In this article we investigate the outer and inner mass distributions of the irregular galaxies UGC 4284 and UGC 11861, taking advantage of published HI and H{alpha} high resolution rotation curves and constraining the stellar disk of both galaxies t
hroughout stellar population synthesis studies. In addition we take into account the gas content of both galaxies deriving the HI+He rotation curve. The deduced baryonic rotation curves (star+gas) are inadequate to account for the total mass of UGC 4284 and UGC 11861, for that reason we examine the possibility of dark matter to explain the incongruity between the observed HI and H{alpha} rotation curves of UGC 4284 and UGC 11861 and the derived baryonic rotation curves. We consider NFW, Burkert, DiCintio, Einasto, and the Stadel dark matter halos, to analyse the dark matter content of UGC 4284 and UGC 11861. The principal results of this work are that cored dark matter models better reproduce the dark matter H{alpha} and HI rotation curves of UGC 11861 and the dark matter HI rotation curve of UGC 4284, while, the H{alpha} rotation curve of UGC 4284 is better reproduced by a cuspy DiCintio DM model. In general, cored exponential two-parameters models Einasto and Stadel, give better fits than Burkert. This trend, as well as to confirm past results, presents for the first time a comparison between two different exponential dark matter models, Einasto and Stadel, in an attempt to better constrain the range of possible exponential dark matter models applied to real galaxies.
Azimuthal age/color gradients across spiral arms are a signature of long-lived spirals. From a sample of 19 normal (or weakly barred) spirals where we have previously found azimuthal age/color gradient candidates, 13 objects were further selected if
a two-armed grand-design pattern survived in a surface density stellar mass map. Mass maps were obtained from optical and near-infrared imaging, by comparing with a Monte Carlo library of stellar population synthesis models that allowed us to obtain the mass-to-light ratio in the J band, (M/L)_J, as a function of (g-i) versus (i-J) color. The selected spirals were analyzed with Fourier methods in search for other signatures of long-lived modes related to the gradients, such as the gradient divergence toward corotation, and the behavior of the phase angle of the two-armed spiral in different wavebands, as expected from theory. The results show additional signatures of long-lived spirals in at least 50% of the objects.
We have carried out a search for gas-rich dwarf galaxies that have lower rotation velocities in their outskirts than MOdified Newtonian Dynamics (MOND) predicts, so that the amplitude of their rotation curves cannot be fitted by arbitrarily increasin
g the mass-to-light ratio of the stellar component or by assuming additional undetected matter. With presently available data, the gas-rich galaxies UGC 4173, Holmberg II, ESO 245-G05, NGC 4861 and ESO 364-G029 deviate most from MOND predictions and, thereby, provide a sample of promising targets in testing the MOND framework. In the case of Holmberg II and NGC 4861, we find that their rotation curves are probably inconsistent with MOND, unless their inclinations and distances differ significantly from the nominal ones. The galaxy ESO 364-G029 is a promising target because its baryonic mass and rotation curve are similar to Holmberg II but presents a higher inclination. Deeper photometric and HI observations of ESO 364-G029, together with further decreasing systematic uncertainties, may provide a strong test to MOND.
In this work we extend the study on the mass distribution of the spiral galaxy NGC 5278, performing 1D and 2D bulge-disk decomposition to determine which components constitute the baryonic mass in this galaxy. Our analysis does not detect any bulge,
instead we find a bright source, probably related with the central AGN, and an exponential disk. We fix the stellar disk contribution to the rotation curve (RC) with broad band photometric observations and population synthesis models, to obtain 2D mass distribution of the stellar disk. In the particular case of NGC 5278, we find that the typical assumption of considering the mass-to-luminosity ratio (M/L) of the disk as constant along the galactocentric radius is not valid. We also extract a baryonic RC from the mass profile, to determine the inability of this baryonic RC and also of the baryonic RC with more and less 30% disk mass (in order to consider the disk mass errors) to fit the entire RC. We perform the RC decomposition of NGC 5278 considering the baryonic RC and four types of dark matter halos: Hernquist; Burkert; Einasto and Navarro, Frenk & White. Our results show that Hernquist halo models better our observed RC in the case of determined disk mass ($M_d=5.6times 10^{10}$ solar masses) and also with 30% less disk mass. In the case of 30% more disk mass the cored Einasto (n < 4) halo is the best fitting model.
Assuming that density waves trigger star formation, and that young stars preserve the velocity components of the molecular gas where they are born, we analyze the effects that non-circular gas orbits have on color gradients across spiral arms. We try
two approaches, one involving semi-analytical solutions for spiral shocks, and another with magnetohydrodynamic (MHD) numerical simulation data. We find that, if non-circular motions are ignored, the comparison between observed color gradients and stellar population synthesis models would in principle yield pattern speed values that are systematically too high for regions inside corotation, with the difference between the real and the measured pattern speeds increasing with decreasing radius. On the other hand, image processing and pixel averaging result in systematically lower measured spiral pattern speed values, regardless of the kinematics of stellar orbits. The net effect is that roughly the correct pattern speeds are recovered, although the trend of higher measured $Omega_p$ at lower radii (as expected when non-circular motions exist but are neglected) should still be observed. We examine the Martinez-Garcia et al. (2009) photometric data and confirm that this is indeed the case. The comparison of the size of the systematic pattern speed offset in the data with the predictions of the semi-analytical and MHD models corroborates that spirals are more likely to end at Outer Lindblad Resonance, as these authors had already found.
Azimuthal color (age) gradients across spiral arms are one of the main predictions of density wave theory; gradients are the result of star formation triggering by the spiral waves. In a sample of 13 spiral galaxies of types A and AB, we find that 10
of them present regions that match the theoretical predictions. By comparing the observed gradients with stellar population synthesis models, the pattern speed and the location of major resonances have been determined. The resonance positions inferred from this analysis indicate that 9 of the objects have spiral arms that extend to the outer Lindblad resonance (OLR); for one of the galaxies, the spiral arms reach the corotation radius. The effects of dust, and of stellar densities, velocities, and metallicities on the color gradients are also discussed.
