Press Release

Discovery of Huge Gas Cloud Expanding around Galaxy

April 15, 2002

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Object Name: Active Galaxy NGC 4388
Telescope: Subaru Telescope / Prime Focus
Instrument: Suprime-Cam
Filter: [OIII] narrow-band (0.50micron), V (0.55 micron), Halpha (0.66micron)
Color: Blue ([OIII] narrow-band), Green (V) , Red (Halpha)
Date: UT 2001 March 24, April 24, 26
Exposure: 60 min ([OIII] narrow-band) , 15 min (V), 160 min (Halpha)
Field of View: approx. 11.6 arcmins x 5 arcmins
Orientation: North up, east left
Position: RA(J2000.0)=12h30.8m, Dec(J2000.0)=+12d23m (Virgo)

Observations with Suprime-Cam on Subaru Telescope have detected for the first time long filaments of ionized hydrogen gas extending 110,000 light years above the disk of a galaxy. In the new image, the gas shows up in red and purple and appears to burst out of the center of the galaxy, reaching as far as the upper-left corner of the image. This galaxy, called NGC 4388, belongs to the Virgo Cluster, a large group of galaxies some 60 million light years from our own Milky Way galaxy. These observations provide new clues to how galaxies evolve inside the dense environment of a cluster of galaxies.

NGC 4388 belongs to a class of galaxies known as "active galaxies"; these objects are believed to harbor at their centers super-massive black holes that are a million times heavier than our Sun and are swallowing gas from their host galaxies in a process called accretion. Accretion produces vast amounts of energy which can outshine the light from all the stars in the galaxy and excite gas so that it loses an electron and becomes ionized. When hydrogen recombines with the electron, it emits light at a specific wavelength, allowing astronomers to see it.

It was previously known that NGC 4388 had ionized gas extending 10,000 light years from the center of the galaxy, but these new results are puzzling as the energy produced by the black hole can only ionize gas for a distance of 50,000 light years. An additional source of ionization may be required to explain the more extended ionized hydrogen gas. As a comparison, the ionized gas in NGC 4388 extends ten times further than the ionized gas in M 82 that is seen in the image taken by FOCAS on Subaru Telescope and released to the public in March 2000.

As for the original source of the hydrogen gas, there are several processes that can occur in a galaxy cluster that may be responsible. One possibility is ram pressure stripping, in which the gas that was originally inside NGC 4388 gets stripped away as it passes through gas that is associated with the cluster as a whole. Another possibility is that NGC 4388 may have swallowed a nearby dwarf galaxy.

In the near future, astronomers hope to take spectra of the light emitted by the ionized gas to determine its velocity and details of its ionization state. Understanding the origin of the hydrogen gas in NGC 4388 and how it becomes ionized will contribute to the larger understanding of how galaxies evolve in the dense environment of a galaxy cluster. Clusters contain anywhere between a few dozen galaxies to several thousand galaxies, and are part of the fundamental pattern of how matter is distributed throughout the Universe. The Virgo Cluster is the cluster closest to the Milky Way.

Michitoshi Yoshida who analyzed this data says: "I never expected to see such extended ionized gas around NGC 4388. This can really help us understand the origin of gas surrounding galaxies, and its physical state."

These results were published in the Astrophysical Journal on March 1st.

Notes: The large streaks of light from the bright stars in the image are artifacts called "blooming" and are not real. When too much light hits a digital camera in one spot, the light spills over onto other areas. If you look at the image carefully, you can see several small streaks. These are solar system objects that were moving during the observation.

(*) NGC: Abbreviation of New General Catalog of Nebula and Cluster of Stars published in 1888



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