Showing papers by "Ralph Schönrich published in 2009"

Chemical evolution with radial mixing

Abstract: Models of the chemical evolution of our Galaxy are extended to include radial migration of stars and flow of gas through the disc. The models track the production of both iron and α-elements. A model is chosen that provides an excellent fit to the metallicity distribution of stars in the Geneva-Copenhagen survey (GCS) of the solar neighbourhood and a good fit to the local Hess diagram. The model provides a good fit to the distribution of GCS stars in the age-metallicity plane, although this plane was not used in the fitting process. Although this model's star formation rate is monotonically declining, its disc naturally splits into an α-enhanced thick disc and a normal thin disc. In particular, the model's distribution of stars in the ([O/Fe], [Fe/H]) plane resembles that of Galactic stars in displaying a ridge line for each disc. The thin-disc's ridge line is entirely due to stellar migration, and there is the characteristic variation of stellar angular momentum along it that has been noted by Haywood in survey data. Radial mixing of stellar populations with high σ z from inner regions of the disc to the solar neighbourhood provides a natural explanation of why measurements yield a steeper increase of σ z with age than predicted by theory. The metallicity gradient in the interstellar medium is predicted to be steeper than in earlier models, but appears to be in good agreement with data for both our Galaxy and external galaxies. The models are inconsistent with a cut-off in the star formation rate at low gas surface densities. The absolute magnitude of the disc is given as a function of time in several photometric bands, and radial colour profiles are plotted for representative times.

Origin and structure of the Galactic disc(s)

Abstract: We examine the chemical and dynamical structure in the solar neighbourhood of a model Galaxy that is the endpoint of a simulation of the chemical evolution of the Milky Way in the presence of radial mixing of stars and gas. Although the simulation's star formation rate declines monotonically from its unique peak and no merger or tidal event ever takes place, the model replicates all known properties of a thick disc, as well as matching special features of the local stellar population such as a metal-poor extension of the thin disc that has high rotational velocity. We divide the disc by chemistry and relate this dissection to observationally more convenient kinematic selection criteria. We conclude that the observed chemistry of the Galactic disc does not provide convincing evidence for a violent origin of the thick disc, as has been widely claimed.

Properties of M31 - I. Dust. Basic properties and a discussion about age-dependent dust heating

Abstract: Aims. Observations acquired by the Spitzer Space Telescope and improvements to theoretical modeling of dust emission properties are used to discuss the distribution of dust and its characteristics in the closest neighbor spiral galaxy M31. These data are then used with GALEX FUV, NUV, and SDSS images to study the age dependence of the dust heating process.Methods. Spitzer IRAC/MIPS maps of M31 were matched together and compared to dust emission models allowing us to constrain the dust mass, the intensity of the mean radiation field, the abundance of polycyclic aromatic hydrocarbon (PAH) particles. The total infrared emission (TIR) was analyzed as a function of UV and optical colors and compared to predictions of models that consider age-dependent dust heating.Results. We demonstrate that cold-dust component emission dominates the infrared spectral energy distribution of M31. The mean intensity of the radiation field heating the dust is low (typically , where is the value in the solar neighborhood). Because of a lack of submillimeter observations, the dust mass is only weakly constrained by the infrared spectrum, but we derived a lower limit of M dust ≳ 1.1 107 with a best-fit model value of M dust = 7.6 107 , in good agreement with expectations from CO and HI measurements. Across the spiral-ring structure of M31, we show that a fraction >3% of the total dust mass is in PAHs. UV and optical colors are correlated with the total infrared to far ultraviolet (TIR/FUV) ratios in ~670 pc-sized regions over the disk of M31, although deviating from the relationship between infrared excess and ultraviolet spectral slope (referred as IRX-β relationship) for starburst galaxies. In particular, redder regions have lower values of the TIR/FUV ratio for a fixed color. Considering the predictions of models that account for the dust-heating age dependence, we found that in 83% of the regions analyzed across the 10 kpc ring, more than 50% of the energy absorbed by the dust is rediated at λ > 4000 A and that dust in M31 appears mainly heated by populations a few Gyr old even across the star-forming ring. We also found that the attenuation varies radially peaking close to 10 kpc and decreases more rapidly with radius in the inner regions of M31 than in the outer regions in agreement with previous studies. We finally derived the attenuation map of M31 at 6″/px resolution (~100 pc/px along the plane of M31).

Dust properties in M31.I. Basic properties and a discussion on age-dependent dust heating

Abstract: Context. Aims. Recent observations derived from the Spitzer Space Telescope and improvements in theoretical modeling of dust emission properties are used to discuss the distribution of dust and i ts characteristics in the closest neighbor spiral galaxy M3 1. Together with GALEX FUV, NUV, and SDSS images we studied the age dependenceof the dust heating process. Methods. Spitzer IRAC/MIPS maps of M31 were matched together and compared to dust emission models allowing to constrain the dust mass, the intensity of the mean radiation field, the abundance of Po lycyclic Aromatic Hydrocarbons (PAH) particles. The total infrared emission (TIR) was analyzed in function of UV and Optical colors and compared to predictions of models which consider the agedependent dust heating. Results. We demonstrate that cold-dust component emission dominates the infrared spectral energy distribution of M31. The mean intensity of the radiation field heating the dust is low (typi cally U 3% of the total dust mass is in PAHs. UV and optical colors are correlated to total infrared to far ultraviolet ( TIR/FUV) ratios in∼ 670 pc-sized regions overall the disk of M31, although deviating from the relationship between infrared excess and ultraviolet spectral slope (referred as IRX-β relationship) for starburst galaxies. In particular, redder regions show lower values of the TIR/FUV ratio for a fixed color. Considering the predictions of mo dels that account for the dust-heating age dependence we derived that in 83% of the regions analyzed across the 10kpc ring more than 50% of the energy absorbed by the dust is rediated atλ> 4000A and that dust in M31 appears mainly heated by populations a few Gyr old even across the star-forming ring. We also found that the attenuation is varying radially peaking near 10kpc and decreasing faster in the inner regions of M31 than in the outer regions in agreement with previous studies. We finally derived the attenuation m ap of M31 at 6 ′′ /px resolution (∼ 100 pc/px along the plane of M31). Conclusions.

Physics and Structure of the Galactic disc(s)

Abstract: The model of Schonrich & Binney (2009) offers new ways to understand the chemo-kinematic structure of the solar neighbourhood in the light of radial mixing. The combination of chemical information with rich kinematic data reveals a still hardly explored abundance of interconnections and structures from which we can learn about both the physics and history of our Galaxy. Large upcoming datasets can be used to improve estimates of central parameters, to shed light on the Galaxy's history and to explore the unexpected way of understanding the well-known division of the Galactic disc yielded by the new model.