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Register a new userStellar abundance gradients in galactic disks. I. Method and spectral line gradients
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S. D. Ryder AAO
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We describe the technique of absorption line imaging of galaxy disks using the Taurus Tunable Filter on the Anglo-Australian Telescope and demonstrate its sensitivity to the behaviour of spectral features associated with Mg and Fe. Radial profiles of Mg2 and Fe5270 line-strengths are presented for a sample of eight face-on spiral galaxies spanning a range of Hubble types. Signatures of phenomena including merger-induced star formation, HII rings and galactic bars are also reported. This study demonstrates the capacity of tunable filters to measure Mg and Fe line-strengths across the face of spiral galaxies, which can ultimately reveal clues about the star formation history and chemical evolution.
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The relationship between abundances and orbital parameters for 235 F- and G-type intermediate- and low- mass stars in the Galaxy is analyzed. We found that there are abundance gradients in the thin disk in both radial and vertical directions (-0.116 dex/kpc and -0.309 dex/kpc respectively). The gradients appear to be flatter as the Galaxy evolves. No gradient is found in the thick disk based on 18 thick disk stars. These results indicate that the ELS model is mainly suitable for the evolution of the thin disk, while the SZ model is more suitable for the evolution of the thick disk. Additionally, these results indicate that in-fall and out-flow processes play important roles in the chemical evolution of the Galaxy.
It has been suggested that the high metallicity generally observed in active galactic nuclei (AGNs) and quasars originates from ongoing star formation in the self-gravitating part of accretion disks around the supermassive black holes. We designate this region as the star forming (SF) disk, in which metals are produced from supernova explosions (SNexp) while at the same time inflows are driven by SNexp-excited turbulent viscosity to accrete onto the SMBHs. In this paper, an equation of metallicity governed by SNexp and radial advection is established to describe the metal distribution and evolution in the SF disk. We find that the metal abundance is enriched at different rates at different positions in the disk, and that a metallicity gradient is set up that evolves for steady-state AGNs. Metallicity as an integrated physical parameter can be used as a probe of the SF disk age during one episode of SMBH activity. In the SF disk, evaporation of molecular clouds heated by SNexp blast waves unavoidably forms hot gas. This heating is eventually balanced by the cooling of the hot gas, but we show that the hot gas will escape from the SF disk before being cooled, and diffuse into the BLRs forming with a typical rate of $sim 1sunmyr$. The diffusion of hot gas from a SF disk depends on ongoing star formation, leading to the metallicity gradients in BLR observed in AGNs. We discuss this and other observable consequences of this scenario.
We determine the radial abundance distributions across the disks of fourteen irregular galaxies of the types Sm and Im (morphological T types T = 9 and T =10) as traced by their HII regions. The oxygen and nitrogen abundances in HII regions are estimated through the Te method or/and with the counterpart method (C method). Moreover, we examine the correspondence between the radial abundance gradient and the surface brightness profile. We find that irregular galaxies with a flat inner profile (flat or outwardly increasing surface brightness in the central region) show shallow (if any) radial abundance gradients. On the other hand, irregular galaxies with a steep inner profile (with or without a bulge or central star cluster) usually show rather steep radial abundance gradients. This is in contrast to the widely held belief that irregular galaxies do not usually show a radial abundance gradient.
We review the observational evidences and the possible theoretical explanations for the abundance gradients in the Galactic disk. In particular, we discuss the implications of abundance gradients and gradients of abundance ratios on the mechanism for the formation of the Galaxy. We conclude that an {bf inside-out} formation of the Galaxy, and in particular of the Galactic disk, where the innermost regions are assumed to have formed much faster than the outermost ones, represents the most likely explanation for abundance gradients and we predict that the abundance gradients along the Galactic disk have increased with time.
In this paper, we study the formation and chemical evolution of the Milky Way disc with particular focus on the abundance patterns ([$alpha$/Fe] vs. [Fe/H]) at different Galactocentric distances, the present-time abundance gradients along the disc and the time evolution of abundance gradients. We consider the chemical evolution models for the Galactic disc developed by Grisoni et al. (2017) for the solar neighborhood, both the two-infall and the one-infall ones, and we extend our analysis to the other Galactocentric distances. In particular, we examine the processes which mainly influence the formation of the abundance gradients: the inside-out scenario, a variable star formation efficiency, and radial gas flows. We compare our model results with recent abundance patterns obtained along the Galactic disc from the APOGEE survey and with abundance gradients observed from Cepheids, open clusters, HII regions and PNe. We conclude that the inside-out scenario is a key ingredient, but cannot be the only one to explain abundance patterns at different Galactocentric distances and abundance gradients. Further ingredients, such as radial gas flows and variable star formation efficiency, are needed to reproduce the observed features in the thin disc. The evolution of abundance gradients with time is also shown, although firm conclusions cannot still be drawn.
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