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Outflowing winds of multiphase plasma have been proposed to regulate the buildup of galaxies, but key aspects of these outflows have not been probed with observations. By using ultraviolet absorption spectroscopy, we show that \"warm-hot\" plasma at 10 5.5 kelvin contains 10 to 150 times more mass than the cold gas in a post-star burst galaxy wind. This wind extends to distances > 68 kiloparsecs, and at least some portion of it will escape. Moreover, the kinematical correlation of the cold and warm-hot phases indicates that the warm-hot plasma is related to the interaction of the cold matter with a hotter (unseen) phase at »10⁶ kelvin. Such multiphase winds can remove substantial masses and alter the evolution of post-star burst galaxies.

Starburst galaxies exhibit in their central regions a highly increased rate of supernovae, the remnants of which are thought to accelerate energetic cosmic rays up to energies of approximately 10¹⁵ electron volts. We report the detection of gamma rays--tracers of such cosmic rays--from the starburst galaxy NGC 253 using the High Energy Stereoscopic System (H.E.S.S.) array of imaging atmospheric Cherenkov telescopes. The gamma-ray flux above 220 billion electron volts is F = (5.5 ± 1.0stat ± 2.8sys) x 10⁻¹³ cm⁻² s⁻¹, implying a cosmic-ray density about three orders of magnitude larger than that in the center of the Milky Way. The fraction of cosmic-ray energy channeled into gamma rays in this starburst environment is five times as large as that in our Galaxy.

Bursting onto the scene The starburst in Henize 2-10, a relatively nearby blue compact dwarf galaxy, has attracted the attention of astronomers for decades, in part because of its prodigious rate of star formation — ten times that of the Large Magellanic Cloud. Now a study of Henize 2-10 at centimetre radio wavelengths and in the near-infrared reveals a compact radio source at its centre that is spatially coincident with a hard X-ray source. This points to the presence of an actively accreting massive black hole, but one not associated with a bulge, a nuclear star cluster or any other well-defined nucleus. This means that Henize 2-10 may reflect an early phase of black hole and galaxy evolution that has not been observed previously. Henize 2-10 is a nearby dwarf starburst galaxy that may be similar to galaxies in the infant Universe. It is reported that Henize 2-10 contains a compact radio source at the dynamical centre of the galaxy that is spatially coincident with a hard X-ray source, from which it is concluded that Henize 2-10 harbours an actively accreting central black hole with a mass of approximately one million solar masses. The results confirm that nearby star-forming dwarf galaxies can indeed form massive black holes, and by implication so can their primordial counterparts. Supermassive black holes are now thought to lie at the heart of every giant galaxy with a spheroidal component, including our own Milky Way 1 , 2 . The birth and growth of the first ‘seed’ black holes in the earlier Universe, however, is observationally unconstrained 3 and we are only beginning to piece together a scenario for their subsequent evolution 4 . Here we report that the nearby dwarf starburst galaxy Henize 2-10 (refs 5 and 6 ) contains a compact radio source at the dynamical centre of the galaxy that is spatially coincident with a hard X-ray source. From these observations, we conclude that Henize 2-10 harbours an actively accreting central black hole with a mass of approximately one million solar masses. This nearby dwarf galaxy, simultaneously hosting a massive black hole and an extreme burst of star formation, is analogous in many ways to galaxies in the infant Universe during the early stages of black-hole growth and galaxy mass assembly. Our results confirm that nearby star-forming dwarf galaxies can indeed form massive black holes, and that by implication so can their primordial counterparts. Moreover, the lack of a substantial spheroidal component in Henize 2-10 indicates that supermassive black-hole growth may precede the build-up of galaxy spheroids.

