High-resolution [C II] imaging of HDF850.1 reveals a merging galaxy at z=5.185

New high-resolution maps with the IRAM Interferometer of the redshifted [C II] 158 micron line and the 0.98mm dust continuum of HDF850.1 at z = 5.185 show the source to have a blueshifted northern component and a redshifted southern component, with a projected separation of 0.3 arcsec, or 2 kpc. We interpret these components as primordial galaxies that are merging to form a larger galaxy. We think it is the resulting merger-driven starburst that makes HDF850.1 an ultraluminous infrared galaxy, with an L(IR) of 1E13 Lsun. The observed line and continuum brightness temperatures and the constant line-to-continuum ratio across the source imply (1) high [C II] line optical depth, (2) a [C II] excitation temperature of the same order as the dust temperature, and (3) dust continuum emission that is nearly optically thick at 158 microns. These conclusions for HDF850.1 probably also apply to other high-redshift submillimeter galaxies and quasar hosts in which the [C II] 158 micron line has been detected, as indicated by their roughly constant [C II]-to-158 micron continuum ratios, in sharp contrast to the large dispersion in their [C II]-to-FIR luminosity ratios. In brightness temperature units, the [C II] line luminosity is about the same as the predicted CO(1-0) luminosity, implying that the [C II] line can also be used to estimate the molecular gas mass, with the same assumptions as for CO.Comment: Accepted by Astronomy and Astrophysic

Motor proteins traffic regulation by supply-demand balance of resources

In cells and in vitro assays the number of motor proteins involved in biological transport processes is far from being unlimited. The cytoskeletal binding sites are in contact with the same finite reservoir of motors (either the cytosol or the flow chamber) and hence compete for recruiting the available motors, potentially depleting the reservoir and affecting cytoskeletal transport. In this work we provide a theoretical framework to study, analytically and numerically, how motor density profiles and crowding along cytoskeletal filaments depend on the competition of motors for their binding sites. We propose two models in which finite processive motor proteins actively advance along cytoskeletal filaments and are continuously exchanged with the motor pool. We first look at homogeneous reservoirs and then examine the effects of free motor diffusion in the surrounding medium. We consider as a reference situation recent in vitro experimental setups of kinesin-8 motors binding and moving along microtubule filaments in a flow chamber. We investigate how the crowding of linear motor proteins moving on a filament can be regulated by the balance between supply (concentration of motor proteins in the flow chamber) and demand (total number of polymerised tubulin heterodimers). We present analytical results for the density profiles of bound motors, the reservoir depletion, and propose novel phase diagrams that present the formation of jams of motor proteins on the filament as a function of two tuneable experimental parameters: the motor protein concentration and the concentration of tubulins polymerized into cytoskeletal filaments. Extensive numerical simulations corroborate the analytical results for parameters in the experimental range and also address the effects of diffusion of motor proteins in the reservoir.Comment: 31 pages, 10 figure

Submillimetre Cosmology at High Angular Resolution

Over the last decade observations at submillimetre (submm) and millimetre (mm) wavelengths, with their unique ability to trace molecular gas and dust, have attained a central role in our exploration of galaxies at all redshifts. Due to the limited sensitivities and angular resolutions of current submm/mm telescopes, however, only the most luminous objects have been uncovered at high redshifts, with interferometric follow-up observations succeeding in resolving the dust and gas reservoirs in only a handful of cases. The coming years will witness a drastic improvement in the current situation, thanks to the arrival of a new suite of powerful submm observatories (single-dish and interferometers) with an order of magnitude improvement in sensitivity and resolution. In this overview I outline a few of what I expect to be the major advances in the field of galaxy formation and evolution that these new ground-breaking facilities will facilitate.Comment: (10 pages, 3 figures. Invited talk, proceedings for the 3rd ARENA Conference "An astronomical observatory at CONCORDIA (Antarctica) for the next decade", 11-15 May 2009, Frascati, Ital

A Kiloparsec-Scale Hyper-Starburst in a Quasar Host Less than 1 Gigayear after the Big Bang

