Search

Author(s): D. P. Marrone (corresponding author) [1]; J. S. Spilker [1]; C. C. Hayward [2, 3]; J. D. Vieira [4]; M. Aravena [5]; M. L. N. Ashby [3]; M. B. [...]

Major galaxy mergers are thought to play an important part in fuelling the growth of supermassive black holes.sup.1. However, observational support for this hypothesis is mixed, with some studies showing a correlation between merging galaxies and luminous quasars.sup.2,3 and others showing no such association.sup.4,5. Recent observations have shown that a black hole is likely to become heavily obscured behind merger-driven gas and dust, even in the early stages of the merger, when the galaxies are well separated.sup.6-8 (5 to 40 kiloparsecs). Merger simulations further suggest that such obscuration and black-hole accretion peaks in the final merger stage, when the two galactic nuclei are closely separated.sup.9 (less than 3 kiloparsecs). Resolving this final stage requires a combination of high-spatial-resolution infrared imaging and high-sensitivity hard-X-ray observations to detect highly obscured sources. However, large numbers of obscured luminous accreting supermassive black holes have been recently detected nearby (distances below 250 megaparsecs) in X-ray observations.sup.10. Here we report high-resolution infrared observations of hard-X-ray-selected black holes and the discovery of obscured nuclear mergers, the parent populations of supermassive-black-hole mergers. We find that obscured luminous black holes (bolometric luminosity higher than 2 × 10.sup.44 ergs per second) show a significant (P < 0.001) excess of late-stage nuclear mergers (17.6 per cent) compared to a sample of inactive galaxies with matching stellar masses and star formation rates (1.1 per cent), in agreement with theoretical predictions. Using hydrodynamic simulations, we confirm that the excess of nuclear mergers is indeed strongest for gas-rich major-merger hosts of obscured luminous black holes in this final stage.High-resolution infrared observations of hard-X-ray-selected black holes show an excess of late-stage mergers in obscured luminous black holes compared with inactive galaxies of similar stellar masses and star formation rates., Author(s): Michael J. Koss [sup.1] [sup.2] , Laura Blecha [sup.3] , Phillip Bernhard [sup.2] , Chao-Ling Hung [sup.4] , Jessica R. Lu [sup.5] , Benny Trakthenbrot [sup.6] [sup.7] , Ezequiel [...]

Photoelectric heating--heating of dust grains by far-ultraviolet photons--has long been recognized as the primary source of heating for the neutral interstellar medium (1). Simulations of spiral galaxies (2) have shown some indication that photoelectric heating could suppress star formation; however, simulations that include photoelectric heating have typically shown that it has little effect on the rate of star formation in either spiral galaxies (3,4) or dwarf galaxies (5), which suggests that supernovae are responsible for setting the gas depletion time in galaxies (6-8). This result is in contrast with recent work (9-13) indicating that a star formation law that depends on galaxy metallicity--as is expected with photoelectric heating, but not with supernovae--reproduces the present-day galaxy population better than does a metallicity-independent one. Here we report a series of simulations of dwarf galaxies, the class of galaxy in which the effects of both photoelectric heating and supernovae are expected to be strongest. We simultaneously include spaceand time-dependent photoelectric heating in our simulations, and we resolve the energy-conserving phase of every supernova blast wave, which allows us to directly measure the relative importance of feedback by supernovae and photoelectric heating in suppressing star formation. We find that supernovae are unable to account for the observed (14) large gas depletion times in dwarf galaxies. Instead, photoelectric heating is the dominant means by which dwarf galaxies regulate their star formation rate at any given time, suppressing the rate by more than an order of magnitude relative to simulations with only supernovae., To investigate whether the depletion times in dwarf galaxies, which are longer than those for Milky-Way-like galaxies by more than an order of magnitude (14,15), are set by the momentum [...]

One of the key questions in observational cosmology is the identification of the sources responsible for ionization of the Universe after the cosmic 'Dark Ages', when the baryonic matter was neutral. The currently identified distant galaxies are insufficient to fully reionize the Universe by redshift z ≅ 6 (refs 1-3), but low-mass, star-forming galaxies are thought to be responsible for the bulk of the ionizing radiation (4-6). As direct observations at high redshift are difficult for a variety of reasons, one solution is to identify local proxies of this galaxy population. Starburst galaxies at low redshifts, however, generally are opaque to Lyman continuum photons (7-9). Small escape fractions of about 1 to 3 per cent, insufficient to ionize much surrounding gas, have been detected only in three low-redshift galaxies (10,11). Here we report far-ultraviolet observations of the nearby low-mass star-forming galaxy J0925+1403. The galaxy is leaking ionizing radiation with an escape fraction of about 8 per cent. The total number of photons emitted during the starburst phase is sufficient to ionize intergalactic medium material that is about 40 times as massive as the stellar mass of the galaxy., So-called 'Green Peas' (GPs), low-mass compact galaxies with very active star formation (12-15), may be promising candidates for sources of escaping ionizing radiation. The GP galaxy J0925+1403 was selected from [...]

