Object of the Week March 8, 2015 - NGC 4449 Train Wreck Galaxies
NGC 4449, UGC 7592, PGC 40973, MCG 7-26-9, Caldwell 21
Dwarf Irregular
Type: IB
RA: 12h 28m 10.9s
Dec: +44 05’ 33”
Size: 5.2’ x 3.3’
Mag: 10.0V 9.5B
Mean Surface Brightness: 20.7 mag/arcsec2
NGC 4449B, UGC 7577, PGC 40904, MCG 7-26-6
Dwarf Irregular
Type: Irr
Size: 3.8’ x 2.5’
Mag: 12.9B
Mean Surface Brightness: 23.5 mag/arcsec2
I really enjoy the opportunity to contribute to the Deep Sky Forum’s Object of the Week. While I sometimes find the actual act of writing the article a bit challenging at times (writer’s block perhaps?), to me each article becomes a personal catalyst for exploring one DSO in depth. I begin with some preliminary research to select an interesting subject. That is followed by more extensive research and observation of the DSO if I haven’t observed it yet. I then begin to weave the facts and mysteries into an article that hopefully draws your attention as well as serving as a catalyst for you.
Not surprisingly, authoring OOTW articles has taken me on journeys of exploration I would have never anticipated. Take for example, my recent January article about NGC 1360. Who would have imagined that this planetary is dissolving, right now, right in front of our eyes.
Let's get back to this week’s OOTW, NGC 4449. When I selected this galaxy I would have never imagined the fascinating journey of exploration on which I was embarking. You ready to go? Buckle up and hang on for the ride!
It started innocently enough. Combing through some my old logs book I happened upon this entry from a trip several years back to observe on Barbarella, Jimi Lowery’s 48” dob in Southwest Texas. The brief logbook notation which caught my eye began with these simple words “NGC 4449 train wreck galaxies…”
But before we go down that path (or train track), let’s visit the basics about NGC 4449.
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NGC 4449 Digital Sky Survey
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NGC 4449 Digital Sky Survey 2
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NGC 4449 Hubble Space Telescope
William Herschel discovered NGC 4449 on April 27, 1788. His discovery notes state: “very bright, considerably large, much extended, double or bifid (either a double nebula, or a single nebula split into two lobes), well-resolved (clearly consisting of stars), 9th-magnitude star 5 arcmin to east."
This was clearly one bright and large galaxy. As a historical side note, despite its brightness, size, and northerly declination, for some unknown reason Charles Messier never cataloged it.
NGC 4449 is an irregular dwarf galaxy located in the M94 or Canes Venatici I Group. Using its generally accepted distance of 12 million light years and its apparent size of 6.2’ by 4.4’, the galaxy reaches about 20 thousand light years across. Its size and structure is quite similar to that of the Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way. The most significant difference, however, is that NGC 4449 is a starburst galaxy due to its high rate of star formation, which is twice that of the LMC.
Radio observations of NGC 4449 in the H II line show that the galaxy is embedded in a large envelope or halo of neutral hydrogen gas that extends 14 times larger than the optical diameter of the galaxy. This halo shows distortions and irregularities likely caused by interactions with nearby galaxies. A disk like feature and a lopsided arm structure have been identified in the halo and were believed to be the remnants of a galaxy merger sometime in its distant past. Despite these radio observations, there were no supporting observations visually, even through the deep, prying eyes of the Hubble Space Telescope.
Okay, so far, so good – actually pretty normal background material these days for extragalactic DSOs; but now the story takes an unusual turn. The first turn was as I reviewed numerous articles and observing reports I came across quite a collection of different common names for NGC 4449, not something I had encountered before. The most recurring references were: Box Galaxy (B.Wilson), Hourglass Galaxy (S.O’Meara), Lobster Galaxy (O.Brazell), Train Wreck Galaxies (J.Lowery), and The LMC for Everyone Else (unattributed).
I would expect each name paints a vivid and unique picture in your mind of NGC 4449. It certainly did for me. Yet I keep coming back to the logbook notation that caught my eye originally – train wreck galaxies. This name, and only this one, created this a burning question that I had to get answered – is this one galaxy or many? This is where the story becomes really interesting, where the unexpected happens.
