New Image and The Atlas of Peculiar Galaxies

This recently completed image from SkyPi Remote Observatory was featured on the Amateur Astronomy Photo of the Day website (AAPOD2) for May 31, 2024

A view toward the constellation Coma Berenices reveals foreground stars within our own Milky Way Galaxy. The two prominent galaxies lie far beyond our own, approximately 41 million light-years distant. Also known as NGC 4747, the smaller odd-shaped galaxy at left is Arp159, or the 159th entry in the Arp Atlas of Peculiar Galaxies. The tidal streams projecting away from that galaxy indicate strong gravitational interactions in its past. NGC 4725 is at right and is much larger, about 100,000 light years across. At first glance NGC 4725 appears to be a normal spiral galaxy, with a central yellowish core consisting of cool, older stars giving way to younger hot blue stars along its’ spiral arms. However it has only one spiral arm extending from its’ core!

Galaxy NGC 4712 is seen at upper right. It is about ¾ the size of NGC 4725 but is about 5 times further away from us!

Capture info:
Location: SkyPi Remote Observatory
Telescope: Orion Optics UK AG14 (F3.8)
Mount: 10 Micron GM3000
Camera: QHY 268M
Data: LRGB, approximately 7,6,5,5 hours respectively
Processing: Pixinsight

For the full resolution version click on the thumbnail under “My astroimages” at lower right on this page

So what is a peculiar galaxy and what is the “Arp Atlas of Peculiar Galaxies”?

To answer this question we need to go back in time about 100 years. It was 1923, the night of October 5. Carnegie astronomer Edwin P. Hubble (you’ve heard of that name?) took a photographic plate of the Andromeda Galaxy (Messier 31) with the Hooker 100-inch telescope at Mount Wilson Observatory in Pasadena, CA. Astronomers believed at the time that M31 was just an oval cloud of hydrogen gas, dust and stars within our own galaxy! Hubble’s discovery of the first Cepheid variable star inside of M31 established beyond any doubt that M31 was not a nebula, but a separate galaxy from our own!

Mount Wilson Observatory in Pasadena CA was home to the largest telescope in the world from 1917 until 1949. The Hooker 100″ reflector owned that title until the 200″ telescope at Mt Palomar was completed. During the early part of the 20th century many of the world-reknowned scientists and astronomers went to Mt Wilson to conduct research there. This was the birthplace of modern astronomy and cosmology and is where Hubble figured out our galaxy was not alone!

The 100″ Hooker telescope at Mt Wilson. From the early 1900’s until the 1940’s or so this was the largest telescope in the world and was the instrument that enabled Hubble to determine that the “Andromeda nebula” was actually a distant galaxy outside our own!

In the summer of 2003, I had the good fortune to be able to spend 2 weeks at Mt Wilson where I did a spectroscopy project. In the basement of the observatory housing the Hooker 100″ telescope were the lockers of the scientists but it was very dark when I tried to take this picture of Edwin Hubble’s locker!

Think about this for a second. In 1922 which is not really that long ago, astronomers believed our own galaxy was all there was! That was it. Just our own Milky Way galaxy was the whole universe!
Hubble’s discovery was one of the most transformative in the history of science. Marcia Bartuziak, an author, journalist and professor emeritus at MIT once said “It’s as if we had been living on one square yard of dirt on the Earth and suddenly realizing, ‘Oh my God, there are mountains and rivers and lakes and oceans and continents.’”

What followed was an explosion in galactic and extragalactic research over the next few decades. Hubble was able to identify many galaxies and in 1926 published a morphological classification scheme for galaxies often colloquially known as the Hubble tuning-fork diagram because the shape in which it is traditionally represented resembles a tuning fork.

Edwin Hubble’s classification of galaxy morphology and evolution showing the progression of elliptical galaxies “E” toward more organized types of spiral galaxies “S”. They diverge into two arms as you see here depending on whether the spiral galaxies have a central “bar” or not. The result is sometimes referred to as Hubble’s “tuning fork” diagram which was challenged by Halton Arp.

The tuning fork scheme divided regular galaxies into three broad classes – ellipticals, lenticulars and spirals – based on their visual appearance (originally on photographic plates).

Ok, this was simple enough, but perhaps too simple. In the late 1950’s and early ‘60’s astronomer Halton Arp was studying galaxies with the “new” Hale 200” telescope at Mount Palomar which saw first light in 1949, thus supplanting the Mt Wilson telescope as the largest in the world!

During the course of Arp’s research, he discovered many galaxies that did not seem to fit into Hubble’s neat “tuning fork”. He saw examples of galaxies with odd and distorted shapes, others that seemed to interact or even merge together. They were “peculiar” for sure! Arp became convinced that astronomers understood little about how galaxies change over time.

The 200″ telescope at Palomar Mountain in San Diego County, California, was where Halton Arp did his observations of peculiar galaxies in the early 60’s. Owned and operated by California Institute of Technology, research is still being done there but of course is limited by the severe light pollution in the area.

In 1966 Arp published the famous “Atlas of Peculiar Galaxies”. This atlas was intended to provide images that would give astronomers data from which they could study the evolution of galaxies.

This is the photograph of ARP 159 in the atlas. It is described by ARP as “disturbed with interior absorption. A very faint plume extends NE”.

Many of these galaxies are way too far away and difficult to resolve with amateur telescopes, but not all. They still represent a fascinating area of photographic study for the amateur!

Thanks for reading!

DrDave