The Magellanic impact: Collision between the outer Galactic HI disk and the leading arms of the Magellanic stream

We show that collisions between the outer Galactic HI disk and the leading arms (LAs) of the Magellanic stream (MS) can create giant HI holes and chimney-like structures in the disk. Based on the results of our N-body simulations on the last 2.5 Gyr evolution of the Large and Small Magellanic Clouds (LMC and SMC, respectively) interacting with the Galaxy, we investigate when and where the LAs can pass through the Galactic plane after the MS formation. We then investigate hydrodynamical interaction between LAs and the Galactic HI disk (``the Magellanic impact'') by using our new hydrodynamical simulations with somewhat idealized models of the LAs. We find that about 1-3% of the initial gas mass of the SMC, which consists of the LAs, can pass through the outer part (R=20-35 kpc) of the Galactic HI disk about 0.2 Gyr ago. We also find that the Magellanic impact can push out some fraction (~1%) of the outer Galactic HI disk to form 1-10 kpc-scale HI holes and chimney-like bridges between the LAs and the disk.Comment: 13pages, 4 figures, accepted by ApJ

Disordered Regimes of the one-dimensional complex Ginzburg-Landau equation

I review recent work on the ``phase diagram'' of the one-dimensional complex Ginzburg-Landau equation for system sizes at which chaos is extensive. Particular attention is paid to a detailed description of the spatiotemporally disordered regimes encountered. The nature of the transition lines separating these phases is discussed, and preliminary results are presented which aim at evaluating the phase diagram in the infinite-size, infinite-time, thermodynamic limit.Comment: 14 pages, LaTeX, 9 figures available by anonymous ftp to amoco.saclay.cea.fr in directory pub/chate, or by requesting them to [email protected]

Metallicity and Physical Conditions in the Magellanic Bridge

We present a new analysis of the diffuse gas in the Magellanic Bridge (RA>3h) based on HST/STIS E140M and FUSE spectra of 2 early-type stars lying within the Bridge and a QSO behind it. We derive the column densities of HI (from Ly\alpha), NI, OI, ArI, SiII, SII, and FeII of the gas in the Bridge. Using the atomic species, we determine the first gas-phase metallicity of the Magellanic Bridge, [Z/H]=-1.02+/-0.07 toward one sightline, and -1.7<[Z/H]<-0.9 toward the other one, a factor 2 or more smaller than the present-day SMC metallicity. Using the metallicity and N(HI), we show that the Bridge gas along our three lines of sight is ~70-90% ionized, despite high HI columns, logN(HI)=19.6-20.1. Possible sources for the ongoing ionization are certainly the hot stars within the Bridge, hot gas (revealed by OVI absorption), and leaking photons from the SMC and LMC. From the analysis of CII*, we deduce that the overall density of the Bridge must be low (<0.03-0.1 cm^-3). We argue that our findings combined with other recent observational results should motivate new models of the evolution of the SMC-LMC-Galaxy system.Comment: Accepted for publication in the Ap

Hole Structures in Nonlocally Coupled Noisy Phase Oscillators

We demonstrate that a system of nonlocally coupled noisy phase oscillators can collectively exhibit a hole structure, which manifests itself in the spatial phase distribution of the oscillators. The phase model is described by a nonlinear Fokker-Planck equation, which can be reduced to the complex Ginzburg-Landau equation near the Hopf bifurcation point of the uniform solution. By numerical simulations, we show that the hole structure clearly appears in the space-dependent order parameter, which corresponds to the Nozaki-Bekki hole solution of the complex Ginzburg-Landau equation.Comment: 4 pages, 4 figures, to appear in Phys. Rev.

The mass-metallicity gradient relation of early-type galaxies

We present a newly observed relation between galaxy mass and radial metallicity gradients of early-type galaxies. Our sample of 51 early-type galaxies encompasses a comprehensive mass range from dwarf to brightest cluster galaxies. The metallicity gradients are measured out to one effective radius by comparing nearly all of the Lick absorption-line indices to recent models of single stellar populations. The relation shows very different behaviour at low and high masses, with a sharp transition being seen at a mass of ~ 3.5 x 10^10 M_sun (velocity dispersion of ~140 km/s, M_B ~ -19). Low-mass galaxies form a tight relation with mass, such that metallicity gradients become shallower with decreasing mass and positive at the very low-mass end. Above the mass transition point several massive galaxies have steeper gradients, but a clear downturn is visible marked by a broad scatter. The results are interpreted in comparison with competing model predictions. We find that an early star-forming collapse could have acted as the main mechanism for the formation of low-mass galaxies, with star formation efficiency increasing with galactic mass. The high-mass downturn could be a consequence of merging and the observed larger scatter a natural result of different merger properties. These results suggest that galaxies above the mass threshold of ~ 3.5 x 10^10 M_sun might have formed initially by mergers of gas-rich disc galaxies and then subsequently evolved via dry merger events. The varying efficiency of the dissipative merger-induced starburst and feedback processes have shaped the radial metallicity gradients in these high-mass systems.Comment: 5 pageg, 3 figures, accepted by ApJ Lette

