Details: NGC 7727 was discovered by the famous German-British astronomer William Herschel on November 27, 1785. It is only 4.7 by 3.5 arcminutes in size as seen from earth, or one tenth the apparent size of the full moon. It has an apparent magnitude of 10.6 or about 50 times fainter than the faintest star visible to the human eye. NGC 7727 is between 90 to 100 million light years away, therefore when light left this galaxy, only dinosaurs and no humans, ruled Earth. It is about 60,000 light-years across, but leaves traces of billions of stars behind at distances greater than that.

Galaxies Just Merged: NGC 7727 is not a normal galaxy, but a peculiar lenticular galaxy and two separate galaxies that have already merged. The peculiar galaxy classification means it is of unusual size, shape or composition, and only 5-10% of galaxies belong to this class. Lenticular denotes a type of galaxy classification that lies between an elliptical galaxy (E) class having no galaxy spiral arms and a spiral galaxy (S) class with spiral arms. There is clear evidence of this merger in that the distorted outer structure is clearly loops of material caused by the galaxies passing each other multiple times. Also there are faint tidal features left over from the gravitational distortion between galaxies and a very chaotic stellar distributions remains compared to galaxies that have not recently merged. The merger is believed to have happened about 1-2 billion years ago when life was first forming on planet earth, but relatively recent in galactic timescales!

Two Supermassive Black Holes: NGC 7727 is a well known somewhat unique galaxy among astronomers in that the remnants of its two galactic nuclei, each contain a supermassive black hole or (SMBH) nearing the end of their merger. SMBHs are the most massive type of black hole with a mass on the order of hundreds of thousands to billions of times the mass of our Sun (M_☉). Note that the center of all black holes are a gravitational singularity, meaning they are all infinitesimally small and with infinite density. Black holes, however, do vary in mass from about 3 solar masses, to billions of solar masses. Scientists measure the size of black holes not by the size of the singularity, which is zero and thereby meaningless, but by the size of the event horizon, or the boundary at a distance from the black hole singularity beyond which no signal or “event” can ever reach an observer. This is due to the gravitational well created by the black hole beyond which no matter or light can escape! The event horizon diameter of a 3 solar mass black hole is about 9 kilometers, meaning the mass of 3 of our Suns fitting into the size of a small city! The event horizon diameter of the largest black hole discovered is 590 billion kilometers, meaning the mass of 100 billion Suns fitting into 100 times the distance between the Sun and Pluto, still a small size of only 1/70^th the distance to our Sun’s nearest stellar neighbor!

The first of the two super massive black holes in NGC 7727 contains about 154 million solar masses. This is the dominant nucleus here visible at the center of the above image as a bright yellow core. The second super massive black hole in comparison is only a pale 7 million solar masses and also visible here to the left and slightly above the dominant one. It is also one of the smallest SMBH nuclei ever measured directly. These SMBHs are only separated by 1600 light-years, making them the closest known pairs of SMBHs known. In about 250 million years the two black holes will merge to form a 160 million solar mass black hole and produce some of the greatest most powerful gravitational waves, however at such a great distance the effect on earth will be miniscule.

As the galaxies merge, they follow the trails of their SMBHs to form galaxy tendrils, loops, or shells of material left behind by the gravitational interaction. These loops visible and annotated here contain billions to tens of billions of suns each!

A Dead Galaxy: The merger of any two galaxies will likely never cause two stars to collide due to the vast distances between stars. However, these mergers do disrupt normal galaxy behavior in star formation activity, availability of interstellar gas and dust, and hence stellar population distribution. In NGC 7727, there is now very little star formation, almost no gas or dust for future star formation since it was mostly expelled form the galaxy, and hence all the stars are of older population. This will of course change over time as older stars die and create new gas sources for future stars. Also the scattered and distributed interstellar gas and dust will eventually begin to coalesce and form new gas sources.

Astronomers love to study galaxies like NGC 7727 as textbook examples of galactic mergers, transforming spiral galaxies into lenticular and eventually elliptical galaxies. Also NGC 7727 demonstrates how supermassive black hole binary systems form and what galaxies look like between the final galactic merger and final black hole merger. Studies by the Hubble Space Telescope show a well resolved SMBH double nucleus, however studies at multiple wavelengths of light show very little activity in the merged galactic nucleus despite the ongoing merger between the black holes.

Annotations: In the image above, hover a mouse or curser over the image to show annotations of NGC 7727, with several enlarged insets identifying interesting features! Starting at the top and left of the image are two distant galaxies likely 100s of millions of light years away. Proceeding downward are the remnant of the early encounter between the two galaxies. On the right side of the image is an inset showing the double nucleus and identifying each of the super massive black holes. Finally, in the center is a path marked where the smaller, less massive secondary galaxy looped around the dominant galaxy. Of course, due to gravity the galaxies actually revolved around each other, however the barycenter or middle point of this revolving was much closer to the central dominant galaxy.

Below Images: The image below is the same image as above but processed to remove all stars. Since most of the stars in the image are in our own galaxy, this is what the galaxy would appear outside of our galaxy, if no stars existed along our line of sight. Using a mouse to hover over the image brings all the stars back. In the second image below is a zoomed in version of NGC 7727. Hovering over the image makes the stars disappear. Finally, the last image below is a comparison with the current generation of processing to my old generation 1 processing. This is the same data, same set of preprocessed images, but with improved processing techniques. Hovering the mouse over the image fades from the old image to the new processing!

Object Statistics: Constellation: Aquarius, Right Ascension: 23^h 39^m 52^s, Declination: -12° 17' 34”, Apparent Magnitude: 10.6, Size: 4’.7 x 3’.5 (60,000 light-years diameter), Distance 89-100 million light-years from Earth.