November and December dark moon nights are dwarf galaxy high season. Barnard’s Galaxy NGC 6822, IC 5152, WLM, Sculptor, Fornax, and IC 1613 are easy in a 6-inch at a good dark site with mag 6 seeing. Phoenix is do-able in a 7”. Carina is the most fiendish dwarf galaxy in the sky for visual observers: you’ll be examining tea leaves and consulting psychics before you actually see the thing. That said, it CAN be done, but half a lifetime of observing ultra-faint extended objects is required homework.
Around 2 a.m. in November & midnight in Dec, you’ll be able to add Sextans A, Sext B, and the Leo I Dwarf near Regulus. That makes 11 dwarfs in one night. So . . . would you like to make it a baker’s dozen?
The above 11 dwarfs are an eyepiece big brass band compared with the Pegasus Dwarf, DDO216. It’s not listed on many charts. Alvin Huey’s “
Observing Local Group Members” is the best visual observer’s guide. (Be sure to also get his “
Globular Clusters”, it’s the perfect eyepiece companion for GC buffs.) Out under the stars, start with the two similar-mag stars 66 & 70 Peg, S of the Square and somewhat left of the centreline. Go N on the alt axis 15 arcmins till you find an easy arrow-shaped asterism of mag 8.5 – 10 stars. Go 50’ further N until you find a shallow bow of five mag 9 to 12 stars, with the brightest of them marking each end. Huey plots it very well. Imagine it a bow that’s just released an arrow. Another 20’ northward and sure enough, there’s the arrow. It is a Y of four mag 13 – 14 stars and is fairly easy because it’s the only asterism in the field at 100 to 120x. Use averted to look left of the arrow’s “feathers” and wait for a very faint glow. Wait. Wait some more. In a 7” and 8” Mak, the Peg Dwarf shows as a few tremulous glows per minute even in my black skies. The visual patch is almost the size of the Arrow asterism, but off-centre and perpendicular to it. The Dwarf is a Patience Special: you just have to wait and wait for that one or two second glow to rise out of the dark night’s curtain, revealing a soft, very faint glow. One or two seconds and pffft it’s gone again. Small wonder: the galaxy’s total luminosity is 12.6, but it’s spread over an ellipse 8’ by 2.5’. I didn’t list it as a confirm till I’d unequivocally held its glow for a couple of seconds two or three times in five minutes, on three separate pointings each night, for three nights in a row. It’s memorable once you do get a confirmed sighting, if for no other reason because so few people ever go after it and even fewer catch it. You’re seeing one of the loneliest galaxies in the Local Group. It’s not even a Milky Way dwarf but rather the uttermost outlier of M31, 1.5 million light years away. That puts the Peg Dwarf at the very limit of M31’s gravitational grasp. There’s not enough orbital data on it to calculate the total orbit time, but it’s likely in the 100 to 250 million year category. It is 3 million light years from us, and >100,000 ly from nearest other galaxy.
If the Pegasus Dwarf was a confirm, take a deep breath and go after Andromeda VI. It’s easier to star hop to it using Huey and WikiSky than to try describing the steppingstone sequence in words. You’ll need two hours of very patient waiting to catch m-a-y-b-e half a dozen unequivocal glimpses. You’ll probably hear a lot more owls than you’ll see hints of And VI. It was discovered only in 1998 and shows its stuff well in its
Wiki article. And VI is a very faint speckle ball with a distracting asterism of 10 mag 14-16 stars directly adjacent to it. I tried for it on the best night of the dark cycle, Nov. 14, using using an 8-inch Mak at 120 and 218x. I can’t call my sighting of it a confirm because, while I saw an extremely faint round glow four times across the span of an hour, I can’t definitively say I wasn’t seeing the six <mag 15 stars in the asterism, which in an 8” scope would very easily look like a soft faint ball.
Once you’re done looking, step back and have an unaided-eye look at the long line between M31 and the southern line of the Great Square where the Peg Dwarf is located. The Peg Dwarf seems awfully far away from the galaxy, and it indeed is. Andromeda VI roughly halfway between the Peg Dwarf and M31. If it’s a good dark night you can see M31’s disc as perhaps 2—2.5° of oval fuzz. The Peg Dwarf is a huge 30° away. A 2.5° galaxy has tentacles that extend 20 times further out, to the Peg Dwarf. Gravity, the weakest force in the universe that we can feel, manages to hold on all the way out there.
If it seems a coincidence that the Peg Dwarf and And VI are in a straight line toward M31, it’s no coincidence at all. Just two years ago in 2013,
Rodrigo Ibata and a team of 14 other dwarf galaxy specialists announced that over half of M31’s dwarf galaxies lie on a thin plane that is the same plane as M31’s disc.* Within a year that team and others showed that many of the Milky Way’s and the M81 galaxy’s dwarfs also lie on thin planes coincident with the parent galaxy’s disc. Even more mysterious, most of those dwarfs are rotating in the same direction as the disc. The more enthusiastic of the “planar alignment” devotees suggest that any dwarf that is NOT on the plane is a captured dwarf from outside the system. The discovery has created a minor industry of computer modelers, theorists, yea-sayers and nay-sayers, all of them devoted to getting to the bottom of what are indubitable observations. Grant writers are having a field day. No one, so far, has come up with a satisfactory explanation (meaning scads of equations) why these planes should exist. Yet there they are, including the Pegasus Dwarf and Andromeda VI we can spot in our modest little scopes in the back yard.
Now go find them and then you can tell all your pals that you’re part of the Planar Alignment Club and watch them look at their wristwatch as think up an excuse to leave you alone to think it over.
_____
* Go to the linked discovery paper, scroll down to p.14, and there’s a gray cube with a representation of the M31 dwarf array as red dots in the middle. Click on the cube. It becomes interactive and you can use your mouse to rotate it in all directions, until you find the one very spot where all the red member dwarfs assemble along a perfect line.
