Top : 1 × 1 contour plot ( g band) of N50. The faintest and brightest contours displayed are 25 and 20 mag arcsec − 1 , respectively, with 0.25 mag arcsec − 1 separation between concentric contour levels. North is up and East to the left. Bottom left : greyscale plot after subtraction of a Sérsic model. Orientation and scale are the same as in the upper panel. Bottom right : enlarged view (15 × 15 ) of the central region with identification for the knots (see Table 1) 

Contexts in source publication

Context 1

... conditions and other internal bias sources (e.g. ongoing star formation) act in facts in the sense of disturbing galaxy morphology making any fitting procedure somewhat dependent on galaxy apparent size and on environment conditions as well. In spite of any standard criterion to single out the bulge component, it is clear however that by simply ne- glecting the problem one would more likely tend to predict too “spiked” galaxy profiles preferring lower values of N (Andredakis et al. 1995). We will turn back on this point and its impact on the L − N relationship in Sect. 4. The galaxy N50 was originally classified as dEpec, N/BCDring by Ferguson & Sandage (1990). Because of poor spatial resolution, C99 failed to detect any BCD feature in this object eventually appearing as a normal dE, based also on its ( B − V ) 0 = 0 . 76 (cf. Caldwell & Bothun 1987). In any case, this would make N50 an interesting object because it is, along with N42, one of the two brightest dEs with convex profile (i.e., N > 1) in the C99 sample. To better assess its evolutionary properties, we collected new observations of N50 with the EFOSC2 cam- era at the ESO 3.6 m telescope in La Silla, Chile, on the nights of April 16 and 17, 1999, as a part of a study of the low surface brightness galaxy population of the NGC 5044 Group. A detailed description of the observations and image processing will be given in a forthcoming paper (Buzzoni et al. 2001). Direct images of this galaxy were obtained under sub-arcsec seeing conditions in the g , r , i , and z bands of the Gunn system (Schneider et al. 1983). Data reduction has been accom- plished using the IRAF 2 package achieving a ± 0 . 001 mag internal error, while external magnitude uncertainty from standard zero points amounted to ± 0 . 03 mag. Surface brightness profiles have been obtained in the four bands down S ∼ 28 mag arcsec − 2 . Direct imaging has been complemented also with long- slit spectroscopy between 4300 and 6300 A at 6 A FWHM wavelength resolution. Supplementary spectroscopic observations have been also carried out in the range 3500– 5400 A at 8 A FWHM resolution with a Boller & Chivens spectrograph at the 2.15 m telescope of the CASLEO ob- servatory in San Juan, Argentina on April 9, 1997. Figure 2 ( top panel ) shows a 1 × 1 g band contour plot of N50 (the stellar PSF FWHM is 0 . 96). The central r ≤ 3 . 5 (400 h − 0 1 pc) region shows several knots surround- ing a central cusp. These features are better seen after subtraction of a Sérsic model ( bottom left panel ), and even a probable dust lane can be appreciated west of the nucleus. An enlarged map that identifies the central knots is also reported in the figure ( bottom right ). Table 1 reports a full summary of the galaxy photometry, including aperture magnitudes and detailed measurements of the single knots. For the latter features, we tried different clean-up procedures to subtract the smooth galaxy contribution; however, a plain subtraction of the lo- cal “background” measured around each source eventually revealed the best choice. The internal photometric error amounted in this case to ± 0 . 004 mag in each band. The g surface brightness profile of N50 is shown in Fig. 3. Error bars including photon noise and sky level uncertainty have been taken into account in the plot. The bottom panel shows the g i colour profile (after slightly degrading the i frame to the g PSF to consistently sample surface luminosity). Note in the figure the blue colour bump about 3 from the centre due to the knotty ring. A smooth colour gradient is also evident along galaxy radius with the outermost regions sensibly bluer than the centre. This colour gradient affects the value of the parameter N , which increases monotonically from N = 1 . 39 in g to N = 1 . 