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Morphology and colours

The R-band image of the galaxy, with filamentary structures at the periphery of the low surface brightness region, is typical of dwarfs with strong SF activity. Curiously, the appearance of HS 0822+3542 is morphologically similar, on the same angular scales, to that of SBS 0335-052E (Melnick et al. [1992]; Thuan et al. [1997]), including some arcs and filaments. The integrated (B-V) and (V-R) colours of HS 0822+3542 are very blue ($0\fm32$ and $0\fm17$, respectively), similar to those of SBS 0335-052E.

Below, we describe the surface brightness (SB) distribution of HS 0822+3542. We show in Fig. 4 the SB profiles in B, V and R-bands, which look very similar, with some deviations in R-band in the middle part of the profile. This is presumably due to additional H$\alpha$emission from the filamentary structure (see Fig. 1). The B-band SB profile (Fig. 5) indicates the presence of two components: the central bright compact body and an exponential disc, dominating the light in the outer part of the profile (hereafter LSB component). The parameters of both components, derived from the fitting of the SB profiles in B, V and R, are given in Table 4.

The corresponding exponential scale lengths are $\alpha_{\rm E}^B$ = 86$\pm$1 pc, $\alpha_{\rm E}^V$ = 86$\pm$2 pc and $\alpha_{\rm E}^R$ = 84$\pm$2 pc. A comparison with the same parameters derived for the LSB component of other BCGs (Papaderos et al. [1996]; Papaderos et al. [1998]; Doublier et al. [1999a]) shows that the scale length of HS 0822+3542 is smaller than that of any BCG studied in the cited works. Even the ``ultracompact'' dwarf galaxy POX 186 (Doublier et al. [1999b]) has a scale length more than twice larger (180 pc). However HS 0822+3542 is not the only extreme case in its small disc ``size''. At least two galaxies have comparable or smaller scale lengths: GR 8 (Mateo [1998]) and Tol 1116-325 (Telles et al. [1997]).

Integrating the SB profile of the underlying exponential disc we obtain its total B magnitude:

$$B_{\rm disc} = \mu_{\rm E,0} - 5\log(\alpha_{\rm E}) - 1.995,$$ (2)

where $\alpha_{\rm E}$ is in arcsec.

With the resulting $B_{\rm disc}$ = 1822 we can estimate also the luminosity of current SF burst and the corresponding brightening of the galaxy. The brightening is quite modest, about 30%. The light of current burst corresponds to $B_{\rm burst}$ = 1946 and a luminosity of M_B = -111.

Since the scale lengths for the underlying disc in all three bands are identical within small uncertainties (1-1.5%), we can accept as a first approximation that the scale length of the disc is a unique one for B, V and R, equal to the average $<\alpha_{\rm E}>$ = 1.40$\pm$0.02 arcsec, and corresponding to 85$\pm$1 pc. Hence, the underlying disc has no colour gradient, and its colours (B-V) $_{\rm disc}$and (V-R) $_{\rm disc}$can be approximated by the colours of its central SB

$$(B-V)_{\rm disc} = \mu_{\rm E,0}^B - \mu_{\rm E,0}^V = 0\fm10\pm0\fm06,$$

$$(V-R)_{\rm disc} = \mu_{\rm E,0}^V - \mu_{\rm E,0}^R = 0\fm29\pm0\fm09,$$

respectively. The disc colours can also be obtained from its integral B, V and R magnitudes (column 7 in Table 4):

$$(B-V)_{\rm disc} = 0\fm12\pm0\fm10,$$

$$(V-R)_{\rm disc} = 0\fm22\pm0\fm16,$$

consistent with the estimates above. The uncertainties of the latter colours are derived from the errors of the integrated disc magnitudes, from the propagation of errors in equation (2). They are $0\fm07$, $0\fm08$and $0\fm14$ for B, V and R, respectively.

A close coincidence of the scale lengths of BCG LSB components in the different filters was found by Papaderos et al. ([1998]) for SBS 0335-052E. The same correlation for disc scale lengths in different bands was shown to exist for a sample of 19 BCGs (Doublier et al. [1999a]). However, the colours $(B-R)_{\rm disc}$ for BCGs with small scale lengths from Doublier et al. are redder and lie in the range of 061-147, with an average $(B-R)_{\rm disc}$ = 111$\pm$030. The most similar to HS 0822+3542 in this colour (with $(B-R)_{\rm disc}$ = 061) is SBS 0940+544, with 12+log(O/H) = 7.43$\pm$0.01 (Izotov & Thuan [1999]).

