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

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

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&

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&

3D global simulations of a cosmic-ray-driven dynamo in dwarf galaxies

Star-forming dwarf galaxies can be seen as the local proxies of the high-redshift building blocks of more massive galaxies according to the current paradigm of the hierarchical galaxy formation. They are low-mass objects, and therefore their rotation speed is very low. Several galaxies are observed to show quite strong magnetic fields. These cases of strong ordered magnetic fields seem to correlate with a high, but not extremely high, star formation rate. We investigate whether these magnetic fields could be generated by the cosmic-ray-driven dynamo. The environment of a dwarf galaxy is unfavourable for the large-scale dynamo action because of the very slow rotation that is required to create the regular component of the magnetic field. We built a 3D global model of a dwarf galaxy that consists of two gravitational components: the stars and the dark-matter halo described by the purely phenomenological profile proposed previously. We solved a system of magnetohydrodynamic (MHD) equations that include an additional cosmic-ray component described by the fluid approximation. We found that the cosmic-ray-driven dynamo can amplify the magnetic field with an exponential growth rate. The $e$-folding time is correlated with the initial rotation speed. The final mean value of the azimuthal flux for our models is of the order of few $\mu$G and the system reaches its equipartition level. The results indicate that the cosmic-ray-driven dynamo is a process that can explain the magnetic fields in dwarf galaxies.Comment: 6 pages, 4 figures, accepted for publication in A&

Cosmic-ray driven dynamo in the interstellar medium of irregular galaxies

Irregular galaxies are usually smaller and less massive than their spiral, S0, and elliptical counterparts. Radio observations indicate that a magnetic field is present in irregular galaxies whose value is similar to that in spiral galaxies. However, the conditions in the interstellar medium of an irregular galaxy are unfavorable for amplification of the magnetic field because of the slow rotation and low shearing rate. We investigate the cosmic-ray driven dynamo in the interstellar medium of an irregular galaxy. We study its efficiency under the conditions of slow rotation and weak shear. The star formation is also taken into account in our model and is parametrized by the frequency of explosions and modulations of activity. The numerical model includes a magnetohydrodynamical dynamo driven by cosmic rays that is injected into the interstellar medium by randomly exploding supernovae. In the model, we also include essential elements such as vertical gravity of the disk, differential rotation approximated by the shearing box, and resistivity leading to magnetic reconnection. We find that even slow galactic rotation with a low shearing rate amplifies the magnetic field, and that rapid rotation with a low value of the shear enhances the efficiency of the dynamo. Our simulations have shown that a high amount of magnetic energy leaves the simulation box becoming an efficient source of intergalactic magnetic fields.Comment: 9 pages, 6 figure