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6 February, 2017

An Intrinsically Very Luminous Lensed High-Redshift Galaxy

An international team led by researchers from the Instituto de Astrofísica de Canarias (IAC) and the University of La Laguna (ULL) has discovered one of the brightest non-active galaxies in the early universe.

This galaxy, called BG1429+1202, is located at a redshift of 2.82 and it is a gravitationally lensed Lyman-Alpha Emitter (LAE) from the Bells Gallery project. Although, in general, LAEs are faint, BG1429+1202 is intrinsically very luminous. Additionally, its flux is boosted by a factor of about nine by gravitational lensing by a massive elliptical galaxy in the line of sight at redshift 0.55.

BG1429+1202 was selected for observations using ACAM at the William Herschel Telescope (WHT) from a large sample of lensed LAE candidates discovered using the Sloan Digital Sky Survey BOSS spectroscopic database. "This is one of the few known cases of galaxies with a very high apparent brightness and also an intrinsically high luminosity", says Rui Marques-Chaves, a doctoral student at the IAC-ULL.

To study this system, astronomers applied for a few hours of Director's Discretionary Time (DDT) at the WHT. They expected to detect a very bright continuum and Lyman-alpha emission from the blue features around the foreground galaxy visible on the Sloan Digital Sky Survey images, and that's actually what they already saw in the first 15-minute ACAM spectrum. BG1429+1202 is so bright that it can even be detected on the photographic images of the Digital Sky Survey.

Motivated by the promising results of the WHT observations, the team applied for DDT time at the Gran Telescopio Canarias (GTC). Observations were carried out just one month after the WHT data were obtained, and provided higher signal-to-noise imaging and low-resolution spectroscopy of the brightest lensed features.

WHT and GTC imaging and spectra of BG1429+1202 as well as colour image from the Dark Energy Camera Legacy Survey (DECaLS). The locations of the long-slits of the spectroscopic observations with WHT/ACAM and GTC/OSIRIS as well as the BOSS fibre are shown. Large format: JPG.

Lens modeling of this system revealed the main properties of the high-redshift LAE. Ismael Pérez-Fournon, from the IAC and ULL and coordinator of this project says, "Its luminosity and the star formation rate are much higher in the rest-frame UV continuum and in the Lyman-alpha line than in typical LAEs at high redshift. With telescopes such as the GTC or the WHT we can observe these high-redshift galaxies because they are gravitationally lensed. If they weren't, then we would need a future telescope, such as the Extremely Large European Telescope (E-ELT) or the Thirty Meter Telescope (TMT), to study them in detail."

To select lensed-LAEs like BG1429+1202, astronomers analysed around a million and a half spectra of galaxies obtained using the Sloan Telescope. Lyman-alpha emission was detected from galaxies much further away than their lenses in 187 cases, of which 21 have been observed with the Hubble Space Telescope. Those observations confirm that the majority of these objects are indeed gravitationally lensed.

"Via these same techniques, future data sets from projects such as WHT/WEAVE and the Dark Energy Spectroscopic Instrument (DESI) will lead to the discovery of many more types of lensed systems at all redshifts", concludes Adam Bolton, Associate Director of the NOAO and an author on this research.

More information:

Rui Marques-Chaves, Ismael Pérez-Fournon, Yiping Shu, Paloma I. Martínez-Navajas, Adam S. Bolton, Christopher S. Kochanek, Masamune Oguri, Zheng Zheng, Shude Mao, Antonio D. Montero-Dorta, Matthew A. Cornachione, and Joel R. Brownstein, 2017, "Discovery of a very bright and intrinsically very luminous, strongly lensed Ly-alpha emitting galaxy at z = 2.82 in the BOSS Emission-Line Lens Survey", ApJL, 834, L18 [ ADS ].

"Discovered one of the brightest distant galaxies so far known", IAC Press Release, 17 Jan 2017.

"17 New Strong Gravitational Lenses Discovered by NAOC Researchers Using HST Data", NAOC Press Release.

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Last modified: 05 February 2017