63 in z , sampling the range 8 ≤ r ≤ 28 . The location of N50 in a g − r vs. g − i colour diagram is shown in Fig. 4. In the main panel of the figure we compared galaxy integrated colours with the locus of Main Sequence stars (based on the Vilnius spectral atlas of Straizys & Sviderskiene 1972), as well as with the theoretical colours for 15 Gyr template galaxies of different morphological type according to the three-zone synthesis models of Buzzoni (1998, 2000). As expected, N50 colours are slightly bluer than high-mass ellipticals, and intermediate between E and Sa Hubble types. A more detailed match of the population synthesis pre- dictions with the N50 colour profile and with the nuclear knotty features is attempted in the insert panel of Fig. 4. Buzzoni’s (1989) simple stellar population (SSP) models, computed for a Salpeter IMF, red horizontal branch morphology, and different metallicity ([Fe/H] = − 0 . 25 , 0 . 0, and +0 . 30) are reported, tracking evolution from 5 to 15 Gyrs. N50 aperture photometry at 2 . 5, 10 , and 30 is displayed together with individual photometry of the visible knots according to the identification number in Table 1 (see also the reference map in Fig. 2). A substantial agreement seems to exist between theoretical models and observations within the zero-point uncertainty in the magnitude scale. An old (10–15 Gyr) stellar population with slightly sub-solar metallicity ([Fe/H] ∼ − 0 . 2) appears to be the main component in N50 but a mild [Fe/H] radial gradient might also exist inducing the blueing colour drift along galaxy radius. Quite interestingly, nuclear knots reveal a much larger (and statistically significant) spread in colour. Knot #1 (the nucleus?) appears indeed even redder than the galaxy core, as do those lying close to the apparent dust lane (#8) visible one arcsec west of the nucleus (cf. Fig. 2). Dust reddening might be “patchy” on the N50 central region, with the west area (corresponding to knots # 1, 6, 7, near the dust lane # 8) slightly more obscured [∆ E ( B − V ) ∼ 0 . 05] than the east side (i.e., about knots # 3 and 4). Data in Fig. 4 are not corrected for our own Galaxy reddening (which however should not exceed E ( B − V ) ∼ 0 . 03 according to Burstein & Heiles 1982). In addition, one should also consider a little blue shift of all the galaxy data by ∆( g − r ) 0 . 012 and ∆( g − i ) 0 . 015 to take into account for k-correction. Even correcting for these effects, it seems likely however that the whole stellar population in N50 should consist of stars older than 5 Gyr, and only a much enhanced (super-solar) metallicity should be in- voked to predict a younger age. The spread in age among the galaxy stellar population is even more evident from the analysis of the integrated spectrum, shown in Fig. 5. Galaxy spectral energy distribution has been obtained by matching the CASLEO data in the range 3500 ≤ λ ≤ 4400 A with ESO observations (4400 ≤ λ ≤ 6300 A), adding then the monochromatic fluxes from the integrated g , r , i , and z magnitudes. Gunn photometry also set the absolute flux calibration reproducing galaxy energy distribution within the central 15 aperture. Two SSP models from the Buzzoni (1989) data set, with [Fe/H] = − 0 . 25 and age 5 and 15 Gyrs, are superposed on the plot. The older model fits well along the opti- cal and red wavelength while it lacks UV luminosity below 4500 A. On the contrary, the 5 Gyr stellar population pro- vides a good fit to the ultraviolet but it would predict too blue Gunn colours. Accounting for SSP luminosity evolution, in a simple interpretative scheme assuming a mix of these two main stellar components, this allows us to estimate that the old (15 Gyr) population comprises about 3/4 of the total mass of the galaxy. The redshift of N50, as derived from our spectra, amounts to cz = 2391 ± 97 km s − 1 (that is z = 0 . 0080 ± 0 . 0003). This yields a distance to the galaxy of 23 . 9 h − 0 1 Mpc and a distance modulus ( m − M ) = 31 . 9 − 5 log( h 0 ). From the model fit of Fig. 5, a bolo- metric correction to g of ( Bol − g ) = − 1 . 01 can be obtained, leading for N50 to M bol = − 17 . 7 + 5 log( h 0 ). This is L = 9 . 3 10 8 h − 2 L or M = − 16 . 1 + 5 log( h ). Estimating a theoretical M/L bol 5 1 from the relevant SSP models of Buzzoni (1989) (once accounting for the whole stellar mass by integrating the Salpeter IMF) then one obtains M tot ∼ 4 . 