Object of the Week March 31, 2019 – NGC 3801 A Galaxy Caught in the Act of Dying
NGC 3801, UGC 6635, PGC 36200, MCG 3-30-40
Class: Lenticular Galaxy
Type: S0/a pec
Constellation: Leo
RA: 11h 40m 16.9s
DEC: +17° 43’ 40”
Mag: 13.20B
Size: 2.6’ x 1.4’
NGC 3801 Group, WBL 347
Class: Galaxy Cluster
Members: NGC 3801, NGC 3802, NGC 3803, NGC 3790, MCG 3-30-35
RA: 11h 40m 17s
DEC: +17° 44m 24s
In preparation for this article I found myself rummaging through my observing notes of the past decade in search for an object worthy enough of being the Object of the Week. Never mind that I have been inconsistent over the years in recording observations and then when I do they are often not as descriptive as I would have liked in hindsight. So, as I was paging through my notes from the night of March, 23, 2012 I come across this entry: “NGC 3801 + 5 other galaxies all in same FOV; E17”. At some point later in time I added the names of the other 5 galaxies: NGC 3802, NGC 3803, NGC 3807, NGC 3790, and MCG 3-30-35. That’s all folks, no further description, no sketch, nada.
With no recollection of my observation of NGC 3801 and its galaxy cluster, I dug into my extensive astronomy library expecting to find an abundance of information. To my surprise I found one reference to it, in the Observing Handbook and Catalogue of Deep-Sky Objects by Luginbuhl and Skiff. Moving on I then googled it, but once again finding only a few references for this galaxy. After reading one paper published in the Monthly Notices of the Royal Astronomical Society published at nearly the same time I was observing this galaxy, there was no doubt in my mind that NGC 3801 is a perfect candidate for OOTW.
According to this paper two spiral galaxies slammed together a billion years or so ago, their stars flinging one another into new, random orbits and their gas clouds colliding to form shock waves that gave birth, for a little while, to new, massive, hot stars. The new stars burned through their fuel very rapidly and exploded as supernovas, driving some of the leftover gas — the stuff new stars are made of — out toward the galaxy's fringes.
This alone wouldn't have shut off further star formation. But just like all spiral galaxies, including the Milky Way, the two that crashed to form NGC 3801 had supermassive black holes lurking in their cores. In the shakeup following the collision, some gas clouds would have been sent spiraling into these cosmic vacuum cleaners. In their frenzied effort to gobble up the gas, the black holes would have emitted high-speed jets of matter, and these would drive off the remaining gas.
Using the Galaxy Evolution Explorer telescope, or GALEX; the Spitzer Space Telescope; the Chandra X-ray Observatory; and the Hubble Space telescope, the research team found several smoking guns to confirm this scenario. GALEX, which records ultraviolet light, and Spitzer, which works in the infrared, both detected telltale signs that the star formation triggered by the merger is still going on, but at a very low level. And the Hubble has picked up gas being pushed out of the galaxy by recent supernovas.
The truly massive shock waves rippling through gas clouds heated by the black holes, meanwhile, are generating X-rays, which can be spotted only with the Chandra observatory. The emissions are speeding away from NGC 3801's center at a frenetic 2 million m.p.h. (3.2 million km/h), or close to it. At that rate, any star-forming gas that's still hanging around will be driven out of the galaxy in about 10 million years — mere seconds from now in cosmic terms. The whole process of transforming two young, vigorous galaxies into a single, feeble geezer takes about a billion years, which is still pretty fast considering that the spirals had probably been alive and healthy for 10 billion years or so before their unfortunate encounter.
ngc3801 SDSS.jpg
Courtesy of SDSS
NGC_3801_GALEX (1).jpg
Courtesy of Galaxy Evolution Explorer telescope (GALEX)
The following diagram and timeline, courtesy NASA/JPL-Caltech, visually sketches out the transition stages NGC 3801 is making from a vigorous spiral galaxy to a quiescent elliptical galaxy whose star-forming days are long past.
leafblower2.jpg
(1) Astronomers think the galaxy NGC 3801 formed around a billion years ago when two young, star-making spiral galaxies, perpendicular to one another, collided.
(2) The remnant of this merger possessed a central core rotating in a "north-south" direction compared to the "east-west" direction of the surrounding disk.
(3) The mashing together of gas and dust during the collision and merger unleashed a burst of star formation around 300 million years ago. The shortest-lived, biggest stars blew up in a few million years as supernovae. Winds and shocks from these explosions dissipated some of the cold gas in the galaxy needed to form new stars. Star formation began to decline in NGC 3801 accordingly.
