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...... a flux may be contributed by an unsubtracted continuum. The large aperture flux at the K band appears to be dominated by stellar An extragalactic supernebula emission, while much of the L 0 flux is from nebular dust. Our confined by photometry and mapping confirm that most, if not all, of the Br a and at least 30% of the Br g emission in NGC5253 emerges from a source coincident with the brightest K-band continuum source. J. L. Turner*, S. C. Beck†, L. P. Crosthwaite*‡, J. E. Larkin*, I. S. McLean* & D. S. Meier*§ Having both Brackett lines allows us to correct the line fluxes for and obtain a good estimate of the true, extinction-free * Department of Physics and Astronomy, UCLA, Los Angeles, California recombination line flux, and thus the total ionizing flux. For an obs obs 90095-1562, USA intrinsic SBra=SBrg flux ratio of 2.8 (ref. 12), a temperature of † School of Physics and Astronomy, Raymond and Beverly Sackler Faculty of Exact 12,000 K (ref. 13), and a Rieke and Lebofsky14 extinction law, we Sciences, Tel Aviv University, Ramat Aviv, Israel obtain extinctions of A a ¼ 0.8 mag at 4.05 mm and A g ¼ 2.0 mag at ‡ Astute Networks, 16868 Via Del Campo Court, Suite 200, San Diego, California 2.17 mm. The corresponding optical extinction is Av ¼ 18 mag. 92127, USA Optical Balmer recombination lines give Av ¼ 3 (ref. 13). The § Department of Astronomy, 1002 West Street, University of Illinois, apparent contradiction between the low optical extinction and Urbana, Illinois 61801, USA higher extinction is resolved if the extinction is high and ...... internal to the nebula. The extinction-corrected Brackett line fluxes Little is known about the origins of globular clusters, which obs 216 22 obs 216 22 are SBra ¼ 7:1 £ 10 Wm and SBrg ¼ 2:5 £ 10 Wm for the contain hundreds of thousands of in a volume only a few 00 inner ,1.5 . These fluxes predict a 15-GHz free–free flux of light years across. Radiation and winds from luminous 24 ^ 8 mJy: the observed 15 GHz flux in this region, corrected for young stars should disperse the -forming gas and disrupt the opacity2,is24^ 3 mJy (ref. 4). The Lyman continuum rate required formation of the cluster. Globular clusters in our cannot to maintain the ionization of the supernebula is provide answers; they are billions of years old. Here we report the N ¼ (4 ^ 1) £ 1052 s21, equivalent to 4,000 O7 stars. This agrees measurement of infrared hydrogen recombination lines from a Lyc with centimetre-wave and millimetre-wave free–free fluxes1,2,4 and young, forming super in the dwarf galaxy NGC5253. radio recombination lines15 (Owens Valley millimetre-wave fluxes2 The lines arise in gas heated by a cluster of about one million give a value of 6,000 O7 stars from a larger aperture). Both the stars, including 4,000–6,000 massive, hot ‘O’stars1,2. It is so young excellent agreement of the Brackett line fluxes and radio fluxes and that it is still enshrouded in gas and dust, hidden from optical the observed compactness of the emission argue that the Brackett view1,3–5. The gases within the cluster seem bound by gravity, line emission arises from the radio ‘supernebula’. which may explain why the windy and luminous O stars have not The velocity information presented here is new; these spectra yet blown away those gases. Young clusters in ‘starbursting’ in the local and distant Universe may also be gravita- tionally confined and cloaked from view. NGC5253 is host to hundreds of large star clusters6, including dozens of extremely bright, young super star clusters7 (SSCs). Only a few million years old8,9, these clusters are found in the central “starburst”9 region of the nearby (3.8 Mpc) galaxy. Subarcsecond radio and infrared imaging reveal3–5 a bright “supernebula” within the starburst, optically invisible, probably powered by a young SSC1,2. The required to ionize the supernebula is (0.8– 9 1.2) £ 10 L (, where the subscript ( refers to the , within a 1– 2 pc (ref. 1) region—perhaps the most concentrated star-forming luminosity known. To verify the nature of the supernebula and to study