A&A 378, L45-L48 (2001)
DOI: 10.1051/0004-6361:20011265

Deep spectroscopy of the low-metallicity blue compact dwarf galaxy SBS 0335-052[*]

Y. I. Izotov 1 - F. H. Chaffee 2 - D. Schaerer 3

1 - Main Astronomical Observatory, Ukrainian National Academy of Sciences, Golosiiv, Kyiv 03680, Ukraine
2 - W. M. Keck Observatory, 65-1120 Mamalahoa Hwy., Kamuela, HI 96743, USA
3 - Observatoire Midi-Pyrénées, Laboratoire d'Astrophysique, UMR 5572, 14 Av. E. Belin, 31400 Toulouse, France

Received 1 August 2001 / Accepted 10 September 2001

The results of deep long-slit spectroscopy of the extremely low-metallicity blue compact dwarf (BCD) galaxy SBS 0335-052 are presented. Down to intensity levels of
10$^{-3 \ldots -4}$ of HH$\beta $, unprecedented for spectroscopy of extra-galactic giant H  II regions, we detect numerous weak permitted and forbidden nebular lines in the brightest part of the galaxy. With varying degrees of confidence, the detections include lines of high-ionization ions like Fe4+-Fe6+, implying very hard ionizing radiation. Two broad emission features, possibly from Wolf-Rayet stars, and stellar He II $\lambda $4200 absorption are seen in the same region. The large spatial extent of He II $\lambda $4686 emission (implying the presence of sufficient ionizing photons with energies above 54 eV) and the spatial distribution of the electron temperature suggest that at least some part of the hard radiation is associated with shocks. Extended H$\alpha $ emission is detected over $\sim$6-8 kpc, a much larger area than in previous studies, suggesting that hot ionized gas is spread out far away from the central ionizing clusters. This shows that nebular line and continuous emission can significantly modify the colours of these extended regions and must be taken into account in studies of the underlying stellar population.

Key words: galaxies: fundamental parameters - galaxies: starburst - galaxies: individual (SBS 0335-052)

1 Introduction

Since its discovery by Izotov et al. (1990) as a very metal-poor galaxy the blue compact dwarf (BCD) galaxy SBS 0335-052 (SBS - the Second Byurakan Survey) has been studied extensively. The oxygen abundance in SBS 0335-052 is 12 + log O/H = 7.30 (Melnick et al. 1992; Izotov et al. 1997, 1999) and places it after I Zw 18 as the second most metal-deficient BCD. The properties of SBS 0335-052 as a probable young galaxy are of great interest for cosmology (e.g., Izotov et al. 1997). Therefore, detailed studies of this galaxy can shed light on the formation and the properties of the high-redshift primeval galaxies. Using deep spectroscopic observations we concentrate in the present paper on two problems: (1) the origin of very hard radiation at wavelengths shorter 228 Å. This radiation is indicated by the presence of the strong He II $\lambda $4686 emission line (Izotov et al. 1997) and implies that some other weak emission lines of high-ionization species could be present in the spectrum of SBS 0335-052. (2) the analysis of the properties of extended emission around SBS 0335-052. This problem is of great importance in understanding the evolutionary status of SBS 0335-052. In particular, we aim to understand how important gaseous emission is in SBS 0335-052 and how far it extends from the ionizing clusters.

...width=7.92cm}\par\psfig{figure=Dh013_f1c.eps,angle=270,width=7.92cm}\end{figure} Figure 1: The spectrum of the brightest region with line identifications. On each panel a continuum is drawn by hand to guide the eye. 2$\sigma $ error in the continuum placement is shown in the upper panel.
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\par\hspace*{0.0cm}\psfig{figure=Dh013_f2a.eps,angle=270,width=7.9cm} \hspace*{0.0cm}\psfig{figure=Dh013_f2b.eps,angle=270,width=8.8cm}\end{figure} Figure 2: a) The spatial distributions along the slit of the relative fluxes of the emission lines and continuum near H$\beta $. The fluxes of the H$\beta $ emission line and continuum are normalized to their maximum values, while the fluxes of [O III] $\lambda $4363 and He II $\lambda $4686 are relative to the flux of H$\beta $. The slit is oriented at position angle PA = $-30^\circ $. Origin is set at the maximum flux of the H$\beta $ emission line. b) The spatial distribution of the electron temperature $T_{\rm e}$ and oxygen abundance 12 + log O/H.
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2 Observations and data reduction

