Revealing the Superfluid Lambda Transition in the Universal Thermodynamics of a Unitary Fermi Gas

We have observed the superfluid phase transition in a strongly interacting Fermi gas via high-precision measurements of the local compressibility, density and pressure down to near-zero entropy. Our data completely determine the universal thermodynamics of strongly interacting fermions without any fit or external thermometer. The onset of superfluidity is observed in the compressibility, the chemical potential, the entropy, and the heat capacity. In particular, the heat capacity displays a characteristic lambda-like feature at the critical temperature of $T_c/T_F = 0.167(13)$. This is the first clear thermodynamic signature of the superfluid transition in a spin-balanced atomic Fermi gas. Our measurements provide a benchmark for many-body theories on strongly interacting fermions, relevant for problems ranging from high-temperature superconductivity to the equation of state of neutron stars.Comment: 11 pages, 8 figure

Cold gas in elliptical galaxies

We explore the evolution of the cold gas (molecular and neutral hydrogen) of elliptical galaxies and merger remnants ordered into a time sequence on the basis of spectroscopic age estimates. We find that the fraction of cold gas in early merger remnants decreases significantly for ~1-2 Gyr, but subsequent evolution toward evolved elliptical systems sees very little change. This trend can be attributed to an initial gas depletion by strong star-formation which subsequently declines to quiescent rates. This explanation is consistent with the merger picture for the formation of elliptical galaxies. We also explore the relation between HI-to-H2 mass ratio and spectroscopic galaxy age, but find no evidence for a statistically significant trend. This suggests little net HI to H2 conversion for the systems in the present sample.Comment: 11 pages, 6 figures, accepted for publication by MNRA

A Magnetohydrodynamic Model for the Formation of Episodic Jets

Episodic ejection of plasma blobs have been observed in many black hole systems. While steady, continuous jets are believed to be associated with large-scale open magnetic fields, what causes the episodic ejection of blobs remains unclear. Here by analogy with the coronal mass ejection on the Sun, we propose a magnetohydrodynamical model for episodic ejections from black holes associated with the closed magnetic fields in an accretion flow. Shear and turbulence of the accretion flow deform the field and result in the formation of a flux rope in the disk corona. Energy and helicity are accumulated and stored until a threshold is reached. The system then loses its equilibrium and the flux rope is thrust outward by the magnetic compression force in a catastrophic way. Our calculations show that for parameters appropriate for the black hole in our Galactic center, the plasmoid can attain relativistic speeds in about 35 minutes.Comment: 8 pages, 2 figures; the finalized version to appear in MNRA

HST/ACS observations of shell galaxies: inner shells, shell colours and dust

AIM:Learn more about the origin of shells and dust in early type galaxies. METHOD: V-I colours of shells and underlying galaxies are derived, using HST Advanced Camera for Surveys (ACS) data. A galaxy model is made locally in wedges and subtracted to determine shell profiles and colours. We applied Voronoi binning to our data to get smoothed colour maps of the galaxies. Comparison with N-body simulations from the literature gives more insight to the origin of the shell features. Shell positions and dust characteristics are inferred from model galaxy subtracted images. RESULT: The ACS images reveal shells well within the effective radius in some galaxies (at 1.7 kpc in the case of NGC 5982). In some cases, strong nuclear dust patches prevent detection of inner shells. Most shells have colours which are similar to the underlying galaxy. Some inner shells are redder than the galaxy. All six shell galaxies show out of dynamical equilibrium dust features, like lanes or patches, in their central regions. Our detection rate for dust in the shell ellipticals is greater than that found from HST archive data for a sample of normal early-type galaxies, at the 95% confidence level. CONCLUSIONS: The merger model describes better the shell distributions and morphologies than the interaction model. Red shell colours are most likely due to the presence of dust and/or older stellar populations. The high prevalence and out of dynamical equilibrium morphologies of the central dust features point towards external influences being responsible for visible dust features in early type shell galaxies. Inner shells are able to manifest themselves in relatively old shell systems.Comment: accepted by A&A; 36 Figures, 25 pages. A version with full resolution Figures can be found here: http://www.astro.rug.nl/~sikkema/shells.p

An optical/NIR survey of globular clusters in early-type galaxies. I. Introduction and data reduction procedures

Context: The combination of optical and near-infrared (NIR) colours has the potential to break the age/metallicity degeneracy and offers a better metallicity sensitivity than optical colours alone. Previous studies of extragalactic globular clusters (GCs) with this colour combination, however, have suffered from small samples or have been restricted to a few galaxies. Aims: We compile a homogeneous and representative sample of GC systems with multi-band photometry to be used in subsequent papers where ages and metallicity distributions will be studied. Methods: We acquired deep K-band images of 14 bright nearby early-type galaxies. The images were obtained with the LIRIS near-infrared spectrograph and imager at the William Herschel Telescope (WHT) and combined with optical ACS g and z images from the Hubble Space Telescope public archive. Results: For the first time GC photometry of 14 galaxies are observed and reduced homogeneously in this wavelength regime. We achieved a limiting magnitude of K~20-21. For the majority of the galaxies we detect about 70 GCs each. NGC4486 and NGC4649, the cluster-richest galaxies in the sample contain 301 and 167 GCs, respectively. We present tables containing coordinates, photometry and sizes of the GCs available.Comment: A&A accepted, 18 pages, 13 figure

Constraints on mass loss and self-enrichment scenarios for the globular clusters of the Fornax dSph