its dynamics, we observed the Brackett a and g recombination lines of hydrogen at 4.05 mm and 2.17 mm using the NIRSPEC10 on the Keck Telescope on 11 March 2000. Spectra were taken through a 0.579 00 £ 24 00 slit at a spectral resolution of R < 25,000, or about 12 km s21. SCAM, a 256 £ 256 array camera within NIRSPEC, simultaneously imaged the slit location on the galaxy at the K band (2.2 mm), allowing us to pinpoint where the spectra were taken. The seeing was 0.55 00 –0.8 00 . The 2.2 mm broadband image of NGC5253 reveals hundreds of bright infrared star clusters, shown in Figs 1 and 2. From their optical colours, these SSCs are estimated to be only ,2–50 Myr in age8,9. We find that the brightest infrared source does not coincide with any of the optical clusters. The brightest 2.2-mm source is offset 00 00 by ,0.3 ^ 0.1 to the northwest of the youngest8 optical source. Figure 1 Infrared–optical colour view of the young SSCs of NGC5253. The l ¼ 2.2 mm This visually obscured K-band source is the location of the strong image ( channel) was constructed from SCAM images made during the night. The Brackett line emission that we observe from the supernebula. infrared image is combined with an optical image from the (HST) Figure 2 shows the image of the slit position with the strongest ( and green channels). Seeing was ,0.7–0.8 00 for the infrared image; the HST image Brackett line emission and the corresponding Brackett a echello- was convolved to match. The brightest K-band source appears as an extended red source gram. Line plots of both Brackett lines are shown in Fig. 3. to the north of the dust lane. A gaussian fit to this source gives a size (FWHM) of obs ^ 00 00 Continuum-subtracted line fluxes are SBra ¼ð3:4 1Þ £ 1.10 £ 0.9 , position angle 368. From smaller clusters we estimate the point spread 216 22 obs ^ 217 22 00 ^ 00 10 Wm and SBrg ¼ð3:9 1Þ £ 10 Wm for a region function for the SCAM image to be 0.75 0.2 . If the K-band source is gaussian, this 00 within ,3 of the continuum peak. For comparison, ref. 11 would imply a source size of ,0.8 00 £ 0.5 00 , along position angle 408. This size is obs ^ 216 22 obs ^ 00 measures SBra ¼ð7:0 1Þ £ 10 Wm and SBrg ¼ð1:5 1Þ £ uncertain because of variable seeing. The entire SCAM image is 46 square. The 10216 Wm22 in a 10 00 £ 20 00 aperture; as much as half of their Br orientation of the image is east to the left, north up.

NATURE | VOL 423 | 5 JUNE 2003 | www.nature.com/nature © 2003 Nature Publishing Group 621 letters to nature probe the dynamics of the nebula at high spatial and spectral Brackett size because both scale with emission measure; VLA images resolution. The Brackett line profiles were fitted with gaussians, show that the nebula is 0.9 pc £ 1.8 pc in size (0.05 00 £ 0.1 00 , ^0.02 00 ) 21 with line centroids at v LSR ¼ 377 ^ 2kms and full-widths at (ref. 1). We assume that the star cluster exciting the nebula lies half-maxima (FWHMs) of 76 ^ 1kms21, consistent with Ha (ref. within the supernebula, otherwise the implied excitation luminosity 16) and radio recombination lines15. For gas at 12,000 K, this would be higher than the total observed IRAS luminosity of the 9 linewidth is supersonic. Supersonic gas motions are expected in entire galaxy, 1.8 £ 10 L ( (ref. 5). For a radius of 0.5–0.9 pc, the nebulae: in addition to nebular expansion there is also the inter- escape velocity is ,25–30 km s21 for a cluster of O stars alone; 21 action of the nebular gas with winds from the massive stars in the vesc < 50–70 km s for an IMF cut-off at 1 M (; and vesc < 85– 21 cluster. For comparison, compact H II regions in our Galaxy around 110 km s for a cluster IMF extending below 1 M (. Gravity must small groups of O stars have linewidths of up to 64 km s21 (ref. 17); play a significant role in the dynamics of this nebula, slowing its nebulae around individual O stars in the Wolf–Rayet phase can have expansion, if not halting it. Virial linewidths are only slightly smaller 21 linewidths of 50–200 km s (ref. 18). than vesc, so the nebula could be in gravitational equilibrium. The first implication of the linewidth of 75 km s21 for the If the ‘supernebula’ stage of SSC formation should prove to be ‘supernebula’ is that if the nebula were actually expanding at the common and long-lived, it may have implications for the detection implied speed of ,38 km s21, then the mean radius of ,0.7 pc (ref. of Lyman a and other lines in star-forming galaxies. 