Alignments of Voids in the Cosmic Web

We investigate the shapes and mutual alignment of voids in the large scale matter distribution of a LCDM cosmology simulation. The voids are identified using the novel WVF void finder technique. The identified voids are quite nonspherical and slightly prolate, with axis ratios in the order of c:b:a approx. 0.5:0.7:1. Their orientations are strongly correlated with significant alignments spanning scales >30 Mpc/h. We also find an intimate link between the cosmic tidal field and the void orientations. Over a very wide range of scales we find a coherent and strong alignment of the voids with the tidal field computed from the smoothed density distribution. This orientation-tide alignment remains significant on scales exceeding twice the typical void size, which shows that the long range external field is responsible for the alignment of the voids. This confirms the view that the large scale tidal force field is the main agent for the large scale spatial organization of the Cosmic Web.Comment: 10 pages, 4 figures, submitted to MNRAS, for high resolution version, see http://www.astro.rug.nl/~weygaert/tim1publication/voidshape.pd

The EFIGI catalogue of 4458 nearby galaxies with morphology II. Statistical properties along the Hubble sequence

The EFIGI catalogue of 4458 galaxies provides a reference database of the morphological properties of nearby galaxies, with 16 shape attributes describing their various dynamical components, their texture and environment, and with a dense sampling of all Hubble types. This catalogue allows us to derive a quantitative description of the Hubble Sequence in terms of the specific morphological features of the various types. The variations of the EFIGI morphological attributes with type confirm that the visual Hubble sequence is a decreasing sequence of bulge-to-total ratio and an increasing sequence of disk contribution to the total flux. There is nevertheless a large dispersion of approximately 5 types for a given bulge-to-total ratio, due to the fact that the Hubble sequence is primarily based on the strength and pitch angle of the spiral arms, independently from the bulge-to-total ratio. The grand spiral design is also related to a steep decrease in visible dust from types Sb to Sbc-Sc. In contrast, the scattered and giant HII regions show different strength variation patterns; hence, they do not appear to directly participate in the establishment of the Hubble sequence. The distortions from a symmetric profile also incidentally increase along the sequence. Bars and inner rings are frequent and occur in 41% and 25% of disk galaxies resp. Outer rings are twice less frequent than inner rings, and outer pseudo-rings occur in 11% of barred galaxies. Finally, we find a smooth decrease in mean surface brightness and intrinsic size along the Hubble sequence. The largest galaxies are cD, Ellipticals and Sab-Sbc spirals, whereas Sd and later spirals are nearly twice smaller. S0 are intermediate in size, and Im, cE and dE are confirmed as small objects. Dwarf spiral galaxies of type Sa to Scd are rare in the EFIGI catalogue, we only find 2 such objects.Comment: Accepted for publication in Astronomy and Astrophysics, 22 pages, 10 tables, 19 colour figures. Data available at http://www.efigi.or

Are Dwarf Galaxies Dominated by Dark Matter?

Mass models for a sample of 18 late-type dwarf and low surface brightness galaxies show that in almost all cases the contribution of the stellar disks to the rotation curves can be scaled to explain most of the observed rotation curves out to two or three disk scale lengths. The concept of a maximum disk, therefore, appears to work as well for these late-type dwarf galaxies as it does for spiral galaxies. Some of the mass-to-light ratios required in our maximum disk fits are high, however, up to about 15 in the R-band, with the highest values occurring in galaxies with the lowest surface brightnesses. Equally well-fitting mass models can be obtained with much lower mass-to-light ratios. Regardless of the actual contribution of the stellar disk, the fact that the maximum disk can explain the inner parts of the observed rotation curves highlights the similarity in shapes of the rotation curve of the stellar disk and the observed rotation curve. This similarity implies that the distribution of the total mass density is closely coupled to that of the luminous mass density in the inner parts of late-type dwarf galaxies.Comment: Accepted for publication in the Astrophysical Journa

The rotation curves shapes of late-type dwarf galaxies

We present rotation curves derived for a sample of 62 late-type dwarf galaxies that have been observed as part of the Westerbork HI Survey of Spiral and Irregular Galaxies (WHISP) project. The rotation curves were derived by interactively fitting model data cubes to the observed cubes, taking rotation curve shape, HI distribution, inclination, and the size of the beam into account. This makes it possible to correct for the effects of beam smearing. The dwarf galaxies in our sample have rotation-curve shapes that are similar to those of late-type spiral galaxies, in the sense that their rotation curves, when expressed in units of disk scale lengths, rise as steeply in the inner parts and start to flatten at two disk scale lengths. None of the galaxies in our sample have solid-body rotation curves that extend beyond three scale lengths. The logarithmic outer rotation curve slopes are similar between late-type dwarf and spiral galaxies. Thus, whether the flat part of the rotation curve is reached seems to depend more on the extent of the rotation curve than on its amplitude. We also find that the outer rotation curve shape does not strongly depend on luminosity, at least for galaxies fainter than M_R~-19. We find that in spiral galaxies and in the central regions of late-type dwarf galaxies, the shape of the central distribution of light and the inner rise of the rotation curve are related. This implies that galaxies with stronger central concentrations of light also have higher central mass densities, and it suggests that the luminous mass dominates the gravitational potential in the central regions, even in low surface brightness dwarf galaxies.Comment: 22 pages, 2009 A&A 493, 87

