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Gaseous halos of star-forming spiral galaxies

A popular introduction to an exciting field of astronomical research: gaseous halos of spiral galaxies.

INTRODUCTION

Before dealing with gaseous halos, what they are (or what they are not), what they consist of, and why they are important objects to study, first a few words on the objects part of which they form: spiral galaxies.

Spiral galaxies

Do you know a spiral galaxy? No? YES, you do! You might just not be aware of it... Our Milky Way, the galaxy in which we live, is a SPIRAL galaxy.

Which properties characterize a galaxy as a spiral? Spiral galaxies are known to consist of several morphological components. They have a thin, rotating disk, a central thicker region (their bulge), and stellar globular cluster systems. Within their disks, there are spiral arms, which give them their name. All these components, or parts thereof, namely primarily stars residing in them, are quite easily accessible to us, because they emit optical light. This is the light that we are, by nature, equipped to be able to see - with our eyes.

However, galaxies also contain matter that our eyes are not equipped to detect, for example gas within the galaxies. Another component invisible to us is a galactic halo. One distinguishes two completely different halo components: massive, dark matter halos and gaseous halos.

DARK MATTER HALOS

The halos themselves consist of different kinds of matter. Most well-known are the massive, dark halos of matter that cannot (yet) be detected by us. They are only known to exist, because they are so massive that they make galaxies spin quickly - more quickly than all visible mass could possibly explain.

GASEOUS HALOS of spiral galaxies

The less massive part of galaxy halos is the gas found there. This gas, which is not one homogeneous entity in itself, but comprises several components, is visible via different emission processes, but it can also be found as an absorber of light from background sources. Most of the information given here deals with the emission characteristics of different phases of gaseous halos around spiral galaxies.

First of all: what do they look like?

As an example, have a look at the newly detected radio halo of the southern edge-on spiral galaxy NGC 7090: Contours of equal intensity of radio emission are overlaid on a grey-scale representation of an optical image of this galaxy. One can see that radio emission is arising from practically all of the galaxy disk, plus more emission from way beyond it, perpendicular to the disk plane. This emission beyond the optically visible galaxy is the halo. It is most obviously visible in the image taken at an observing frequency of 1.43 GHz, which corresponds to a wavelength of 21 cm. At 2.45 GHz (13 cm wavelength) there is less emission, which can be explained based on the physical properties of the particles producing the radio photons.

What do gaseous halos consist of?
And how can we observe them?

Just like the interstellar medium (ISM) in galaxy disks, gaseous halos consist of a whole jumble of various phases of gas, with different properties, such as density, pressure, temperature, clumpiness, etc. These are listed in the following. In the central column of the table exemplary observing techniques are listed. There are others, not described here, that may for some applications be just as important as the ones listed in the table. The right column tabulates typical temperatures for the individual gas phases. The names of the most prominent tracers of gas in galaxy halos are written in boldface font.

Gas
phase
Observing
technique
Typical
temperature
Hot ionized gas X-ray imaging and spectroscopy 106 K
Warm ionized gas Optical emission lines 104 K
Warm neutral gas HI line emission 3000 K
Cold neutral gas CO line emission 20 K
Dust Far-infrared emission 20 K
Cosmic rays Radio continuum emission N/A
Magnetic fields Polarized radio continuum N/A

A 'panchromatic' view - piecing together the puzzle

Different wavebands tell us different parts of the same story. It is literally true that, by observing emission arising from different astrophysical processes, we view celestial objects "in a different light". Gathering information about all phases of the ISM above, by observing their typical emission, is the only way for us to obtain a general overview of their properties that we can then compare with our theoretical predictions and mathematical calculations.

Yet more emission processes should in fact be included in such a global observational study so as to include the properties of the stars in galaxies, which interact with the ambient gaseous medium (and actually dominate its properties). Therefore, as a means of understanding celestial objects as a whole, multi-waveband observations are the most powerful tool available to us. Just by looking at the pictures in the picture gallery one can judge how different galaxies can look in different wavebands.

To be or not to be? How are gaseous halos created?
Which galaxies have one, which don't?

Different processes can contribute to the presence of gas outside the thin disks of spiral galaxies. The most important ones are:

My own research concentrates on the influence of massive star formation on the properties of the ISM in spiral galaxies and on the creation and maintenance of gaseous halos around them. This is the reason why the information given on these pages is biased towards that field.

If not massive, why are gaseous halos important?

Halos are not only interesting because of their potential role in an attempt to explain where the missing "dark" matter might reside in galaxies. Gaseous halos are important constituents of spirals as well, because they are an integral part of the ISM in galaxies with high star formation rates. They are the sites of metal-enriched gas and therefore a repository of material from which future generations of stars can form, once (part of) the gas falls back onto the galaxy disk, cools and thereby condensates. Thereby, gaseous halos are an important factor in the chemical evolution of galaxies. This is not only true for galaxies at the present time, but even more so at an earlier time in the Universe, at which we know high-level star formation to have been more common than now.

By comparing the properties of current-day galaxies with those of galaxies in the past (which we see when looking at distant, i.e. redshifted, objects), we can assess the general chemical evolution of galaxies over time. If, as suggested by many observations, galaxies were more active in the past than now, gaseous halos of spirals must also have played a more important role. Together with gas expelled by massive starbursts in current-day elliptical galaxies, the gas expelled by starbursts in spirals into the intergalactic medium might be an important contributor to the enrichment of intergalactic space with heavy elements. These are found by us by detecting their absorption lines against the background light from bright distant sources like quasars.

Gravitationally heated hot gaseous halos

There is an entirely different type of gaseous halos around elliptical galaxies. Different, because they are caused by a different creation mechanism. In ellipticals, gas is not heated by the energy and radiation of massive star formation, but by the gravitational force of their enormous masses. One can imagine the gas being heated up by the pressure working on it by the massive galaxy pulling everything towards it. Although the source of energy is entirely different, one can compare this effect with the heating up of a pump in which air is compressed before it is released through a valve into a tyre. Such halos of elliptical galaxies are not described here any further.