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Register a new userCoupling local to global star formation in spiral galaxies: the effect of differential rotation
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Star formation is one of the key factors that shapes galaxies. This process is relatively well understood from both simulations and observations on a small local scale of individual giant molecular clouds and also on a global galaxy-wide scale (e.g. the Kennicutt-Schmidt law). However, there is still no understanding on how to connect global to local star formation scales and whether this connection is at all possible. Here we analyze spatially resolved kinematics and the star formation rate density $Sigma_{SFR}$ for a combined sample of 17 nearby spiral galaxies obtained using our own optical observations in H$alpha$ for 9 galaxies and neutral hydrogen radio observations combined with a multi-wavelength spectral energy distribution analysis for 8 galaxies from the THINGS project. We show that the azimuthally averaged normalized star formation rate density in spiral galaxies on a scale of a few hundred parsecs is proportional to the kinetic energy of giant molecular cloud collisions due to differential rotation of the galactic disc. This energy is calculated from the rotation curve using the two Oort parameters A and B as $log (Sigma_{SFR} / SFR_{tot}) propto log[2 A^2+ 5 B^2]$. The total kinetic energy of collision is defined by the shear velocity that is proportional to A and the rotational energy of a cloud proportional to the vorticity B. Hence, shear does not act as a stabilizing factor for the cloud collapse thus reducing star formation but rather increases it by boosting the kinetic energy of collisions. This result can be a tool through which one can predict a radial distribution of star formation surface density using only a rotation curve.
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We investigate the star formation histories (SFHs) of massive red spiral galaxies with stellar mass $M_ast>10^{10.5}M_odot$, and make comparisons with blue spirals and red ellipticals of similar masses. We make use of the integral field spectroscopy from the SDSS-IV/DR15 MaNGA sample, and estimate spatially resolved SFHs and stellar population properties of each galaxy by applying a Bayesian spectral fitting code to the MaNGA spectra. We find that both red spirals and red ellipticals have experienced only one major star formation episode at early times, and the result is independent of the adopted SFH model. On average, more than half of their stellar masses were formed $>$10 Gyrs ago, and more than 90% were formed $>6$ Gyrs ago. The two types of galaxies show similarly flat profiles in a variety of stellar population parameters: old stellar ages indicated by $D4000$ (the spectral break at around 4000AA), high stellar metallicities, large Mgb/Fe ratios indicating fast formation, and little stellar dust attenuation. In contrast, although blue spirals also formed their central regions $>$10 Gyrs ago, both their central regions and outer disks continuously form stars over a long timescale. Our results imply that, massive red spirals are likely to share some common processes of formation (and possibly quenching) with massive red ellipticals in the sense that both types were formed at $z > 2$ through a fast formation process.Possible mechanisms for the formation and quenching of massive red spirals are discussed.
Abridged - We quantify the effect of the galaxy group environment (for 12.5 < log(M_group/Msun) < 14.0) on the star formation rates of the (morphologically-selected) population of disk-dominated local Universe spiral galaxies (z < 0.13) with stellar masses log(M*/Msun) > 9.5. Within this population, we find that, while a small minority of group satellites are strongly quenched, the group centrals, and the large majority of satellites exhibit levels of SFR indistinguishable from ungrouped field galaxies of the same M*, albeit with a higher scatter, and for all M*. Modelling these results, we deduce that disk-dominated satellites continue to be characterized by a rapid cycling of gas into and out of their ISM at rates similar to those operating prior to infall, with the on-going fuelling likely sourced from the group intrahalo medium (IHM) on Mpc scales, rather than from the circum-galactic medium on 100kpc scales. Consequently, the color-density relation of the galaxy population as a whole would appear to be primarily due to a change in the mix of disk- and spheroid-dominated morphologies in the denser group environment compared to the field, rather than to a reduced propensity of the IHM in higher mass structures to cool and accrete onto galaxies. We also suggest that the inferred substantial accretion of IHM gas by satellite disk-dominated galaxies will lead to a progressive reduction in their specific angular momentum, thereby representing an efficient secular mechanism to transform morphology from star-forming disk-dominated types to more passive spheroid-dominated types.
