Friday 15 October 2010

Title: Gas accretion as the origin of chemical abundance gradients in distant galaxies


In Cresci et al. (arXiv: 1010.2534) they find 'inverse' metallicity gradients for three rotationally supported star-forming galaxies at z=3. They conclude that the central gas has been diluted by the accretion of promordial gas, as predicted by 'cold flow' models. In Jones et al. (arXiv: 1010.1538) they find a 'normal' metallicity gradient for a lensed z=2 star-forming galaxy. While the physical gradient is considerably steeper than that observed in local galaxies, in terms of the effective radius at that epoch, the gradient is similar. They conclude that subsequent growth occurs in an inside-out manner with the inner metallicity gradient diminished over time due to radial mixing and enrichment from star formation.

MEASUREMENT OF A METALLICITY GRADIENT IN A Z = 2 GALAXY:

Jones et al. present near-infrared imaging spectroscopy of the strongly-lensed z = 2.00 galaxy SDSSJ120601.69+514227.8 (‘the Clone arc’).



FIG. 1: (Left) Hubble Space Telescope color composite image of the Clone arc. A1-5 represent multiple images of the z=2.00 source; the critical curve is shown in red. The OSIRIS pointings are indicated by the two green rectangles offset by 1.8 arcseconds in the East-West direction. Foreground lensing galaxies are labeled as G1-4. (Right) Distribution of the key emission line fluxes in multiple images A3-5 in units of 10−18 erg/s/cm2 (see text for discussion of optimum samplig). North is up and East is to the left. The critical line passes through the A3 and A4 components such that a small part of the arc (including A5) is imaged 4 times in the OSIRIS field. The remainder of A3/A4 is imaged twice in the OSIRIS field.



From these and other results they conclude:

We find a strong radial gradient in both the [Nii]/H and [Oiii]/H ratios indicating a metallicity gradient of −0.27±0.05 dex kpc−1 with central metallicity close to solar. This suggests that subsequent growth occurs in an inside-out manner with the inner metallicity gradient diminished over time due to radial mixing and enrichment from star formation.