High Surface Brightness Spiral Galaxies Research Articles

Magnetic fields in Local Group dwarf irregulars

We wish to clarify whether strong magnetic fields can be effectively generated in typically low-mass dwarf galaxies and to assess the role of dwarf galaxies in the magnetization of the Universe. We performed a search for radio emission and magnetic fields in an unbiased sample of 12 Local Group (LG) irregular and dwarf irregular galaxies with the 100m Effelsberg telescope at 2.64 and 4.85GHz. Magnetic fields in LG dwarfs are three times weaker than in the normal spirals (<4.2+-1.8muG). The production of total magnetic fields appears to be regulated mainly by the star-formation surface density, with the power-law exponent of 0.30+-0.04, or by the gas surface density (with the exponent 0.47+-0.09). In addition, we find systematically stronger fields in objects of higher global star-formation rate. The dwarf galaxies follow a similar far-infrared relationship (with a slope of 0.91+-0.08) to that determined for high surface brightness spiral galaxies. The magnetic field strength in dwarf galaxies does not correlate with their maximum rotational velocity, indicating a small-scale rather than a large-scale dynamo process. If magnetization of the Universe by galactic outflows is coeval with its metal enrichment, we show that more massive objects (such as Lyman Break Galaxies) can efficiently magnetize the intergalactic medium with a magnetic field strength of about 0.8nG out to a distance of 160-530kpc at redshifts 5-3, respectively. Several times weaker fields and shorter magnetization distances are expected from primordial dwarf galaxies. We also predict that most star-forming local dwarfs might have magnetized their surroundings up to about 0.1muG within about 5kpc distance. Strong magnetic fields (>6muG) are observed only in dwarfs of extreme characteristics while typical LG dwarfs are unsuitable objects for the efficient supply of magnetic fields to the intergalactic medium.

Bulge Evolution in Face-On Spiral and Low Surface Brightness Galaxies

It is an observational fact that bulges of spiral galaxies contain a high fraction of old and metal-rich stars. Following this observational fact, we have investigated the colors of 21 bulges hosted by a selected sample of high surface brightness spiral galaxies and low surface brightness galaxies observed in the B and R optical bands and the J and Ks near-IR bands. Using structural parameters derived from these observations, we obtain evidence that bulges could be formed by pure disk evolution (secular evolution), in agreement with suggestions by some authors. The color profiles, especially the near-IR ones, show null or almost null color gradients, supporting the hypothesis that the disk stellar populations are similar to those present in the bulge and/or that some bulges can be understood as disks with enhanced stellar density (or pseudobulges). In the optical, half of the galaxies present an inverse color gradient, giving additional evidence in favor of secular evolution for the sample investigated. The comparison of the observed colors with those obtained from spectrophotometric models of galaxy evolution suggests that bulges of the selected sample have solar and subsolar metallicities and are independent of the current stellar formation rate. Also, we obtain evidence suggesting that galaxies hosting small bulges tend to be systematically metal-poor compared to those with larger bulges. These results are being checked more carefully with high signal-to-noise ratio spectroscopy.

Mass Models for Spiral Galaxies from Two-dimensional Velocity Maps

We model the mass distributions of 40 high surface brightness spiral galaxies inside their optical radii, deriving parameters of mass models by matching the predicted velocities to observed velocity maps. We use constant mass-to-light disk and bulge models, and we have tried fits with no halo and with three different halo density profiles. The data require a halo in most, but not all, cases, while in others the best fit occurs with negligible mass in the luminous component, which we regard as unphysical. All three adopted halo profiles lead to fits of about the same quality, and our data therefore do not constrain the functional form of the halo profile. The halo parameters display large degeneracies for two of the three adopted halo functions, but the separate luminous and dark masses are better constrained. However, the fitted disk and halo masses vary substantially between the adopted halo models, indicating that even high-quality two-dimensional optical velocity maps do not provide significant constraints on the dark matter content of a galaxy. We demonstrate that data from long-slit observations are likely to provide still weaker constraints. We conclude that additional information is needed in order to constrain the separate disk and halo masses in a galaxy.

The $V_{\rm circ}$-$\sigma_{\rm c}$ relation in high and low surface brightness galaxies

We investigate the relation between the asymptotic circular velocity, V_c, and the central stellar velocity dispersion, sigma_c, in galaxies. We consider a new sample of high surface brightness spiral galaxies (HSB), low surface brightness spiral galaxies (LSB), and elliptical galaxies with HI-based V_c measurements. We find that: 1) elliptical galaxies with HI measurements fit well within the relation; 2) a linear law can reproduce the data as well as a power law (used in previous works) even for galaxies with sigma_c < 70 km/s; 3) LSB galaxies, considered for the first time with this respect, seem to behave differently, showing either larger V_c values or smaller sigma_c values.

Dark Matter within High Surface Brightness Spiral Galaxies

We present results from a detailed dynamical analysis of five high surface brightness, late-type spiral galaxies, NGC 3810, NGC 3893, NGC 4254, NGC 5676, and NGC 6643, which were studied with the aim of quantifying the luminous-to-dark matter ratio inside their optical radii. The galaxies' stellar light distribution and gas kinematics have been observed and compared to hydrodynamic gas simulations that predict the gasdynamics arising in response to empirical gravitational potentials, which are combinations of differing stellar disk and dark halo contributions. The gravitational potential of the stellar disk was derived from near-infrared photometry, color corrected to yield a constant stellar mass-to-light ratio (M/L); for the dark halo, the mass density distribution of an axisymmetric isothermal sphere with a core was chosen. Hydrodynamic gas simulations were performed for each galaxy for a sequence of five different mass fractions of the stellar disk and for a wide range of spiral pattern speeds. These two parameters mainly determine the modeled gas distribution and kinematics. The agreement between the simulated and observed gas kinematics permitted us to conclude that the galaxies with the highest rotation velocities tend to possess very massive stellar disks that dominate the gasdynamics within the optical radius. In less massive galaxies, with a maximal rotation velocity of less than 200 km s-1, the mass of the dark halo at least equals the stellar mass within 2-3 disk scale lengths. The maximal disk stellar mass-to-light ratio in the K band was found to lie at about M/LK ≈ 0.6. Furthermore, the gasdynamic simulations provide a powerful tool for accurately determining the dominant spiral pattern speed for galaxies, independent of a specific density wave theory. It was found that the location of the corotation resonance falls into a narrow range of around three exponential disk scale lengths for all galaxies from the sample. The corotation resonance encloses the strong part of the stellar spiral in all cases. Based on the experience gained from this project, the use of a color correction to account for local stellar population differences is strongly encouraged when properties of galactic disks are studied that rely on their stellar mass distributions.

Morphology and Stellar Populations in the Gas-Rich, Giant LSBGs

AbstractAn unexpected characteristic of low surface brightness galaxies (LSBGs) is that a significant number are massive and possess substantial amounts of atomic gas. We present preliminary results of an ongoing program to obtain BVRIJHK imaging, along with some nuclear spectroscopy, of a well-defined sample of LSBGs which are gas-rich and of similar size to giant, high surface brightness spiral galaxies (HSBGs). These LSBGs span the entire range of Hubble disk morphologies. While their disks are bluer, on average, than comparable HSBGs, the optical morphology of massive LSBGs indicates that many of these systems have undergone previous star formation episodes. They typically have long disk scale lengths, and range from MB = −16 to −22 (H0 = 75 km s−1 Mpc−1). About half of the LSBGs with bulges show evidence of nuclear activity, and ~30% appear to be barred. These massive, gas-rich LSBGs apparently have varied, and often complex, evolutionary histories.