The Blue Compact Dwarf (BCD) galaxy SBS 0335–052 is one of the most metal–deficient galaxies known and one of the best candidates for a young dwarf galaxy in the process of formation. A VLA HI map reveals an unusual structure: the neutral gas is distributed in a very extended disk, about 15 times larger than the Holmberg optical diameter of the BCG. There are two peaks of high density. The eastern peak is close to the position of SBS 0335–052 whereas the western HI peak is associated with a faint compact optical galaxy. A 6–m telescope spectrum of this object shows H, H, and [OIII]5007 emission with a redshift close to that of SBS 0335–052. We suggest that it may be a young and chemically unevolved dwarf galaxy.Peer reviewe

Brinks, E.

Izotov, Y.I.

Knyazev, A.Y.

Lipovetsky, V.A.

Neizvestnyi, S. I.

Pustil'nik, S.A.

Thuan, T.X.

Ugryumov, A. V.

Astronomy Letters

English

University of Hertfordshire Research Archive

arXiv:astro-ph/9606021v1  4 Jun 1996Detection of a Second Optical Component in the HI cloudassociated with the Young Dwarf Galaxy SBS 0335–052: NewData from the 6–m Telescope.S.A.Pustilnik1, V.A.Lipovetsky1,Yu.I.Izotov2, E.Brinks3, T.X.Thuan4,A.Yu.Kniazev1, S.I.Neizvestny1, A.V.Ugryumov11Special Astrophysical Observatory, Russian Academy of Sciences, Nizhnij Arkhyz,Karachai–Circessia, 357147, Russia2Main Astronomical Observatory, Goloseevo, Kiev, Ukraine3NRAO, Socorro, NM, USA (currently on leave at the Departemento de Astronomı´a,Universidad de Guanajuato, Guanajuato, Me´xico)4Astronomy Department, University of Virginia, Charlottesville, VA 22903, USASubmitted to Astronomy Letters (Russia)AbstractThe Blue Compact Dwarf (BCD) galaxy SBS 0335–052 is one of the most metal–deficientgalaxies known and one of the best candidates for a young dwarf galaxy in the process offormation. A VLA HI map reveals an unusual structure: the neutral gas is distributedin a very extended disk, about 15 times larger than the Holmberg optical diameter of theBCG. There are two peaks of high density. The eastern peak is close to the position ofSBS 0335–052 whereas the western HI peak is associated with a faint compact optical galaxy.A 6–m telescope spectrum of this object shows Hα, Hβ, and [OIII]λ5007 emission with aredshift close to that of SBS 0335–052. We suggest that it may be a young and chemicallyunevolved dwarf galaxy.1 IntroductionThe blue compact galaxy SBS 0335–052 was discovered by Izotov et al. (1990) as an ex-tremely low metallicity galaxy, similar to I Zw 18. Its extremely low oxygen abundance[O/H]=7.26 has been confirmed by by Terlevich et al. (1992) who used this galaxy for anew estimate of the primordial helium abundance. An improved value of [O/H]=7.30 andan NTT Hα image was obtained by Melnick et al. (1992). They show that the object hasa double nuclear structure with a separation between the two components of ∼1.0′′. Thefainter component has a higher abundance but it is still very metal poor, ([O/H]=7.64). TheHα image shows a complex structure with filaments and wisps; the hydrogen lines extend toabout 6′′ on either side of the nuclei.Subsequent studies showed that SBS 0335–052 is an unusual HI gas–rich dwarf galaxy(Thuan et al. 1996a), with no indications of an old stellar population, either from highS/N MMT optical spectra (Izotov et al. 1996), or from V and I imaging with HST (Thuanet al. 1995). The images of SBS 0335–052 are generally reminiscent of I Zw 18 (Hunterand Thronson 1995). They show a large arc and several star–forming regions in the centralpart surrounded by extended low surface brightness emission of about 4 kpc in size, withsystems of loops typical for supernova related phenomena in star–forming galaxies. The age1of these gas filaments is expected to be lower than the age of the current star–formationburst (several million years). Alternatively, the blue color of the extended halo could be dueto stellar light, but in this case, the stellar age also has to be less than 100 million years.The structure of the surrounding HI gas is especially interesting since it can indicatehow primeval galaxies collapse and form their first stars. We used the VLA in the C and Dconfigurations to map and study the velocity field of SBS 0335–052 in the 21–cm line. Theobservations were carried out in 1994–1995 for a total of 4.5 hours. The VLA data will bediscussed in detail in a forthcoming paper by Thuan et al. (1996b).2 Optical identificationWhen making the optical identifications we were guided by the integrated HI flux densitymap. The gas structure of this unique young dwarf galaxy is quite unusual. There are twoprominent slightly resolved HI peaks located at roughly equal distance from the center ofthe HI cloud, separated