Pressure Support in Galaxy Disks: Impact on Rotation Curves and Dark Matter Density Profiles

Rotation curves constrain a galaxy's underlying mass density profile, under the assumption that the observed rotation produces a centripetal force that exactly balances the inward force of gravity. However, most rotation curves are measured using emission lines from gas, which can experience additional forces due to pressure. In realistic galaxy disks, the gas pressure declines with radius, providing additional radial support to the disk. The measured tangential rotation speed will therefore tend to lag the true circular velocity of a test particle. The gas pressure is dominated by turbulence, and we evaluate its likely amplitude from recent estimates of the gas velocity dispersion and surface density. We show that where the amplitude of the rotation curve is comparable to the characteristic velocities of the interstellar turbulence, pressure support may lead to underestimates of the mass density of the underlying dark matter halo and the inner slope of its density profile. These effects may be significant for galaxies with rotation speeds <75km/s, but are unlikely to be significant in higher mass galaxies. We find that pressure support can be sustained over long timescales, because any reduction in support due to the conversion of gas into stars is compensated for by an inward flow of gas. However, we point to many uncertainties in assessing the importance of pressure support in galaxies. Thus, while pressure support may alleviate possible tensions between rotation curve observations and LambdaCDM on kiloparsec scales, it should not be viewed as a definitive solution at this time.Comment: Accepted to the Astrophysical Journal; 18 pages including 5 pages of figure

Fossil evidence for spin alignment of SDSS galaxies in filaments

We search for and find fossil evidence that the distribution of the spin axes of galaxies in cosmic web filaments relative to their host filaments are not randomly distributed. This would indicate that the action of large scale tidal torques effected the alignments of galaxies located in cosmic filaments. To this end, we constructed a catalogue of clean filaments containing edge-on galaxies. We started by applying the Multiscale Morphology Filter (MMF) technique to the galaxies in a redshift-distortion corrected version of the Sloan Digital Sky Survey DR5. From that sample we extracted those 426 filaments that contained edge-on galaxies (b/a < 0.2). These filaments were then visually classified relative to a variety of quality criteria. Statistical analysis using "feature measures" indicates that the distribution of orientations of these edge-on galaxies relative to their parent filament deviate significantly from what would be expected on the basis of a random distribution of orientations. The interpretation of this result may not be immediately apparent, but it is easy to identify a population of 14 objects whose spin axes are aligned perpendicular to the spine of the parent filament (\cos \theta < 0.2). The candidate objects are found in relatively less dense filaments. This might be expected since galaxies in such locations suffer less interaction with surrounding galaxies, and consequently better preserve their tidally induced orientations relative to the parent filament. The technique of searching for fossil evidence of alignment yields relatively few candidate objects, but it does not suffer from the dilution effects inherent in correlation analysis of large samples.Comment: 20 pages, 19 figures, slightly revised and upgraded version, accepted for publication by MNRAS. For high-res version see http://www.astro.rug.nl/~weygaert/SpinAlignJones.rev.pd

HI Observations of Giant Low Surface Brightness Galaxies

We have used the Nancay Radio Telescope to obtain new global HI data for 16 giant low surface brightness (LSB) galaxies. Our targets have optical luminosities and disk scale lengths at the high end for spiral galaxies (L_B~10^10 Lsun and h_r>~6 kpc for H_0=75 km/s/Mpc), but they have diffuse stellar disks, with mean disk surface brightnesses ~1 magnitude or more fainter than normal giant spirals. Thirteen of the galaxies previously had been detected in HI by other workers, but the published HI observations were either confused, resolved by the telescope beam, of low signal-to-noise, or showed significant discrepancies between different authors. For the other 3 galaxies, no HI data were previously available. Several of the galaxies were resolved by the Nancay 3.6' E-W beam, so global parameters were derived from multiple-point mapping observations. Typical HI masses for our sample are ~10^10 Msun, with M_HI/L_B=0.3-1.7 (in solar units). All of the observed galaxies have published optical surface photometry, and we have compiled key optical measurements for these objects from the literature. We frequently find significant variations among physical parameters of giant LSB galaxies reported by various workers.Comment: accepted to Astronomy and Astrophysics Supplements; 14 pages; improved table formattin

Hot debris dust around HD 106797

Photometry of the A0 V main-sequence star HD 106797 with AKARI and Gemini/T-ReCS is used to detect excess emission over the expected stellar photospheric emission between 10 and 20 micron, which is best attributed to hot circumstellar debris dust surrounding the star. The temperature of the debris dust is derived as Td ~ 190 K by assuming that the excess emission is approximated by a single temperature blackbody. The derived temperature suggests that the inner radius of the debris disk is ~ 14 AU. The fractional luminosity of the debris disk is 1000 times brighter than that of our own zodiacal cloud. The existence of such a large amount of hot dust around HD 106797 cannot be accounted for by a simple model of the steady state evolution of a debris disk due to collisions, and it is likely that transient events play a significant role. Our data also show a narrow spectral feature between 11 and 12 micron attributable to crystalline silicates, suggesting that dust heating has occurred during the formation and evolution of the debris disk of HD 106797.Comment: Accepted to ApJ Letters, 8 pages, 2 figure