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Q:

Why is the discovery of the hot gas halo around NGC 4631 important?

A:

A galaxy is an ecosystem. Interstellar gas is condensed to form stars. As they evolve, star loose mass and energy in form of stellar winds and radiation. As a massive star dies in a supernova explosion, it heats the surrounding gas to millions of degrees. This gas can then circulate between the disk and halo of a galaxy. Finally, when the gas cools down, it may be condensed again to form new stars. There is a great deal of uncertainties in this picture of the galaxy ecosystem, however. The study of the amount and extent of hot gas in a galaxy is critically important for us to understand how the whole system works. Therefore, the discovery of the hot gas halo around NGC 4631 may lead to a better understanding of the structure and evolution of galaxies.

Q:

What is the energy source of the hot gas?

A:

Supernova explosions are believed to be the major heating mechanism of hot gas in a galaxy. The supernova blastwave can naturally heat the interstellar gas to a temperature of a few million degrees. The hot gas with its high pressure and low density tends to flow out from the galactic disk, just as an atomic bomb explosion produces a mushroom cloud flowing into the upper atmosphere of the Earth. The comparison of the Chandra and Hubble images clearly shows that the hot gas indeed originates in recent massive star forming regions, where the supernova explosions are most frequent.

Q:

Do other galaxies have similar halos?

A:

We don't know yet. NGC 4631 is the very first galaxy that convincingly shows the presence of a diffuse hot gas halo. This galaxy is located in a direction far away from the Galactic plane of the Milky Way so that soft X-ray absorption by cold interstellar gas in our Galaxy is minimal. Also, NGC 4631 is quite active in star formation, which is presumably required to produce enough hot gas to be observable. Galaxies that are not edge-on are also not ideal for detecting halo gas because of confusion with X-ray emission from galactic disks. Most importantly, the Chandra X-ray Observatory provides us for the first time the high spatial resolution capability to separate discrete sources from the diffuse hot gas contribution. Chandra is going to observe other nearby edge-on disk galaxies. So in the near future we will know what kind of galaxies have hot gas halos.

Q:

Is there a hot gas halo around the Milky Way?

A:

This is a million dollar question. The notion that there might be a hot extended corona around our Milky Way was postulated more than four decades ago. Spitzer (1956) proposed the presence of the corona as a way to confine cold gas clouds observed away from the Galactic disk. Since then, various observations have been conducted in various wavelengths to probe the hot gas. While there are indications for the presence of the hot corona (the soft X-ray background and ultraviolet emission/absorption lines), the interpretation of the observations has been controversial. There are two major difficulties in the study of hot gas in our own Galaxy: first, we are in the mist of the hot gas. Second, we cannot see far from the solar neighborhood because of the X-ray absorption by cool gas in the Milky Way. These facts make it difficult for us to appreciate the overall extent and shape of hot gas, let alone its interaction with other components (massive stars and cool interstellar gas) of the Galaxy. Therefore, an outsider's view of a nearby disk galaxy such as NGC 4631 helps us greatly in interpreting observations of our own Milky Way.

Q:

Is NGC 4631 similar to our own Galaxy?

A:

Both the Milky Way and NGC 4631 are disk galaxies and are active in star formation. Unlike so-called starburst galaxies with star formation concentrated in their nuclear regions, both the Milky Way and NGC 4631 show active star formation over much of the galactic disks. But NGC 4631 appears to be smaller (and lighter) and is interacting more vigorously with neighboring galaxies than our own Galaxy. How these differences may affect the amount and overall morphology of the hot gas halo is yet to be determined. This is why we still need observations of galaxies with various properties.