The physics of star formation and the deposition of mass, momentum and energy into the interstellar medium by massive stars ('feedback') are the main uncertainties in modern cosmological simulations of galaxy formation and evolution.sup.1,2. These processes determine the properties of galaxies.sup.3,4 but are poorly understood on the scale of individual giant molecular clouds (less than 100 parsecs).sup.5,6, which are resolved in modern galaxy formation simulations.sup.7,8. The key question is why the timescale for depleting molecular gas through star formation in galaxies (about 2 billion years).sup.9,10 exceeds the cloud dynamical timescale by two orders of magnitude.sup.11. Either most of a cloud's mass is converted into stars over many dynamical times.sup.12 or only a small fraction turns into stars before the cloud is dispersed on a dynamical timescale.sup.13,14. Here we report high-angular-resolution observations of the nearby flocculent spiral galaxy NGC 300. We find that the molecular gas and high-mass star formation on the scale of giant molecular clouds are spatially decorrelated, in contrast to their tight correlation on galactic scales.sup.5. We demonstrate that this decorrelation implies rapid evolutionary cycling between clouds, star formation and feedback. We apply a statistical method.sup.15,16 to quantify the evolutionary timeline and find that star formation is regulated by efficient stellar feedback, which drives cloud dispersal on short timescales (around 1.5 million years). The rapid feedback arises from radiation and stellar winds, before supernova explosions can occur. This feedback limits cloud lifetimes to about one dynamical timescale (about 10 million years), with integrated star formation efficiencies of only 2 to 3 per cent. Our findings reveal that galaxies consist of building blocks undergoing vigorous, feedback-driven life cycles that vary with the galactic environment and collectively define how galaxies form stars. Observations that molecular gas in NGC 300 is spatially uncorrelated with high-mass stars are attributed to rapid evolution, with molecular clouds quickly destroyed by stellar feedback, and low star-formation efficiency., Author(s): J. M. Diederik Kruijssen [sup.1] [sup.2] , Andreas Schruba [sup.3] , Mélanie Chevance [sup.1] , Steven N. Longmore [sup.4] , Alexander P. S. Hygate [sup.1] [sup.2] , Daniel T. [...]

Evidence has mounted in recent decades that outflows of matter and energy from the central few parsecs of our Galaxy have shaped the observed structure of the Milky Way on a variety of larger scales.sup.1. On scales of 15 parsecs, the Galactic Centre has bipolar lobes that can be seen in both the X-ray and radio parts of the spectrum.sup.2,3, indicating broadly collimated outflows from the centre, directed perpendicular to the Galactic plane. On larger scales, approaching the size of the Galaxy itself, [gamma]-ray observations have revealed the so-called 'Fermi bubble' features.sup.4, implying that our Galactic Centre has had a period of active energy release leading to the production of relativistic particles that now populate huge cavities on both sides of the Galactic plane. The X-ray maps from the ROSAT all-sky survey show that the edges of these cavities close to the Galactic plane are bright in X-rays.sup.4-6. At intermediate scales (about 150 parsecs), radio astronomers have observed the Galactic Centre lobe, an apparent bubble of emission seen only at positive Galactic latitudes.sup.7,8, but again indicative of energy injection from near the Galactic Centre. Here we report prominent X-ray structures on these intermediate scales (hundreds of parsecs) above and below the plane, which appear to connect the Galactic Centre region to the Fermi bubbles. We propose that these structures, which we term the Galactic Centre 'chimneys', constitute exhaust channels through which energy and mass, injected by a quasi-continuous train of episodic events at the Galactic Centre, are transported from the central few parsecs to the base of the Fermi bubbles.sup.4. Huge X-ray structures, termed Galactic Centre 'chimneys', extending hundreds of parsecs above and below the Galactic plane, appear to be exhaust channels connecting the Galactic Centre region to the Fermi bubbles., Author(s): G. Ponti [sup.1] [sup.2] , F. Hofmann [sup.1] , E. Churazov [sup.3] [sup.4] , M. R. Morris [sup.5] , F. Haberl [sup.1] , K. Nandra [sup.1] , R. Terrier [...]

