Galaxies Under the Cosmic Microscope: a GMOS-North Study of the Lensed Galaxy #289 in A 2218

The deflection of light by the deep gravitational potential of galaxy clusters can be harnessed to greatly increase the effective collecting area of astronomical telescopes.  Mark Swinbank (Ph.D. student at the University of Durham, U.K.) and his international team used Gemini's Multi-Object Spectrograph with Integral Field Unit (GMOS-IFU) on The Frederick C. Gillett Gemini Telescope (Gemini North) to exploit the rich galaxy cluster A 2218.  They used A 2218 as a magnifying glass to observe distant background galaxies at a comparable level of detail to galaxies at redshift z ~ 0.1.  This work is part of Mark's  Ph. D. project––to explore the Tully-Fisher relation and chemical abundance gradients in distant young galaxies.

Figure 1. Optical image of the massive galaxy cluster A 2218 with the lensed galaxy studied by Gemini indicated with the yellow arrow. This image was obtained with the Hubble Space Telescope.

Swinbank et al., report on their first result of spatially-resolved [OII]3727 nebular gas emission from a lensed galaxy at a redshift z=1.034. This corresponds to an epoch half the present age of the universe. Using a detailed model for the cluster internal mass distribution, taking into account both visible and dark matter, they were able to correct the apparent distorted shape of the galaxy for the lensing by the cluster. The researchers reconstructed the source morphology and derived the velocity field of the interstellar gas in this distant object (Figures 2 and 3).

In addition to distorting images of background objects, gravitational lenses amplify their intensity several times. In the case of the #289, the overall light magnification is a factor of 4.92+/-0.15. This leads to a rest-frame absolute I-band magnitude of MI ^rest = - 22.4+/-0.2, which makes the #289 galaxy something of an equivalent of our neighbor M31 undergoing a relatively intense star forming episode.

Figure 2. On the left is the arc in A 2218 generated by combining three drizzled frames using HST WFPC2 (I, V and B bands). On the right is the reconstructed image of the arc corrected for lens amplification.

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Figure 3. On the left is the GMOS-IFU [OII]3727 emission in shaded blue (scaled for intensity) with the derived velocity field in the galaxy rest frame superposed. The lines, in km/s, map regions of same velocity in the rest-frame of the galaxy.  On the right panel, the isovelocity contours are shown with the image distortion corrected.   The background is not color-coded.  Blue is approaching and red is receding.  Scales are in arcseconds.

The rotational circular velocity, corrected for the inclination in the plane of the sky, is 206+/-18 km/s (see rotation curve in the inset of Figure 4). The galaxy lies very close to the mean Tully-Fisher relation of present day spirals (Figure 4). The Tully-Fisher relation is a relationship that exists between a spiral galaxy's luminosity and its maximum rotation velocity. The behavior of the relation favors a clear preference for hierarchical formation model indicated by the apparent small change in the I-band magnitudes over a large range of redshifts, which is illustrated by arc #289.

Figure 4. The red dot on the left panel plot shows Arc #289 on the rest frame B-Band Tully-Fisher relation, compared to high redshift (z = 0.83) field galaxies and other data sets. The inset shows the rotation curve derived from the [OII]3727 line GMOS-IFU observations; horizontal error bars are ~0.7 arcsec, seeing transformed to source frame. The right is the I-band Tully-Fisher relation (with Arc #289 as the red dot) from the references indicated in the panel.

Gravitational optics provides a powerful tool to study the nature of mass assembly, chemistry and star forming activity in galaxies at early epochs. The methods that have been developed in this Gemini-based study show the potential of IFU observations of gravitational lensed galaxies as a means of studying the spatially resolved properties of high redshift galaxies in a remarkable level of detail.

For more details, see the original paper by A. M Swinbank, J. Smith, R. G. Bower, I. Smail, R. S. Ellis, Graham P. Smith, J.P. Kneib, M. Sullivan & J. Allington-Smith, "Galaxies under the microscope: a GMOS study of the lensed disk-galaxy #289 in A 2218," The Astrophysical Journal, November 20, 2003.

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