3 N50: A link between BCDs and dEs?

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 camera 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 NGC5044 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 accomplished using the IRAF package achieving a $\pm 0.001$ mag internal error, while external magnitude uncertainty from standard zero points amounted to $\pm 0.03$ mag. Surface brightness profiles have been obtained in the four bands down $S_{\rm g} \sim 28$ mag arcsec^-2.

Direct imaging has been complemented also with long-slit spectroscopy between 4300 and 6300 Å at 6 Å FWHM wavelength resolution. Supplementary spectroscopic observations have been also carried out in the range 3500-5400 Å at 8 Å FWHM resolution with a Boller & Chivens spectrograph at the 2.15 m telescope of the CASLEO observatory in San Juan, Argentina on April 9, 1997.

Figure 2 (top panel) shows a $1\hbox {$^\prime $ }\times 1\hbox {$^\prime $ }$ gband contour plot of N50 (the stellar PSF FWHM is $0\hbox{$.\!\!^{\prime\prime}$ }96$). The central $r \le 3\hbox{$.\!\!^{\prime\prime}$ }5$ ( 400 h₀^-1 pc) region shows several knots surrounding 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 local "background'' measured around each source eventually revealed the best choice. The internal photometric error amounted in this case to $\pm 0.004$ mag in each band.

 

 

Table 1: Gunn photometry of N50

	Aperture magnitudes*
radius ['']	g	g-r	g-i	g-z
$2\hbox{$.\!\!^{\prime\prime}$ }5$	17.554	0.436	0.697	0.610
$5\hbox{$.\!\!^{\prime\prime}$ }0$	16.384	0.417	0.666	0.582
$10\hbox {$^{\prime \prime }$ }$	15.628	0.416	0.669	0.582
$15\hbox{$^{\prime\prime}$ }$	15.347	0.416	0.667	0.582
$20\hbox{$^{\prime\prime}$ }$	15.250	0.413	0.661	0.574
$30\hbox {$^{\prime \prime }$ }$	15.213	0.412	0.654	0.575
	Knot photometry*
	g	g-r	g-i	g-z
K-1	19.272	0.482	0.757	0.652
K-2	19.786	0.360	0.572	0.517
K-3	19.862	0.374	0.609	0.520
K-4	19.779	0.381	0.614	0.554
K-5	19.950	0.398	0.628	0.554
K-6	19.797	0.416	0.646	0.590
K-7	20.029	0.429	0.682	0.598
K-8	19.820	0.474	0.755	0.662
K-9	19.750	0.399	0.635	0.551
K-10	19.913	0.409	0.647	0.564

^(*) Internal mag uncertainty is 0.001 for aperture photometry and 0.004 for knot magnitudes. Zero-point external error is 0.03 mag throughout.

Figure 3: Top: the g-band surface brightness profile of N50. Error bars account for photometric errors and uncertainties in the sky level. Bottom: the resulting g-i colour profile of the galaxy

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 $3\hbox{$^{\prime\prime}$ }$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 $8\hbox{$^{\prime\prime}$ }\le r \le 28\hbox{$^{\prime\prime}$ }$.

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 predictions 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.

Figure 4: ( Main panel) - Two-colour diagram comparing N50 integrated g-r and g-i (big open dot) with the stellar Main Sequence from the Vilnius spectral atlas (star markers) and with Buzzoni's (1998, 2000) 15 Gyr galaxy models for different Hubble types, as labelled top left. ( Insert panel) - N50 aperture photometry (solid dots) and individual colours of the visible knots in the galaxy central region (open dots with identification labels according to Table 1). Aperture photometry is for circular spots at $2\hbox{$.\!\!^{\prime\prime}$ }5$, $10\hbox {$^{\prime \prime }$ }$, and $30\hbox {$^{\prime \prime }$ }$ from galaxy centre. Colours become bluer at larger distance. Buzzoni's (1989) SSP evolutionary sequences for 5, 10, and 15 Gyr (in the sense of redder colours) are superposed (thin solid lines). Model metallicity is for [Fe/H] = -0.25 (squares), 0.0 (triangles) and +0.3 (stars). A reddening vector for E(B-V) = 0.03mag is displayed top left in the panel

N50 aperture photometry at $2\hbox{$.\!\!^{\prime\prime}$ }5$, $10\hbox {$^{\prime \prime }$ }$, and $30\hbox {$^{\prime \prime }$ }$ 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] $\sim -0.2$) 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 [ $\Delta E(B-V) \sim 0.05$] 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 $E(B-V) \sim 0.03$according to Burstein & Heiles 1982). In addition, one should also consider a little blue shift of all the galaxy data by $\Delta (g-r) \simeq 0.012$and $\Delta (g-i) \simeq 0.015$ 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 invoked 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 $3500 \leq \lambda \leq 4400$ Å with ESO observations ( $4400 \leq \lambda \leq 6300$ Å), 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 $15\hbox{$^{\prime\prime}$ }$ aperture.

Figure 5: Composite spectral energy distribution of galaxy N50. The CASLEO spectrum, in the range $3500 \leq \lambda \leq 4400$ Å has been matched to ESO observations ( $4400 \leq \lambda \leq 6300$ Å) while integrated g, r, i, and z Gunn magnitudes have been converted into monochromatic fluxes. Absolute flux calibration reproduces galaxy energy distribution within the central $15\hbox{$.\!\!^{\prime\prime}$ }$ aperture. Two SSP models from Buzzoni (1989), with [Fe/H] = -0.25 and age 5 and 15 Gyrs, are superposed on the plot (thin solid lines, the older model is the "redder'' one)

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 optical and red wavelength while it lacks UV luminosity below 4500 Å. On the contrary, the 5 Gyr stellar population provides 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 $cz = 2391 \pm 97$ km s^-1 (that is $z = 0.0080 \pm 0.0003$). This yields a distance to the galaxy of 23.9 h₀^-1 Mpc and a distance modulus $(m-M) = 31.9 -5\log(h_0)$. From the model fit of Fig. 5, a bolometric correction to g of $({\it Bol} - g) = -1.01$ can be obtained, leading for N50 to $M_{\rm bol} = -17.7 +5 \log(h_0)$. This is $L_{\rm bol} = 9.3~10^8 h_0^{-2} \,L_\odot$ or $M_B = -16.1 +5 \log(h_0)$. Estimating a theoretical $M/L_{\rm bol} \sim 5 \pm 1$ from the relevant SSP models of Buzzoni (1989) (once accounting for the whole stellar mass by integrating the Salpeter IMF) then one obtains $M_{\rm tot} \sim 4.5 (\pm 1.0)~10^9h_0^{-2}\,M_\odot$ as a fair estimate of the total (stellar) mass of N50.

   
3.1 Star formation and morphology transition

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 ($\sim$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) classification.

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$\beta$ and 3727 Å [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 connection (Thuan 1985; Evans et al. 1990; Meurer et al. 1992).

Figure 6: Expected B-V and g-r star-burst colour evolution according to Buzzoni (1989, 1998). Three different values for [Fe/H] = -0.3, 0.0 (solid lines), and +0.3 are displayed, as labelled on the B-Vplot. The indicative age distribution of the N50 central knots is also reported in the g-r plot (assuming [Fe/H] = -0.25). Note that, in general, even any massive starburst episode older than 2 Gyr, superposed to a galaxy old stellar component would not be able to turn integrated colours bluer than (B-V) = 0.7 still allowing galaxy to be recognized as a quiescent "standard'' elliptical according to the YC00 colour selection criterium

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-Nrelationship.

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.