Starbursts in Barred Spiral Galaxies. III. Definition of a homogeneous sample of Starburst Nucleus Galaxies

This paper presents optical long-slit spectroscopic observations of 105 barred Markarian IRAS galaxies. These observations are used to determine the spectral type of emission-line regions in the nucleus and along the bar of the galaxies, in order to define a homogeneous sample of Starburst Nucleus Galaxies (SBNGs). Our selection criteria have been very efficient for selecting star-forming galaxies, since our sample of 221 emission-line regions includes 82% nuclear or extranuclear starbursts. The contamination by Seyferts is low (9%). The remaining galaxies (9%) are objects with ambiguous classification (HII or LINER). The dust content and Halpha luminosity increase towards the nuclei of the galaxies. No significant variation of the electron density is found between nuclear and bar HII regions. However, the mean Halpha luminosity and electron density in the bar are higher than in typical disk HII regions. We investigate different mechanisms for explaining the excess of nitrogen emission observed in our starburst nuclei. There is no evidence for the presence of a weak hidden active galactic nucleus in our starburst galaxies. The cause of this excess is probably a selective enrichment of nitrogen in the nuclei of the galaxies, following a succession of short and intense bursts of star formation. Our sample of SBNGs, located at a mean redshift of 0.015, has moderate Halpha (10^41 erg/s) and far infrared (10^10 Lsun) luminosities. The types are distributed equally among early- and late-type giant spirals with a slight preference for Sbc/Sc types because of their barred morphology. The majority (62%) of SBNGs are isolated with no sign of gravitational interaction. In terms of distance, luminosity and level of interaction, SBNGs are intermediate between HII galaxies and luminous infrared galaxies.Comment: Accepted for publication in A&A Supplement Series. 14 pages including 12 figures and 7 table

Tidal Imprints of a Dark Sub-Halo on the Outskirts of the Milky Way II. Perturber Azimuth

We extend our analysis of the observed disturbances on the outskirts of the HI disk of the Milky Way. We employ the additional constraints of the phase of the modes of the observed HI image and asymmetry in the radial velocity field to derive the azimuth of the perturber inferred to be responsible for the disturbances in the HI disk. We carry out a modal analysis of the phase of the disturbances in the HI image and in SPH simulations of a Milky Way-like galaxy tidally interacting with dark perturbers, the relative offset of which we utilize to derive the perturber azimuth. To make a direct connection with observations, we express our results in sun-centered coordinates, predicting that the perturber responsible for the observed disturbances is between -50 \la l \la -10. We show explicitly that the phase of the disturbances in the outskirts of simulated galaxies at the time that best fits the Fourier amplitudes, our primary metric for the azimuth determination, is relatively insensitive to the equation of state. Our calculations here represent our continuing efforts to develop the "Tidal Analysis" method of Chakrabarti \& Blitz (2009; CB09). CB09 employed SPH simulations to examine tidal interactions between perturbing dark sub-halos and the Milky Way. They found that the amplitudes of the Fourier modes of the observed planar disturbances are best-fit by a perturbing dark sub-halo with mass one-hundredth that of the Milky Way, and a pericentric approach distance of $\sim 5-10~\rm kpc$. The overarching goal of this work is to attempt to outline an alternate procedure to optical studies for characterizing and potentially discovering dwarf galaxies -- whereby one can approximately infer the azimuthal location of a perturber, its mass and pericentric distance (CB09) from analysis of its tidal gravitational imprints on the HI disk of the primary galaxy.Comment: submitted to ApJ; 12 pages; higher resolution figures can be found at: http://astro.berkeley.edu/~sukanya/perturbersubmit.pd

Leading Wave as a Component of the Spiral Pattern of the Galaxy

The spiral pattern of the Galaxy identified by analyzing the kinematics of young stars within 3 kpc of the Sun is Fourier decomposed into spiral harmonics. The spiral pattern of the Galaxy is shown to be representable as a superposition of trailing and leading waves with interarm distances of 1.8(+/-0.4) kpc and 4(+/-2) kpc, respectively. Shock waves are probably present only in the portions of the trailing spiral pattern where it crosses the crest of the leading wave. The small interarm distance of the trailing spiral wave (1.8 kpc) can be explained by its evolution - by the decrease in the interarm distance as the wave is displaced toward the inner Lindblad resonance. The Carina arm may be part of this resonance ring.Comment: 17 pages, 4 figures, to be published in Astronomy Letters, 200