A galactic disk is inherently susceptible to the formation of the lopsided mode (Section 3). This mode can be long-lived as evident from the high fraction of galaxies that are observed to show lopsidedness. The dynamical origin and the evolution of these features is a challenging problem as discussed here. But apart from that, does the existence of lopsidedness affect the galaxy in any way? The answer is a resounding yes.
The list of processes whereby a lopsided distribution can affect the evolution of the galactic disk in a significant way include the following:
1. A lopsided distribution can help in the angular momentum transport in the disk and can thus contribute to the secular evolution in the disk. This is especially important given the long-lived nature of the lopsidedness. This can cause a redistribution of matter as studied by Lynden-Bell & Kalnajs (1972). This process is especially important at lower radii because the dynamical timescale is smaller. Hence the net dynamical evolution is likely to occur on timescales less than the age of the galaxy. The details for this process need to be worked out.
2. The fueling of the central active galactic nucleus (AGN) can occur due to the m = 1 motion of the central black hole (Section 4.2) and this could be more effective than the first process above.
3. Lopsidedness could affect the details of galaxy formation. For example, the accretion of mass as mediated by m = 1 would be especially important for the highly disturbed high redshift galaxies.
4. The lopsided distribution results in an azimuthal asymmetry in star
formation in a galactic disk. In a lopsided potential, the effective
disk surface density is shown to be a maximum at
=
0°, corresponding to an overdense
region, while there is an underdense region in the opposite direction along
=
180°. The fractional increase in surface
density at
= 0° is high ~ 0.3 - 0.5 for strongly lopsided galaxies (see
Fig. 1,
Jog 1997).
Thus, the molecular gas in the overdense region could become unstable
and result in enhanced star formation, as shown
for example for the parameters for M101. Further, the enhanced star
formation in the overdense region is argued to give rise to a
preferential formation of massive stars
(Jog & Solomon 1992).
This will result in more HII regions in the overdense region. This
prediction is exactly in agreement
with observations of more HII regions seen along the SW in M101.
Lopsidedness has also been observed in the
H
emission
from the star-forming regions in dwarf irregular galaxies
(Heller et al. 2000).
Such asymmetry is expected to be common in all galaxies
and we suggest future work in this area is necessary.