The host galaxy of the quasar SDSS J114816.64+525150.3 (at redshift z=6.42, when the Universe was <1 billion years old) has an infrared luminosity of 2.2x10^13 L_sun, presumably significantly powered by a massive burst of star formation. In local examples of extremely luminous galaxies such as Arp220, the burst of star formation is concentrated in the relatively small central region of <100pc radius. It is unknown on which scales stars are forming in active galaxies in the early Universe, which are likely undergoing their initial burst of star formation. We do know that at some early point structures comparable to the spheroidal bulge of the Milky Way must have formed. Here we report a spatially resolved image of [CII] emission of the host galaxy of J114816.64+525150.3 that demonstrates that its star forming gas is distributed over a radius of ~750pc around the centre. The surface density of the star formation rate averaged over this region is ~1000 M_sun/yr/kpc^2. This surface density is comparable to the peak in Arp220, though ~2 orders of magnitudes larger in area. This vigorous star forming event will likely give rise to a massive spheroidal component in this system.Comment: Nature, in press, Feb 5 issue, p. 699-70

Modelling the effect of ribosome mobility on the rate of protein synthesis

Translation is one of the main steps in the synthesis of proteins. It consists of ribosomes that translate sequences of nucleotides encoded on mRNA into polypeptide sequences of amino acids. Ribosomes bound to mRNA move unidirectionally, while unbound ribosomes diffuse in the cytoplasm. It has been hypothesized that finite diffusion of ribosomes plays an important role in ribosome recycling and that mRNA circularization enhances the efficiency of translation. In order to estimate the effect of cytoplasmic diffusion on the rate of translation, we consider a Totally Asymmetric Simple Exclusion Process (TASEP) coupled to a finite diffusive reservoir, which we call the Ribosome Transport model with Diffusion (RTD). In this model, we derive an analytical expression for the rate of protein synthesis as a function of the diffusion constant of ribosomes, which is corroborated with results from continuous-time Monte Carlo simulations. Using a wide range of biological relevant parameters, we conclude that diffusion in biological cells is fast enough so that it does not play a role in controlling the rate of translation initiation.Comment: article, 16 pages, 5 figure

The Spectral Energy Distribution of CO lines in M82

We present an analysis of the excitation conditions of the molecular gas in the streamers and the outflow of M82 based on observations obtained at the IRAM 30m telescope. Our analysis of J=1-0 and J=2-1 transitions of CO and 13CO and the CO(3-2) line in 13 regions outside the central starburst disk shows that the gas density within the streamer/outflow system is about an orderof magnitude lower (n(H2) ~ 10^3 cm^-3) than in the central molecular disk. We have used an LVG model and data from the literature to constrain the flux density in each CO transition (the `CO line SED') arising from the streamer/outflow system and the central starburst disk itself. Globally, we find that the CO flux density up to the J=3-2 line is dominated by the diffuse outer regions while lines above the J=5-4 transition are almost exclusively emitted by the central starburst disk. We compare the CO line SED of M82 to CO observations of galaxies at high redshift and suggest that small high-J/low-J CO flux density ratios (observed in some of these sources) are not necessarily caused by a different excitation of the central molecular gas concentration, but may result from an additional, more extended and diffuse gas reservoir around these systems, reminiscent of the situation in M82.Comment: 12 pages, 7 figures, accepted by A&