The observations of SBS 0335-052 were carried out on January 9, 2000 on the Keck II telescope with the low-resolution imaging spectrograph (LRIS) (Oke et al. 1995), using the 300 groove mm-1 grating which provides a dispersion 2.52 Å pixel-1 and a spectral resolution of about 8 Å in first order. The slit was 1 $^{\prime\prime}$$\times$180 $^{\prime\prime}$, centered on the brightest H II region and oriented along the major axis with a position angle PA = -30$^\circ $. No binning along the spatial axis has been done, yielding a spatial sampling of 0 $.\!\!^{\prime\prime}$2 pixel-1. The total exposure time was 60 min, broken into four 15 min exposures. All exposures were taken at an airmass of 1.1. The seeing was 0 $.\!\!^{\prime\prime}$9. Wavelength calibration was provided by Hg-Ne-Ar comparison lamp spectra obtained after each exposure. Two spectrophotometric standard stars, Feige 34 and HZ 44, were observed for flux calibration. The data reduction was made with the IRAF[*] software package. The two-dimensional spectra were bias-subtracted and flat-field corrected. Cosmic-ray removal, wavelength calibration, night sky background subtraction, correction for atmospheric extinction and absolute flux calibration were then performed.

For the analysis of spatial distribution of the brightest emission lines we also use the Keck spectrum of SBS 0335-052 obtained earlier by Izotov et al. (1999) at a position angle of 80$^\circ $.

A distance of 54 Mpc to SBS 0335-052 is adopted throughout this paper (Izotov et al. 1997). At this distance, 1 $^{\prime\prime}$ corresponds to 260 pc.

The spectrum of the brightest region in a 1 $^{\prime\prime}$$\times$2 $.\!\!^{\prime\prime}$2 aperture with secure and tentative line identifications (see below) is shown in Fig. 1. The measured S/N is of the order of 100. Extinction-corrected fluxes of the emission lines normalized to the flux of 10 $^{-4}\times I$(H$\beta $) are shown in Table 1.

Table 1: Extinction-corrected fluxes of the emission lines in the brightest region.

$\lambda_{0}$(Å) Ion
$\lambda_{\rm obs}^{\rm a}$ I($\lambda $)$^{\rm b}$     $\lambda_{0}$(Å) Ion $\lambda_{\rm obs}^{\rm a}$ I($\lambda $)$^{\rm b}$     $\lambda_{0}$(Å) Ion $\lambda_{\rm obs}^{\rm a}$ I($\lambda $)$^{\rm b}$

3727 [O II]
3728 2136     4815 [Fe II] 4814 9     5876 He I 5875 1043
3750 H12 3751 393     4861 H$\beta $ 4861 10000     ? 5917 4
3771 H11 3771 471     4907 [Fe IV]? 4904 22     ? 5932 3
3798 H10 3798 597     4922 He I 4922 94     5958 Si II 5957 8
3820 He I 3820 75     4959 [O III] 4959 10663     5979 Si II 5978 8
3835 H9 3836 798     5007 [O III] 5007 32271     ? 5999 3
3868 [Ne III] 3869 2349     5085 [Fe III] 5080 6     6046 O I 6044 5
3889 H8 + He I 3890 1741     ? 5097 3     6074 He II 6075 3
3926 He I 3927 27     5112 [Fe II] 5112 6     ? 6085 2
? 3943 31     5146 [Fe VI] 5149 15     6102 He II 6100 4
3968 [Ne III] + H7 3969 2478     5159 [Fe II], [Fe VII] 5160 15     ? 6150 4
4009 He I 4009 11     5176 [Fe VI] 5181 7     6170 He II 6169 2
4026 He I 4027 137     5199 [N I] 5198 33     ? 6270 3
4068-76 [S II] 4071 44     ? 5214 3     6300 [O I] 6300 87
4101 H$\delta$ 4102 2604     ? 5235 5     6312 [S III] 6311 51
4121 He I 4122 16     ? 5244 2     6347 Si II 6346 8
4144 He I 4145 20     5262 [Fe II] 5259 7     6363 [O I] 6363 32
4169 He I 4168 11     5271 [Fe III] 5270 242     6407 He II 6407 4
4227 [Fe V] 4228 36     ? 5309 6     ? 6455 9
4287 [Fe II] 4288 18     5335 [Fe VI] 5336 6     6563 H$\alpha $ 6562 27366
? 4314 12     ? 5351 2     6678 He I 6677 260
4340 H$\gamma$ 4341 4753     ? 5369 4     6717 [S II] 6716 197
4363 [O III] 4364 1089     ? 5390 2     6731 [S II] 6731 166
4388 He I 4389 43     5411 He II 5412 13     7002 O I 7002 13
4415 [Fe II] 4416 12     5424 [Fe VI] 5424 2     ? 7038 5
? 4434 10     ? 5436 2     7065 He I 7065 427
4452 [Fe II] 4453 6     5485 [Fe VI] 5482 2     7136 [Ar III] 7135 212
4471 He I 4472 344     ? 5501 5     7171 [Ar IV] 7170 7
4511 N III 4509 7     5517 [Cl III] 5517 13     ? 7195 2
4565 Si III 4566 10     5538 [Cl III] 5537 10     ? 7210 2
? 4581 8     ? 5552 5     ? 7224 2
4603-20 N V?, N III? 4609 32     ? 5580 3     7237 [Ar IV] 7237 3
4640 N III 4641 13     ? 5595 4     7254 O I 7254 10
4658 [Fe III] 4659 43     ? 5608 3     7263 [Ar IV] 7264 5
4686 He II 4686 221     5639 [Fe VI] 5638 3     7281 He I 7280 52
4711 [Ar IV]+He I 4712 182     5677 [Fe VI] 5678 7     7298 He I 7295 5
4740 [Ar IV] 4741 104     ? 5698 2     7320 [O II] 7319 60
4755 [Fe III] 4757 7     5721 [Fe VII] 5721 10     7330 [O II] 7330 28