by about 28 kpc. The location of the eastern peak is close to theposition of SBS 0335–052.In Fig.1 we show the identification map extracted from the Digital Sky Survey (DSS).There is a faint, very compact, slightly elongated galaxy at about 1.5′ West of SBS 0335–052.The observational parameters for both eastern and western optical components are given inTable 1. The position of the W component is very close to the coordinates of the westernHI peak and suggests that the two are physically associated. We estimate the integrated Vluminosity of the W component to be roughly 19m. Its size as measured by its FWHM is2.8′′× 2.2′′.3 SpectroscopyA spectrum of this western component (hereafter SBS 0335–052W) was obtained with the6–m telescope on December 17, 1996. We used the spectrograph SP–124 at the Nasmyth–1focus in combination with a new Tektronix 1024×1024 CCD detector provided by the Astro-nomical Institute at Potsdam (AIP) in the framework of a cooperative program between SAOand the AIP. Since the CCD detector had just been installed for a test run, the instrumentwas not optimized and the quality of the spectrum presented here is not representative ofthe system at its best. The weather conditions were not photometric with fairly poor seeing.A slit size of 35′′×2′′ was used. The observations were carried out in a MIDAS environment,with the use of the package NICE (Kniazev and Shergin 1995).The spectrum covers the 4700÷7000 A˚A˚ range with a resolution of 5.2 A˚ pxl−1. Four600 seconds exposures of variable quality were averaged with appropriate weights to obtainthe final spectrum. The spectrophotometric standards Hiltner 600 and Feige 34 were used tocorrect the linearized spectrum for the spectral response of the total system. The reductionof the raw data was done in MIDAS with the package LONG, adapted for our instrumentby A.Ugryumov.The final spectrum is shown in Fig.2. Only three significant features are seen in thespectrum, Hα, Hβ and [OIII] 5007 A˚. Their equivalent widths are respectively 332, 93 and98 A˚. A very faint feature is seen at the expected position of [OIII] 4959 A˚ with an EW ofabout 25 A˚ which is close to the expected value from the theoretical ratio of the two [OIII]2lines. There is no indication of either the [NII] or [SII] lines near Hα. For comparison wealso obtained the spectrum of SBS 0335–052 in the same night with a total exposure timeof 30 min. The spectra were taken with a position angle of –5◦ for the W component and of+10◦ for SBS 0335–052.It is interesting to estimate the extent of this W component. The region which emits inHα can be traced on our long–slit spectra over ∼13 pixels = 5.2′′, the scale of the spectrumperpendicular to the dispersion direction being 0.4′′ pxl−1. This is about a factor of 3 lessthan the extent of Hα in SBS 0335–052 which covers about 40 pixels, as derived from the6–m telescope spectra obtained under the same conditions.4 DiscussionThe heliocentric velocities of 3990 km s−1 for SBS 0335–052W, and 4110 km s−1 for SBS 0335–052 do not leave any doubt that both objects belong to one system. Both optical velocitiescoincide within the errors with the velocities of the HI gas (see Table 1). Thus in thisunusual system we observe a pair of dwarf galaxies embedded within a massive and hugeHI cloud. All the data indicate that in the eastern HI density peak the formation of thegalaxy SBS 0335–052 started very recently. Its first massive stars were formed during thelast several million years.Table 1. Observed and derived parameters for the SBS 0335–052 systemParameter 0335–052W 0335–052α(1950.0)a ............................... 03h35m09s.57 03h35m15s.15δ(1950.0) ................................. −05◦12′24′′.0 −05◦12′25′′.9V (mag) .................................. 19 17angular size (′′)b ...................... 5.2 16linear size (kpc)c ..................... 1.3 4.1Vopt (km s−1) .......................... 3990±40 4110±20VHI (km s−1)d ......................... 4006± 5 4068± 5F(Hα)×10−16 (erg s−1 cm−2)... 