Molecular gas in QSO host galaxies at z>5

We present observations with the IRAM Plateau de Bure Interferometer of three QSOs at z>5 aimed at detecting molecular gas in their host galaxies as traced by CO transitions. CO (5-4) is detected in SDSSJ033829.31+002156.3 at z=5.0267, placing it amongst the most distant sources detected in CO. The CO emission is unresolved with a beam size of ~1", implying that the molecular gas is contained within a compact region, less than ~3kpc in radius. We infer an upper limit on the dynamical mass of the CO emitting region of ~3x10^10 Msun/sin(i)^2. The comparison with the Black Hole mass inferred from near-IR data suggests that the BH-to-bulge mass ratio in this galaxy is significantly higher than in local galaxies. From the CO luminosity we infer a mass reservoir of molecular gas as high as M(H2)=2.4x10^10 Msun, implying that the molecular gas accounts for a significant fraction of the dynamical mass. When compared to the star formation rate derived from the far-IR luminosity, we infer a very short gas exhaustion timescale (~10^7 yrs), comparable to the dynamical timescale. CO is not detected in the other two QSOs (SDSSJ083643.85+005453.3 and SDSSJ163033.90+401209.6) and upper limits are given for their molecular gas content. When combined with CO observations of other type 1 AGNs, spanning a wide redshift range (0<z<6.4), we find that the host galaxy CO luminosity (hence molecular gas content) and the AGN optical luminosity (hence BH accretion rate) are correlated, but the relation is not linear: L(CO) ~ [lambda*L_lambda(4400A)]^0.72. Moreover, at high redshifts (and especially at z>5) the CO luminosity appears to saturate. We discuss the implications of these findings in terms of black hole-galaxy co-evolution.Comment: Accepted for publication in A&A Letters, 6 pages, 3 figure

Ionized Nitrogen at High Redshift

We present secure [N II]_(205 μm) detections in two millimeter-bright, strongly lensed objects at high redshift, APM 08279+5255 (z = 3.911) and MM 18423+5938 (z = 3.930), using the IRAM Plateau de Bure Interferometer. Due to its ionization energy [N II]_(205 μm) is a good tracer of the ionized gas phase in the interstellar medium. The measured fluxes are S([N II]_(205 μm)) = (4.8 ± 0.8) Jy km s^(–1) and (7.4 ± 0.5) Jy km s^(–1), respectively, yielding line luminosities of L([N II]_(205 μm)) = (1.8 ± 0.3) × 10^9 μ^(–1) L_⊙ for APM 08279+5255 and L([N II]_(205 μm)) = (2.8 ± 0.2) × 10(^9) μ^(–1) L_⊙ for MM 18423+5938. Our high-resolution map of the [N II]_(205 μm) and 1 mm continuum emission in MM 18423+5938 clearly resolves an Einstein ring in this source and reveals a velocity gradient in the dynamics of the ionized gas. A comparison of these maps with high-resolution EVLA CO observations enables us to perform the first spatially resolved study of the dust continuum-to-molecular gas surface brightness (Σ_(FIR)α Σ^N_CO, which can be interpreted as the star formation law) in a high-redshift object. We find a steep relation (N = 1.4 ± 0.2), consistent with a starbursting environment. We measure a [N II]_(205 μm)/FIR luminosity ratio in APM 08279+5255 and MM 18423+5938 of 9.0 × 10^(–6) and 5.8 × 10^(–6), respectively. This is in agreement with the decrease of the [N II]_(205 μm)/FIR ratio at high FIR luminosities observed in local galaxies

Simultaneous magma and gas eruptions at three volcanoes in southern Italy: an earthquake trigger?

In September 2002, a series of tectonic earthquakes occurred north of Sicily, Italy, followed by three events of volcanic unrest within 150 km. On October 28, 2002, Mt. Etna erupted; on November 3, 2002, submarine degassing occurred near Panarea Island; and on December 28, 2002, Stromboli Island erupted. All of these events were considered unusual: the Mt. Etna NE-rift eruption was the largest in 55 yr, the Panarea degassing was one of the strongest ever detected there, and the Stromboli eruption, which produced a landslide and tsunami, was the largest effusive eruption in 17 yr. Here, we investigate the synchronous occurrence of these clustered unrest events, and develop a possible explanatory model. We compute short-term earthquake-induced dynamic strain changes and compare them to long-term tectonic effects. Results suggest that the earthquake-induced strain changes exceeded annual tectonic strains by at least an order of magnitude. This agitation occurred in seconds, and may have induced fluid and gas pressure migration within the already active hydrothermal and magmatic systems