5( ± 1 . 0) 10 9 h − 0 2 M as a fair estimate of the total (stellar) mass of N50. In its overall morphology, N50 is very reminiscent of the dwarf ( M B = − 16 . 71 mag) galaxy Markarian 996, which Thuan et al. (1996) report to have smooth elliptical isophotes, with several bright knots and dust patches in its central ( ∼ 400 pc) region. However, contrary to what we observe in N50, Mrk 996 clearly shows signs of active ongoing star formation in its centre, thus fitting with a nE BCD (nuclear-elliptical blue-compact-dwarf) classifi- cation. The age we derive for the blue knots in N50 (see Fig. 6) suggests that it may have looked very similar to Mrk 996 a few Gyrs ago. Although now observed in its more quiescent evolutionary stage, the H β and 3727 A [OII] emission lines, prominent in our spectrum of the galaxy, still witness the presence of a wealth residual gas. In this sense, N50 probably represents an ideal link in the dE–BCD connec- tion (Thuan 1985; Evans et al. 1990; Meurer et al. 1992). Quite importantly, it is also worth noting from Fig. 6 that even any massive intervening starburst activity older than 2 Gyr, superposed to the galaxy “quiescent” old stellar component, would still maintain integrated colours redder than ( B − V ) = 0 . 7. According to the YC00 colour selection criterium, active dwarf ellipticals would therefore be recognized as “standard” galaxies in the L − N relationship. Despite such a negligible effect on the integrated colours, starburst episodes could however much more strongly affect galaxy surface brightness profiles. In the case of N50, for example, the presence of the bright knotty ring around the centre certainly modulates the Sérsic shape parameter leading to a higher fitting value for N . Each of the three ...

Context 2

... actually has a convex profile”, meaning the three “outliers” N42, N49, and N50. One of the questioned objects in the C99 sample is galaxy N42, for which C99 indicates N = 1 . 43 to be compared with N = 0 . 60 in the YC00 fit. At least two evident weak points emerge, in our opinion, from the YC00 analysis. Contrary to the C99 fitting procedure, that only relied on the S/N > 1 portion of galaxy surface brightness profile (i.e. with I ( r ) > σ SKY ), the new fit extends much farther from galaxy centre. In the outermost regions, photon noise and statistical uncertainty in the sky subtraction begin to dominate causing the output profile to artificially level off at large radii. By itself, this effect works in inducing nominally “concave” (i.e., N < 1) surface brightness profiles throughout in the YC00 fit (see Andredakis et al. 1995 for similar conclusions dealing with bulge deconvolution in spiral galaxies). In addition, even at first glance (cf. Fig. 1) the N42 surface brightness profile reveals at least two distinct components: an inner bulge and a main body extending out to ∼ 50 . This galaxy would therefore need a multi- component scheme (e.g.: Papaderos et al. 1996) to prop- erly decompose its profile. Cellone’s (1999) model for the N42 main component provided an integrated magnitude V T = 15 . 3 mag. After subtraction, this leaves the inner bulge component, with V bulge = 16 . 4, and extending out to r 17 , as evident from Fig. 1. Although this decom- position scheme might probably be not unique, it shows however that the main morphological component of the galaxy, providing about 3/4 of the total V luminosity, is in fact suitably fitted by the original “convex” profile. Our choice is also supported by a χ 2 test on the fitting residuals confirming that a simple Sérsic fit can be ruled out at a 95% confidence level. As a comparison, Fig. 1 ( lower panel ) also shows the profile of the bright dwarf N29. In this case, no change in slope is evident, and a single Sérsic law (with N = 0 . 