Massive stars inject mechanical and radiative energy into the surrounding environment, which stirs it up, heats the gas, produces cloud and intercloud phases in the interstellar medium, and disrupts molecular clouds (the birth sites of new stars.sup.1,2). Stellar winds, supernova explosions and ionization by ultraviolet photons control the lifetimes of molecular clouds.sup.3-7. Theoretical studies predict that momentum injection by radiation should dominate that by stellar winds.sup.8, but this has been difficult to assess observationally. Velocity-resolved large-scale images in the fine-structure line of ionized carbon ([C ii]) provide an observational diagnostic for the radiative energy input and the dynamics of the interstellar medium around massive stars. Here we report observations of a one-square-degree region (about 7 parsecs in diameter) of Orion molecular core 1--the region nearest to Earth that exhibits massive-star formation--at a resolution of 16 arcseconds (0.03 parsecs) in the [C ii] line at 1.9 terahertz (158 micrometres). The results reveal that the stellar wind originating from the massive star [theta].sup.1 Orionis C has swept up the surrounding material to create a 'bubble' roughly four parsecs in diameter with a 2,600-solar-mass shell, which is expanding at 13 kilometres per second. This finding demonstrates that the mechanical energy from the stellar wind is converted very efficiently into kinetic energy of the shell and causes more disruption of the Orion molecular core 1 than do photo-ionization and evaporation or future supernova explosions.Wind from the most massive star in the Trapezium cluster in Orion has carved out a large and expanding cavity around the cluster, bounded by a thin, 2,600-solar-mass shell., Author(s): C. Pabst [sup.1] , R. Higgins [sup.2] , J. R. Goicoechea [sup.3] , D. Teyssier [sup.4] , O. Berne [sup.5] , E. Chambers [sup.6] , M. Wolfire [sup.7] , [...]

The formation, evolution and death of massive stars release large quantities of energy and momentum into the gas surrounding the sites of star formation. This process, generically termed 'feedback', inhibits further star formation either by removing gas from the galaxy, or by heating it to temperatures that are too high to form new stars. Observations reveal feedback in the form of galactic-scale outflows of gas in galaxies with high rates of star formation, especially in the early Universe. Feedback in faint, low-mass galaxies probably facilitated the escape of ionizing radiation from galaxies when the Universe was about 500 million years old, so that the hydrogen between galaxies changed from neutral to ionized--the last major phase transition in the Universe., Although feedback from star formation is a complex phenomenon involving many processes over a wide range of physical scales, its most basic consequences are not difficult to understand: stars form [...]

When cosmic star formation history reaches a peak (at about redshift z ≅ 2), galaxies vigorously fed by cosmic reservoirs (1,2) are dominated by gas (3,4) and contain massive star-forming clumps (5,6), which are thought to form by violent gravitational instabilities in highly turbulent gas-rich disks (7,8). However, a clump formation event has not yet been observed, and it is debated whether clumps can survive energetic feedback from young stars, and afterwards migrate inwards to form galaxy bulges (9-12). Here we report the spatially resolved spectroscopy of a bright off-nuclear emission line region in a galaxy at z = 1.987. Although this region dominates star formation in the galaxy disk, its stellar continuum remains undetected in deep imaging, revealing an extremely young (less than ten million years old) massive clump, forming through the gravitational collapse of more than one billion solar masses of gas. Gas consumption in this young clump is more than tenfold faster than in the host galaxy, displaying high star-formation efficiency during this phase, in agreement with our hydrodynamic simulations. The frequency of older clumps with similar masses (13), coupled with our initial estimate of their formation rate (about 2.5 per billion years), supports long lifetimes (about 500 million years), favouring models in which clumps survive feedback and grow the bulges of present-day galaxies., The high spatial resolution and sensitivity of Hubble Space Telescope (HST) imaging and spectroscopy routinely allows us to resolve giant star-forming regions (clumps) inside galaxies at z ≅ 2, at [...]

Gas clouds in present-day galaxies are inefficient at forming stars. Low star-formation efficiency is a critical parameter in galaxy evolution: it is why stars are still forming nearly 14 billion years after the Big Bang (1) and why star clusters generally do not survive their births, instead dispersing to form galactic disks or bulges (2). Yet the existence of ancient massive bound star clusters (globular clusters) in the Milky Way suggests that efficiencies were higher when they formed ten billion years ago. A local dwarf galaxy, NGC 5253, has a young star cluster that provides an example of highly efficient star formation (3). Here we report the detection of the J = 3 → 2 rotational transition of CO at the location of the massive cluster. The gas cloud is hot, dense, quiescent and extremely dusty. Its gas-to-dust ratio is lower than the Galactic value, which we attribute to dust enrichment by the embedded star cluster. Its star-formation efficiency exceeds 50 per cent, tenfold that of clouds in the Milky Way. We suggest that high efficiency results from the force-feeding of star formation by a streamer of gas falling into the galaxy., The Submillimeter Array image of NGC 5253, shown in Fig. 1, reveals a bright CO(3 → 2) source coincident with the giant cluster and its 'supernebula' (4). 'Cloud D' (ref. [...]