1) of the nebula implies an implausibly short dynamical age of 7,000 There is evidence that Ly a is detectable primarily in galaxies in years. Dynamical ages of compact H II regions in our Galaxy are which the potential absorbing gas is Doppler-shifted out of the path generally too short to explain the numbers of observed nebulae21.It of the Ly a photons20. This would be most likely in galaxies with is thought that confinement mechanisms such as the pressure of superwinds and superbubbles, phenomena which can be caused by dense molecular clouds are responsible for the retardation of their large concentrations of hot and windy O stars21. If a significant expansion. This could lengthen the lifetime of the supernebula fraction of the lifetime of the ionizing phase of an SSC is spent in a phase, if there were molecular gas nearby, which is as yet unde- confined, dust-enshrouded state such as the supernebula, it is likely tected2. that these winds will develop only late in the formation process of The more unusual implication of the linewidth is that although the cluster, perhaps at the occurrence of the first supernovae. By that they are supersonic, these nebular lines are remarkably narrow time the ionizing flux of the cluster is already declining. SSCs may be given the size of the nebula and the high luminosity of its embedded an important mode of in the early Universe. Ly a star cluster. Brackett line, radio and infrared fluxes all require searches for primeval galaxies may therefore fall far short of 9 L OB < (0.8–1.2) £ 10 L ( for excitation of the nebula. For a detecting the true star formation rate, as suggested by obser- cluster with a Salpeter initial mass function (IMF) and a lower vations20,22–24. mass cut-off of 1 M (, the mass in stars corresponding to this To put the supernebula in context, we can compare it to a more 5 luminosity is (5–7) £ 10 M (; if the IMF extends down to stars traditional nebula, 30 Doradus in the . 6 less than 1 M (, the cluster mass may reach 10 M(. The size of the 30 Dor is a large, luminous nebula ionized by an optically visible star radio nebula is well-determined, and should be the same as the cluster with an estimated age of 10 Myr (ref. 25), 150–200 pc in

Figure 3 Spectra of Br a and Br g in NGC5253. The spectra were integrated over the central 3 00 centred on the brightest infrared continuum source. Each grating setting was followed by a calibration A star. On/off slit SCAM images of the standard indicate that 50% Figure 2 Echellogram and slit position for Brackett spectra of the supernebula. The of the light entered the slit. Because the Brackett line emission has a spatial extent 46 00 £ 46 00 SCAM image shows the position of the 0.579 00 £ 24 00 slit on the brightest comparable to the standard star, ,6–9 pixels (0.8 00 –1.3 00 ), calibration using the standard K-band source in NGC5253. In the inset is the Brackett a echellogram, with / should automatically correct for this effect. We estimate an uncertainty of 30% in the line velocity running horizontally and the spatial dimension vertically. The full slit length is 24 00 , fluxes due to variability in seeing. Near-infrared continuum emission from the brightest K- but only 6 00 is shown in the inset. The orientation of the image is such that north is at an band source is so strong that it can be seen even in these highly dispersed spectra. We angle of 2438 (clockwise) from vertical. The Brackett line emission is less than 1.3 00 in measure continuum fluxes of 186 ^ 60 mJy and 12 ^ 4 mJy at L 0 and K, respectively, spatial extent on the echellogram, the same as the standard star. Spectra taken at eight as compared to the 144 mJy and 20 mJy fluxes in these bands observed in ref. 29 with other positions showed weak or no emission. 8–9 00 apertures.

