Details at a distance

Most of my research so far has been on gravitational lenses so on occasion I troll the arxiv for lensing papers.  The paper which caught my eye today was actually posted last week and looks at a distant, lensed star-forming galaxy with the goal of exploring its physical properties.  The authors ask: “[W]hat were the physical conditions inside these galaxies—metallicity, abundance, extinction, stellar effective temperature, electron temperature and density—compared to star-forming galaxies today? How did these physical conditions evolve through episodes of star formation and gas accretion?”

Strong gravitational lensing is an excellent observational method for studying distant galaxies. The lensed background galaxies are magnified (and distorted) by a foreground object, so distant galaxies may be studied in more detail than would otherwise be possible.  For example, I previously discussed a paper which looked at the evolution of the mass-metallicity relation in a sample of lensed galaxies.

In this paper, the authors seek to explore the properties of star-forming galaxies by looking at a lensed galaxy, originally presented by Eva Wuyts & others.  This galaxy has been distorted into a giant arc by the gravitational potential of a foreground galaxy cluster.  They placed a slit over the brightest part of the arc and took an infrared spectra of the galaxy.  Only the average magnification (~17) of the galaxy is known so for now the authors adopt this value, although they anticipate that the selected regions are of higher magnification.  Fortunately, this doesn’t affect most of the results.

Rigby et al. are able to determine properties of this galaxy such as the electron density, various elemental abundances and the star formation rate.  For example, they find that the electron density in this galaxy is not especially higher than densities found in the local Universe.  Very high electron densities have been found in a handful of other lensed galaxies, so this measurement demonstrates the need for further, precise study.  The authors also compare different ways of measuring some of these parameters; in particular they look at the reliability of diagnostics of oxygen abundance.  Several of the methods are extremely sensitive to other parameters so it is hard to gauge their performance.  Some may not be applicable to galaxies at high redshift (and correspondingly, early times) because they were calibrated for nearby galaxies where physical conditions are presumably different.

In this work, only the overall properties of the selected region were studied.  The authors hope to get a bit more detailed and look at the star formation rates, abundances, etc. in different parts of the galaxy.  This requires a better model of the lens, but with high resolution images from HST such work will be possible.

About Elisabeth Newton

Elisabeth was a Harvard graduate student and an astrobites and ComSciCon co-founder and is now a professor at Dartmouth College.

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