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Astron. Astrophys. 351, 903-919 (1999)

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3. Structural parameters

3.1. Central surface brightnesses and scale lengths

Most luminosity profiles clearly reveal the presence of a (HSB) bulge embedded in a low surface brightness disk (Figs. 1, upper panels). The profiles were decomposed into bulge and disk components by taking exponential profiles of the form

[EQUATION]

where [FORMULA] is the central intensity (e.g. [FORMULA] pc-2), h is the scale length of either bulge or disk and n is taken to be 1 for both bulge and disk. Transforming to a logarithmic scale this equation becomes

[EQUATION]

where [FORMULA] is the central surface brightness (mag arcsec-2) of either disk or bulge and r is the distance along the major axis. Typical errors are 15% in scale length and 0.1 mag arcsec-2 in surface brightness. In the past people used [FORMULA] profiles for fitting bulges of all types of spiral galaxies. But it is now known that the bulge exponent correlates well with the morphological type of the galaxy. Ellipticals and S0's have luminosity profiles that follow the [FORMULA] law (de Vaucouleurs 1948), while bulges of Sa and Sb galaxies are better described by [FORMULA] profiles. The bulges of later types can be well represented by exponential ([FORMULA]) luminosity profiles (Andredakis 1997; de Jong 1995, hereafter dJ95). However, the bulges of the galaxies in our sample were all better fitted with exponential profiles, regardless of their exact morphological types. This of course should not come as a surprise given the large difficulties in assigning Hubble types to such low contrast galaxies. Examples of bulge-disk decompositions are shown in Fig. 3.

[FIGURE] Fig. 1a. Top panels contain B and R surface brightness profiles; bottom panels contain radial color profiles for the galaxies ESO-LV 0050050, ESO-LV 4350310 and ESO-LV 4250180. In all cases R-I has been offset by -0.5 mag.

[FIGURE] Fig. 1b. Top panels contain B and R surface brightness profiles; bottom panels contain radial color profiles of the galaxies ESO-LV 0140040, ESO-LV 0540240 and ESO-LV 5650160. In all cases R-I has been offset by -0.5 mag.

[FIGURE] Fig. 1c. Top panels contain B and R surface brightness profiles; bottom panels contain radial color profiles for the galaxies ESO-LV 1590200, ESO-LV 0590090 and ESO-LV 4990110. In all cases R-I has been offset by -0.5 mag.

[FIGURE] Fig. 1d. Top panels contain B and R surface brightness profiles; bottom panels contain radial color profiles for the galaxies ESO-LV 2060140, ESO-LV 5450360 and ESO-LV 1530170. In all cases R-I has been offset by -0.5 mag.

[FIGURE] Fig. 1e. Top panels contain B and R surface brightness profiles; bottom panels contain radial color profiles for the galaxies ESO-LV 3500110, ESO-LV 5520190 and ESO-LV 1220040. In all cases R-I has been offset by -0.5 mag.

[FIGURE] Fig. 1f. Top panels contain B and R surface brightness profiles; bottom panels contain radial color profiles for the galaxies ESO-LV 4220090, ESO-LV 4370420 and ESO-LV 2520100. In all cases R-I has been offset by -0.5 mag.

[FIGURE] Fig. 1g. Top panel contains B and R surface brightness profiles; bottom panel contains radial color profiles for ESO-LV 3740090. R-I has been offset by -0.5 mag.

The structural parameters derived from our fits are listed in Table 2. The histograms of the structural disk parameters, scale length and central surface brightness, for our sample, a HSB sample (dJ95) and a giant LSB sample (Sprayberry et al. 1995) are shown in Fig. 2.

[FIGURE] Fig. 2. Histograms of the B band structural disk parameters [FORMULA] and [FORMULA]. Solid line = bulge LSB galaxies. Dashed line = HSB galaxies (dJ95). Shaded region = giant LSB galaxies (Sprayberry et al. 1995). The histogram of the dJ95 sample is divided by 3. There is no preferred value of either [FORMULA] or [FORMULA]. Note the HSB galaxy in our sample.