54) fits nicely this profile all along its useful range, as confirmed again by the χ 2 statistics. The case of N51 (the second galaxy disputed by YC00) is similar to that of N42, although not so extreme, while for the third object, the previously uncatalogued galaxy N95A, C99 reported an exceedingly low surface brightness ( S 0 ∼ 24 mag arcsec − 2 ) that definitely prevented any reliable fit. For this reason this galaxy was not included in any subsequent analysis. While statistical tests support in our case both the choice of a two-component fit for N42, and a simple Sérsic law for N29, more generally any suitable correction for the bulge contribution in dwarf galaxies may be a non- univocal task. Seeing conditions and other internal bias sources (e.g. ongoing star formation) act in facts in the sense of disturbing galaxy morphology making any fitting procedure somewhat dependent on galaxy apparent size and on environment conditions as well. In spite of any standard criterion to single out the bulge component, it is clear however that by simply ne- glecting the problem one would more likely tend to predict too “spiked” galaxy profiles preferring lower values of N (Andredakis et al. 1995). We will turn back on this point and its impact on the L − N relationship in Sect. 4. The galaxy N50 was originally classified as dEpec, N/BCDring by Ferguson & Sandage (1990). Because of poor spatial resolution, C99 failed to detect any BCD feature in this object eventually appearing as a normal dE, based also on its ( B − V ) 0 = 0 . 76 (cf. Caldwell & Bothun 1987). In any case, this would make N50 an interesting object because it is, along with N42, one of the two brightest dEs with convex profile (i.e., N > 1) in the C99 sample. To better assess its evolutionary properties, we collected new observations of N50 with the EFOSC2 cam- era at the ESO 3.6 m telescope in La Silla, Chile, on the nights of April 16 and 17, 1999, as a part of a study of the low surface brightness galaxy population of the NGC 5044 Group. A detailed description of the observations and image processing will be given in a forthcoming paper (Buzzoni et al. 2001). Direct images of this galaxy were obtained under sub-arcsec seeing conditions in the g , r , i , and z bands of the Gunn system (Schneider et al. 1983). Data reduction has been accom- plished using the IRAF 2 package achieving a ± 0 . 001 mag internal error, while external magnitude uncertainty from standard zero points amounted to ± 0 . 03 mag. Surface brightness profiles have been obtained in the four bands down S ∼ 28 mag arcsec − 2 . Direct imaging has been complemented also with long- slit spectroscopy between 4300 and 6300 A at 6 A FWHM wavelength resolution. Supplementary spectroscopic observations have been also carried out in the range 3500– 5400 A at 8 A FWHM resolution with a Boller & Chivens spectrograph at the 2.15 m telescope of the CASLEO ob- servatory in San Juan, Argentina on April 9, 1997. Figure 2 ( top panel ) shows a 1 × 1 g band contour plot of N50 (the stellar PSF FWHM is 0 . 96). The central r ≤ 3 . 5 (400 h − 0 1 pc) region shows several knots surround- ing a central cusp. These features are better seen after subtraction of a Sérsic model ( bottom left panel ), and even a probable dust lane can be appreciated west of the nucleus. An enlarged map that identifies the central knots is also reported in the figure ( bottom right ). Table 1 reports a full summary of the galaxy photometry, including aperture magnitudes and detailed measurements of the single knots. For the latter features, we tried different clean-up procedures to subtract the smooth galaxy contribution; however, a plain subtraction of the lo- cal “background” measured around each source eventually revealed the best choice. The internal photometric error amounted in this case to ± 0 . 004 mag in each band. The g surface brightness profile of N50 is shown in Fig. 3. Error bars including photon noise and sky level uncertainty have been taken into account in the plot. The bottom panel shows the g i colour profile (after slightly degrading the i frame to the g PSF to consistently sample surface luminosity). Note in the figure the blue colour bump about 3 from the centre due to the knotty ring. A smooth colour gradient is also evident along galaxy radius with the outermost regions sensibly bluer than the centre. This colour gradient affects the value of the parameter N , which increases monotonically from N = 1 . 39 in g to N = 1 . 