One in twenty galaxies 'missing' The observational properties of the hydrogen Lyman-α (Lyα) emission line mean that it is the tool of choice for the study of star-forming galaxies at the highest redshifts. Doubts remain about the interpretation of the data, however, since Lyα photons scatter in the neutral interstellar medium of their host galaxies, so their sensitivity to absorption by interstellar dust may be enhanced such that the Lyα luminosity may be significantly reduced, or even completely suppressed. In order to calibrate this effect in an unbiased sample, Hayes et al . examine Lyα emissions at great distances, using a second feature, Hα. They find significant discrepancies (up to a factor of 20) in the distribution of luminosities, suggesting that only about 5% of Lyα photons escape from star-forming galaxies. This needs to be borne in mind when evaluating previous and future Lyα survey results and means that there are probably many more galaxies at these epochs waiting to be discovered. The main observational signature of star-forming galaxies at the highest redshifts is the Lyman-α (Lyα) emission line. But Lyα photons scatter in the neutral interstellar medium of their host galaxies, and may therefore be greatly absorbed by interstellar dust. It is now shown that the average escaping fraction of Lyα photons from star-forming galaxies at redshift z = 2.2 is just 5 per cent. This implies that numerous conclusions based on Lyα-selected samples will require upwards revision by an order of magnitude. The Lyman-α (Lyα) emission line is the primary observational signature of star-forming galaxies at the highest redshifts 1 , and has enabled the compilation of large samples of galaxies with which to study cosmic evolution 2 , 3 , 4 , 5 . The resonant nature of the line, however, means that Lyα photons scatter in the neutral interstellar medium of their host galaxies, and their sensitivity to absorption by interstellar dust may therefore be greatly enhanced. This implies that the Lyα luminosity may be significantly reduced, or even completely suppressed. Hitherto, no unbiased empirical test of the escaping fraction ( f esc ) of Lyα photons has been performed at high redshifts. Here we report that the average f esc from star-forming galaxies at redshift z = 2.2 is just 5 per cent by performing a blind narrowband survey in Lyα and Hα. This implies that numerous conclusions based on Lyα-selected samples will require upwards revision by an order of magnitude and we provide a benchmark for this revision. We demonstrate that almost 90 per cent of star-forming galaxies emit insufficient Lyα to be detected by standard selection criteria 2 , 3 , 4 , 5 . Both samples show an anti-correlation of f esc with dust content, and we show that Lyα- and Hα-selection recovers populations that differ substantially in dust content and f esc .

Starring black holes The discovery that every nearby galaxy hosts a massive central black hole, with a mass directly proportional to that of its stellar bulge, suggested that the growth of the black hole and stellar bulge of galaxies must be synchronized in some way. These stars are very old, so if there is a link to black hole growth it must have occurred a long time — more than 8 billion years — ago. The distant luminous galaxies known as submillimetre-bright galaxies (SMGs) are at the sort of redshift associated with such great age and a new survey of SMGs reveals simultaneous black-hole and stellar growth, providing the first evidence of a direct link between the major black hole–stellar bulge growth phase of nearby massive galaxies. The tight relationship between the masses of black holes and galaxy spheroids in nearby galaxies 1 implies a causal connection between the growth of these two components. Optically luminous quasars host the most prodigious accreting black holes in the Universe, and can account for ≳30 per cent of the total cosmological black-hole growth 2 , 3 . As typical quasars are not, however, undergoing intense star formation and already host massive black holes (> 10 8 M ⊙ , where M ⊙ is the solar mass) 4 , 5 , there must have been an earlier pre-quasar phase when these black holes grew (mass range ∼(10 6 –10 8 ) M ⊙ ). The likely signature of this earlier stage is simultaneous black-hole growth and star formation in distant (redshift z > 1; >8 billion light years away) luminous galaxies. Here we report ultra-deep X-ray observations of distant star-forming galaxies that are bright at submillimetre wavelengths. We find that the black holes in these galaxies are growing almost continuously throughout periods of intense star formation. This activity appears to be more tightly associated with these galaxies than any other coeval galaxy populations. We show that the black-hole growth from these galaxies is consistent with that expected for the pre-quasar phase.