[FIGURE] Fig. 3. Bulge-disk decompositions. The exponential bulge and disk fits plus the total profiles are shown.


[TABLE]

Table 2. Structural parameters of bulge dominated LSB galaxies.


The distribution of B band disk scale lengths is very broad and has a median value of [FORMULA] = 12.6 kpc. Our LSB sample has a median disk central surface brightness of [FORMULA] = 23.04 B mag arcsec-2. There is no preferred value of either. The one HSB galaxy (ESO-LV 1150280) in our sample (Fig. 2) was not used in all further discussions and conclusions about bulge dominated LSB galaxies. The fact that there is only one HSB disk galaxy in our sample confirms that our selection was succesful in picking LSB disks.

3.2. Effect of seeing on bulge scale lengths

The typical sizes of the bulges in our sample are [FORMULA] and the mean B band bulge scale length is 2.37". The bulges are thus larger than the seeing disk ([FORMULA]) and only the innermost ([FORMULA]) parts are not resolved. In order to investigate the effect of seeing on the fitted bulge scale lengths we used the images of the galaxies with the smallest bulges as scaled models and smoothed these to various resolutions. It appeared that only the innermost one or two points are affected. The fitted scale lengths, which are largely determined by the outerpoints between radii of 2" and [FORMULA], do not change within the stated errors, so we did not attempt to correct for seeing.

3.3. Total magnitudes

Total apparent magnitudes can be determined by integrating Eq. 1 out to infinity and converting to a logarithmic scale:

[EQUATION]

Optical depth effects in the galaxies have not been taken into account, because of the small inclinations. However, we did not use Eq. 3 as [FORMULA] depends on extrapolation of the light profile which introduces uncertainties. Furthermore, it does not take into account bars and rings which are clearly present in some of the galaxies in our sample. To determine total absolute magnitudes we used aperture magnitudes [FORMULA] instead. We summed all the light to [FORMULA] above sky ([FORMULA] B mag arcsec-2) and converted this to magnitudes. We used the distances listed in Table 1 to derive total absolute magnitudes. Fig. 4 shows the distribution of absolute magnitudes [FORMULA] derived in this way. It is evident that low surface brightness does not imply low luminosity. LSB galaxies can be very luminous.

[FIGURE] Fig. 4. Histogram of the total absolute B magnitudes. LSB galaxies can be very luminous.

3.4. Colors and color profiles

Radial color profiles were obtained by subtracting two radial luminosity profiles from each other. The bottom panels of Figs. 1 show color profiles of the colors B-R , B-V and R-I . The errors in the color profiles were estimated by adding in quadrature the errors in the surface brightness profiles. Most galaxies show color gradients and become bluer with increasing radius (declining profiles). It is believed that the dust content in disk LSB galaxies is low (dB95). Despite having no CO and metallicity information of the galaxies in our sample we probably can assume that some bulge LSB galaxies are also dust poor - in ESO-LV 4250180 you can actually see a background galaxy through the spiral arms. Then these color gradients are probably not caused by dust only. The bluer colors in the outer parts could be caused by relatively more light coming from a younger population at larger radii. This suggests that star formation started in the inner parts and progressed outwards.

Total integrated colors were determined by dividing sky-subtracted images and then integrating within ellipses. The same ellipse parameters as for determining the radial luminosity profiles have been used. We determined three total colors. The nuclear color is the color of the inner 10". The luminosity weighted color is the color as measured through a large isophotal aperture covering almost the entire galaxy. The area weighted color is determined from the average of the colors of many pixel-sized apertures over the entire disk. When determining luminosity weighted colors most weight is given to bright regions (nucleus, HII regions) of the galaxy. This is in contrast with area weighted colors where all parts have equal weight and only the area matters. Table 3 gives total colors of our sample. The estimated errors are [FORMULA] mag.


[TABLE]

Table 3. Total colors of bulge dominated LSB galaxies.


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© European Southern Observatory (ESO) 1999

Online publication: November 16, 1999
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