63 in z , sampling the range 8 ≤ r ≤ 28 . The location of N50 in a g − r vs. g − i colour diagram is shown in Fig. 4. In the main panel of the figure we compared galaxy integrated colours with the locus of Main Sequence stars (based on the Vilnius spectral atlas of Straizys & Sviderskiene 1972), as well as with the theoretical colours for 15 Gyr template galaxies of different morphological type according to the three-zone synthesis models of Buzzoni (1998, 2000). As expected, N50 colours are slightly bluer than high-mass ellipticals, and intermediate between E and Sa Hubble types. A more detailed match of the population synthesis pre- dictions with the N50 colour profile and with the nuclear knotty features is attempted in the insert panel of Fig. 4. Buzzoni’s (1989) simple stellar population (SSP) models, computed for a Salpeter IMF, red horizontal branch morphology, and different metallicity ([Fe/H] = − 0 . 25 , 0 . 0, and +0 . 30) are reported, tracking evolution from 5 to 15 Gyrs. N50 aperture photometry at 2 . 5, 10 , and 30 is displayed together with individual photometry of the visible knots according to the identification number in Table 1 (see also the reference map in Fig. 2). A substantial agreement seems to exist between theoretical models and observations within the zero-point uncertainty in the magnitude scale. An old (10–15 Gyr) stellar population with slightly sub-solar metallicity ([Fe/H] ∼ − 0 . 2) appears to be the main component in N50 but a mild [Fe/H] radial gradient might also exist inducing the blueing colour drift along galaxy radius. Quite interestingly, nuclear knots reveal a much larger (and statistically significant) spread in colour. Knot #1 (the nucleus?) appears indeed even redder than the galaxy core, as do those lying close to the apparent dust lane (#8) visible one arcsec west of the nucleus (cf. Fig. 2). Dust reddening might be “patchy” on the N50 central region, with the west area (corresponding to knots # 1, 6, 7, near the dust lane # 8) slightly more obscured [∆ E ( B − V ) ∼ 0 . 05] than the east side (i.e., about knots # 3 and 4). Data in Fig. 4 are not corrected for our own Galaxy reddening (which however should not exceed E ( B − V ) ∼ 0 . 03 according to Burstein & Heiles 1982). In addition, one should also consider a little blue shift of all the galaxy data by ∆( g − r ) 0 . 012 and ∆( g − i ) 0 . 015 to take into account for k-correction. Even correcting for these effects, it seems likely however that the whole stellar population in N50 should consist of stars older than 5 Gyr, and only a much enhanced (super-solar) metallicity should be in- voked to predict a younger age. The spread in age among the galaxy stellar population is even more evident from the analysis of the integrated spectrum, shown in Fig. 5. Galaxy spectral energy distribution has been obtained by matching the CASLEO data in the range 3500 ≤ λ ≤ 4400 A with ESO observations (4400 ≤ λ ≤ 6300 A), adding then the monochromatic fluxes from the integrated g , r , i , and z magnitudes. Gunn photometry also set the absolute flux calibration reproducing galaxy energy distribution within the central 15 aperture. Two SSP models from the Buzzoni (1989) data set, with [Fe/H] = − 0 . 25 and age 5 and 15 Gyrs, are superposed on the plot. The older model fits well along the opti- cal and red wavelength while it lacks UV luminosity below 4500 A. On the contrary, the 5 Gyr stellar population pro- vides a ...

Context 3

... (1990). Because of poor spatial resolution, C99 failed to detect any BCD feature in this object eventually appearing as a normal dE, based also on its ( B − V ) 0 = 0 . 76 (cf. Caldwell & Bothun 1987). In any case, this would make N50 an interesting object because it is, along with N42, one of the two brightest dEs with convex profile (i.e., N > 1) in the C99 sample. To better assess its evolutionary properties, we collected new observations of N50 with the EFOSC2 cam- era at the ESO 3.6 m telescope in La Silla, Chile, on the nights of April 16 and 17, 1999, as a part of a study of the low surface brightness galaxy population of the NGC 5044 Group. A detailed description of the observations and image processing will be given in a forthcoming paper (Buzzoni et al. 2001). Direct images of this galaxy were obtained under sub-arcsec seeing conditions in the g , r , i , and z bands of the Gunn system (Schneider et al. 1983). Data reduction has been accom- plished using the IRAF 2 package achieving a ± 0 . 001 mag internal error, while external magnitude uncertainty from standard zero points amounted to ± 0 . 