Laser Guide Star Adaptive Optics Sharpens Subaru Telescope's Eyesight and Opens a New Vision of the Distant Universe

Figure 1: Laser beam shot from the Subaru Telescope to generate an artificial guide star at a height of 90 km in the upper atmosphere. (Photo by Daniel Birchall, NAOJ)

Figure 2: The Laser Guide Star Adaptive Optics, LGSAO, produces an artificial guide star at an altitude of 90 km in the upper atmosphere, bright enough to serve as a light source to measure atmospheric disturbance, with a wavefront sensor operating at 2000 Hz. A deformable mirror cancels the wavefront error in real time to improve the spatial resolution by an order of magnitude.

Figure 3: LGSAO observation at the Subaru Telescope control room.

Figure 4: Double quasar SDSS J1334+3315 imaged without adaptive optics (upper left) and with adaptive optics (upper right), each image spanning a 10 arc second field of view. The lower panel is enlarged by five times and clearly shows for the first time the lensing galaxy located in between the two quasar images. Click the image to display the larger figure. Retrieve the raw image without labels here.

Figure 5: Test exposures of the gravitationally lensed quasar B1422+231 with adaptive optics (right) and without adaptive optics (left).

Figure 6: A 10 - minute exposure image of the most distant quasar ULAS J1120+0641 (lower right), taken by the Subaru Telescope LGSAO at a wavelength of 2.1 micron. The upper left figure shows the same object observed with the UKIRT telescope with 40 sec exposure as part of the UKIDSS sky survey project, also in the near infrared at 2.2 micron. Field of view is 60 arc seconds. The lower left figure shows a close-up of 8 arcs second field view, the same as the Subaru Telescope LGSAO image. Click the image to display the higher resolution image. The raw image without labels is available through this link.

A research team at NAOJ has begun scientific observations with Subaru Telescope's Laser Guide Star Adaptive Optics (LGSAO) system. The LGSAO uses a powerful laser to illuminate the sodium layer of the atmosphere and create an artificial guide star that shines in the upper atmosphere (fig 1). The LGSAO system integrates the creation of an artificial guide star with adaptive optics that compensates for atmospheric turbulence. This integrated system delivers to the scientific instruments images that are ten times sharper than previous ones (fig 2). Consequently, it will facilitate discoveries of new objects and clarify the details of other faintly observed ones.

Atmospheric turbulence distorts observations with ground-based telescopes. The spatial resolution of images from large, 8 - meter telescopes at the summit of Mauna Kea, known as one of the best sites on earth for astronomical observations, degrades by a factor of ten. Since October 2008, Subaru Telescope's adaptive optics system with 188 control elements (AO188) (Note 1) has opened the possibility for the Subaru Telescope to reach its theoretical capability for high resolution imaging (Note 2). The operation of this powerful system requires measurement of the atmospheric turbulence toward the target object. By monitoring the light from a bright star, one can measure how the turbulent atmosphere distorts the light propagating through it. Such a bright star used to measure the wavefront aberration of light is called a "guide star". However, the instrument has only had access to one percent of all available objects that have sufficiently bright guide stars for such measurements.

In order to increase the number of targets accessible to AO 188, the research team (Note 3, 4) developed an integrated system (LGSAO, fig 2) that combines the Laser Guide Star generation system with the AO 188 system. The resulting system can generate an artificial star bright enough to be used as a light source for measuring and compensating for atmospheric turbulence in any direction where the Subaru Telescope is pointing (Reference 1). The ongoing performance verification tests have confirmed that the integrated system performs as designed (fig 3).

The LGSAO team started their first scientific observations by targeting SDSS J1334+3315, a quasar pair discovered by the Sloan Digital Sky Survey in the constellation Canes Venatici (Note 5). This object consists of two star-like images separated by 0.8 arc seconds and has been thought to be a double image of a distant quasar gravitationally lensed by an unidentified foreground galaxy. A gravitational lens is formed when light from a very distant, bright light source is bent around a massive object between the source object and the observer, creating two or more images. The Subaru LGSAO observation of the quasar clearly revealed the foreground galaxy and demonstrated that its gravitational lensing effect is responsible for the creation of the double quasar image (fig 4)(Reference 2). A short exposure image (fig 5) of another gravitationally lensed quasar system, B1422+231, also demonstrates the striking improvement in image resolution when using LGSAO.

The measured redshift distance (i.e., how much the light has been stretched due to the expansion of the universe) to SDSS J1334+3315 confirms that it is 10.9 billion light years away. The newly discovered lensing galaxy is likely to be at a distance of 5.4 billion light years away, based on three independent estimates. Monitoring observations should confirm the lensing model, which predicts a ten-day time delay for the source brightness variation between the two lensed images. The paper reporting these discoveries will be published in the Astrophysical Journal in July.

Fig. 6 shows a quick look image of the most distant quasar ULAS J1120+641, that was spotted by UKIDSS survey and confirmed of its redshift at z=7.085 by Gemini follow-up spectroscopy as reported in the Nature paper published on 30 June, 2011 (Reference 3). NAOJ team is discussing for possible follow-up observations of this important object and a test exposure was taken by LGSAO (Note 6).

The completion of the Laser Guide Star Adaptive Optics sharpens the vision of the Subaru Telescope in the near infrared by a factor of ten. In addition, it opens new prospects for making high-resolution studies of distant galaxies, quasars, and supernovae, as well as capturing more detailed images of globular clusters in our Galaxy, for which no nearby bright natural guide star is available.

The LGSAO system starts its service for open use observations in July 2011. Many researchers from around the world are looking forward to using this system.

Reference:

1) Hayano et al., SPIE 7736, 21, (2010), "Commissioning status of Subaru laser guide star adaptive optics system"

2) Rusu et al., in press Astrophys.J (2011), "SDSS J133401.39+331534.3: A New Subarcsecond Gravitationally Lensed Quasar"

3) Mortlock,D.J. et al., Nature, 474, 616 (2011), A luminous quasar at a redshift of z=7.085

Notes

Note 1 : Subaru Telescope Press release: 20 Nov. 2006

Note 2 : The theoretical limit for the spatial resolution of the Subaru Telescope, called the diffraction limit, is 0.06 arcsec for infrared light at 2 microns. At this resolution, one can count the number of golf balls at the summit of Mt. Fuji from Tokyo, a distance of about 100 km.

Note 3 : Masanori Iye (Project Representative), Hideki Takami, Yutaka Hayano (Principal Investigator)、Hiroshi Terada, Yosuke Minowa, Masayuki Hattori, Yoshihiko Saito, Shin Oya, Olivier Guyon, Tae-Soo Pyo, Mai Shirahata, Makoto Watanabe, Meguru Itoh, Michihiro Takami, Stephen Colley, Michael Eldred, Mathew Dinkins, Taras Golota, Tom Kane、Vincent Garrel, Christophe Clergeon.

Note 4 : LGSAO project is supported by MEXT Grant-in-Aid for Specially Promoted research (2002-2006), "Laser Guide Star Adaptive Optics", and JSPS Grant-in Aid for Scientific Research (S) (2007-2011) "Laser Guide Star Adaptive Optics."

Note 5 : Observation team for SDSS J1334+3315 includes Cristian Eduard Rusu (Univ. Tokyo), Masamune Oguri (IPMU), Issha Kayo (Toho University), and Naohisa Inada (Nara National College of Technology).

Note 6 : Test observation of ULAS J1120+0641 was made with the assistance of Chris Simpson (Liverpool Jon Moores University) and Takatoshi Shibuya (Graduate Univ. for Advanced Studies).

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