The Far-Ultraviolet Signature of the 'Missing' Baryons in the Local

letters to nature .............................................................. with negative velocities, while the other hemisphere contains mostly HV-O VI absorbers with positive velocity. There are three The far-ultraviolet signature of exceptions: two of these have LSR velocities very close to the 21 the ‘missing’ baryons in the threshold velocity of jvLSRj¼100kms ; and so may well belong to the LV-O VI population. The third lies at very high latitude, where Local Group of galaxies the concept of longitude becomes meaningless. For comparison, the population of LV-O VI absorbers show no velocity segregation in the Fabrizio Nicastro*, Andreas Zezas*, Martin Elvis*, Smita Mathur†, LSR. Fabrizio Fiore‡, Cesare Cecchi-Pestellini*, Douglas Burke*, The red circles in Fig. 2 mark the six HV-O VI absorbers (,18% of Jeremy Drake* & Piergiorgio Casella* the sample) which are tentatively identified with 21-cm high- velocity H I clouds or complexes, either because of spatial and * Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, velocity coincidence (within broad ranges) with entries in the Massachusetts 02138, USA catalogues of refs 17 and 18, or because they are identified as such † Astronomy Department, The Ohio State University, Columbus, Ohio 43210, in the HV-O VI compilation of ref. 4 (in which ,20% of HV-O VI USA are tentatively identified with HV-H I). We stress here that these are ‡ Osservatorio Astronomico di Monteporzio, Via Osservatorio Via Frascati 33, the only six HV-O VI absorbers, in either compilation (ours and ref. Monteporzio-Catone (RM), I-00040 Italy 4), that may be identified with HV-H I clouds with a known distance, ............................................................................................................................................................................. , The number of baryons detected in the low-redshift (z < 1) either in our Galaxy’s inner halo (with distances less than 10 kpc), Universe is far smaller than the number detected in correspond- or between us and the two Magellanic Clouds. To be conservative we do not consider further these six HV-O VI absorbers. ing volumes at higher redshifts. Simulations1–3 of the formation The systematic LSR velocity distribution of HV-O VI is consistent of structure in the Universe show that up to two-thirds of the with matter that is (1) counter-rotating, with respect to the Galaxy ‘missing’ baryons may have escaped detection because of their disk rotation, on orbits external to the Sun’s orbit, (2) at rest in high temperature and low density. One of the few ways to detect the Galactic halo, or (3) at rest in the intergalactic space surround- this matter directly is to look for its signature in the form of ing the Galaxy. The range of radial LSR velocities of the HV-O VI ultraviolet absorption lines in the spectra of background sources ð100 , jv jHV , 550kms21Þ greatly exceeds the range of such as quasars. Here we show that the amplitude of the average LSR 51 observed radial velocities in the Galactic disk or halo, suggesting velocity vector of ‘high velocity’ O VI (O ) absorption clouds that the Galaxy-related options (1) and (2) are unlikely. Moreover, detected in a survey4 of ultraviolet emission from active galactic some of the HV-O VI velocities exceed a plausible measure of the nuclei decreases significantly when the vector is transformed to escape velocity from the Milky Way19. Finally, clouds in the Galaxy’s the frames of the Galactic Standard of Rest and the Local Group halo would probably be rotating on random orbits around the of galaxies. At least 82 per cent of these absorbers are not Galaxy’s centre, as globular clusters do. The peculiar velocities of associated with any ‘high velocity’ atomic hydrogen complex in these clouds along these orbits would tend to randomize the our Galaxy, and are therefore likely to result from a primordial apparent symmetry induced in the LSR by the circular motion of warm–hot intergalactic medium pervading an extended corona the Sun in the Galaxy for matter effectively at rest in the halo, as around the Milky Way or the Local Group. The total mass of observed in globular clusters. An intergalactic origin, with distances baryons in this medium is estimated to be up to ,1012 solar larger than ,100 kpc, is more consistent with the data. masses, which is of the order of the mass required5 to dynamically The LSR is not a rest frame system for the HV-O VI absorbers stabilize the Local Group. (Fig. 2a, Table 1). This is confirmed by translations of the velocity The first evidence for an absorption system due to the warm–hot distribution to other convenient rest frames: the Galactic Standard intergalactic medium has been discovered in the X-ray and ultraviolet spectra of the blazar PKS21552304 (ref. 6), obtained with the Chandra low-energy transmission grating (LETG)7, and the Far- Ultraviolet Spectroscopic Explorer (FUSE)8. But one line of sight is inadequate to fully define this absorbing medium. FUSE obser- vations of active galactic nuclei show two types of ubiquitous z < 0 O VI absorbers, similar to those observed in PKS21552304: (1) low-velocity O VI clouds (LV-O VI, velocity with respect to the Local 21 9–13 Standard of Rest jv LSRj , 100 km s ) ; and (2) high-velocity 21 4,14–16 O VI clouds (HV-O VI, jv LSRj . 