03 mag. Surface brightness profiles have been obtained in the four bands down S ∼ 28 mag arcsec − 2 . Direct imaging has been complemented also with long- slit spectroscopy between 4300 and 6300 A at 6 A FWHM wavelength resolution. Supplementary spectroscopic observations have been also carried out in the range 3500– 5400 A at 8 A FWHM resolution with a Boller & Chivens spectrograph at the 2.15 m telescope of the CASLEO ob- servatory in San Juan, Argentina on April 9, 1997. Figure 2 ( top panel ) shows a 1 × 1 g band contour plot of N50 (the stellar PSF FWHM is 0 . 96). The central r ≤ 3 . 5 (400 h − 0 1 pc) region shows several knots surround- ing a central cusp. These features are better seen after subtraction of a Sérsic model ( bottom left panel ), and even a probable dust lane can be appreciated west of the nucleus. An enlarged map that identifies the central knots is also reported in the figure ( bottom right ). Table 1 reports a full summary of the galaxy photometry, including aperture magnitudes and detailed measurements of the single knots. For the latter features, we tried different clean-up procedures to subtract the smooth galaxy contribution; however, a plain subtraction of the lo- cal “background” measured around each source eventually revealed the best choice. The internal photometric error amounted in this case to ± 0 . 004 mag in each band. The g surface brightness profile of N50 is shown in Fig. 3. Error bars including photon noise and sky level uncertainty have been taken into account in the plot. The bottom panel shows the g i colour profile (after slightly degrading the i frame to the g PSF to consistently sample surface luminosity). Note in the figure the blue colour bump about 3 from the centre due to the knotty ring. A smooth colour gradient is also evident along galaxy radius with the outermost regions sensibly bluer than the centre. This colour gradient affects the value of the parameter N , which increases monotonically from N = 1 . 39 in g to N = 1 . 63 in z , sampling the range 8 ≤ r ≤ 28 . The location of N50 in a g − r vs. g − i colour diagram is shown in Fig. 4. In the main panel of the figure we compared galaxy integrated colours with the locus of Main Sequence stars (based on the Vilnius spectral atlas of Straizys & Sviderskiene 1972), as well as with the theoretical colours for 15 Gyr template galaxies of different morphological type according to the three-zone synthesis models of Buzzoni (1998, 2000). As expected, N50 colours are slightly bluer than high-mass ellipticals, and intermediate between E and Sa Hubble types. A more detailed match of the population synthesis pre- dictions with the N50 colour profile and with the nuclear knotty features is attempted in the insert panel of Fig. 4. Buzzoni’s (1989) simple stellar population (SSP) models, computed for a Salpeter IMF, red horizontal branch morphology, and different metallicity ([Fe/H] = − 0 . 25 , 0 . 0, and +0 . 30) are reported, tracking evolution from 5 to 15 Gyrs. N50 aperture photometry at 2 . 5, 10 , and 30 is displayed together with individual photometry of the visible knots according to the identification number in Table 1 (see also the reference map in Fig. 2). A substantial agreement seems to exist between theoretical models and observations within the zero-point uncertainty in the magnitude scale. An old (10–15 Gyr) stellar population with slightly sub-solar metallicity ([Fe/H] ∼ − 0 . 2) appears to be the main component in N50 but a mild [Fe/H] radial gradient might also exist inducing the blueing colour drift along galaxy radius. Quite interestingly, nuclear knots reveal a much larger (and statistically significant) spread in colour. Knot #1 (the nucleus?) appears indeed even redder than the galaxy core, as do those lying close to the apparent dust lane (#8) visible one arcsec west of the nucleus (cf. Fig. 2). Dust reddening might be “patchy” on the N50 central region, with the west area (corresponding to knots # 1, 6, 7, near the dust lane # 8) slightly more obscured [∆ E ( B − V ) ∼ 0 . 05] than the east side (i.e., about knots # 3 and 4). Data in Fig. 4 are not corrected for our own Galaxy reddening (which however should not exceed E ( B − V ) ∼ 0 . 03 according to Burstein & Heiles 1982). In addition, one should also consider a little blue shift of all the galaxy data by ∆( g − r ) 0 . 012 and ∆( g − i ) 0 . 