3.2. More Distant Dwarf Irregular Galaxies
The other Local Group dIrrs are more distant from the dominant spirals, and fairly isolated. Interactions may still occur, but if this happens the interaction partners tend to be gas clouds rather than galaxies. Generally, star formation activity and gas content decrease with galaxy mass, but the detailed star formation histories and ISM properties of the dIrrs present a less homogeneous picture.
NGC6822, a dIrr at a distance of ~ 500 kpc, is
embedded in an elongated HI cloud with numerous shells
and holes. Its total HI mass is 1.1 × 108
M, ~ 7%
of its total mass. The masses of individual CO clouds reach up to
(1 - 2) × 105
M
(Petitpas & Wilson
1998),
while the estimated H2 content is 15% of the
HI mass
(Israel 1997),
and the dust-to-gas mass ratio is ~ 1.4 × 10-4
(Israel, Bontekoe, &
Kester 1996).
NGC6822 contains many HII regions. Its
huge supershell (2.0 × 1.4 kpc) was likely caused by the passage of
and interaction with a nearby 107
M
HI cloud and does not show signs of expansion
(de Blok & Walter
2000).
The older stars in IC10 describe an elliptical, extended halo
(Letarte et al. 2002)
distinct from the elongated HI distribution. The
latter, however, is traced closely by a population of young blue stars
(~ 180 Myr) that appear to have formed following the interaction with
the passing HI cloud
(de Blok & Walter
2003;
Komiyama et al. 2003)
some 300 Myr ago. In NGC6822, the HI distribution is thus
only slightly more extended than the stellar loci.
The HI of IC10 (distance 660 kpc) is 7.2 times more
extended than its Holmberg radius
(Tomita, Ohta, &
Saito 1993).
While the inner part of the neutral hydrogen of IC10 is a regularly rotating disk full of
shells and holes, the outer HI gas is counter-rotating
(Wilcots & Miller
1998).
IC10 is currently experiencing a massive starburst,
which is possibly triggered and fueled by an infalling HI
cloud
(Saito et al. 1992;
Wilcots & Miller
1998).
IC10 contains a nonthermal superbubble that may be
the result of several supernova explosions
(Yang & Skillman
1993).
The masses of the CO clouds in IC10 appear to be
as high as up to 5 × 106
M
(Petitpas & Wilson
1998),
which would indicate that more than 20% of this
galaxy's gas mass is molecular. Owing to the high radiation field and the
destruction of small dust grains, the ratio of far-infrared
[CII] to CO 1-0 emission is a factor 4 larger than in the
Milky Way
(Bolatto et al. 2000),
resulting in small CO cores surrounded by large
[CII]-emitting envelopes
(Madden et al. 1997).
Two H2O masers were detected in dense clouds in IC10, marking
sites of massive star formation
(Becker et al. 1993).
The internal dust content of
IC10 is high, and its properties prompted
Richer et al. (2001)
to suggest
that this galaxy should actually be classified as a blue compact dwarf.
Less detailed information is available for the ISM in the other Local Group
dIrrs, which do not appear to be involved in ongoing interactions and which
are evolving fairly quiescently. The HI in these dIrrs
may be up
to 3 times more extended than the optical galaxy and is clumpy on scales
of 100 to 300 pc. The most massive clumps reach ~ 106
M.
HI concentrations tend to be close to
HII regions. Some dIrrs contain cold HI clouds
associated with molecular gas.
The total HI masses are usually < 109
M
, and
less than 107
M
for
transition-type dwarfs. The center of the HI distribution
coincides roughly with the optical center of the dIrrs, although the
HI may show a central depression surrounded by an
HI ring or arc (e.g.,
SagDIG, LeoA), possibly a consequence of star formation, or the
HI may be off-centered (e.g.,
Phoenix;
St-Germain et al. 1999).
In low-mass dIrrs there are no signatures of rotation, but these may be
obscured by expanding shells and bubbles. Further details are given in
Lo, Sargent, & Young
(1993),
Young & Lo (1996,
1997),
Elmegreen & Hunter
(2000),
and Young et al. (2003).
Lower gravitational pull and the lack of shear in the absence of differential rotation imply that HI shells may become larger and are long-lived (Hunter 1997). Diameters, ages, and expansion velocities of the HI shells increase with later Hubble type (Walter & Brinks 1999) and scale approximately with the square root of the galaxy luminosity (Elmegreen et al. 1996). Shell-like structures, HI holes, or off-centered gas may be driven by supernovae and winds from massive stars following recent star formation episodes or tidal interactions.
For a review on nebular abundances in Irrs, see the contribution by Garnett (2004). Here it should only be mentioned that the effective yields in Irrs computed from gas-phase abundances are lower than those in the main stellar disks of spirals. Lower effective yields are also correlated with lower rotational velocities (Garnett 2002). This is interpreted as preferential metal loss through winds in the more shallow potential wells of Irrs and dIrrs, but may also be due to lower astration levels (e.g., Pilyugin & Ferrini 2000). For a review of the general ISM properties in Local Group dwarf galaxies, see Grebel (2002a).