622 © 2003 Nature Publishing Group NATURE | VOL 423 | 5 JUNE 2003 | www.nature.com/nature letters to nature extent and with a gas density of 20–100 cm23 (ref. 26). 30 Dor is 200 ...... times larger and 100–1,000 times less dense than the supernebula in NGC5253, and its exciting star cluster is 10–100 times less A strong decrease in Saturn’s massive than the cluster in NGC5253 (ref. 27). The escape velocity 21 equatorial jet at cloud level for 30 Dor is less than the thermal sound speed of 10 km s so its gas motions are determined by winds and turbulence rather than by A. Sa´nchez-Lavega*,S.Pe´rez-Hoyos*, J. F. Rojas†, R. Hueso* gravity28. How the supernebula in NGC5253 will free itself from & R. G. French‡ ‘gravitational bondage’ and evolve into an extended, diffuse nebula like 30 Dor will probably depend on the evolution of its underlying * Departamento Fı´sica Aplicada I, Escuela Superior de Ingenieros, Universidad del star cluster. A Paı´s Vasco, Alameda Urquijo s/n, 48013 Bilbao, Spain † Departamento Fı´sica Aplicada I, EUITI, Universidad del Paı´s Vasco, Received 27 November 2002; accepted 28 April 2003; doi:10.1038/nature01689. Plaza Casilla s/n, 48013 Bilbao, Spain 1. Turner, J. L., Beck, S. C. & Ho, P. T. P. The radio supernebula in NGC5253. Astrophys. J. 532, ‡ Department of Astronomy, Wellesley College, Wellesley, Massachusetts 02481, L109–L112 (2000). USA 2. Meier, D. S., Turner, J. L. & Beck, S. C. Molecular gas and star formation in NGC5253 revisited. Astron...... J. 124, 877–885 (2002). 3. Beck, S. C., Turner, J. L., Ho, P. T. P., Kelly, D. & Lacy, J. H. The central star cluster of the star-forming The atmospheres of the giant Jupiter and Saturn have dwarf galaxy NGC5253. Astrophys. J. 457, 610–615 (1996). a puzzling system of zonal (east–west) winds alternating in 4. Turner, J. L., Ho, P. T. P. & Beck, S. C. The radio properties of NGC5253 and its unusual HII regions. latitude, with the broad and intense equatorial jets on Saturn Astron. J. 116, 1212–1220 (1998). 5. Gorjian, V., Turner, J. L. & Beck, S. C. Infrared emission from the radio supernebula in NGC5253: A having been observed previously to reach a velocity of about 21 1 proto-? Astrophys. J. 554, L29–L32 (2001). 470 m s at cloud level . Globally, the location and intensity of 6. Caldwell, N. & Phillips, M. M. Star formation in NGC5253. Astrophys. J. 338, 789–803 (1989). Jupiter’s jets are stable in time to within about ten per cent2,3, but 7. Gorjian, V. WFPC2 imaging of the NGC5253. Astron. J. 1120, 1886–1893 (1996). little is known about the stability of Saturn’s jet system. The long- 8. Calzetti, D. et al. Dust and recent star formation in the core of NGC5253. Astron. J. 114, 1834–1849 (1997). term behaviour of these winds is an important discriminator 4–9 9. Tremonti, C. A., Calzetti, D., Leitherer, C. & Heckman, T. M. Star formation in the field and clusters of between models for giant- circulations . Here we report NGC5253. Astrophys. J. 555, 322–337 (2001). that Saturn’s winds show a large drop in the velocity of the 10. McLean, I. S. et al. Performance and results with the NIRSPEC echelle spectrograph on the Keck II 21 Telescope. Proc. SPIE 4008, 1048–1055 (2000). equatorial jet of about 200 m s from 1996 to 2002. By contrast, 11. Kawara, K., Nishida, M. & Phillips, M. M. Brackett alpha and gamma observations of starburst and the other measured jets (primarily in the southern hemisphere) Seyfert galaxies. Astrophys. J. 337, 230–235 (1989). appear stable when compared to the Voyager wind profile of 12. Storey, P. J. & Hummer, D. G. Recombination line intensities for hydrogenic atoms. IV. Total 1980–81. recombination coefficients and machine-readable tables for Z ¼ 1to8.Mon. Not. R. Astron. Soc. 272, 41–48 (1995). The first precise measurements of Saturn’s zonal wind velocity 13. Walsh, J. R. & Roy, J.-R. Optical spectroscopic and abundance mapping of the amorphous galaxy profile were performed during the Voyager encounters in 1980–81 NGC5253. Mon. Not. R. Astron. Soc. 239, 297–324 (1989). (refs 10, 11). Previous wind measurements, obtained by tracking 14. Rieke, G. H. & Lebofsky, M. J. The interstellar extinction law for 1 to 13 microns. Astrophys. J. 288, individual ‘spots’ in the atmosphere, are scarce, but are in general 618–621 (1985). 12 15. Mohan, N. R., Anantharamaiah, K. R. & Goss, W. M. Very Large Array observations of the H92a line agreement with the Voyager data . Cloud motions measured on from NGC5253 and Henize 2–10: Ionized gas around super star clusters. Astrophys. J. 557, 659–670 Voyager images revealed a strong and broad equatorial jet (peak (2001). velocity ,470 m s21, spanning planetographic latitudes ^408), 16. Martin, C. L. & Kennicutt, R. C. Jr Soft X-ray emission from NGC5253 and the ionized interstellar medium. Astrophys. J. 447, 171–183 (1995). twice as broad and four times more intense than that of Jupiter. 