100 km s ) . To investigate the nature of these absorbers we have examined (see Methods section) a sample of active galactic nuclei using publicly available FUSE data. This has allowed us to clearly identify the HV-O VI component with diffuse gas in an extended Galactic corona or in the Local Group. The O VI velocity distribution in the LSR (Fig. 1, dashed histo- 21 gram) shows a narrow peak between ^100 km s (LV-O VI) with a much broader, roughly symmetric distribution (Fig. 1, solid histo- 21 gram) extending to ^550 km s (HV-O VI). The bimodality of this distribution suggests that LV- and HV-O VI systems belong to two Figure 1 The velocity range of HV-O VI absorbers greatly exceeds typical rotational different populations of absorbers, as previously pointed out in refs velocities in the Galaxy. Histogram of the HV-O VI (solid line) and LV-O VI (dashed line) 9 and 14. Here we concentrate on the HV-O VI absorbers, and use velocity distributions in the Local Standard of Rest (LSR). Among the 54 objects of our the LV-O VI absorbers only as a comparison sample. A complete sample, 45 (83%) show at least one clear O VI absorption component at z < 0 at our discussion of both LVand HV systems is deferred to a forthcoming detection thresholds (7 out of the remaining 9 objects have poor-quality FUSE spectra, paper (F.N., A.Z., M.E. & P. Casella, manuscript in preparation). with detection thresholds of equivalent width $200 mA˚ ). Four show multiple LV and HV A plot in Galactic coordinates (latitude, b; longitude, l) of the LSR absorption. 38 lines of sight show LV-O VI absorption (70% of the sample), and 32 (59% of velocity distributions for the HV (Fig. 2a) absorption systems shows the sample) show HV-O VI components, with 22 objects showing both. Only four lines of that the hemisphere with 08#l # 1808 contains only HV-O VI lines sight show multiple LV or HV absorption. NATURE | VOL 421 | 13 FEBRUARY 2003 | www.nature.com/nature © 2003 Nature Publishing Group 719 letters to nature of Rest (GSR) and the Local Group Standard of Rest (LGSR). The motion of the filament, towards our Galaxy, in the direction of M31. symmetry present in the LSR velocity distribution of the HV-O VI This is similar to what is observed for the velocity field HV-O VI systems (Fig. 2a) disappears in the LGSR (Fig. 2b) and instead absorbers. The inverse of the mean GSR velocity vector of the HV- HV 21 appears random, suggesting that the LGSR is the rest frame for the O VI absorbers (Table 1) has kjvGSRj l ¼ 264kms ; and Galactic HV-O VI absorbers. The amplitude of the HV-O VI average velocity coordinates of l ¼ 768, b ¼ 2388, fairly close to M31: l ¼ 1218, vector decreases monotonically from the LSR to the GSR, to the b ¼ 2228. HV 21 LGSR, at which point it is only kjvLGSRj l ¼ 32kms (Table 1), Estimates of the dynamical mass of our Local Group exceed the much smaller than the corresponding velocities in the LSR, and measured mass of its visible constituents by more than a factor of close to the FUSE resolution (,20 km s21 at 1,032 A˚ ). At the same two (see, for example, ref. 5). A total mass of ,1012 solar masses time, the direction of the average vector becomes very poorly would be needed to stabilize, dynamically, the Local Group. From constrained (Table 1). Both results suggest that GSR and LGSR the combined analysis of the absorption features from highly are preferred rest frames for the HV-O VI absorbers. This locates the ionized gas in the ultraviolet (HV-O VI) and X-ray (O VII,OVIII population of HV absorbers in intergalactic space, possibly associ- and Ne IX) spectra of the blazar PKS21552304, it has been6 con- ated with an extended Galactic corona or Local Group medium. cluded that this absorber has to fill the intergalactic space surround- We checked the validity of this method against our control ing the Galaxy. This system has an electron density of the order of 6 23 sample of LV-O VI absorbers, for which a nearby Galactic origin ne < ð4–6Þ £ 10 cm ; an equivalent hydrogen column density of 9–13 19 21 22 has been claimed .

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