A Gaseous Group with Unusual Remote Star Formation

We present VLA 21-cm observations of the spiral galaxy ESO 481-G017 to determine the nature of remote star formation traced by an HII region found 43 kpc and ~800 km s^-1 from the galaxy center (in projection). ESO 481-G017 is found to have a 120 kpc HI disk with a mass of 1.2x10^10 Msun and UV GALEX images reveal spiral arms extending into the gaseous disk. Two dwarf galaxies with HI masses close to 10^8 Msun are detected at distances of ~200 kpc from ESO 481-G017 and a HI cloud with a mass of 6x10^7 Msun is found near the position and velocity of the remote HII region. The HII region is somewhat offset from the HI cloud spatially and there is no link to ESO 481-G017 or the dwarf galaxies. We consider several scenarios for the origin of the cloud and HII region and find the most likely is a dwarf galaxy that is undergoing ram pressure stripping. The HI mass of the cloud and Halpha luminosity of the HII region (10^38.1 erg s^-1) are consistent with dwarf galaxy properties, and the stripping can trigger the star formation as well as push the gas away from the stars.Comment: 19 pages, 4 figures, accepted by PAS

A search for ultra-compact dwarf galaxies in the NGC 1023 group of galaxies

We present a photometric search for UCD candidates in the nearby galaxy group NGC 1023 (d=11 Mpc) -- the poorest environment searched for UCDs yet --, based on wide field imaging with CFHT. After photometric and morphological selection, we obtain a sample of 21 UCD candidates with -12<M_V<-11 mag, if located at NGC 1023's distance. From spectroscopy taken at Calar Alto observatory, we identify the UCD candidate in closest projection to NGC 1023 as an emission line background galaxy. Our photometric data show that in the NGC 1023 group, the mass spectrum of analogs to Fornax/Virgo UCD is restricted to about 1/4 of the maximum Fornax/Virgo UCD mass. More spectroscopy is needed to further constrain the mass range of UCDs in this galaxy group.Comment: 5 pages, 4 figures, to appear in the proceedings of ESO Astrophysics Symposia: "Groups of Galaxies in the Nearby Universe", eds. I. Saviane, V. Ivanov, J. Borissov

Spectroscopic evolution of dusty starburst galaxies

By using a one-zone chemical and spectrophotometric evolution model of a disk galaxy undergoing a dusty starburst, we investigate, numerically, the optical spectroscopic properties in order to explore galaxy evolution in distant clusters. We adopt an assumption that the degree of dust extinction (represented by $A_V$) depends on the ages of starburst populations in such a way that younger stars have larger $A_V$ (originally referred to as selective dust extinction by Poggianti & Wu 2000). In particular, we investigate how the time evolution of the equivalent widths of [OII]$\lambda$3727 and H$\delta$ is controlled by the adopted age dependence. This leads to three main results: (1) If a young stellar population (with the age of $\sim$ $10^6$ yr) is more heavily obscured by dust than an old one ($>$ $10^8$ yr), the galaxy can show an ``e(a)'' spectrum characterized by strong H$\delta$ absorption and relatively modest [OII] emission. (2) A dusty starburst galaxy with an e(a) spectrum can evolve into a poststarburst galaxy with an a+k (or k+a) spectrum 0.2 Gyr after the starburst and then into a passive one with a k-type spectrum 1 Gyr after the starburst. This result clearly demonstrates an evolutionary link between galaxies with different spectral classes (i.e., e(b), e(a), a+k, k+a, and k). (3) A dusty starburst galaxy can show an a+k or k+a spectrum even in the dusty starburst phase if the age-dependence of dust extinction is rather weak, i.e., if young starburst populations with different ages ($\le$ $10^7$ yr) are uniformly obscured by dust.Comment: 27 pages 12 figures,2001,ApJ,in pres