17. De Pree, C. G., Wilner, D. J., Goss, W. M., Welch, W. J. & McGrath, E. Ultracompact HII regions in The equator is also the place where infrequent but violent eruptions W49N at 500 AU scales: Shells, winds, and the water maser source. Astrophys. J. 540, 308–315 (2000). (the ‘Great White Spots’) occur13–15. As Saturn has a large obliquity 18. Chu, Y. H. Ring nebulae around massive stars throughout the HR diagram. In IAU Symposium 212 of 26.78, and the shadow of the rings produces strong insolation (eds van der Hucht, K. A., Herrero, A. & Esteban, C.) (in the press). 19. Wood, D. O. S. & Churchwell, E. Massive stars embedded in molecular clouds: Their population and changes in the equatorial region, induced effects during Saturn’s distribution in the Galaxy. Astrophys. J. 340, 265–272 (1989). year (29.4 terrestrial years) could be expected at cloud level where 20. Kunth, D. et al. HSTstudy of Lyman-alpha emission in star-forming galaxies: the effect of neutral gas the winds are measured16. The zonal jet measurements rely on the flows. Astron. Astrophys. 334, 11–20 (1998). detection and tracking of cloud features, and because of the lower 21. Tenorio-Tagle, G., Silich, S. A., Kunth, D., Terlevich, E. & Terlevich, R. The evolution of superbubbles and the detection of Lya in star-forming galaxies. Mon. Not. R. Astron. Soc. 309, 332–342 (1999). contrast, size and number of such features in Saturn than in Jupiter, 22. Meier, D. L. & Terlevich, R. Extragalactic HII regions in the UV. Implications for primeval galaxies. this can only be performed at present using the Hubble Space Astrophys. J. 246, L109–L113 (1981). Telescope (HST). The Voyager data were obtained during the 23. Hartmann, L., Huchra, J. P. & Geller, M. J. How to find galaxies at high redshift. Astrophys. J. 287, northern hemisphere spring (1980–81), and HST high-quality 487–491 (1984). 24. Steidel, C. C., Giavalisco, M., Pettini, M., Dickinson, M. & Adelberger, K. L. Spectroscopic images of Saturn are available for the period 1994–2002, during confirmation of a population of normal star-forming galaxies at redshifts z . 3. Astrophys. J. 462, Saturn’s southern hemisphere spring and early summer. Both L17–L21 (1996). epochs are well placed to look for long-term changes. 25. Walborn, N. R. & Barba´,R.H.inNew Views of the Magellanic Clouds (eds Chu, Y.-H., Suntzeff, N., Hesser, J. & Bohlender, D.) 213 (Astronomical Society of the Pacific, San Francisco, 1999). The HST images analysed here were obtained between 1996 and 26. Mills, B. Y., Turtle, A. J. & Watkinson, A. A radio model of the 30 Doradus region. Mon. Not. R. Astron. 2002 (3–6 days per year) using the Wide Field Planetary Camera 2 Soc. 185, 263–276 (1978). (WFPC2) at its maximum spatial resolution. A series of filters from 27. Kennicutt, R. C. Jr & Chu, Y.-H. Giant HII regions and the rormation of populous clusters. Astron. J. 255 to 1,042 nm were used, including at times a filter isolating the 95, 720–730 (1988). 28. Melnick, J., Tenorio-Tagle, G. & Terlevich, R. Supersonic gas motion in extragalactic HII regions. Mon. 890-nm methane absorption band (dates and filters for each Not. R. Astron. Soc. 302, 677–683 (1999). campaign are given in Supplementary Information). In total, 29. Moorwood, A. F. M. & Glass, I. S. Infrared emission and star formation in NGC5253. Astron. about 100 images were selected, navigated on the planetary limb Astrophys. 115, 84–89 (1982). and photometrically calibrated. Images obtained at of Acknowledgements We thank T. Glassman and E. Greisen for assistance with the data and M. Jura 439 nm, 675 nm, 814 nm and 890 nm showed the clouds to have for discussions. This research is supported by the US National Science Foundation, the Israel high contrast (enhanced by an ‘unsharp masking’ processing), and Academy Center for Multi- Astronomy, and the Laboratory of Astronomical Imaging were used for target identification and tracking. Image pairs or at the University of Illinois. triplets obtained with the same filter, separated by a temporal interval ranging from 0.5 to 8 days, were used for the tracking. Competing interests statement The authors declare that they have no competing financial interests. Following the ring-plane crossing epoch in 1995, Saturn’s orbital obliquity and ring projection limited our visibility of the northern Correspondence and requests for materials should be addressed to J.T. ([email protected]). hemisphere, so most features were observed from latitudes þ308 to

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