Hi Ron, PGC & All,
Thanks for posting that link Ron -- wonderful image of a true giant of the cosmos.
Quote:
Originally Posted by pgc hunter
bloody hell that's alot of globulars!
Any idea on the actual size of the galaxy?
|
The approximate answer to this can be found resonably easily. According to the RC3* the vital statistics are:
ESO 306-17 = MCG -7-12-9 = PGC 17570 Galaxy *
RA: 05h 40m 06.6s Dec: -40° 50' 13"
Mag: 13.3 (P) S.B.: --- B-V: --- Size: 2.4'x1.4'
Class: E+3 P.A.: 177 Inclination: --- R.V.: +10734
The R/V (Recessional Velocity) of +10734 indirectly indicates a distance to be a tic under 493,000,000ly assuming the Hubble Constant (Ho) (H subscript zero) is 71km/sec/Mpc.
Once you have this approximate distance and the angular size on the sky you can get a good estimate on the diameter (d) usng:
d = 2 pi A D / 360
where
A = angular size of galaxy in degrees
D = distance to galaxy in light-years
d = diameter of galaxy in light years
Now, the size in the RC3* figures (above) is based on its apparent size on the survey plate down to a certain isophote level -- usually where the surface brightness of the eg drops below 25 magnitudes/arcsecond square. This new image is at least somewhat deeper than the survey plate, so it appears bigger than the image on the survey plate.
Using the new image and comparing it to certain landmarks on the plate viewed with Megastar and doing a bit of "clicking", I'd say a conservative angular size in our sky is something like 3.0 to 3.2 arcminutes -- not 2.4 arc-minutes as in the RC3* figures above.
So, plugging in a fairly conservative 3.0 arcminutes angular size on our celestial sphere and using the formula above gives ESO 306-17 an approximate diameter of 430,000-odd light-years. A less conservative (probably more realistic) 3.5 arc-minutes yields approximately 500,000 light-years diameter. -- that's your basic big-boy
.
What's more you should be able to see this in a 8"-10" 'scope from a good rural site without undue difficulty.
Compare that in size with Andromeda (estimates are now 220,000 to 250,000ly but lighter than the Milky Way because it's less dense) and the Milky Way at about 120,000ly.
But, you must remember that comparing spirals with ellipticals (this is an E+3) is a bit like comparing apples with lemons. As a guesstimate (we don't know the likely star-density or how much dark-matter there is), ESO 306-17 would be, maybe, 10 to 20 times more massive than the Milky Way -- possibly (somewhat) larger. Absolute bolometric magnitude for the whole eg around the (minus) -24.5 mark -- odd ...
Best,
Les D