3 Photometric analysis

   
3.1 Morphology and colour distribution

Star formation activity in SBS 0940+544 takes place primarily in the unresolved high-surface-brightness H II region labeled a in Fig. 1a. This region with an absolute V magnitude of -13.6 mag accounts for 80% of the total H$\alpha$ emission and for nearly 1/4 of the total V light of the BCD within the 25 V mag arcsec^-2 isophote. A chain of faint point-like sources superimposed on an asymmetric curved arm originating from region a is traceable to $\sim$10 $^{\prime \prime }$ further to the northeast. This feature, labeled b in Fig. 1a, is not detected in H$\alpha$. Another feature is marginally visible to the SW side of the elongated low-surface-brightness (LSB) main body of the galaxy, at a surface brightness level 25.5 V mag arcsec^-2 (region southwards of feature d in Fig. 1a). Judging from the continuum-subtracted H$\alpha$ map (Fig. 1b), star formation in SBS 0940+544 is not exclusively confined to region a but also occurs in the LSB host. A secondary H$\alpha$-emitting region is visible close to the geometrical center of the outer isophotes (region c in Fig. 1a).

Evidence of recent star-formation activity along the major axis of the cometary LSB body of the BCD is also seen in the V-I and V-R colour maps (Fig. 2). The V-I and V-R colours in region c are respectively $\sim$0.43 mag and $\sim$0.16 mag. The V-R colour index remains nearly constant at $\sim$0.2 mag over a large part of the LSB body out to its southeastern extreme end (position +13 $^{\prime \prime }$, -13 $^{\prime \prime }$ in Fig. 2b) and increases to $\sim$0.34 mag at the faint ( $\mu_V\approx 24.0{-}24.5$ mag arcsec^-2) southern region d (position $\sim$+5 $^{\prime \prime }$, -14 $^{\prime \prime }$). The V-I colour shows a similar general, although less well-defined trend, varying from $\sim$0.4-0.5 mag in the main body to $\sim$0.65 mag in region d and getting bluer again (0.36 mag) close to the southeastern tip of the LSB host. The smooth colour change with decreasing intensity in the LSB body is accompanied by a clear isophotal twist between 23.5 and 27 V mag arcsec^-2, with the position angle PA changing from -40$^{\circ }$ to -10$^{\circ }$ and an average gradient of 8.5$^{\circ }$ mag^-1. Likewise, the ellipticity $10\times(1-\frac{b}{a})$ of the LSB host, a and b denoting respectively the semimajor and semiminor axis of the ellipse fitted to a given isophote, decreases from 5.5 at 24 V mag arcsec^-2 to 3.9 at 27 V mag arcsec^-2.

Region a shows extraordinarily blue colours: V-I = -0.5 mag and V-R = -0.2 mag. Doublier et al. (1997) have obtained B and R photometry of SBS 0940+544. They mistook region a for a Galactic foreground star. Spectra of the object show without doubt that it is an extremely compact H II region with very strong emission lines (Fig. 5) at the redshift of SBS 0940+544. Izotov et al. (1991) first derived element abundances in this H  II region, followed by Izotov et al. (1994) who named it SBS 0940+544N.

3.2 Surface photometry and colours

As remarked in Sect. 3.1 (see also Sects. 4 and 5), signs of low-level ongoing or recent star formation are present all over the inner part of the main body of SBS 0940+544. In order to disentangle the stellar from the gaseous emission and better constrain the surface brightness and colour distribution of its faint underlying LSB host, we performed surface photometry in all available broad-band filters in two steps. First we fitted the intensity distribution and subtracted both the dominant region a and all adjacent point sources (designated by b, cf. Fig. 1a). We then derived surface brightness profiles (SBPs) for the main body using methods described in Papaderos et al. (1996b). SBPs computed in this way (Fig. 3a) trace the luminosity of the cometary LSB host of the BCD and the superimposed star-forming zone extending from region c to the SE corner. The radial intensity distribution of the LSB host was approximated by adjusting different fitting laws to the outer part of the SBPs. We then computed SBPs for the whole BCD, and by subtracting from them the best model for the intensity distribution of the LSB host, we obtained the surface brightness distribution of the discrete and diffuse starburst sources located in the northwestern part and main body (cf. Fig. 3b).