015 to take into account for k-correction. Even correcting for these effects, it seems likely however that the whole stellar population in N50 should consist of stars older than 5 Gyr, and only a much enhanced (super-solar) metallicity should be in- voked to predict a younger age. The spread in age among the galaxy stellar population is even more evident from the analysis of the integrated spectrum, shown in Fig. 5. Galaxy spectral energy distribution has been obtained by matching the CASLEO data in the range 3500 ≤ λ ≤ 4400 A with ESO observations (4400 ≤ λ ≤ 6300 A), adding then the monochromatic fluxes from the integrated g , r , i , and z magnitudes. Gunn photometry also set the absolute flux calibration reproducing galaxy energy distribution within the central 15 aperture. Two SSP models from the Buzzoni (1989) data set, with [Fe/H] = − 0 . 25 and age 5 and 15 Gyrs, are superposed on the plot. The older model fits well along the opti- cal and red wavelength while it lacks UV luminosity below 4500 A. On the contrary, the 5 Gyr stellar population pro- vides a good fit to the ultraviolet but it would predict too blue Gunn colours. Accounting for SSP luminosity evolution, in a simple interpretative scheme assuming a mix of these two main stellar components, this allows us to estimate that the old (15 Gyr) population comprises about 3/4 of the total mass of the galaxy. The redshift of N50, as derived from our spectra, amounts to cz = 2391 ± 97 km s − 1 (that is z = 0 . 0080 ± 0 . 0003). This yields a distance to the galaxy of 23 . 9 h − 0 1 Mpc and a distance modulus ( m − M ) = 31 . 9 − 5 log( h 0 ). From the model fit of Fig. 5, a bolo- metric correction to g of ( Bol − g ) = − 1 . 01 can be obtained, leading for N50 to M bol = − 17 . 7 + 5 log( h 0 ). This is L = 9 . 3 10 8 h − 2 L or M = − 16 . 1 + 5 log( h ). Estimating a theoretical M/L bol 5 1 from the relevant SSP models of Buzzoni (1989) (once accounting for the whole stellar mass by integrating the Salpeter IMF) then one obtains M tot ∼ 4 . 5( ± 1 . 0) 10 9 h − 0 2 M as a fair estimate of the total (stellar) mass of N50. In its overall morphology, N50 is very reminiscent of the dwarf ( M B = − 16 . 71 mag) galaxy Markarian 996, which Thuan et al. (1996) report to have smooth elliptical isophotes, with several bright knots and dust patches in its central ( ∼ 400 pc) region. However, contrary to what we observe in N50, Mrk 996 clearly shows signs of active ongoing star formation in its centre, thus fitting with a nE BCD (nuclear-elliptical blue-compact-dwarf) classifi- cation. The age we derive for the blue knots in N50 (see Fig. 6) suggests that it may have looked very similar to Mrk 996 a few Gyrs ago. Although now observed in its more quiescent evolutionary stage, the H β and 3727 A [OII] emission lines, prominent in our spectrum of the galaxy, still witness the presence of a wealth residual gas. In this sense, N50 probably represents an ideal link in the dE–BCD connec- tion (Thuan 1985; Evans et al. 1990; Meurer et al. 1992). Quite importantly, it is also worth noting from Fig. 6 that even any massive intervening starburst activity older than 2 Gyr, superposed to the galaxy “quiescent” old stellar component, would still maintain integrated colours redder than ( B − V ) = 0 . 7. According to the YC00 colour selection criterium, active dwarf ellipticals would therefore be recognized as “standard” galaxies in the L − N relationship. Despite such a negligible effect on the integrated colours, starburst episodes could however much more strongly affect galaxy surface brightness profiles. In the case of N50, for example, the presence of the bright knotty ring around the centre certainly modulates the Sérsic shape parameter leading to a higher fitting value for N . Each of the three “outliers” that depart from the L N relation in the NGC 5044 sample studied by C99 (namely N42, N49, and N50), displays a different kind of pecu- liarity. As shown in Sect. 2, N42 has a normal colour ( B − V = 0 . 75), but its morphology reveals the presence of a central bulge that should be accounted for in a multi- component fitting model. On the other hand, N49 is a very blue ( B − V = 0 . 49) irregular galaxy (C99), and cannot therefore be included in our analysis of the dwarf elliptical population. N50 takes only apparently the look of a standard dE object. In the previous section we showed that in spite of its quite normal B − V , this galaxy shows the ...