$^{\rm a}\lambda_{\rm obs}$ = $\lambda_{\rm measured}$/(1+z), where $\lambda_{\rm measured}$ is the measured wavelength
of the emission line, z = 0.0135 is the redshift of SBS 0335-052.
$^{\rm b}$ I($\lambda $) is normalized to the flux of 10 $^{-4}\times I$(H$\beta $).

3 Weak nebular emission and stellar absorption lines

One of main results (see Fig. 1) is the detection of numerous weak permitted and forbidden emission lines, which are - to the best of our knowledge - detected for the first time in the spectrum of an extra-galactic giant H  II region. Lines with fluxes down to $\sim$0.1% of HH$\beta $ are clearly detected. For weaker lines the identifications are tentative and the measured fluxes are fairly uncertain, with relative errors $\sim$100%, while the relative errors of the fluxes for the strongest lines are $\sim$1%. Thirty unidentified emission features with fluxes 0.02%-0.12% that of H$\beta $ are also shown in Table 1 and labeled by "?''.

Some permitted lines, e.g. O I $\lambda $6046, $\lambda $7002, $\lambda $7254, Si II $\lambda $5958, $\lambda $5979, $\lambda $6347, are likely of fluorescent origin and produced by absorption of the intense UV radiation (Esteban et al. 1998). Other lines (broad features at $\lambda $4620 and $\lambda $5700-5850) can be stellar in origin and likely produced by Wolf-Rayet stars. Weak He II $\lambda $4200 absorption is also detected, which is produced in the atmospheres of O stars. Although our detection level is comparable to that in deep spectra of Orion and planetary nebulae, optical recombination lines found in those objects (e.g. Esteban et al. 1998; Liu et al. 2000) are not detected here. This is likely due to the much lower metallicity of SBS 0335-052.

\par\hspace*{0.0cm}\psfig{figure=Dh013_f3a.eps,angle=0,width=7.65cm} \hspace*{0.0cm}\psfig{figure=Dh013_f3b.eps,angle=0,width=7.65cm} \end{figure} Figure 3: a) Distribution along the slit of the equivalent widths of H$\alpha $ (solid line), H$\beta $ (dashed line) and [O III] $\lambda $5007 (dotted line) emission lines. The slit is oriented at position angle -30$^\circ $. Thick solid line shows the range of the reliable EW(H$\alpha $). Outside this range the continuum is very weak, corresponging to R band surface brightness fainter 25 mag arcsec-2. b) Same as a) but slit is oriented at position angle 80$^\circ $.
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Several emission lines indicate the presence of intense hard radiation at energies above 54 eV ($\lambda <$ 228 Å) and possibly even above 75-99 eV. The well-known He II $\lambda $4686 line and also He II $\lambda $5411 are detected. We confirm the presence of the forbidden [Fe V] $\lambda $4227 line, previously discovered in SBS 0335-052 and another low-metallicity BCD (Tol 1214-277) by Fricke et al. (2001). In addition several weak [Fe VI] and possibly [Fe VII] emission lines are detected in our spectrum. The ionization potentials corresponding to these Fe lines are 54.8, 75.0, and 99 eV respectively; the He+ emitting region is therefore expected to be associated with the emission of [Fe  V-VII]. The [Ar  V] $\lambda $7006 line is likely detected. However, it is blended with O  I $\lambda $7002 emission line. The possible identification of [Ar  V] $\lambda $6435 line by Fricke et al. (2001) appears very uncertain from our spectrum.

