The same components of the ISM originally detected in the disk of the Milky Way were later also found in external galaxies. Observations of external galaxies of Hubble types similar to that of the Milky Way (Sbc) have shown that our Galaxy is a ``normal'' specimen of its kind, including its SFR.
5.1 Starburst vs. normal galaxies
Galaxies with general similarities in several key parameters like Hubble type, total mass, and maximum rotation velocity, can still be entirely different in various other respects. One such important quantity is the current SFR, which can range from close to 0 to a few 10 M yr-1. The former - which means no observable recent high-mass SF - leads to old stellar populations (II) and low emission levels in most wavebands (except those tracing gas in low excitation conditions, i.e., the neutral atomic and molecular phases, and old stars). The latter - which characterizes a starburst (Soifer et al. 1987b) - leads to the presence of hot young stars (population I) with blue color. SF is going on at such a high rate that it cannot be maintained over a Hubble time (i.e., the age of the Universe), implying that it must be a transient phenomenon. In a more restrictive definition, there is a second condition characterizing a starburst: a concentration of the high-level SF in a circumnuclear area of 1 kpc diameter (Lehnert & Heckman 1996a).
Intermediate levels of SF are characteristic of galaxies which are called ``normal''. Their gas consumption rates are too low to exhaust the gas supply in less than a Hubble time, but the SFR is still >> 0. This leads to the co-existence of different stellar populations, with a range of different ages and thus colors. The Milky Way galaxy and M31 are typical examples of normal galaxies.
The distinction between starburst and normal galaxies is not always straightforward. Some galaxies, e.g., NGC 4631 and NGC 4666, have both circumnuclear starbursts and normal SFRs in their outer disks. Therefore, I will separate late-type spiral galaxies in the following sections into three categories:
|(1)||pure starburst galaxies (with a 1 kpc diameter circumnuclear starburst),|
|(2)||normal galaxies with wide-spread SF in their disks, and|
|(3)||mixed types with both circumnuclear starburst and widespread SF in their outer disks.|
In each category, the observational evidence for the existence of a gas halo will be presented in detail for one galaxy, while all others will be mentioned more briefly.