Monday, May 21, 2007

Galactic Nebulae near M82 and the Nature of the M82 Cap



[Image credit: Mandel-Wilson, http://www.galaxyimages.com/UNP_IFNebula.html. The image shows a snapshot of one small part of the IFN nebulosity near M81 (largest elliptical blob) and M82 (smaller blob to the right and slightly up from M81).]

APOD (astronomy picture of the day) is often spectacular, but rarely presents something a professional astronomer won't already have heard of.

But Integrated Flux Nebulae (IFN) were something I'd never heard of before. Basically these are large (in angular extent) Milky-Way reflection nebulae near or associated with the North Polar Spur (itself an interesting topic, most probably part of a very nearby supernova remnant, although some have argued that it is a much larger, more distant, part of the Milky Way's disk/halo interface. If I have time I will discuss this in a another post). They are visible in the optical via the scattering of starlight by the dust grains contained within the gas clouds making up the nebula. Apparently the dust clouds were already known from the IR-based dust maps of Schlegel, Finkbeiner and Davis (1998), but the first optical images of them were obtained by the amateur astronomers behind the Mandel-Wilson Catalogue of Unexplored Nebulae, of which the IFN website is a part.

That there is a lot of dust and optical nebulosity associated with the NPS is perhaps not too surprising, but what what hit me was that these objects were right in the vicinity of M81 and M82, objects whose environment I thought I knew everything about given the large amount of professional study of M82's wind and the M81/M82/NGC 3077 tidal streamers. These results raise the question of whether some of the nebulosity associated with the M81 group and M82's wind may be confused foreground emission from the NPS IFN... Note how much structure there is even in the dust images shown at the IFN website. The foreground extinction toward the M81 group is clearly not going to be uniform.

Here I will post a very rough initial attempt at answering the issue of whether IFN-related gunk is actually responsible for what are currently thought to be very extended parts of M82's wind. Of specific interest is the M82 northern ridge or cap, seen in H-alpha, UV and X-ray emission (Devine & Bally 1999, Lehnert Heckman & Weaver 1999, Hoopes et al 2005). The image below is the GALEX NUV (red) and FUV (blue) color composite of M82 from Hoopes et al (2005).

The heliocentric velocity of M82's nucleus is ~200 km/s (redshifted with respect to us), although the warm ionized gas associated with the inner wind has velocities of ~ 50 km/s and ~350 km/s. Thus the near-side (blue-shifted with respect to M82) H-alpha emission is already close in line-of-sight velocity to the velocities expected of material within our Galaxy. Devine & Bally (1999) show that the cap exhibits a monotonic trend in LOS velocity from ~0 to ~150 km/s (i.e. blueshifted by 200 to 50 km/s w.r.t M82).


If the cap were instead Galactic and associated with the NPS then its angular length of 3 arcminutes would correspond to a physical extent of about ~0.8 pc (d/1 kpc), where d is the assumed distance between us and the NPS material. Typical estimates of the distance to the NPS are only 50 to 200 pc, the cap would be < 0.2 pc in length if it were this close. To have a 200km/s velocity change in a gaseous cloud over a distance of only <1 pc would highly unusual (and rather unphysical for a small cloud within a supernova remnant).

Although I do not know the exact line of sight velocity of the NPS or IFN in this region, Savage et al (1997) find the LOS velocities for UV absorption lines in the NPS toward 3D 273 to be ~-60 to ~-5 km/s w.r.t. heliocentric, quite different from the LOS velocity of the cap.

Furthermore, the cap displays behaviour very similar to the rest of the superwind, specifically simultaneous soft X-ray, H-alpha and NUV/FUV emission, with a soft X-ray/H-alpha flux ratio ~1 which is typical of superwinds (e.g. see Strickland et al 2002). Material associated with a cool dusty or photoionized cloud that is part of the NPS would not be expected to be detected as soft X-ray source or similar surface brightness to the superwind (although the presence of more widely-distributed lower surface brightness X-ray emission from the NPS might be expected), nor have that particular wind-like SX/H-alpha flux ratio.

This leads me to conclude that the Cap is very unlikely to be associated with the Galactic NPS, and most likely to be a genuine feature of M82's wind.


Nevertheless, I'm pleasantly surprised to learn something new about the M81/M82 region. It is also another nice example of the genuinely valuable contribution that amateur astronomers can still make.


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