The origin of hard radiation in giant H II regions has been debated for many years. Possible sources for this radiation include shocks, X-ray binaries, or hot Wolf-Rayet stars (cf. Garnett et al. 1991; Schaerer 1996). The spatial distribution of He II $\lambda $4686, [O III] $\lambda $4363, H$\beta $, and the adjacent continuum (see Fig. 2a) show some evidence for the presence of shocks, at least in a limited area of SBS 0335-052. Indeed, in the NW direction He  II $\lambda $4686/HH$\beta $ remains relatively strong[*] out to the position of the ionized gas shell seen in HST images at a distance $\sim$6 arcsec from the two central clusters 1 and 5 of Thuan et al. (1997; cf. also Papaderos et al. 1998). Although expected, the other high excitation lines of [Fe  V-VII] are too faint to be detected away from the main clusters. Furthermore, an increase of the electron temperature is observed away from the main clusters towards the shell (Fig. 2b), which may result from additional heating due to shocks. Both the extent of the region with He  II $\lambda $4686/HH$\beta $ $\sim$ 0.06 and the increase of $T_{\rm e}$ at large distance from the main ionizing clusters, where geometric dilution will greatly reduce the local ionization parameter, are suggestive of shocks in the area within and out to the shell. On the other hand, in the region centered on the main clusters we cannot distinguish between shocked and photoionized gas, as nearby stellar objects with hard ionizing spectra cannot be excluded. In passing we note that if shocks are present, the oxygen abundance gradient derived assuming a pure photoionized H  II region model (Fig. 2b) may not be real. Observations of the spatial distribution of various high excitation lines (including [Fe  V-VII] and [Ne  V] $\lambda $3426 which is expected to be strong from shock models; e.g. Dopita & Sutherland 1996) as well as a detailed modeling are required to establish more firmly the importance of shocks on the spectrum of SBS 0335-052.

4 Extended nebular emission

Melnick et al. (1992) first obtained an H$\alpha $ image of SBS 0335-052 and found that the ionized gas extends out to $\sim$7 arcsec, or over a region of $\sim$2 kpc in diameter. The deep long-slit Keck spectra of SBS 0335-052 allow us to trace ionized gas emission over a much larger region. In Fig. 3 we show spatial distributions of the equivalent widths in two nearly perpendicular directions of the brightest emission lines H$\beta $, H$\alpha $ and [O III] $\lambda $5007. H$\alpha $ emission is detected over 32 arcsec in the direction of PA = $-30^\circ $ and over 26 arcsec in the direction of PA = 80$^\circ $. This corresponds to a linear size of 8 $\times$ 6 kpc, roughly the size of the 26 mag arcsec-2 R band isophote (Lipovetsky et al. 1999), or more than 3 times larger than that obtained from the H$\alpha $ image by Melnick et al. (1992).

Very high equivalent widths of the emission lines in SBS 0335-052 (Fig. 3) imply that the contribution of the ionized gas to the total light is important and dominate in the regions with the largest equivalent widths. This finding shows, as already pointed out in several earlier studies (e.g. Krüger et al. 1995; Izotov et al. 1997; Papaderos et al. 1998), that both nebular continuum and line emission must be taken into account in photometric studies of stellar populations in the extended regions of BCDs. Obviously, the contribution of the emission lines to the total light depends on their relative strengths and redshift. H II region models predict a decrease of [O III] $\lambda $5007/H$\beta $ with distance from the center due to a decreasing ionization parameter. This is observed in SBS 0335-052. If one assumes that the extended emission in SBS 0335-052 is only gaseous, then the V-I colour is changed from ${\sim}-0.6$ mag in the center, where [O III] $\lambda $5007/H$\beta $ $\sim$ 3.3, to $\sim$0.0 mag in outer regions, where [O  III] $\lambda $5007/H$\beta $ $\sim$ 1.5. In other words, even without stellar emission, the distribution of gaseous emission mimics a contribution of red stars increasing with distance. For analysis of stellar populations a proper removal of ionized gas emission is therefore crucial.

Y. I. I. has been partly supported by INTAS 97-0033 grant, and acknowledges support from the "Kiev project'' of the Université Paul Sabatier of Toulouse. He is grateful for the hospitality of the Midi-Pyrénées Observatory where this work was conducted. The authors further wish to extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, none of the observations presented herein would have been possible.



Copyright ESO 2001