After one of the most fun and productive summers of astro-imaging at Chiefland in the 9-year history of my observatory, I now look forward to some cool, dry air filtering in from the Great North giving us seriously transparent skies. The Fall Chiefland Star Party starts in 4 days and the 10-day forecast is already very exciting with lows in the lower 50s Saturday and Sunday nights and clear! As the southern Milky Way bids farewell, my attention turns to the incredible variety of galaxies presenting themselves. The next two emails highlight two of these galaxies, namely Bode’s Galaxy M81 and NGC5907 in Draco, both of which exhibit profound gravitational phenomena.
The attached M81LRGB image is a 25 hour image consisting of 17 hours of 1×1 luminance and 8 hours of red, green and blue binned 2×2. The luminance has been deconvolved using CCDStack and DDP processed in Photoshop to tame the hub. ZOOM IN and see the details! The stars in the spiral arms are resolved and there is detail in the HII regions both in M81 and dwarf galaxy Holmberg IX. Double diffraction spikes are seen in both double stars SAO15020 and SAO15017! Very few images of M81 display this. A close-up of this “double-double” is attached resolving both double star pairs WDS J09551+6854AB & CD! Not only is this M81 image resolved, it is deep! Check out the faint integrated flux / galactic cirrus dust in the background. The circular dust cloud to the upper right of M81 has been named Arp’s Loop. Arp’s Loop is arguably cirrus dust surrounding our galaxy and is in the foreground (~30,000LY) compared to M81 (13 million LY). Another peculiarity is the vertical lines seen tracing across the right (north) edge of M81s nuclear hub. I first noticed this as a teenager in images of M81 taken with the 200-inch Hale telescope. I recall Tony Hallas explaining to me, while we were processing an M81 image we took at the 2003 Winter Star Party, that these lines were dust related to tidal interactions of M81 with other nearby galaxies including M82. We have all accepted this explanation for years as gospel. Recently, however, Tony Hallas, Jay Gabany and others raised doubts about the explanation of the lines across M81 and Arp’s Loop in a fantastic article published in Astronomy and Astrophysics in 2010 (PDF link below)! While local dust can be seen wrapping around the galaxy hub, the vertical lines likely represent foreground superimposed galactic cirrus rather than gravitational tidal streams at M81!!! I inverted my M81 image (like they did in the article) and drew lines through Arp’s Loop intersecting M81. Those dark vertical streaks appear aligned with Arp’s Loop and are probably all the same cirrus dust. Tony calls his observatory in the article “HOA” Hallas Observatory Annex! Gotta love it! Disappointing in the article, though, is the fact that both images in Figure 1 are mirror-reversed, something the editor should have picked up! Tony’s image in Fig. 5 is anatomically-correct, naturally. Satellite tidal dwarf galaxy Holmberg IX at magnitude 16.5 is fully resolved in my attached close-up image and shows a large HII region as well as many distant background galaxies some of which can be seen through Holmberg IX!!
M81 has shown up in plenty of previous “Astronomy Picture of the Day” (APODs) – links below. Perhaps my favorite is Ken Crawford’s, although it is mirror-reversed! Likewise for the professional Suburu image below! Too picky, you say?? I attached an image of my son that I deliberately mirror-reversed. It is important to orient correctly so his shirt reads “UF” and not “FU”! Could lead to misunderstanding! Tomorrow, I will send a deep NGC5907. …..clear skies ….. Bill W.
P.S. – Hard to digest::: Page 2 of the article below states that the point source limiting visual magnitude of Tony’s HOA image is 27!!!!! Is that possible? Could they be referring to surface brightness measured as 27 arcsec>-2 instead??? Point source magnitude 27 seems too deep even for HOA! Could this be correct??? Any ideas??
http://www.aanda.org/articles/aa/pdf/2010/08/aa14085-10.pdf – Fantastic article featuring the work of Tony Hallas and Jay Gabany! READ!
https://apod.nasa.gov/apod/ap130416.html — April 16, 2013 APOD
https://apod.nasa.gov/apod/ap160203.html — February 3, 2016 DEEP M81-82!
https://apod.nasa.gov/apod/ap141119.html — November 19, 2014 Suburu image (mirror-reversed)
https://apod.nasa.gov/apod/ap151017.html — October 17, 2015 APOD – Ken Crawford beaut!
http://www.usno.navy.mil/USNO/astrometry/optical-IR-prod/wds/WDS – Washington Double Star Catalog