Recently, high-dispersion spectroscopy has demonstrated conclusively that four of the five globular clusters (GCs) in the Fornax dwarf spheroidal galaxy are very metal-poor with [Fe/H]<-2. The remaining cluster, Fornax 4, has [Fe/H]=-1.4. This is in stark contrast to the field star metallicity distribution which shows a broad peak around [Fe/H]=-1 with only a few percent of the stars having [Fe/H]<-2. If we only consider stars and clusters with [Fe/H]<-2 we thus find an extremely high GC specific frequency, SN=400, implying by far the highest ratio of GCs to field stars known anywhere. We estimate that about 1/5-1/4 of all stars in the Fornax dSph with [Fe/H]<-2 belong to the four most metal-poor GCs. These GCs could, therefore, at most have been a factor of 4-5 more massive initially. Yet, the Fornax GCs appear to share the same anomalous chemical abundance patterns known from Milky Way GCs, commonly attributed to the presence of multiple stellar generations within the clusters. The extreme ratio of metal-poor GC- versus field stars in the Fornax dSph is difficult to reconcile with scenarios for self-enrichment and early evolution of GCs in which a large fraction (90%-95%) of the first-generation stars have been lost. It also suggests that the GCs may not have formed as part of a larger population of now disrupted clusters with an initial power-law mass distribution. The Fornax dSph may be a rosetta stone for constraining theories of the formation, self-enrichment and early dynamical evolution of star clusters.Comment: 4 pages, 2 figures, accepted for A&A Letter

Large atom number dual-species magneto-optical trap for fermionic 6Li and 40K atoms

We present the design, implementation and characterization of a dual-species magneto-optical trap (MOT) for fermionic 6Li and 40K atoms with large atom numbers. The MOT simultaneously contains 5.2x10^9 6Li-atoms and 8.0x10^9 40K-atoms, which are continuously loaded by a Zeeman slower for 6Li and a 2D-MOT for 40K. The atom sources induce capture rates of 1.2x10^9 6Li-atoms/s and 1.4x10^9 40K-atoms/s. Trap losses due to light-induced interspecies collisions of ~65% were observed and could be minimized to ~10% by using low magnetic field gradients and low light powers in the repumping light of both atomic species. The described system represents the starting point for the production of a large-atom number quantum degenerate Fermi-Fermi mixture

Thickening of galactic disks through clustered star formation

(Abridged) The building blocks of galaxies are star clusters. These form with low-star formation efficiencies and, consequently, loose a large part of their stars that expand outwards once the residual gas is expelled by the action of the massive stars. Massive star clusters may thus add kinematically hot components to galactic field populations. This kinematical imprint on the stellar distribution function is estimated here by calculating the velocity distribution function for ensembles of star-clusters distributed as power-law or log-normal initial cluster mass functions (ICMFs). The resulting stellar velocity distribution function is non-Gaussian and may be interpreted as being composed of multiple kinematical sub-populations. The notion that the formation of star-clusters may add hot kinematical components to a galaxy is applied to the age--velocity-dispersion relation of the Milky Way disk to study the implied history of clustered star formation, with an emphasis on the possible origin of the thick disk.Comment: MNRAS, accepted, 27 pages, 9 figure

Central Structural Parameters of Early-Type Galaxies as Viewed with HST/NICMOS

We present surface photometry for the central regions of a sample of 33 early-type (E, S0, and S0/a) galaxies observed at 1.6 microns (H band) using the Hubble Space Telescope (HST). We employ a new technique of two-dimensional fitting to extract quantitative parameters for the bulge light distribution and nuclear point sources, taking into consideration the effects of the point-spread function. Parameterizing the bulge profile with a ``Nuker'' law, we confirm that the central surface-brightness distributions largely fall into two categories, each of which correlates with the global properties of the galaxies. ``Core'' galaxies tend to be luminous ellipticals with boxy or pure elliptical isophotes, whereas ``power-law'' galaxies are preferentially lower luminosity systems with disky isophotes. Unlike most previous studies, however, we do not find a clear gap in the distribution of inner cusp slopes; several objects have inner cusp slopes (0.3 < gamma < 0.5) which straddle the regimes conventionally defined for core and power-law type galaxies. The nature of these intermediate objects is unclear. We draw attention to two objects in the sample which appear to be promising cases of galaxies with isothermal cores that are not the brightest members of a cluster. Unresolved nuclear point sources are found in about 50% of the sample galaxies, roughly independent of profile type, with magnitudes in the range m^{nuc}_H = 12.8 to 17.4 mag, which correspond to M_H^{nuc} = -12.8 to -18.4 mag. (Abridged)Comment: To appear in The Astronomical Journal. Latex, 24 pages and 17 JPEG image

Plasmoids in Reconnecting Current Sheets: Solar and Terrestrial Contexts Compared

Magnetic reconnection plays a crucial role in violent energy conversion occurring in the environments of high electrical conductivity, such as the solar atmosphere, magnetosphere, and fusion devices. We focus on the morphological features of the process in two different environments, the solar atmosphere and the geomagnetic tail. In addition to indirect evidence that indicates reconnection in progress or having just taken place, such as auroral manifestations in the magnetosphere and the flare loop system in the solar atmosphere, more direct evidence of reconnection in the solar and terrestrial environments is being collected. Such evidence includes the reconnection inflow near the reconnecting current sheet, and the outflow along the sheet characterized by a sequence of plasmoids. Both turbulent and unsteady Petschek-type reconnection processes could account for the observations. We also discuss other relevant observational consequences of both mechanisms in these two settings. While on face value, these are two completely different physical environments, there emerge many commonalities, for example, an Alfven speed of the same order of magnitude, a key parameter determining the reconnection rate. This comparative study is meant as a contribution to current efforts aimed at isolating similarities in processes occurring in very different contexts in the heliosphere, and even in the universe.Comment: 21 pages, 9 figures, in press at J. Geophys. Res. (Space Physics), for the special NESSC section on Comparative Aspects of Magnetic Reconnectio