Figure 3: a) Surface brightness profiles (SBPs) of the main body of SBS 0940+544 after subtraction of the brightest H II region a and the adjacent point sources b (cf. Fig. 1a). For the sake of clarity, the R and I SBPs have been shifted vertically by -1 and -2 mag respectively. The modeled surface brightness distributions of the underlying LSB host in the V band corresponding to a Sérsic fitting law with an exponent $\eta =2$ (cf. Eq. (1)) and a modified exponential distribution with (b, q)=(3.0, 0.9) (Eq. (2)) are shown respectively by the thin/black and thick/grey curves. b) SBPs of the total emission of SBS 0940+544. The meaning of the symbols is the same as in the left panel. Small open circles show the surface brightness distribution of the light in excess of the model for the LSB host (thick/grey curve) in the V band. The effective radius ${r_{\rm eff}}$ of the V band SBP and the isophotal radii P25 and E25 of the starburst and LSB component at 25 V mag arcsec-2 are indicated.

 

 

Table 1: Structural properties of the starburst - and LSB component of SBS 0940+544.

Band	$\mu_{\rm E,0}$	$\alpha$	$m_{\rm LSB}^{\rm fit}$	P25	mP25	E25	mE25	$m_{\rm LSB}$	$m_{\rm SBP}$	$m_{\rm tot}$	${r_{\rm eff}}$, r80
	mag arcsec-2	arcsec	mag	kpc	mag	kpc	mag	mag	mag	mag	kpc
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)	(11)	(12)
V	$20.46 \pm 0.12$	$2.51 \pm 0.05$	16.46	0.82	17.89	1.36	17.53	17.34	$16.81 \pm 0.02$	16.83	0.60, 1.05
R	$20.14 \pm 0.10$	$2.52 \pm 0.05$	16.13	0.76	18.34	1.46	17.1	16.97	$16.68 \pm 0.02$	16.67	0.68, 1.13
I	$19.83 \pm 0.13$	$2.50 \pm 0.06$	15.85	0.83	17.89	1.54	16.87	16.76	$16.45 \pm 0.04$	16.48	0.69, 1.13

The SBPs of the main body (Fig. 3a) provide an upper limit to the intensity distribution of the underlying unresolved older stellar population of the LSB host, since part of the light for R^* 8 $^{\prime \prime }$ comes from region c and adjacent young sources. An exponential fitting law provides a reasonable approximation to the SBPs for mag arcsec^-2, for radii <10 $^{\prime \prime }$, however, it predicts a higher intensity than the one observed. This is also the case for the SBPs derived for the total emission of the BCD (Fig. 3b). This type of convex profile with an exponential distribution in the outer parts, and leveling off in the inner part (within 1-3 scale lengths) is not rare among low-luminosity dwarf ellipticals (e.g. Binggeli & Cameron 1991), dwarf irregulars (Patterson & Thuan 1996; Makarova 1999; van Zee 2000) and blue low surface brightness galaxies (Rönnback & Bergvall 1994; Bergvall et al. 1999), and has been reported in a few BCDs (e.g. Vennik et al. 1996; Papaderos et al. 1996b; Telles et al. 1997; Papaderos et al. 1999; Fricke et al. 2001). The observed profile shape suggests that a Sérsic (1990) profile of the form

$$% I(R^*) = I_0\,\exp\left(-\frac{R^*}{\alpha}\right)^{\eta}$$ (1)

with $\eta$ > 1 provides a better approximation to the light of the LSB host than a pure exponential ($\eta$ = 1).

An alternative fitting formula which well reproduces an exponential distribution flattening inwards has been discussed by Papaderos et al. (1996b):

$$% I(R^*) = I_0\,\exp\left( -\frac{R^*}{\alpha}\right) \big[1-q\,\exp(-P_3(R^*))\big],$$ (2)

where P₃(R^*) is

$$% P_3(R^*) = \left(\frac{R^*}{b\,\alpha}\right)^3+\left(\frac{R^*}{\alpha}\,\frac{1-q}{q}\right)\cdot$$ (3)

Near the center, such an intensity distribution depends on the relative central intensity depression $q=\Delta I/I_0$, where I₀ is the central intensity obtained by extrapolation of the outer exponential slope with the scale length $\alpha$ to R^*=0, and $b \alpha$ is the cutoff-radius where the central flattening occurs.

Expressions (1) and (2) were fitted to the SBPs of the main body for radii $R^*\geq 9$ $^{\prime \prime }$, where the colours become nearly constant (Fig. 4). Fits with a Sérsic profile give $\eta$ = 2.0 $\pm$ 0.1 and, as illustrated for the V profile in Fig. 3a, are nearly indistinguishable from those using Eq. (2) with b=3.0 and q=0.9. The V surface brightness distribution of the residual light in excess of the fit to the emission of the main body (Eq. (2)) is shown by small circles in the right panel of Fig. 3.