(4) Around five million years ago, the supermassive black hole at NGC 3801's core started to flare up, sending out jets of fast-moving particles from the surrounding material. Today we see the twin jets bent into an “S” shape from the rotation of the galactic disk. More powerful jets from the black hole appear to have started up in the past one or two million years but have not yet emerged from the galaxy’s core.
(5) In the next few million years, the stronger jets will send out shock waves into the disk of NGC 3801. The remaining gas and dust will be swept up and pushed out of the galaxy. will have transformed into an elliptical "red and dead" galaxy, full of old, red stars.
(6) In about 10 million years or so, the expanding shock waves will have cleared out the remaining cold gas in NGC 3801 and star formation will cease. NGC 3801 will have transformed into an elliptical "red and dead" galaxy, full of old, red stars.
N3801_Group_finder.jpg
mikewood_ngc3801group_04022017_small.jpg
Courtesy of Mike Wood, Webb Deep-Sky Society
Deep Sky Forum OOTW contributor and Webb Deep-Sky Society Galaxy Section Director, Owen Brazell, wrote about the NGC 3801 galaxy group several years ago. Selected portions of his article follow.
“The group has quite a chequered discovery history, and the usual naming challenges. The brightest galaxy in the group is NGC 3801 and it was discovered by William Herschel, along with NGC 3790. Even the NGC sleuths seem to disagree about who discovered NGC 3806. This could have been either William Herschel or, more probably, John Herschel.
Although NGC 3806 was bright enough to be seen by William at the time he was using his 18.7” reflector in Newtonian mode rather than in front view mode and Wolfgang Steinicke suggests the extra light loss may have contributed to him not seeing it. Some confusion has also reined over NGC 3806 being numbered as NGC 3807 but this is actually a star seen by the team at Birr. They did however discover the other two galaxies in the field, NGC 3802 and NGC 3803.
NGC 3801 is classified as a S0 (lenticular galaxy) but as the Hubble image shows it has some very strange dust clouds in it which would be unusual for a S0 galaxy including one at right angles to the main axis.
The group is regarded as a physical system, or at least some of them are, and have been given the designation WBL 347 (The Catalog of Nearby Poor Clusters of Galaxies of White et al. 1999), which lists 5 galaxies in the group.
Most of the galaxies here are regarded as lenticulars except for NGC 3806 which is a face on spiral.
Interestingly none of these galaxies appear in Night Sky Observers' Guide (NSOG) or other popular resources, although it is in the AL Galaxy groups and clusters list.”
Mike Wood provided this visual observation (and sketch above) of the NGC 3801 Galaxy Group with a 20" dob at 181X.
"I began my sketch of the view with the 13 ethos. Twenty minutes into the sketch, thinking I was done, I caught a hint of of fuzz, North of the obvious bright star. This of course was NGC 3806 which I had totally forgotten about. It is definitely the hardest of the four galaxies to see.
Notes
NGC 3801: easy, immediately obvious and with a distinct core.
NGC 3802: slightly fainter than NGC 3801, smaller and elongated, hint of a core at 470 times magnification.
NGC 3790: elongated and smaller still.
NGC 3806: easy to miss as it's large and faint, circular patch of fuzz.
This group provided a very rewarding 45 minutes of observing".
Andrew Robertson provided the following visual observation and sketch of NGC 3801 and NGC 3806.
"Didn’t look at the charts beforehand and it was a full month ago that I observed the group with the 24" so couldn’t remember what was where, just had two numbers in my head; 3801 and 3806. Sent the scope to NGC 3801 with 40mm Pentax in giving just x89. Immediately saw NGC 3801 but it was faint and nothing else seen. Put the 27mm Pan in giving x135 and started suspecting a faint galaxy directly above it (NGC 3802). Switched to 22mm Pan (x162) and confirmed this AV2. I then kept to the 22mm Pan and 20mm Pentax (x179) alternating between the two eyepieces and couldn’t decide which was best so I think this was the optimum power for the conditions which I should say weren’t bad at this point; 21.1 SQM and NELM 5.5. I was using a diagonal and they were on the meridian so North was up and West to the left in the eyepiece. After some scrutiny I picked up a galaxy to the left (NGC 3790), I think this was a tad easier in the 20mm Pentax (AV3). Couldn’t detect anything else. So I then sent the scope to NGC 3806 whilst looking in the eyepiece. NGC 3801 moved to the lower left and slightly below centre I was picking up NGC 3806 AV3. I think it was just too close to the edge to pick up when NGC 3801 was in the centre".
andrewrobertson_ngc3801group_03022017_large.jpg
Courtesy Andrew Robertson, Webb Deep-Sky Society
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