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We aim to use statistical analysis of a large number of various galaxies to probe, model, and understand relations between different galaxy properties and magnetic fields. We have compiled a sample of 55 galaxies including low-mass dwarf and Magellanic-types, normal spirals and several massive starbursts, and applied principal component analysis (PCA) and regression methods to assess the impact of various galaxy properties on the observed magnetic fields. According to PCA the global galaxy parameters (like HI, H2, and dynamical mass, star formation rate (SFR), near-infrared luminosity, size, and rotational velocity) are all mutually correlated and can be reduced to a single principal component. Further PCA performed for global and intensive (not size related) properties of galaxies (such as gas density, and surface density of the star formation rate, SSFR), indicates that magnetic field strength B is connected mainly to the intensive parameters, while the global parameters have only weak relationships with B. We find that the tightest relationship of B is with SSFR, which is described by a power-law with an index of 0.33+-0.03. The observed weaker associations of B with galaxy dynamical mass and the rotational velocity we interpret as indirect ones, resulting from the observed connection of the global SFR with the available total H2 mass in galaxies. Using our sample we constructed a diagram of B across the Hubble sequence which reveals that high values of B are not restricted by the Hubble type. However, weaker fields appear exclusively in later Hubble types and B as low as about 5muG is not seen among typical spirals. The processes of generation of magnetic field in the dwarf and Magellanic-type galaxies are similar to those in the massive spirals and starbursts and are mainly coupled to local star-formation activity involving the small-scale dynamo mechanism.
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Starburst galaxies have elevated star formation rates (SFRs) for their stellar mass. In Ellison et al. (2018) we used integral field unit (IFU) maps of star formation rate surface density (Sigma_SFR) and stellar mass surface density (Sigma_*) to show that starburst galaxies in the local universe are driven by SFRs that are preferentially boosted in their central regions. Here, we present molecular gas maps obtained with the Atacama Large Millimeter Array (ALMA) observatory for 12 central starburst galaxies at z~0 drawn from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey. The ALMA and MaNGA data are well matched in spatial resolution, such that the ALMA maps of molecular gas surface density (Sigma_H2) can be directly compared with MaNGA maps at kpc-scale resolution. The combination of Sigma_H2, Sigma_* and Sigma_SFR at the same resolution allow us to investigate whether central starbursts are driven primarily by enhancements in star formation efficiency (SFE) or by increased gas fractions. By computing offsets from the resolved Kennicutt-Schmidt relation (Sigma_H2 vs. Sigma_SFR) and the molecular gas main sequence (Sigma_* vs. Sigma_H2), we conclude that the primary driver of the central starburst is an elevated SFE. We also show that the enhancement in Sigma_SFR is accompanied by a dilution in O/H, consistent with a triggering that is induced by metal poor gas inflow. These observational signatures are found in both undisturbed (9/12 galaxies in our sample) and recently merged galaxies, indicating that both interactions and secular mechanisms contribute to central starbursts.
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We analyze the intrinsic velocity dispersion properties of 648 star-forming galaxies observed by the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey, to explore the relation of intrinsic gas velocity dispersions with star formation rates (SFRs), SFR surface densities ($rm{Sigma_{SFR}}$), stellar masses and stellar mass surface densities ($rm{Sigma_{*}}$). By combining with high z galaxies, we found that there is a good correlation between the velocity dispersion and the SFR as well as $rm{Sigma_{SFR}}$. But the correlation between the velocity dispersion and the stellar mass as well as $rm{Sigma_{*}}$ is moderate. By comparing our results with predictions of theoretical models, we found that the energy feedback from star formation processes alone and the gravitational instability alone can not fully explain simultaneously the observed velocity-dispersion/SFR and velocity-dispersion/$rm{Sigma_{SFR}}$ relationships.
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