Table 1 summarizes the derived photometric properties of the LSB host and the star-forming component of SBS 0940+544. Columns 2 and 3 give respectively the central surface brightness $\mu_{\rm E,0}$ and exponential scale length $\alpha\hbox{$^{\prime\prime}$ }$ of the LSB host as obtained from linear fits to the SBPs for $R^*\geq 9$ $^{\prime \prime }$ and weighted by the photometric uncertainty of each point. The corresponding integrated magnitude $m_{\rm LSB}^{\rm fit}$ of a pure exponential distribution $\mu_{{\rm E},0}-5\log(\alpha\hbox{$^{\prime\prime}$ })-2$ is listed in Col. 4. As discussed above, the surface brightness distribution of the LSB host becomes significantly shallower for R^*<9 $^{\prime \prime }$ than that predicted by inwards extrapolation of the exponential slope observed at the outskirts of SBS 0940+544. Therefore, for the case under study $m_{\rm LSB}^{\rm fit}$ overestimates the apparent magnitude of the LSB host ($\approx$ $m_{\rm LSB}$; Col. 9). Columns 5 through 9 list quantities obtained from profile decomposition, with the intensity distribution of the LSB host modeled by Eq. (2). Columns 5 and 7 give respectively the radial extent P₂₅ and E₂₅ of the starburst and LSB components, determined at a surface brightness level of 25 mag arcsec^-2 (cf. Fig. 3b). The apparent magnitudes of each component within P₂₅ and E₂₅ are listed in Cols. 6 and 8. Column 9 gives the apparent magnitude of the LSB component in each band within the photometric radius of 15 $.\!\!^{\prime\prime}$5, as derived from integration of the modeled distribution Eq. (2). The total magnitude of the BCD as inferred from SBP integration out to the same radius and by summing up the flux within a polygonal aperture is given in Cols. 10 and 11, respectively. Column 12 lists the effective radius ${r_{\rm eff}}$ and the radius r₈₀ which encircles 80% of the galaxy's total flux.

Figure 4: V-R and V-I colour profiles of the main body of SBS 0940+544 derived by subtraction of the surface brightness profiles in Fig. 3a. The radial range corresponding to the surface brightness interval between 22 and 23 V mag arcsec-2 is indicated by vertical lines. The average colours of the bright H  II region a (cf. Fig. 1a) ($V-R \sim$ -0.2 mag and -0.5 mag ) are indicated by horizontal lines.

Figure 5: Spectra of the brightest H II region a in SBS 0940+544 obtained with: a) the Keck II telescope and b) the MMT. The lower spectra are the observed spectra downscaled by a factor of 100.

The V-R and V-I colour profiles of the LSB host of SBS 0940+544 as derived by subtraction of the corresponding SBPs (Fig. 3a) are shown in Fig. 4. Both colour profiles show very weak gradients for  mag arcsec^-2, and become nearly constant for R^*>9 $^{\prime \prime }$. Beyond that radius the colour profiles (Fig. 4) give average colour indices $V-R= 0.33 \pm 0.04$ mag and V-I = 0.58 $\pm$ 0.03 mag for the LSB component. These values are consistent with the integrated colours V-R = 0.37 mag and V-I = 0.58 mag, determined from the apparent magnitudes of the modeled intensity distribution of the LSB host within R^*=15 $^{\prime \prime }$ (Table 1, Col. 9).

We have compared our results to those obtained for the B and R profiles by Doublier et al. (1997, 1999). The agreement is not satisfactory. The exponential scale length $\alpha$ of the LSB host derived here (Table 1) is very similar in all bands (V, R and I) being $\sim$2 $.\!\!^{\prime\prime}$5 (320 pc). Although Doublier et al. (1997) also report an exponential intensity decrease in B and R for radii 3 $^{\prime \prime }$, they derive for the distance assumed here a B and R exponential scale length of $\sim$1 kpc, 3 times greater than our value. A striking disagreement is also evident from a comparison of the colour profiles derived here and those presented in Doublier et al. (1997). The B-R colour profile of Doublier et al. (1997) shows a maximum of $\sim$0.5 mag at $R^*\sim 0$ $^{\prime \prime }$, then decreases monotonically with increasing radius to 0.2 mag. Our colour profile shows the opposite trend: very blue colours at small radii and nearly constant, moderately red V-R and V-I colours in the outskirts of SBS 0940+544.