, beginning with a set of constraints, and with each iteration more free parameters are included, based on the previous iteration. The lens model translates clumps observed in the lensed image into their physical size and position as they would be seen in an un-lensed image. The clumps are then traced onto the HST image, and a resolved image of clumps in produced, as illustrated below.

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Model reconstruction of star-forming clumps is shown on the left. The model is then traced onto the HST image of the lensed galaxy to resolve clumpy structure, shown on the right.

The clumpy morphology of star-formation seen in nearby galaxies is normally not resolved at the time when most of the stars in the universe were formed.  From the lens model, star-forming clumps are detected with sizes down to 30 pc. SGAS J1110 provides the sharpest view ever seen of a galaxy at redshift z ~ 2 , revealing for the first time star-formation at cosmic noon on spatial scales well below 100 pc. These findings challenge the theoretical picture of 1 kpc scales being the critical size scale for star-formation.

22% of rest frame UV light from SGAS J1110 comes from more than twenty star-forming clumps, with measured sizes ranging from 30 – 50 pc. A distribution of clump sizes shows that most clumps have a size of 30 – 40 pc. These measurements indicate that at the time of cosmic noon, star-formation occurred on much smaller scales than previously assumed, scales that would not normally be resolvable without magnification form gravitational lensing. However, current sample sizes of lensed galaxies are too small to generalize for the larger galaxy population at high redshifts. Nonetheless it is exciting to be able to see star formation on such scales in a galaxy far, far away. Combining natural phenomena of the universe with computer modelling allows us to see what the Universe tries to hide.

-Umar