We study numerically large-scale magnetic field evolution and its enhancement in gaseous disks of spiral galaxies. We consider a set of models with the various spiral pattern parameters and the initial magnetic field strength with taking into account gas self-gravity and cooling/heating. In agreement with previous studies, we find out that galactic magnetic field is mostly aligned with gaseous structures, however small-scale gaseous structures (spurs and clumps) are more chaotic than the magnetic field structure. In spiral arms magnetic field strongly coexists with the gas distribution, in the inter-arm region we see filamentary magnetic field structure. Simulations reveal the presence of the small-scale irregularities of the magnetic field as well as the reversal of magnetic field at the outer edge of the large-scale spurs. We provide evidences that the magnetic field in the spiral arms has a stronger mean-field component, and there is a clear inverse correlation between gas density and plasma-beta parameter, compared to the rest of the disk with a more turbulent component of the field and an absence of correlation between gas density and plasma-beta. We show the mean field growth up to 3-10$mu G$ in the cold gas during several rotation periods (500-800 Myr), whereas ratio between azimuthal and radial field is equal to 4/1. Mean field strength increases by a factor of 1.5-2.5 for models with various spiral pattern parameters. Random magnetic field component can reach up to 25 % from the total strength. By making an analysis of the time-depended evolution of radial Poynting flux we point out that the magnetic field strength is enhanced stronger at the galactic outskirts which is due to the radial transfer of magnetic energy by the spiral arms pushing the magnetic field outward. Our results also support the presence of sufficient conditions for development of MRI at distances >11 kpc.
We investigate the impact of spiral structure on global star formation using a sample of 2226 nearby bright disk galaxies. Examining the relationship between spiral arms, star formation rate (SFR), and stellar mass, we find that arm strength correlates well with the variation of SFR as a function of stellar mass. Arms are stronger above the star-forming galaxy main sequence (MS) and weaker below it: arm strength increases with higher $log,({rm SFR}/{rm SFR}_{rm MS})$, where ${rm SFR}_{rm MS}$ is the SFR along the MS. Likewise, stronger arms are associated with higher specific SFR. We confirm this trend using the optical colors of a larger sample of 4378 disk galaxies, whose position on the blue cloud also depends systematically on spiral arm strength. This link is independent of other galaxy structural parameters. For the subset of galaxies with cold gas measurements, arm strength positively correlates with HI and H$_2$ mass fraction, even after removing the mutual dependence on $log,({rm SFR}/{rm SFR}_{rm MS})$, consistent with the notion that spiral arms are maintained by dynamical cooling provided by gas damping. For a given gas fraction, stronger arms lead to higher $log,({rm SFR}/{rm SFR}_{rm MS})$, resulting in a trend of increasing arm strength with shorter gas depletion time. We suggest a physical picture in which the dissipation process provided by gas damping maintains spiral structure, which, in turn, boosts the star formation efficiency of the gas reservoir.
Using a suite of isolated $L_star$ galaxy simulations, we show that global depletion times and star-forming gas mass fractions in simulated galaxies exhibit systematic and well-defined trends as a function of the local star formation efficiency per freefall time, $epsilon_{rm ff}$, strength of stellar feedback, and star formation threshold. We demonstrate that these trends can be reproduced and explained by a simple physical model of global star formation in galaxies. Our model is based on mass conservation and the idea of gas cycling between star-forming and non-star-forming states on certain characteristic time scales under the influence of dynamical and feedback processes. Both the simulation results and our model predictions exhibit two limiting regimes with rather different dependencies of global galactic properties on the local parameters. When $epsilon_{rm ff}$ is small and feedback is inefficient, the total star-forming mass fraction, $f_{rm sf}$, is independent of $epsilon_{rm ff}$ and the global depletion time, $tau_{rm dep}$, scales inversely with $epsilon_{rm ff}$. When $epsilon_{rm ff}$ is large or feedback is very efficient, these trends are reversed: $f_{rm sf} propto epsilon_{rm ff}^{-1}$ and $tau_{rm dep}$ is independent of $epsilon_{rm ff}$ but scales linearly with the feedback strength. We also compare our results with the observed depletion times and mass fractions of star-forming and molecular gas and show that they provide complementary constraints on $epsilon_{rm ff}$ and the feedback strength. We show that useful constraints on $epsilon_{rm ff}$ can also be obtained using measurements of the depletion time and its scatter on different spatial scales.
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