Author(s): Ignacio Martn-Navarro (corresponding author) [1, 2]; Jean P. Brodie [2]; Aaron J. Romanowsky [1, 3]; Toms Ruiz-Lara [4, 5]; Glenn van de Ven [2, 6] Supermassive black holes, with [...]

Author(s): R. Maiolino (corresponding author) [1, 2]; H. R. Russell [3]; A. C. Fabian [3]; S. Carniani [1, 2]; R. Gallagher [1, 2]; S. Cazzoli [4]; S. Arribas [4]; F. [...]

Stellar-mass black holes are found in X-ray-emitting binary systems, where their mass can be determined from the dynamics of their companion stars (1-3). Models of stellar evolution have difficulty producing [...]

Author(s): Raul Jimenez (corresponding author) [1]; Zoltan Haiman (corresponding author) [2] Four recent observational results have challenged our understanding of high-redshift galaxies, as they require the presence of far more [...]

Author(s): Michael J. West (corresponding author) [1]; Patrick Côté [2]; Ronald O. Marzke [3]; Andrés Jordán [2] One way to study the history of galaxies is by observing those that [...]

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., The starburst in Henize 2-10, a relatively nearby (9 megaparsecs, ~30 million light years) blue compact dwarf galaxy, has attracted the attention of astronomers for decades (6-10). Stars are forming [...]

Author(s): J. Stuart B. Wyithe (corresponding author) [1]; Abraham Loeb (corresponding author) [2] A large number of faint galaxies, born less than a billion years after the Big Bang, have [...]

Author(s): J. L. Turner (corresponding author) [1]; S. C. Beck [2]; L. P. Crosthwaite [1, 3]; J. E. Larkin [1]; I. S. McLean [1]; D. S. Meier [1, 4] Little [...]

Author(s): Daniel Stern (corresponding author) [1]; Peter Eisenhardt [1]; Hyron Spinrad [2]; Steve Dawson [2]; Wil van Breugel [3]; Arjun Dey [4]; Wim de Vries [3]; S. A. Stanford [3, [...]

The detection of photons sufficiently energetic to ionize neutral hydrogen, coming from a compact, star-forming galaxy, offers clues to how the first generation of galaxies may have reionized hydrogen gas in the early Universe. See Letter p.178 Observation of a 'reionizing' galaxy The early Universe went through a period known as the cosmic 'Dark Ages', when matter was largely transparent to radiation and transformed to neutral gas. Later, some 800 million years after the Big Bang, it was ionized again. Which sources were responsible for this re-ionization? Low-mass, star-forming galaxies are prime candidates, but are hard to observe. Here Yuri Izotov et al. present far-ultraviolet observations of a nearby low-mass star-forming galaxy that can be considered a proxy for the reionizing galaxy population. The galaxy, J0925+1403, is leaking ionizing radiation with an escape fraction of ~8 per cent. The total number of photons emitted during the starburst phase is sufficient to ionize intergalactic medium material that is about 40 times as massive as the stellar mass of the galaxy., Author(s): Dawn K. Erb [sup.1] Author Affiliations: (1) Department of Physics, Dawn K. Erb is at the Leonard E. Parker Center for Gravitation, Cosmology and Astrophysics, University of Wisconsin-Milwaukee, Milwaukee, [...]

Most of the ordinary matter in the Universe is found not in stars, but in the diffuse gas between galaxies: the intergalactic medium (IGM), which is mainly hydrogen. This gas [...]

On page 66 of this issue, Reines and collaborators (1) report a most unlikely discovery: a big black hole in the centre of a very small galaxy. The low-mass (or [...]

Author(s): Simon F. Portegies Zwart (corresponding author) [1, 2]; Edward P. J. van den Heuvel (corresponding author) [1, 3] Supernova SN 2006gy in the galaxy NGC 1260 is the most [...]

Supernova SN 2006gy in the galaxy NGC 1260 is the most luminous Recorded (1-4). Its progenitor might have been a very massive (>100[M.sub.[??]] where [M.sub.[??]] is the mass of the [...]

Arising from: S. R. Kulkarni et al. Nature 447, 458-460 (2007) An anomalous transient in the early Hubble-type (S0) galaxy Messier 85 (M85) in the Virgo cluster was discovered by [...]

Author(s): Francesca Matteucci (corresponding author) [1] Most of the deuterium in the Universe was thought to be created during the first three minutes after the Big Bang, along with other [...]