Astron. Astrophys. 358, 845-849 (2000)
2. Photometric evidence for a spiral structure
IC3328 is known as an early-type dwarf galaxy in the Virgo cluster,
morphologically classified as dE1,N (Virgo Cluster Catalog 856,
Binggeli et al. 1985). The redshift of
km s-1 made this galaxy a
probable cluster member and a good candidate for a more refined
distance determination based on the Surface Brightness Fluctuations
(SBF) method. Candidates for the SBF method should have smooth and
symmetric light distributions, and show no obvious dust or
star-forming regions.
The first step in applying the SBF method is to determine the mean
2-D surface brightness distribution by suitable averaging and then
subtracting it from the galaxy image, leaving just the fluctuating
part which arises from the Poisson distribution of the stellar
sources, ie. the fluctuations due to unresolved stars in the galaxy.
However in the case of IC3328 the residuals were most surprising. The
composite of three 400 sec R-band images of IC3328 obtained at
the VLT with the FORS1 multi-mode instrument in Service Mode on July
13, 1999 under excellent seeing conditions
( ) is shown in the left panel of
Fig. 1. The surface brightness "fluctuations" obtained after
subtracting the mean is shown in the right panel.
![[FIGURE]](img4.gif) |
Fig. 1. The deep R-band CCD image of IC3328 (left panel) illustrates the overall morphology of this as dE1,N classified galaxy: a smooth radially decreasing light distribution with a centrally located nucleus. After the subtraction of the axis symmetric component, the residual image (right panel) reveals a prominent 2-armed spiral structure with a possible central bar.
|
Clearly "fluctuations" is not the right word to describe the
exceptionally regular spiral whose amplitude is so low that it
remained invisible in the original exposure. The spiral structure is
confined to the inner of the galaxy
which has an isophotal radius of =80".
It appears that we have discovered the real nature of IC3328: a
seemingly dust-free disk galaxy with unusually faint spiral
structure.
The initial analysis of the light distribution in IC3328 followed
the traditional route, based on standard IRAF procedures. The nuclear
offsets determined by fitting ellipses to the light distribution were
perfectly normal (top panels of Fig. 2). The first signs of
unusual behaviour came from the ellipticity and position angle
variations of the ellipse fits. The coherent wiggles at smaller radii
( ) suggested interesting, but low
level structure, which could be seen as a spiral pattern in the
contour maps.
![[FIGURE]](img13.gif) |
Fig. 2. The coordinate offset and between the nucleus and the center of isophotal ellipses, ellipticity, and position angle (counterclockwise from north) shown as functions of radius.
|
Contour maps, and in particular the smoothed contour maps
reconstructed from the ellipse fits are powerful tools for revealing
faint structure, but are not immediately suitable for quantitative
analysis of the light distribution. The presence of a smooth spiral
indicates that we are dealing with a disk or a disk embedded in a
spheroidal mass distribution. The most straight-forward analysis
consists of guessing the orientation of the disk and expanding the
deprojected light distribution in a Fourier series in the azimuthal
angle :
![[EQUATION]](img16.gif)
The amplitude of the two-armed spiral,
, is quite sensitive to changes in
inclination, and less so to the position angle; a wrong inclination
will produce modulations in the amplitude. An inclination of
(which corresponds to the
ellipticity ), and the position angle
of produced the smoothest two-armed
component. The amplitude and phase of this spiral is shown in
Fig. 3. The fractional amplitude of
is extremely low.
![[FIGURE]](img22.gif) |
Fig. 3. Azimuthal phase (top panel) and amplitude (central panel) of the spiral as a function of radius. The logarithmic spiral approximation is shown by the sloping dotted line. The fractional amplitude of the spiral is plotted in the bottom panel.
|
The one-armed, or terms become
important close to the center ( ).
In the interval the phase
is well approximated by a straight
line. Such a phase variation corresponds to a two-armed logarithmic
spiral inclined at to a circle. The
angular winding of the arms is . In
these respects IC3328 resembles a Sb or Sbc galaxy very similar to M51
(Danver 1942; Kennicutt 1981), but without obvious gas, dust or bright
HII regions.
The surface brightness profile derived from
is shown in Fig. 4. It can be
approximated by two straight lines (exponentials), with the cross-over
occuring at , which is also the place
where the spiral pattern ends. The end of the spiral can be seen in
the flattening of the phase in Fig. 3. The R surface
brightness profile has the same characteristics as the B
profile, which has been classified as type IIIb by Binggeli &
Cameron (1991, hereafter BC91).
![[FIGURE]](img34.gif) |
Fig. 4. The R-band surface brightness profile of VCC0856 exhibits a distinct bi-linearity, characteristic for a type IIIb profile (see BC91). The transition occurs at precisely where the spiral fades out.
|
The total apparent magnitude, ,
computed from is 13.17. The half
light radius , and the mean surface
brightness within the effective radius
mag arcsec-2. The
best-fitting line (exponential) of the inner part has a central
surface brightness of mag
arcsec-2 and a scale length
.
© European Southern Observatory (ESO) 2000
Online publication: June 20, 2000
helpdesk.link@springer.de  |