Some galaxies die young.
A galaxy is a vast structure made up of stars, dark matter, gas, dust and other elements. They are held together by mutual gravitation. Galaxies' main source of visible light is usually from stars. Most galaxies have stars that are born from clouds made up of gas and dust. This ensures that these galaxies will continue to shine for billions of years.
However, if a galaxy runs low on gas, it cannot make more stars. Massive blue stars are the first to die, and over time, the galaxy reddens and eventually runs out star-making gas. We call it dead when it happens. Commonly, "Red and Dead" refers to the color of low-mass stars.
The rate at which stars form varies from one galaxy to the next. While the Milky Way produces approximately 2-3 times the Sun’s star mass each year, some galaxies produce more. This rate varies depending on the location of the galaxy. Galaxy clusters can contain thousands of galaxies. In many of these clusters, the star formation rate within the member galaxies of the cluster is very low.
NGC 4567 is one of two Virgo Cluster galaxies that are undergoing a severe collision. ALMA observations of gas (orange), superposed on a Hubble Space Telescope photo of the galaxies, (blue), help astronomers to see how the collision affects the gas relative to the stars in that galaxy. Photo credit: ALMA (ESO/NAOJ/NRAO)./S. Dagnello (NRAO)
This means that something is causing star birth to be quenched in these galaxies. There are many things that can do this. Ram pressure stripping is one of the most common. This involves removing the internal gas from a galaxy as it travels through a cluster. It's basically like opening your windows while driving to let out the polluted air that your dog ate after you gave it some burritos. The galaxy can't make star if its internal gas is gone.
There are also other possible causes. A big galaxy can pass another, common in cluster environments where they are packed together quite closely. The gravity of one galaxy can strip the gas from the other, which further suppresses star formation. A galaxy may pass near another galaxy, and this can cause it to stir up its gas, so that the internal gas is heated enough to prevent star formation. The intra-cluster medium, which is a thin, hot gas between cluster galaxies, can heat up the cold gas within a galaxy to prevent it from collapsing into stars. To stop a galaxy from making more stars, other processes may also be involved.
These processes all change the gas content of a galaxy in different ways. Astronomers can use this information to help them understand how to qunch it.
To show their relative positions, galaxies observed with ALMA for VERTICO (orange; enlarged 20 times for clarity) were superposed on top of a ROSAT image(blue) showing hot gas in Virgo Cluster. Photo credit: ALMA (ESO/NAOJ/NRAO). Dagnello (NRAO)/Bohringer et al. (ROSAT All-Sky Survey)
To get a grip on this, a team of astronomers used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the Virgo Cluster, the nearest galaxy cluster to us. Virgo lies 55 million light-years away and contains many hundreds, if not thousands, of galaxies. It is approximately 8 to so million light-years across. This is in contrast to our Local Group, which is only 3 million light-years across, and only three large galaxies, including ours. The cluster's total mass is 200 trillion times that of the Sun.
This is a bit unusual, as the clusters that make up this galaxy are still actively forming star stars. It also means they still have gas we can observe. This gas is mostly hydrogen but carbon monoxide (CO), is also very common. Although cold hydrogen is difficult to see, CO emits light at wavelengths ALMA can clearly see. The astronomers then observed 51 galaxies in Virgo and were able map the locations of CO gas in 49 of these galaxies to scales of a few hundred light years, a fraction of a tenth of their actual size.
NGC 4254, a fairly normal spiral galaxy with a face-on in the Virgo cluster, is currently making new stars. ALMA observations of gas (orange), superposed on a Hubble Space Telescope picture (blue), can be used to establish a baseline for how other galaxies within the cluster lose their ability to make stars. Photo credit: ALMA (ESO/NAOJ/NRAO)./S. Dagnello (NRAO)
The project, called VERTICO for Virgo Environment Traced In CO -- is just beginning. They published a paper about their method, how they used it, and the reasons for it. This data will be made available to the community for astronomers to examine the cold gas in these galaxies to see what's there.
For example, we know that star formation quenching is caused by ram pressure stripping. But is it widespread? It is efficient? It works on very small scales. Recent research has shown that gas can sometimes fall back onto the galaxy, encouraging star formation. This is only a fraction of the gas that's lost. But how much? How do other processes affect individual galaxies within a cluster?
NGC 4921, a spiral galaxy located in the Coma galaxy cluster is currently losing gas. It is passing through material between galaxies. ALMA images of gas (orange), superposed on Hubble images, show the areas where gas is being reabsorbed into the galaxy. This triggers star formation. Photo credit: ALMA (ESO/NAOJ/NRAO). Dagnello (NRAO), NASA/ESA/Hubble/K. Cook (LLNL), L. Shatz
VERTICO's goal is to answer these questions. Although it may take some time, the data is there. Astronomers should spend a day field-tracking.
You may wonder, "What about our galaxy?" The Milky Way is located in the Local Group. It doesn't contain a lot material among galaxies. We aren't soaring through it as fast as Virgo Cluster galaxies. (The overall gravity here is lower because there are fewer big galaxies). The Milky Way's star-forming glory days were billions of years ago so we are in decline... for a while. In four-and-a-half-billion years or so we'll collide and merge with the Andromeda galaxy, and we both have a lot of gas lying around to make stars. It should be exciting to witness the fireworks as star birth rates rise by large factors.
Make sure to mark your calendars.