In Fig. 6 we show the distributions of observed (B-V) (filled squares) and (V-R) (open circles) colours, as functions of the effective radius. Mean disc colours are shown by filled and open rectangles to the right of the observed colour profiles. Note that the (V-R) colours are shifted by 10 in Fig. 6, to minimize confusion. Note also that (V-R) is even bluer for the outermost part of disc (R^* >3): 016$\pm$012.

Effective sizes for the central gaussian component in the different bands also coincide, within small uncertainties. We can therefore think of this component as a body with a homogeneous colour distribution. We note also, that this compact gaussian component is significantly larger than we could expect for a point-like source convolved with the point spread function (PSF). The FWHM for stellar images measured on the CCD frames near the galaxy is 125, whereas it is $\alpha_{\rm G}$=16 (Table 4) for the bright central region. The deconvolved FWHM for this component amounts to $\approx$ 10. Comparing with the structure of star-forming regions in other BCGs, it is natural to assign the enhanced optical emission in the central part of HS 0822+3542 to a young massive-star cluster formed in the current SF burst and its associated H II region. The characteristic linear radius of this complex is $\sim$ 30 pc, comparable to the size of the star cluster R136 in LMC (Walborn [1991]). Accordingly, the absolute B magnitude of this bright component -111 is near the lower limit of the range found with HST for super-star clusters by O'Connel et al. ([1994]) in two nearby star-bursting galaxies.

The total size of HS 0822+3542 out to the $\mu_{B}$=25 mag arcsec^-2 isophote can be approximated by an ellipse with major and minor axes 148 and 74, or 900 by 450 pc, respectively.

As we have shown above, the SB distribution of HS 0822+3542 can be fitted by two components. While the brighter and more compact central component seemingly corresponds to the complex of young massive stars formed during the current SF burst and its associated H II region, the underlying exponential disc can consist of older stars. The latter could be formed either during a previous much earlier SF episode, or relatively recently, in a precursor of the current SF burst, which could be significantly displaced ($\sim$150-200 pc) from the underlying disc centre. If the current SF burst and the previous SF activity are causally connected, the propagation time of the SF wave from the underlying disc centre, with a typical velocity of 10 km s^-1 (e.g., Zenina et al. [1997]) would be only 15-20 Myr. In order to check this option one could search for the He I absorption features of early B stars in the underlying disc.

To check for emission from older stellar populations, which may reside in the region outside the current SF burst, we compare the colours of underlying disc with those predicted for various models, as well as with similar parameters for other young galaxy candidates.

The very blue colours of the disk, after correcting for the extinction in the Galaxy (B-V)₀ = $0\fm05\pm0\fm06$and (V-R)₀ = $0\fm26\pm0\fm09$are reasonably consistent with the predictions for an instantaneous SF burst with a Salpeter IMF, a metallicity of 1/20 $Z_{\odot}$ and an age of $\sim$ 100 Myr (Leitherer et al. [1999]). If prolonged star formation is assumed, then the age of the oldest stellar population in the extended disc can be larger than that of the instantaneous burst. The observed colours can also be explained by continuous SF with a constant star formation rate from 500 Myr to 20 Myr ago. However, older stellar populations with an age of 10 Gyr are excluded. The derived age is somewhat lower for the outermost part of the of the disc with a bluer (V-R) colour in comparison to its average value. Integrated colours of ionized gas are about (B-R) = 06-08 (Izotov et al. [1997a]), what is redder than our observed values in the underlying disc. Therefore if some gas emission inputs to the integrated disc colours, then the true colours of stellar component should be even bluer than measured.

These colours are very similar to those of the underlying nebulosity in SBS 0335-052. The latter colours are shown to be well explained by the radiation of ionized gas and A-stars with the ages of no more than 100 Myr, created in the current SF episode. This SF episode is prolonged and may represent a propagating SF wave (Papaderos et al. [1998]), similar to what is suggested for another candidate young galaxies I Zw 18 (Izotov et al. [2000]) and CG 389 (Thuan et al. [1999b]). This similarity of LSB disc colours of HS 0822+3542 and SBS 0335-052E suggests that up to the radial distances of $\sim$300 pc the input of stellar population with the ages larger than 100 Myr to the disc radiation is undetectable.

As a first approximation, the colours of the underlying disc do not contradict the possibility that the current burst is the first SF episode in this galaxy. However since the emission of ionized gas can add significantly to the total radiation from the volume within 300-400 pc, further photometric and spectroscopic observations of HS 0822+3542 are required to account properly for the contribution of gaseous emission.