Two different scientific discoveries announced in late 2011 lead to a totally different understanding of NGC 4449. The first paper is Dwarfs Gobbling Dwarfs: A Stellar Tidal Stream Around NGC 4449 And Hierarchical Galaxy Formation on Small Scales. Here researchers announced the discovery a small companion galaxy to NGC 4449, aptly named NGC 4449B. Interestingly this faint companion was independently discovered by two teams of astronomers both using amateur collaborators for the deep wide field images.
The dwarf galaxy NGC 4449B has been stretched into a comet-like “S” shape by the gravitational pull from NGC 4449. The companion had remained undetected for so long because it is more than 10 times fainter than the natural brightness of the night sky and some 1,000 times fainter than our own Milky Way galaxy. The gigantic “S” is so large that if one end were placed at the center of the Milky Way, the other end would reach all the way to the sun’s position. In fact, NGC 4449B is the largest dwarf galaxy known in the “local group” that also includes the Milky Way and the Andromeda galaxy. This stretched “S” is a faint stellar tidal stream located in the halo of NGC 4449, which is considered the on-going disruption of a smaller dwarf galaxy by a larger dwarf (an LMC/SMC analogue).
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NGC 4449 & NGC 4449B
NGC 4449 Tidal Stream (inset)
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NGC4449 and its companion dwarf galaxy NGC 4449B
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NGC 4449 and NGC 4449B discovery photo by Subaru telescope
The title of a National Geographic News article about this discovery summarizes it best: Dwarf Galaxy Found Secretly Feasting on Smaller Dwarf – New pictures confirm smudge of stars is dim galaxy being consumed. You got to love it, a headline straight out a New York City tabloid.
At about the same time, a second paper announces Cluster 77 in NGC 4449: The Nucleus of a Satellite Galaxy Being Transformed Into A Globular Cluster? In brief, researchers, using the HST, find a globular cluster inside NGC 4490, which appears associated with two tails of blue stars. One of the tails is associated with a concentration of HI and infrared emission, which appears as part of a much longer stream wrapping around the galaxy. These properties suggest that the globular cluster is the nucleus of a former gas-rich satellite galaxy, still surrounded by remains of its former parent galaxy, undergoing tidal disruption by NGC 4490.
What can the amateur see? Because NGC 4449 is so bright and large it yields detail even in an 8” telescope. Under dark skies the galaxy is a distinctly rectangular shape. Depending on aperture two or more bright spots are visible. These are the HII star-forming regions. In larger aperture scopes additional HII regions, a dark lane, and structure in the outer halo are visible. In Jimi’s 48” telescope, I easily detected four very bright HII star-forming regions. The galaxy and its halo exhibited extensive mottling. If I saw a dark lane, I made no note of it at the time.
Luginbuhl and Skiff in their Observing Handbook and Catalogue of Deep-Sky Objects share these comparative observations of NGC 4449. “This bright irregular galaxy is visible in 6 cm [2.4 in] as a broad 2’ brightening. 15 cm [5.9 in] shows a fairly even gradation of light. Viewed in 25 cm [9.8], the galaxy appears elongated NE-SW with interesting bright spots near the center of the elongated core. Averted vision shows an arm curving NW from the 3’ x 1’ oval halo. The halo extends to 6’ x 2.5’ in 30 cm [11.8 in]. The arm NW is seen well”.
I do believe Train Wreck Galaxies is altogether a fitting name for NGC 4449, NGC 4449B, and CL77. Two dwarf galaxies merging, the larger one consuming the smaller one through tidal disruption and forming a globular cluster. Wow!
By the way if you google “train wreck 4449” you’ll get hits about the Southern Pacific 4449, the only surviving example of Southern Pacific Railroad's (SP) GS-4 class of steam locomotives. The GS-4 was one good-looking steam locomotive. The wreck of the SP 4449 occurs in the movie Tough Guys, starring Burt Lancaster, Kirk Douglas, Eli Wallach, Charles Durning, Dana Carvey and Darlanne Fluegel.
SP 4449.jpg
Southern Pacific SP 4449
Now it’s time for you to give it a go. Let us know what name you think best fits your observation, or perhaps you’ll come up with even a better name.
“GIVE IT A GO AND LET US KNOW”
GOOD LUCK AND GREAT VIEWING!
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