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.

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.

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 :

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.

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).

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
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