Press Release

Star Formation Activities at the Outskirts of Spiral Galaxy NGC 6946 - Young College Students Celebrate the 10th Birthday of Subaru Telescope -

September 21, 2009

This beautiful image of spiral galaxy NGC 6946 (Fig.1) was obtained by the Subaru Observation Experience Program Team, a group of 10 young, qualified undergraduate students from various universities in Japan. The Subaru Observation Experience Program gives undergraduate students an opportunity to consider a career path as astronomers by conducting and experiencing real observations with the Subaru Telescope.

The team used Subaru's Suprime-Cam on the night of August 27, 2008 to study star formation activities in NGC 6946. The team's observations resulted in an image that is unique in its width and depth: It spans an area four times wider than that covered by recent studies and goes deep enough to detect tiny, star forming regions. Because Suprime-Cam is equipped with a dedicated filter that efficiently extracts hydrogen emission lines (Hα), it highlights star forming regions.

Although the team members were allotted only 40 minutes of observation time, they identified 1413 HII regions and became the first to discover star forming regions more than 60,000 light years out from the galaxy center, where astronomers have thought that no significant star forming activities would be found. This finding indicates that star forming activities can occur in very remote regions of the galaxy, where almost no stars are visible (Fig.2).

The team members also compared the distribution of HII regions with a map of neutral hydrogen (HI) traced by a 21cm radio line, based on previously published findings. They found that most 'HI holes', where the HI emission is locally weak, are surrounded by HII regions, as is shown in Fig.3. This result indicates that the local density of neutral hydrogen would be one of triggering conditions for active star formation.

Overall, the findings from this young team's observations reveal a picture of star forming activities at the fringes of a galaxy, for which no intensive studies have been conducted. The data that the team collected can be used as a fundamental data set for investigating star forming activities at the outskirts of galaxies.

The team's results will be presented at the autumn bi-annual meeting of the Astronomical Society of Japan, which will be held in Yamaguchi.

[Observation Conditions]

Object: NGC 6946
Telescope: Subaru Telescope (8.2m effective aperture) Prime Focus
Instrument: Suprime-Cam
Filter: B (0.45um), V (0.55um), IA651 (0.651um)
Color Composite: Blue (B), Green (V), Red (IA651)
Observation Data: Sep. 3 (B), Aug. 28 (V), Aug 28 (IA651), 28 (IA651), 2008 UT
Exposure Times 720 sec (B), 540 sec (V), 1155 sec (IA651)
Orientation: North is up, East is left
Coordinates: R.A.=20h35m, Dec=+60d09m (Cepheus)

Figure 1

Figure 1:The color image of NGC 6946. NGC 6946 is a suitable galaxy to study star forming activity in the galaxy from the center to the outskirts, because we can view it from the top.

Over all view (Field of view: 32' x 25')

Over all view (Field of view: 32' x 25') High resolution file for download

Close-up view of galaxy (Field of view: 11' x 9.7')

Figure 2

Figure 2:Close-up view of HII regions at the outskirts. HII regions identified in this study are marked by yellow lines.


Figure 3

Figure 3:The spatial distribution of the 'HI hole' (black ellipse with number) and HII regions (red cross). The figure clearly shows that many HII regions trace the edges of 'HI holes'. This indicates that the local density of neutral hydrogen would be one of triggering conditions of active star formation.




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