Gas accretion in galactic disks

Evidence for the accretion of material in spiral galaxies has grown over the past years and clear signatures can be found in HI observations of galaxies. We describe here new detailed and sensitive HI synthesis observations of a few nearby galaxies (NGC 3359, NGC 4565 and NGC 6946) which show that indeed accretion of small amounts of gas is taking place. These should be regarded as examples illustrating a general phenomenon of gas infall in galaxies. Such accretion may also be at the origin of the gaseous halos which are being found around spirals. Probably it is the same kind of phenomenon of material infall as observed in the stellar streams in the halo and outer parts of our galaxy and M 31Comment: To appear in the proceedings of the symposium "Extraplanar Gas", ASP Conference series, editor R. Braun, 8 pages + 5 figure

Star Formation and Gas Accretion in Nearby Galaxies

In order to quantify the relationship between gas accretion and star formation, we analyse a sample of 29 nearby galaxies from the WHISP survey which contains galaxies with and without evidence for recent gas accretion. We compare combined radial profiles of FUV (GALEX) and IR 24 {\mu}m (Spitzer) characterizing distributions of recent star formation with radial profiles of CO (IRAM, BIMA, or CARMA) and HI (WSRT) tracing molecular and atomic gas contents to examine star formation efficiencies in symmetric (quiescent), asymmetric (accreting), and interacting (tidally disturbed) galaxies. In addition, we investigate the relationship between star formation rate and HI in the outer discs for the three groups of galaxies. We confirm the general relationship between gas surface density and star formation surface density, but do not find a significant difference between the three groups of galaxies.Comment: Accepted for publication in MNRAS, 18 pages, 9 figures, 1 tabl

First Detection of Molecular Gas in the Shells of CenA

Shells are faint arc-like stellar structures, which have been observed around early type galaxies and are thought to be the result of an interaction. HI gas has recently been detected in shells, a surprising result in view of the theoretical predictions that most of the gas should decouple from stars and fall into the nucleus in such interactions. Here we report the first detection of molecular gas (CO) in shells, found 15kpc away from the center of NGC5128 (CenA), a giant elliptical galaxy that harbors an active nucleus (AGN). The ratio between CO and HI emission in the shells is the same as that found in the central regions, which is unexpected given the metallicity gradient usually observed in galaxies. We propose that the dynamics of the gas can be understood within the standard picture of shell formation if one takes into account that the interstellar medium is clumpy and hence not highly dissipative. The observed metal enrichment could be due to star formation induced by the AGN jet in the shells. Furthermore our observations provide evidence that molecular gas in mergers may be spread out far from the nuclear regions.Comment: Accepted for publication in Astronomy & Astrophysics Letters, (Vol. 356), 4 pages + 1 color figur

Star formation and the interstellar medium in low surface brightness galaxies. II. Deep CO observations of low surface brightness disk galaxies

We present deep, pointed $^{12}$CO($J=2-1$) observations of three late-type LSB galaxies. The beam-size was small enough that we could probe different environments (\HI maximum, \HI mininum, star forming region) in these galaxies. No CO was found at any of the positions observed. We argue that the implied lack of molecular gas is real and not caused by conversion factor effects. The virtual absence of a molecular phase may explain the very low star formation rates in these galaxies.Comment: 9 pages, 3 figure, uses aa.cls. Typos in Tables and text correcte

Distribution and kinematics of atomic and molecular gas inside the Solar circle

The detailed distribution and kinematics of the atomic and the CO-bright molecular hydrogen in the disc of the Milky Way inside the Solar circle are derived under the assumptions of axisymmetry and pure circular motions. We divide the Galactic disc into a series of rings, and assume that the gas in each ring is described by four parameters: its rotation velocity, velocity dispersion, midplane density and its scale height. We fit these parameters to the Galactic HI and CO (J=1-0) data by producing artificial HI and CO line-profiles and comparing them with the observations. Our approach allows us to fit all parameters to the data simultaneously without assuming a-priori a radial profile for one of the parameters. We present the distribution and kinematics of the HI and H2 in both the approaching (QIV) and the receding (QI) regions of the Galaxy. Our best-fit models reproduces remarkably well the observed HI and CO longitude-velocity diagrams up to a few degrees of distance from the midplane. With the exception of the innermost 2.5 kpc, QI and QIV show very similar kinematics. The rotation curves traced by the HI and H2 follow closely each other, flattening beyond R=6.5 kpc. Both the HI and the H2 surface densities show a) a deep depression at 0.5<R<2.5 kpc, analogous to that shown by some nearby barred galaxies, b) local overdensities that can be interpreted in terms of spiral arms or ring-like features in the disk. The HI (H2) properties are fairly constant in the region outside the depression, with typical velocity dispersion of 8.9+/-1.1 (4.4+/-1.2) km/s, density of 0.43+/-0.11 (0.42+/-0.22) cm-3 and HWHM scale height of 202+/-28 (64+/-12) pc. We also show that the HI opacity in the LAB data can be accounted for by using an `effective' spin temperature of about 150 K: assuming an optically thin regime leads to underestimate the HI mass by about 30%.Comment: 23 pages, 24 figures. Accepted by A&

Influence of hole size on the extraordinary transmission through subwavelength hole arrays

We show that the extraordinary transmission of light through an array of square subwavelength holes is strongly influenced by the size of the holes. For small, square holes (air fraction below 20%), the dependence of the normalized transmission (transmissivity) on hole width greatly exceeds the expectations on the basis of conventional aperture theory. For larger holes, the transmissivity saturates. Moreover, the positions of the transmission maxima shift when the size is varied