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Register a new userEquipartition magnetic fields and star formation rates in normal galaxies at sub-kpc scales
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We studied the total magnetic field strength in normal star-forming galaxies estimated using energy equipartition assumption. Using the well known radio--far infrared correlation we demonstrate that the equipartition assumption is valid in galaxies at sub-kpc scales. We find that the magnetic field strength is strongly correlated with the surface star formation rate in the galaxies NGC 6946 and NGC 5236. Further, we compare the magnetic field energy density to the total (thermal + turbulent) energy densities of gas (neutral + ionized) to identify regions of efficient field amplification in the galaxy NGC 6946. We find that in regions of efficient star formation, the magnetic field energy density is comparable to that of the total energy density of various interstellar medium components and systematically dominates in regions of low star formation efficiency.
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(Abridged) We present a comprehensive analysis of the relationship between star formation rate surface density (SFR SD) and gas surface density (gas SD) at sub-kpc resolution in a sample of 18 nearby galaxies. We use high resolution HI data from THINGS, CO data from HERACLES and BIMA SONG, 24 micron data from the Spitzer Space Telescope, and UV data from GALEX. We target 7 spiral galaxies and 11 late-type/dwarf galaxies and investigate how the star formation law differs between the H2-dominated centers of spiral galaxies, their HI-dominated outskirts and the HI-rich late-type/dwarf galaxies. We find that a Schmidt-type power law with index N=1.0+-0.2 relates the SFR SD and the H2 SD across our sample of spiral galaxies, i.e., that H2 forms stars at a constant efficiency in spirals. The average molecular gas depletion time is ~2*10^9 yrs. We interpret the linear relation and constant depletion time as evidence that stars are forming in GMCs with approximately uniform properties and that the H2 SD may be more a measure of the filling fraction of giant molecular clouds than changing conditions in the molecular gas. The relationship between total gas SD and SFR SD varies dramatically among and within spiral galaxies. Most galaxies show little or no correlation between the HI SD and the SFR SD. As a result, the star formation efficiency (SFE = SFR SD / gas SD) varies strongly across our sample and within individual galaxies. We show that in spirals the SFE is a clear function of radius, while the dwarf galaxies in our sample display SFEs similar to those found in the outer optical disks of the spirals. Another general feature of our sample is a sharp saturation of the HI SD at ~9 M_sol/pc^2 in both the spiral and dwarf galaxies.
We study the propagation of star formation based on the investigation of the separation of young star clusters from HII regions nearest to them. The relation between the separation and U-B colour index (or age) of a star cluster was found. The average age of star clusters increases with the separation as the 1.0-1.2 power in the separation range from 40 to 200 pc and as the 0.4-0.9 power in the range of 100-500 pc in the galaxies with symmetric morphology. The galaxies with distorted asymmetric disc structure show more complex and steeper (power >1.2 at separations from 40 to 500 pc) dependence between the age and the separation. Our results confirm the findings of previous studies on the dominant role of turbulence in propagation of the star formation process on spatial scales up to 500 pc and on time scales up to 300 Myr. On a smaller scale (=<100 pc), other physical processes, such as stellar winds and supernova explosions, play an important role along with turbulence. On the scale of stellar associations (100-200 pc and smaller), the velocity of star formation propagation is almost constant and it has a typical value of a few km/s.
We present the relation between the star formation rate surface density, $Sigma_{rm SFR}$, and the hydrostatic mid-plane pressure, P$_{rm h}$, for 4260 star-forming regions of kpc size located in 96 galaxies included in the EDGE-CALIFA survey covering a wide range of stellar masses and morphologies. We find that these two parameters are tightly correlated, exhibiting smaller scatter and strong correlation in comparison to other star-forming scaling relations. A power-law, with a slightly sub-linear index, is a good representation of this relation. Locally, the residuals of this correlation show a significant anti-correlation with both the stellar age and metallicity whereas the total stellar mass may also play a secondary role in shaping the $Sigma_{rm SFR}$ - P$_{rm h}$ relation. For our sample of active star-forming regions (i.e., regions with large values of H$alpha$ equivalent width), we find that the effective feedback momentum per unit stellar mass ($p_ast/m_ast$),measured from the P$_{rm h}$ / $Sigma_{rm SFR}$ ratio increases with P$_{rm h}$. The median value of this ratio for all the sampled regions is larger than the expected momentum just from supernovae explosions. Morphology of the galaxies, including bars, does not seem to have a significant impact in the $Sigma_{rm SFR}$ - P$_{rm h}$ relation. Our analysis suggests that self regulation of the $Sigma_{rm SFR}$ at kpc scales comes mainly from momentum injection to the interstellar medium from supernovae explosions. However, other mechanism in disk galaxies may also play a significant role in shaping the $Sigma_{rm SFR}$ at local scales. Our results also suggest that P$_{rm h}$ can be considered as the main parameter that modulates star formation at kpc scales, rather than individual components of the baryonic mass.
Star formation rate density, $Sigma_{rm SFR}$, has shown a remarkable correlation with both components of the baryonic mass at kpc scales (i.e., the stellar mass density, and the molecular gas mass density; $Sigma_{ast}$, and $Sigma_{rm mol}$, respectively) for galaxies in the nearby Universe. In this study we propose an empirical relation between $Sigma_{rm SFR}$ and the baryonic mass surface density ($Sigma_{rm b}$ =$Sigma_{rm mol,Av}$ + $Sigma_{ast}$; where $Sigma_{rm mol,Av}$ is the molecular gas density derived from the optical extinction, Av) at kpc scales using the spatially-resolved properties of the MaNGA survey - the largest sample of galaxies observed via Integral Field Spectroscopy (IFS, $sim$ 8400 objects). We find that $Sigma_{rm SFR}$ tightly correlates with $Sigma_{rm b}$. Furthermore, we derive an empirical relation between the $Sigma_{rm SFR}$ and a second degree polynomial of $Sigma_{rm b}$ yielding a one-to-one relation between these two observables. Both, $Sigma_{rm b}$ and its polynomial form show a stronger correlation and smaller scatter with respect to $Sigma_{rm SFR}$ than the relations derived using the individual components of $Sigma_{rm b}$. Our results suggest that indeed these three parameters are physically correlated, suggesting a scenario in which the two components of the baryonic mass regulate the star-formation activity at kpc scales.
By resimulating a region of a global disc simulation at higher resolution, we resolve and study the properties of molecular clouds with a range of masses from a few 100s M$_{odot}$ to $10^6$ M$_{odot}$. The purpose of our paper is twofold, i) to compare the ISM and GMCs at much higher resolution compared to previous global simulations, and ii) to investigate smaller clouds and characteristics such as the internal properties of GMCs which cannot be resolved in galactic simulations. We confirm the robustness of cloud properties seen in previous galactic simulations, and that these properties extend to lower mass clouds, though we caution that velocity dispersions may not be measured correctly in poorly resolved clouds. We find that the properties of the clouds and ISM are only weakly dependent on the details of local stellar feedback, although stellar feedback is important to produce realistic star formation rates and agreement with the Schmidt-Kennicutt relation. We study internal properties of GMCs resolved by $10^4-10^5$ particles. The clouds are highly structured, but we find clouds have a velocity dispersion radius relationship which overall agrees with the Larson relation. The GMCs show evidence of multiple episodes of star formation, with holes corresponding to previous feedback events and dense regions likely to imminently form stars. Our simulations show clearly long filaments, which are seen predominantly in the inter-arm regions, and shells.
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