Spherical Branes

We construct new solutions of ten-dimensional supergravity with sixteen supercharges which describe the backreaction of D$p$-branes with spherical worldvolume. These solutions are holographically dual to the $(p+1)$-dimensional maximally supersymmetric Yang-Mills theory on $S^{p+1}$. The finite size of the sphere provides an IR cut-off for the gauge theory which is manifested in the supergravity solution as a smooth cap-off of the geometry. In the UV the size of the sphere plays no role and the backgrounds asymptote to the well-known supergravity solutions that describe the near-horizon limit of flat D$p$-branes. We compute the on-shell action of our spherical brane solutions and show that it is in agreement with recent supersymmetric localization results for the free energy of maximal SYM theory on $S^{p+1}$.Comment: 57 pages, 4 figures; v2: minor corrections, appendix adde

Discovery of a tidal dwarf galaxy in the Leo Triplet

We report discovery of a dwarf galaxy in the Leo Triplet. Analysis of the neutral hydrogen distribution shows that it rotates independently of the tidal tail of NGC 3628, with a radial velocity gradient of 35-40 km s$^{-1}$ over approximately 13 kpc. The galaxy has a very high neutral gas content, explaining large part of its total dynamic mass - suggesting small dark matter content. As it is located at the tip of the gaseous tail, this strongly suggests its tidal origin. Should it be the case, it would be one of the most confident and closest (to the Milky Way) detections of a tidal dwarf galaxy and, at the same time, a most detached from its parent galaxy ($\approx$140 kpc) object of this type.Comment: 6 pages, 4 figures; The Astrophysical Journal, in pres

Outflow or galactic wind: The fate of ionized gas in the halos of dwarf galaxies

Context: H\alpha images of star bursting irregular galaxies reveal a large amount of extended ionized gas structures, in some cases at kpc-distance away from any place of current star forming activity. A kinematic analysis of especially the faint structures in the halo of dwarf galaxies allows insights into the properties and the origin of this gas component. This is important for the chemical evolution of galaxies, the enrichment of the intergalactic medium, and for the understanding of the formation of galaxies in the early universe. Aims: We want to investigate whether the ionized gas detected in two irregular dwarf galaxies (NGC 2366 and NGC 4861) stays gravitationally bound to the host galaxy or can escape from it by becoming a freely flowing wind. Methods: Very deep H\alpha images of NGC 2366 and NGC 4861 were obtained to detect and catalog both small and large scale ionized gas structures down to very low surface brightnesses. Subsequently, high-resolution long-slit echelle spectroscopy of the H\alpha line was performed for a detailed kinematic analysis of the most prominent filaments and shells. To calculate the escape velocity of both galaxies and to compare it with the derived expansion velocities of the detected filaments and shells, we used dark matter halo models. Results: We detected a huge amount of both small scale (up to a few hundred pc) and large scale (about 1-2 kpc of diameter or length) ionized gas structures on our H\alpha images. Many of the fainter ones are new detections. The echelle spectra reveal outflows and expanding bubbles/shells with velocities between 20 and 110 km/s. Several of these structures are in accordance with filaments in the H\alpha images. A comparison with the escape velocities of the galaxies derived from the NFW dark matter halo model shows that all gas features stay gravitationally bound.Comment: 15 pages, 13 figures, accepted for publication in A&

The kinematics of the diffuse ionized gas in NGC 4666

The global properties of the interstellar medium with processes such as infall and outflow of gas and a large scale circulation of matter and its consequences for star formation and chemical enrichment are important for the understanding of galaxy evolution. In this paper we studied the kinematics and morphology of the diffuse ionized gas (DIG) in the disk and in the halo of the star forming spiral galaxy NGC 4666 to derive information about its kinematical properties. Especially, we searched for infalling and outflowing ionized gas. We determined surface brightness, radial velocity, and velocity dispersion of the warm ionized gas via high spectral resolution (R ~ 9000) Fabry-P\'erot interferometry. This allows the determination of the global velocity field and the detection of local deviations from this verlocity field. We calculated models of the DIG distribution and its kinematics for comparison with the measured data. In this way we determined fundamental parameters such as the inclination and the scale height of NGC 4666, and established the need for an additional gas component to fit our observed data. We found individual areas, especially along the minor axis, with gas components reaching into the halo which we interpret as an outflowing component of the diffuse ionized gas. As the main result of our study, we were able to determine that the vertical structure of the DIG distribution in NGC 4666 is best modeled with two components of ionized gas, a thick and a thin disk with 0.8 kpc and 0.2 kpc scale height, respectively. Therefore, the enhanced star formation in NGC 4666 drives an outflow and also maintains a thick ionized gas layer reminiscent of the Reynold's layer in the Milky Way.Comment: 12 pages, 10 figures, 3 table

A search for extended radio emission from selected compact galaxy groups

Context. Studies on compact galaxy groups have led to the conclusion that a plenitude of phenomena take place in between galaxies that form them. However, radio data on these objects are extremely scarce and not much is known concerning the existence and role of the magnetic field in intergalactic space. Aims. We aim to study a small sample of galaxy groups that look promising as possible sources of intergalactic magnetic fields; for example data from radio surveys suggest that most of the radio emission is due to extended, diffuse structures in and out of the galaxies. Methods. We used the Effelsberg 100 m radio telescope at 4.85 GHz and NRAO VLA Sky Survey (NVSS) data at 1.40 GHz. After subtraction of compact sources we analysed the maps searching for diffuse, intergalactic radio emission. Spectral index and magnetic field properties were derived. Results. Intergalactic magnetic fields exist in groups HCG 15 and HCG 60, whereas there are no signs of them in HCG 68. There are also hints of an intergalactic bridge in HCG 44 at 4.85 GHz. Conclusions. Intergalactic magnetic fields exist in galaxy groups and their energy density may be comparable to the thermal (X-ray) density, suggesting an important role of the magnetic field in the intra-group medium, wherever it is detected.Comment: 13 pages, 4 figures, 3 tables, accepted for publication in A&

Probing The Multiphase Interstellar Medium Of The Dwarf Starburst Galaxy NGC 625 With FUSE Spectroscopy

We present new FUSE spectroscopy of the dwarf starburst galaxy NGC 625. These observations probe multiple phases of the interstellar medium, including the coronal, ionized, neutral and molecular gas. This nearby (D = 3.9 +/- 0.2 Mpc) system shows a clear detection of outflowing coronal gas as traced by OVI 1032 Angstrom absorption. The centroid of the OVI profile is blueshifted with respect to the galaxy systemic velocity by ~ 30 km/sec, suggesting a low-velocity outflow. The implied OVI velocity extent is found to be 100 +/- 20 km/sec, which is fully consistent with the detected HI outflow velocity found in radio synthesis observations. We detect multiple lines of diffuse H2 absorption from the ISM of NGC 625; this is one of only a few extragalactic systems with FUSE detections of H2 lines in the Lyman and Werner bands. We find a potential abundance offset between the neutral and nebular gas that exceeds the errors on the derived column densities. Since such an offset has been found in multiple dwarf galaxies, we discuss the implications of a lower-metallicity halo surrounding the central star forming regions of dwarf galaxies. The apparent offset may be due to saturation of the observed OI line, and higher S/N observations are required to resolve this issue.Comment: ApJ, in press; full-resolution version may be obtained at http://www.astro.umn.edu/~cannon/n625.fuse.p