H II regions in Messier 33
The Sc galaxy M 33 is apart from our Milky Way and the Andromeda Nebula the third largest galaxy in the Local Group of galaxies. It´s distance is 809 kpc, so that the apparent diameter of 65 arcmin leads to a real diameter of about 15.3 kpc. The equatorial coordinates (J2000) are: RA = 01 h 33,9 min, DEC = +30 ° 39 ‘.
M 33 shows a mean color index B-V = 0.55 mag. This means a blue color in the galaxy´s spiral arms, resulting from the huge amount of young and hot stars and stellar associations, which were born only 4 to 6 million years ago. The former starburst was accompanied by an enormous number of H II regions, tracing the spiral arm structure. In many cases these H II regions present a bubble-like structure, caused by the stellar winds of central Wolf Rayet and/or O-type stars. But it should be emphasized that some of the H II regions also show strong emission in [O III].
The surrounding of M 33 can be characterized by faint cirrus clouds, being produced by the Milky Way. But there are also some extremely faint areas of outlying stars, detected in 2014 with the 4-m telescope of Hawaii. These faint features are presented in an inverted view.
TBG member Fabian Neyer has got this new image with an apochromatic refractor TEC 140 (f/7.2). The field of view is 121′ x 81′. Although this is quite small, we can see the galaxy clearly being resolved into individual stars. Fabian used a CCD camera SBIG STL-11000M with Baader filters. The exposure time was 10.3-7.5-4.7-5.2-6.5 h for L(Hα+[NII])RGB, this means a total of 34.2 h. The individual exposures were taken in 17 nights between october 2012 and october 2014. In order to get more pronounced H II regions, the Hα-emission was isolated by subtracting the red continuum and following addition to the R image.
Special literature regarding Messier 33:
Courtès G. et al.: H-alpha survey of M33 with the six-meter telescope. Morphology of the general diffuse emission, evidence for a chaotic medium of bubbles and filaments; A&A 174, 28-56 (3/1987) (link to publication)
Drissen L., Shara M.M., Moffat A.F.J.: Ring nebulae around Wolf-Rayet stars in M33; AJ 101, 1659-1662, 1947-1951 (5/1991) (link to publication)
Hunter D.A.: Imaging and spectroscopy of ionized shells in M33 and M31; AJ 108, 1658-1666 (1994) (link to publication)
Hunter D.A. Et al.: The Wolf-Rayet star population in the most massive giant H II regions of M33; ApJ 456, 174 (1/1996) (link to publication)
Pellerin A.: Massive stellar content of giant H II regions in M33 and M101; AJ 131, 849-858 (2/2006) (link to publication)
McConnachie A.W. et al.: The Photometric Properties of a Vast Stellar Substructure in the Outskirts of M33; ApJ 723, 1038-1052 (2010) (link to publication)
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