NGC 2179 (TW)
This is an excellent example of a tightly wound nuclear dust spiral.
It appears to begin as a two arm spiral near the nucleus, but then at
larger scales the two dust lanes merge into a one arm spiral that
remains coherent for at least three revolutions about the nucleus.
The PV diagram of NGC 2179 is the prototype of the Type I
class. The two-dimensional shapes of the emission lines
are such that they gives the erroneous impression of two
distinct velocity components. One apparent velocity
component has the form of a highly tilted straight line
rising from zero velocity in the galaxy center (i.e., a
faster-rising rotation curve). The other component is a
less-tilted straight line (i.e., a slower-rising rotation
curve) superimposed on the first. Both lines naively appear
to imply solid-body rotation in the inner parts of this
galaxy; both lines culminate as the radius increases to
almost the same maximum velocity; the slower-rising
rotation curve shows a flat portion in its outer regions.
Rather than being of two different physical origins, we
have shown that properly accounting for the seeing, slit
width, and pixel size effects, these two apparently
solid-body rotation curves can be modelled as the velocity
field of a thin gaseous disk rotating in the combined
gravitational potential of central point-like mass and an
extended stellar disk (see Bertola et al. 1998 for details).
Sep 26/27, 1978
103aO + GG385
NGC 2179 is a multiple-armed Sa galaxy of
later type than the Sa galaxies on the preceding
pages. The arms are almost circular. They are
thin and show a few isolated probable HII regions
along the upper left spiral arm. The filamentary
multiple-arm pattern winds through the well-defined
disk. The galaxy is among the earliest
non-smooth-armed, multiple-armed type (MAS),
whose later examples are NGC 2775 (Sa; Hubble
Atlas, p. 10; panels 78, 87, S12 here) and NGC
4699 (Sab or Sa; Hubble Atlas, p. 16; panels 78,
87, 118, S12 here).
(r): 0.55 x 0.4, (R'): 0.95 x 0.6.
Overexposed elongated nucleus, faint (r'): 0.95 x 0.55,
very faint extended corona. Several stars superposed.
=ESO 555- G 38
Bright diffuse nucleus, smooth nebulosity
with traces of spiral arcs or whorls in outer parts.