GASP and MaNGA Surveys Shed Light on the Enigma of the Gas Metallicity Gradients in Disk Galaxies

Making use of both MUSE observations of 85 galaxies from the survey GASP (GAs Stripping Phenomena in galaxies with MUSE) and a large sample from MaNGA (Mapping Nearby Galaxies at Apache Point Observatory survey), we investigate the distribution of gas metallicity gradients as a function of stellar mass for local cluster and field galaxies. Overall, metallicity profiles steepen with increasing stellar mass up to 1010.3 M ⊙ and flatten out at higher masses. Combining the results from the metallicity profiles and the stellar mass surface density gradients, we propose that the observed steepening is a consequence of local metal enrichment due to in situ star formation during the inside-out formation of disk galaxies. The metallicity gradient−stellar mass relation is characterized by a rather large scatter, especially for 109.8 < M ⋆/M ⊙ < 1010.5, and we demonstrate that metallicity gradients anti-correlate with the galaxy gas fraction. Focusing on the galaxy environment, at any given stellar mass, cluster galaxies have systematically flatter metallicity profiles than their field counterparts. Many subpopulations coexist in clusters: galaxies with shallower metallicity profiles appear to have fallen into their present host halo sooner and have experienced the environmental effects for a longer time than cluster galaxies with steeper metallicity profiles. Recent galaxy infallers, like galaxies currently undergoing ram pressure stripping, show metallicity gradients more similar to those of field galaxies, suggesting they have not felt the effect of the cluster yet.

Extragalactic Magnetism with SOFIA (Legacy Program) - II: A Magnetically Driven Flow in the Starburst Ring of NGC 1097*

Galactic bars are frequent in disk galaxies and they may support the transfer of matter toward the central engine of active nuclei. The barred galaxy NGC 1097 has magnetic forces controlling the gas flow at several kpc scales, which suggest that magnetic fields (B-fields) are dynamically important along the bar and nuclear ring. However, the effect of the B-field on the gas flows in the central kpc scale has not been characterized. Using thermal polarized emission at 89 μm with HAWC+/SOFIA, here, we measure that the polarized flux is spatially located at the contact regions of the outer bar with the starburst ring. The linear polarization decomposition analysis shows that the 89 μm and radio (3.5 and 6.2 cm) polarization traces two different modes, m, of the B-field: a constant B-field orientation and dominated by m = 0 at 89 μm, and a spiral B-field dominated by m = 2 at radio. We show that the B-field at 89 μm is concentrated in the warmest region of a shock driven by the galactic-bar dynamics in the contact regions between the outer bar with the starburst ring. Radio polarization traces a superposition of the spiral B-field outside and within the starburst ring. According to Faraday rotation measures between 3.5 and 6.2 cm, the radial component of the B-field along the contact regions points toward the galaxy's center on both sides. We conclude that gas streams outside and within the starburst ring follow the B-field, which feeds the black hole with matter from the host galaxy.

The Effects of Inertial Forces on the Dynamics of Disk Galaxies

When dealing with galactic dynamics, or more specifically, with galactic rotation curves, one basic assumption is always taken: the frame of reference relative to which the rotational velocities are given is assumed to be inertial. In other words, fictitious forces are assumed to vanish relative to the observational frame of a given galaxy. It might be interesting, however, to explore the outcomes of dropping that assumption; that is, to search for signatures of non-inertial behavior in the observed data. In this work, we show that the very discrepancy in galaxy rotation curves could be attributed to non-inertial effects. We derive a model for spiral galaxies that takes into account the possible influence of fictitious forces and find that the additional terms in the new model, due to fictitious forces, closely resemble dark halo profiles. Following this result, we apply the new model to a wide sample of galaxies, spanning a large range of luminosities and radii. It turns out that the new model accurately reproduces the structures of the rotation curves and provides very good fittings to the data.

The Effects of Inertial Forces on the Dynamics of Disk Galaxies

When dealing with galactic dynamics, or more specifically, with galactic rotation curves, one basic assumption is always taken: the frame of reference relative to which the rotational velocities are given is assumed to be inertial. In other words, fictitious forces are assumed to vanish relative to the observational frame of a given galaxy. It might be interesting, however, to explore the outcomes of dropping that assumption; that is, to search for signatures of non-inertial behavior in the observed data. In this work, we show that the very discrepancy in galaxy rotation curves could be attributed to non-inertial effects. We derive a model for spiral galaxies that takes into account the possible influence of fictitious forces and find that the additional terms in the new model, due to fictitious forces, closely resemble dark halo profiles. Following this result, we apply the new model to a wide sample of galaxies, spanning a large range of luminosities and radii. It turns out that the new model accurately reproduces the structures of the rotation curves and provides very good fittings to the data.

Quantifying the fine structures of disk galaxies with deep learning: Segmentation of spiral arms in different Hubble types

Context. Spatial correlations between spiral arms and other galactic components such as giant molecular clouds and massive OB stars suggest that spiral arms can play vital roles in various aspects of disk galaxy evolution. Segmentation of spiral arms in disk galaxies is therefore a key task when these correlations are to be investigated. Aims. We therefore decomposed disk galaxies into spiral and nonspiral regions using the code U-Net, which is based on deep-learning algorithms and has been invented for segmentation tasks in biology. Methods. We first trained this U-Net with a large number of synthesized images of disk galaxies with known properties of symmetric spiral arms with radially constant pitch angles and then tested it with entirely unknown data sets. The synthesized images were generated from mathematical models of disk galaxies with various properties of spiral arms, bars, and rings in these supervised-learning tasks. We also applied the trained U-Net to spiral galaxy images synthesized from the results of long-term hydrodynamical simulations of disk galaxies with nonsymmetric spiral arms. Results. We find that U-Net can predict the precise locations of spiral arms with an average prediction accuracy (Fm) of 98%. We also find that Fm does not depend strongly on the numbers of spiral arms, presence or absence of stellar bars and rings, and bulge-to-disk ratios in disk galaxies. These results imply that U-Net is a very useful tool for identifying the locations of spirals arms. However, we find that the U-Net trained on these symmetric spiral arm images cannot predict entirly unknown data sets with the same accuracy that were produced from the results of hydrodynamical simulations of disk galaxies with nonsymmetric irregular spirals and their nonconstant pitch angles across disks. In particular, weak spiral arms in barred-disk galaxies are properly segmented. Conclusions. These results suggest that U-Net can segment more symmetric spiral arms with constant pitch angles in disk galaxies. However, we need to train U-Net with a larger number of more realistic galaxy images with noise, nonsymmetric spirals, and different pitch angles between different arms in order to apply it to real spiral galaxies. It would be a challenge to make a large number of training data sets for such realistic nonsymmetric and irregular spiral arms with nonconstant pitch angles.