Virgo Galaxy Cluster Mosaic

Another step in what will likely be a lifelong project, here’s a two-panel mosaic of a large chunk of the Virgo Galaxy Cluster shot over three years. There’s not a lot to shoot with a 1,000mm telescope in the spring—what we call “galaxy season”—except for galaxies. So I decided to focus on one of the most prominent galaxy clusters in the sky. To capture the entire expanse of the Virgo Galaxy Cluster will take several more years. But it’s fun to see it take shape one year at a time.

Virgo Galaxy Cluster

This image depicts a nice chunk of the Virgo Galaxy Cluster. Containing more than 2,000 galaxies (56 of which are readily visible in this image) around 30 to 50 million light years away, the Virgo Cluster exerts tremendous gravitational influence on its members and neighbors, including what astronomers call our own “Local Group” of galaxies. Current evidence suggests that eventually, the Virgo cluster will swallow up the Local Group.

The Virgo cluster is home to 16 galaxies listed in the Messier catalogue, but this image contains only five of them—Messiers 84, 86, 87, 89, and 90 (M84, M86, M87, M89, and M90 as well as NGC 4374, NGC 4406, NGC 4486, NGC 4552, and NGC 4569, respectively). But there are many other prominent galaxies in the field—most notably a pair called The Eyes (NGC 4435 & 4438).

Markarian’s Chain

One prominent feature in this image (on the right side) is a spectacular formation of galaxies called Markarian’s Chain. The Chain includes M84 and M86 (but not M87 below, or M89 and M90 on the left side of the image). It starts far to the right and curves upward and to the left.

Markarian’s Chain is named for Benjamin Markarian, an Armenian astrophysicist who, in 1961, discovered that the group of aforementioned galaxies shared a common motion. Just adjacent to M86 on its left are “The Eyes”—two interacting galaxies catalogued as NGC 4435 and 4438.

While I’m certainly not the first to expose it, one of the things that is unusual about this image is that it reveals not only red foreground emissions of hydrogen gas, likely in our own galaxy, but also what is often referred to “The Bridge” between The Eyes and M86. This was discovered in 2007 at Kitt Peak by Jeffrey Kenney and his team using the Mayall four-meter telescope. The Bridge has been characterized as tidal stripping of hydrogen from galaxies that are interacting gravitationally.

The Bridge is extremely faint. To achieve this capture required 280 hours of time on the telescope using a specialized hydrogen filter. The exposure time for the rest of the panel containing Markarian’s Chain and M87, however, still amounted to more than 400 hours, making this one of my most arduous projects.

Messier 87

This image includes the (relatively) famous galaxy Messier 87 (M87). It’s the large elliptical galaxy at the bottom just right of center. M87 is gigantic—containing several trillion stars (compared to a few hundred billion in the Milky Way) and around 15,000 globular clusters (compared to somewhere north of 100 orbiting our galaxy). M87 is famous for the jet of synchrotron radiation emanating from its central black hole (not visible in this image).

But M87 is even more famous for having given up the very first image ever taken of a black hole—a black hole that has 6.5 billion times the mass of our sun (compared to the Milky Way’s central black hole that weighs in at a paltry four million suns). In 2017, the Event Horizon Telescope (EHT) Collaboration released an image of M87’s central black hole (called M87*) using data gathered from radio telescopes around the world. The technique the EHT team used with telescopes located all over the planet effectively turned the Earth into a giant radio telescope able to resolve the core of a galaxy 50 million light years away.

Messier 89

Messier 89 (M89) is the amorphous blob on the left side of the image below center. It has more than 2,000 globular clusters orbiting it. Many of the “stars” you see in the image close to it may actually be globular clusters. It also contains a complex of distinct shells that may be evidence of interaction or merger with smaller galaxies in the past. A protuberance that extends up to 150,000 light years from the galaxy’s center, perhaps also the result of past interactions with other galaxies.

Messier 90 and Ram-Pressure Stripping

On the left side of the image above center, you can see a spiral galaxy, Messier 90 (M90), showing red tendrils trailing off to the right. These tendrils are more than 300,000 light years long and are the result of a phenomenon called ram-pressure stripping.

Galaxy clusters contain hot, X-ray emitting gas called intra-cluster medium. As M90 speeds through this gas, it peels the light hydrogen gas out of the galaxy much the same way a strong wind might blow dust off the hood of your car. This interaction causes some of that hydrogen to ionize, which makes it visible to us when we employ a special hydrogen filter and a good bit of patience and perseverance. I previously got a closer image of this with my larger scope.

Red Stuff

You may notice that the background of the image contains patches of deep  burgundy color. It represents a phenomenon called extended red emission (ERE). ERE tends to occur when particles of polycyclic aromatic hydrocarbons fluoresce with a deep red color due to the presence of ultraviolet radiation from nearby stars. This differs from hydrogen material emitting its own red light due to ionization—as in the Bridge or in emission nebulas.

I'm grateful to Steve Mandel for putting me in contact with Dr. Adolf Witt at the University of Toledo who went to great lengths to help me verify that the burgundy color you see in the background is really there and not just an artifact of the photographic process.

There are at least five sources of red hues that appear in this image. So what are they? This is my understanding:

(1) the red that’s a component of starlight that finds its way into the continuum;

(2) red from foreground ionized hydrogen in our own galaxy;

(3) red from fluorescing aromatic hydrocarbons in and around our galaxy, called extended red emission (ERE);

(4) red from hydrogen drawn out of The Eyes and M86 that is ionized due to their gravitational interaction; and

(5) red from hydrogen pulled out of M90 and ionized due to ram-pressure stripping.

It’s common knowledge among astrophotographers that ionized hydrogen can be isolated in an image with a process called continuum subtraction—subtracting the natural red channel from the ionized hydrogen channel. What I learned in this process is that ERE can be isolated by applying continuum subtraction in reverse-—subtracting the ionized hydrogen channel from the red channel.

Integrated Flux Nebula

Aside from all the red stuff in the background, a grayish dust is also present through the frame. This is due to the presence of integrated flux nebula material.

Integrated flux nebulas (IFNs) are the result of extremely diffuse gas and dust at or beyond the perimeter of the galaxy reflecting the light of all the stars in the Milky Way. IFNs are very faint and are extremely hard to capture with a telescope, often requiring dozens or even hundreds of hours of exposure time to unravel.