Under the support from the Strategic Priority Research Program of the Chinese Academy of Sciences (Pilot B Program): The emergence of cosmological structures, Dr. Yiping Shu and Prof. Shude Mao from the National Astronomical Observatories, Chinese Academy of Sciences (NAOC) and their collaborators recently discovered a sample of 17 new strong gravitational lens systems from data collected by the Hubble Space Telescope (HST). This will be a unique sample for studying the nature of dark matter and high-redshift Lyα emitters.
Dark matter is believed to contribute more than 80% of the total mass in our Universe as suggested by numerous observational facts. However, unlike the ordinary matter we usually see in daily life, dark matter does not emit any light and therefore appears to be completely invisible to most of our detection instruments. The only way of detecting dark matter is through its gravitational effects. One such unique probe is the strong gravitational lensing phenomenon in which the light rays from a distant object (the “source”) are deflected by the gravity of an intervening object (the “lens”) and eventually form distorted, multiple images. “The image separations can provide a robust and accurate measurement of the total mass of the lens object, including dark matter,” comments Bolton.
Various experiments have been carried out to further determine the nature of dark matter, one of which is to constrain the properties of dark-matter subhalos with the aid of strong gravitational lensing. Dark-matter subhalos are small satellites around galaxies that primarily consist of dark matter. Their abundance and mass distribution provide direct evidence of the nature of dark matter. “In this experiment, the capability of detecting low-mass dark-matter subhalos is crucial because predictions by different dark-matter models are only different for low-mass subhalos,” adds Kochanek. Previous studies have shown that the smaller the background source is, the lower the mass limit can be achieved. “However, background source sizes in previous strong-lens samples are still larger than required for a decisive conclusion on the nature of dark matter,” comments Oguri.
In order to push the detection limit downward, Shu and collaborators specifically selected 21 strong-lens candidates involving redshift 2-3 Lyα emitters as the background sources from approximately one million galaxies. Lyα emitters are young galaxies that emit strong Lyα emission due to the transition of the electron of a hydrogen atom from the first excited state to the ground state. “Compared to previous samples, Lyα emitters are intrinsically smaller by a factor of 3 to 5, and therefore can push the detection limit down by roughly an order of magnitude,” comments Shu. Follow-up HST imaging observations confirmed that 17 of the 21 candidates are definite strong lenses (see Figure 1). “This is also the very first discovery of galaxy-scale strong lenses made by Chinese astronomers using the HST data,” adds Mao.
By analyzing the HST data, Shu and collaborators found that the background Lyα emitters are generally irregular and clumpy. “With the aid of lensing magnification, we can resolve the Lyα emitters in exceptional detail with sizes from a hundred to several thousand light years,” comments Perez-Fournon. “Future follow-up observations of the Lyα emitters will expand our understanding of how galaxies form and evolve,” adds Zheng. This specially designed, new strong-lens sample promises to be an invalueable resource for studying high-redshift Lyα emitters and the nature of dark matter.
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