53±10 891±15a The coordinates were obtained from the DSS using MIDAS.b The Hα extension was used as the object’s size.c We adopted H◦=75 km s−1 Mpc −1.d HI parameters (Thuan et al. 1996b).The absolute magnitude of the western component is MV ∼ -14.5m and its linear diameteris only about 1.3 kpc, taking the Hα extension as the size, which places it in the range ofvery small blue compact dwarf galaxies. Its emission line spectrum indicates that there is acurrent starburst in this galaxy as well. No strong low–excitation line is seen in the red partof the spectrum indicating, possibly, a high temperature and a low metal abundance. Thestrong [OIII] and Hβ lines seen do not contradict such a possibility. Our spectrum did notcover the region shortward of 4700 A˚, so we have no information on the range of possibletemperatures in the HII region. But if it turns out that its temperature is high then the lowintensity of [OIII] 5007 A˚ relative to that of Hβ could be interpreted as an indication of a3very low abundance of oxygen. Clearly a high S/N spectrum of the western component isnecessary to elucidate its oxygen abundance and evolutionary status.There are two possible models to explain this system. Our preferred hypothesis whichtakes into account all available data, is that the eastern and western parts of SBS 0335–052 are two very young low metallicity objects embedded in one huge HI cloud havingapproximately the same young age. A second scenario describing the nature of this system isalso possible, but seems to be not very probable. If the metallicity of the western componentturns out to indicate that it is an older dwarf, we then witness an encounter of the dwarfwith a pristine huge HI–cloud. In that case the tidal disturbance of this encounter hastriggered the gas collapse in the eastern part of the HI cloud and the subsequent dwarfgalaxy formation (SBS 0335–052 itself). Additional spectral observations and a detailedstudy of the HI kinematics of this unique system will be necessary to make a choice betweenthese two models.As for the possible tidal effect of another galaxy, indeed, such a galaxy, the Scd spiralNGC 1376, is located at an angular distance of about 9′ to the West of the SBS 0335–052system. Its velocity, determined from HI observations (Shostak 1975) is 4155±5 km s−1.The projected distance from the center of the SBS 0335–052 HI cloud is about 135 kpc.However the role of this galaxy is far from evident. If the radial velocities of SBS 0335–052and NGC 1376 are due purely to Hubble flow, their relative distance is about 1500 kpc, andit would be hard to ascribe the very recent process of galaxy formation in the SBS 0335–052system to an influence exerted by NGC 1376.The only known BCD which is more metal–deficient than SBS 0335–052 is I Zw 18. Italso has a double–component structure. But in I Zw 18, the two components embeddedwithin an HI cloud are separated by only ∼ 300 pc. This is very different from the situationin SBS 0335–052. The newly discovered western component lies at a projected distanceof 28 kpc and without the VLA map (Thuan et al. 1996b), we could not have guessedthat SBS 0335–052 and the its western component were parts of one and the same physicalsystem, spanning a total size 64 kpc.Better knowledge of the kinematics of the neutral and ionized gas in this key dwarf galaxysystem in formation will allow us to shed some light on the processes occurring at the epochof galaxy formation, and thus to predict better the properties of primeval galaxies.The SAO authors are grateful to their German colleagues from the Astrophysical Institutein Potsdam for the gift of a new CCD at the 6–m telescope which made possible theseobservations. Partial financial support for this international collaboration was made possibleby NATO collaborative research grant 921285 and by INTAS grant No 94–2285.References1. Hunter D.A., Thronson H.A., 1995, Ap.J., 452, 2382. Izotov Yu.I., Lipovetsky V.A., Guseva N.G., Kniazev A.Yu., Stepanian J.A., 1990,Nature, 343, 238.3. Izotov Yu.I., Chaffee F., Foltz C., Guseva N.G., Lipovetsky V.A., 1996 (in preparation).4. Kniazev A.Yu., Shergin V.S., 1995, Special Astrophysical Obs. Technical Report No.239.45. Melnick J., Heydari–Malayeri M., Leisy P., 1992, A.Ap., 253, 16.6. Shostak G.S., 1975, Ap.J., 189, L1.7. Terlevich E., Terlevich R., Skillman E., Stepanian J., Lipovetsky V., 1992, in Elementsand Cosmos, eds.M.G.Edmunds, R.J.Terlevich, p.21, Cambridge, University Press.8. Thuan T.X., Izotov Yu.I., Lipovetsky V.A., 1995, in Proc.of 11–th IAP Meeting, Paris.9. Thuan T.X., Lipovetsky V.A., Martin J.–M., Pustilnik S.A., 1996a, A.Ap. Suppl.Ser.(submitted).10. Thuan T.X., Brinks E., Pustilnik S.A., Lipovetsky V.A., Izotov Yu.I., 1996b (in prepa-ration).5Figure 1: Identification chart from the DSS, 340′′×170′′ in size. SBS 0335–052 and the faintcompact object 1.5′ to West are marked.Figure 2: 6–m telescope spectra of SBS 0335–052W (a) and SBS 0335–052 (b); verticalarrows show the position of emission lines of Hβ, [OIII] 5007 A˚, and Hα.6

Special Astrophysical Obs.

Detection of a second optical component in the H I cloud of the young dwarf galaxy SBS 0335-052 : New observations with the 6-m telescope

Detection of a second optical component in the H I cloud of the young dwarf galaxy SBS 0335-052 : New observations with the 6-m telescope

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