Thinking outside the galaxy Spitzer Space Telescope observations of ultraluminous infrared galaxies show that the molecular hydrogen (H 2 ) emission in these objects — often considered an indicator of star formation — originates not from starburst activity deep within the galaxies but from outside the galaxies' dusty central regions. These objects are among the most luminous in the local Universe and are thought to be powered by intense star formation. These observations by Nadia Zakamska point to an alternative origin for the H 2 , however. Zakamska proposes that the H 2 emission is produced by shocks in the surrounding material, which are excited by interactions with nearby galaxies. Ultraluminous infrared galaxies are among the most luminous objects in the local Universe and are thought to be powered by intense star formation. In these objects, the rotational lines of molecular hydrogen (H 2 ) observed at mid-infrared wavelengths are not affected by dust obscuration, but the source of excitation has been unknown. Here it is found that H 2 emission originates outside the obscured regions; it is proposed that H 2 emission traces shocks in the surrounding material that are excited by interactions with nearby galaxies. Ultraluminous infrared galaxies are among the most luminous objects in the local Universe and are thought to be powered by intense star formation 1 , 2 . It has been shown that in these objects the rotational spectral lines of molecular hydrogen observed at mid-infrared wavelengths are not affected by dust obscuration 3 , but left unresolved was the source of excitation for this emission. Here I report an analysis of archival Spitzer Space Telescope data on ultraluminous infrared galaxies and demonstrate that dust obscuration affects star formation indicators but not molecular hydrogen. I thereby establish that the emission of H 2 is not co-spatial with the buried starburst activity and originates outside the obscured regions. This is unexpected in light of the standard view that H 2 emission is directly associated with star-formation activity 3 , 4 , 5 . I propose the alternative view that H 2 emission in these objects traces shocks in the surrounding material that are excited by interactions with nearby galaxies. Large-scale shocks cooling by means of H 2 emission may accordingly be more common than previously thought. In the early Universe, a boost in H 2 emission by this process may have accelerated the cooling of matter as it collapsed to form the first stars and galaxies, and would make these first structures more readily observable 6 .

Analysis of the deepest available images of the sky, obtained by the Hubble Space Telescope, reveals a large number of candidate high-redshift galaxies. A catalogue of 1,683 objects is presented, with estimated redshifts ranging from z = 0 to z > 6. The high-redshift objects are interpreted as regions of star formation associated with the progenitors of present-day normal galaxies, at epochs that may reach back 95% of the time to the Big Bang.

Magnetic fields may play an important role in the star-formation process, especially in the central regions of ‘starburst’ galaxies where star formation is vigorous. But the field directions are very difficult to determine in the dense molecular gas out of which the stars form, so it has hitherto been impossible to test this hypothesis. Dust grains in interstellar clouds tend to be magnetically aligned, and it is possible to determine the alignment direction based on the polarization of optical light due to preferential extinction along the long axes of the aligned grains 1 . This technique works, however, only for diffuse gas, not for the dense molecular gas. Here we report observations of polarized thermal emission from the aligned dust grains in the central region of M82, which directly traces 2 the magnetic field structure (as projected onto the plane of the sky). Organized field lines are seen around the brightest star-forming regions, while in the dusty halo the field lines form a giant magnetic bubble possibly blown out by the galaxy's ‘superwind’.

Ultraluminous infrared galaxies have been known for more than a decade, but the source of their very large far-infrared luminosities remains controversial. It may reflect a quasar-like active nucleus surrounded by a torus of dense gas and dust, the latter absorbing the energetic photons from the nuclear region and re-emitting at infrared wavelengths 1 , or a huge burst of massive-star formation in dense dusty clouds of molecular gas close to the nucleus 2 , which heats the surrounding dust. A number of ultraluminous galaxies are also a source of OH-megamaser emissions (intense laser-like spectral lines at microwave frequencies), an observation that may hold important clues as to the main power source in these galaxies. A general feature of many models 3,4 is that the masers are pumped radiatively by the absorption of infrared photons. Identifying the source of the maser pump may therefore indicate whether the ultimate energy source is a burst of star formation, or an active nucleus. Here we report the detection of a strong mid-infrared OH absorption line in the prototypical megamaser and ultraluminous galaxy 5 , Arp220. We find that the power absorbed in this line alone is sufficient to pump the megamaser emission seen at radio wavelengths. Moreover, the warm, extended nature of the pumping region is suggestive of a starburst origin for the ultraluminous infrared emissions.