The SAURON Deep Field: Investigating the Diffuse Lyman-α Halo of Blob1 in SSA 22
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ING Newsletter No. 7, December 2003

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The SAURON Deep Field: Investigating the Diffuse Lyman-α Halo of “Blob1” in SSA 22

R. G. Bower1, S. L. Morris1, R. Bacon2, R. Wilman1, M. Sullivan1, S. Chapman3, R. L. Davies4, P. T. de Zeeuw5

1: Physics Department, University of Durham. 2: CRAL-Observatoire, Lyon. 3: California Institute of Technology. 4: Dept. of Astrophysics, University of Oxford. 5: Sterrewacht Leiden.

Recent studies of star-forming objects in the early universe, measuring their clustering properties and determining their luminosity functions, have shown that these galaxies are key to understanding the star formation and metal enrichment history of the universe and the role of galactic “super-winds” in regulating the conversion of baryons into stars.

In this article, we describe how, using the SAURON integral field spectrograph, we study the formation of the most massive galaxies in the Universe. The primary target is the bright Ly-α emission line halo in the conspicuous SSA 22 super-cluster at z=3.07–3.11 (Steidel et al., 2000). The highly-obscured very luminous submillimeter galaxy found by SCUBA near the centre of this halo probably is an example of a forming massive elliptical galaxy (Chapman et al., 2001).

Using SAURON, we can map the three-dimensional velocity structure of  the SSA 22 ‘blob1’ halo. This allows us to probe the nature of the ionised gas surrounding the SCUBA source, gaining insight into the origin of the diffuse halo (is it primordial material infalling onto the central object, or material expelled during a violent star burst), the mass of its dark matter halo, and the energetics of any super-wind being expelled from the galaxy. We can also trace the large scale structure surrounding the central source, and investigate whether similar haloes are surrounding other galaxies in the field. The answers to these questions will allow us to understand how galaxy formation is regulated in massive galaxies in the high-redshift Universe. They offer key insight into the “feedback” process and will help explain why less than 10% of the baryon content of the universe ever forms into stars (the “cosmic cooling crisis”; Cen & Ostriker, 1999; Balogh et al., 2001).

This is new ground for the SAURON instrument. Although it was designed to study the dynamics and stellar populations of nearby elliptical galaxies, we will show that it can very effectively be used to study low surface brightness emission features only detectable in long integrations. These observations offer a fore-taste of the deep field observations that can be made with the VIMOS and MUSE integral field spectrographs on 8m telescopes.

The Data-Cube

The SAURON instrument is a high throughput integral field spectrograph (Bacon et al., 2001) that is currently operated on the William Herschel Telescope. It was designed and built by a partnership between Lyon, Durham and Leiden with the main objective of studying the dynamics and stellar populations of early-type galaxies (de Zeeuw et al., 2002). It combines a wide field (41"×33" sampled at 0.95") with a relatively high spectral resolution (4Å FWHM, equivalent to σ = 100 km s–1 in the target rest frame). The instrument achieves this by compromising on the total wavelength coverage, which is limited to 4810 to 5400Å. This spatial and spectral sampling ensure that low surface brightness features are not swamped by read-out noise. However, the limited spectral coverage means that it is only possible to study the Ly-α emission from systems at redshifts between z=2.95 and 3.45. Fortunately, the SSA 22 supercluster lies within this redshift range. The sky background is devoid of strong night sky emission in the Sauron wavelength range. For these observations, the SAURON grating was upgraded with a VPH unit giving an overall system throughput of 20%.

Sauron was used to observe the SSA 22 source for a total of 9 hours, spread over 3 nights in July 2002. The raw data was reduced using the XSauron software. The extraction procedure uses a model for the instrumental distortions to locate each of the spectra, and to then extract them using optimal weighting. The extraction process takes into account the flux overlap between adjacent spectra. To remove small flat-field and sky subtraction residuals, a super-flat was created using the eighteen 30 min individual exposures. This procedure improved the flat field accuracy up to 1% RMS. Each individual datacube was then registered to a common spatial location using the faint star in the south east of the field and then merged into the final data-cube. To produce the map of Ly-α emission, we subtract the continuum, using a low order polynomial fit to the full wavelength range. The end result is a 3-D (x, y, λ) map of the Ly-α emission from the region.


Three dimensional data of this type must be carefully visualised in order to extract the maximum information from the data. We started by creating a colour projection of the data cube shown in Figure 1. In this view, the red, green and blue colour channels have been created from the data in the wavelength ranges 4976.05, 4964.75, 4988.70. Each channel is 5.75Å wide (350 km s–1 in the system rest frame). The image has also been smoothed spatially with a Gaussian of 1" width. We have marked the positions of the Lyman break galaxies, C11 and C15, identified by Steidel et al. (1996) and the location of the sub-mm source identified by Chapman et al. (2003) (see below). The data-cube can alternatively be viewed as a sequence of wavelength slices as shown in Figure 2, or these slices can be combined together to make an animation.

Figure 1
Figure 1. A colour representation of the wavelength shifts of Ly-α emission in the diffuse halo of SSA 22 ‘blob1’. A simple interpretation of the image is that red, green and blue channels represent the red-shifted and blue-shifted motions of the ionised material in the halo. The positions of the two Lyman break galaxies C11 and C15 are marked, along with the position of the submillimeter source (SMM). The area shown is 37"×46". [ JPEG | TIFF ]

Figure 2
Figure 2. A sequence of contour plots showing the changing morphology of the Ly-α emission at different wavelengths. The velocity step between each map is 208 km s–1, with each slice combining a 5.75Å wavelength range so that alternate panels show independent data. Crosses mark the positions of Lyman break galaxies and the submillimeter source. The grid squares have a spacing of 8". [ JPEG | TIFF ]

Many striking structures can be clearly seen in the main halo. The overall width of the emission is very broad (~1500 km s–1 FWHM) but separate emission structures can be identified. If we interpret the wavelength shift as a Doppler shift, the systems differ in velocity by a few hundred km s–1. There is significant velocity asymmetry in the emission region around the Lyman break galaxy C11 and across the main halo. The morphology of the diffuse emission also becomes clear in these velocity slices: particularly interesting is the depression seen near the centre of the halo (this is partially filled by redshifted emission), and the diffuse extension of the halo towards the nearby Lyman break galaxy C15. C15 itself is centred in a separate but much smaller halo. There is a clear E–W velocity shift across this ‘mini-halo’. We discuss each of these features below.

The optical counter-part of the sub-mm source has been identified by Chapman et al. (2003) after detecting the associated CO emission. To locate  the emission relative to the SCUBA source more precisely, we aligned the IFU data cube and the HST STIS image of Chapman et al. using the locations of the alignment star and the Lyman break galaxies C11 and C15. Figure 3 shows the STIS image overlayed with the contours of the total Ly-α emission. This clearly shows the location of the sub-mm source close to the centre of the ‘cavity’ in the emission structure.

Figure 3
Figure 3. A deep STIS image of the SSA 22 ‘blob1’ region showing the position for the SCUBA counterpart (Chapman et al., 2003) relative to the total Ly-α emission (contours). The sub-mm source may lie in a 3-D cavity in the emission (compare contours with Figure 1). The Lyman break galaxies C15 and C11 are marked. Their distinct haloes are clearly seen in the 3-D data set. [ JPEG | TIFF ]


Below we divide our results into the separate features seen in our data and discuss some ideas for how we might interpret them. The interpretation is complicated because Ly-α is a resonant line. Thus shifts in the feature can appear both because of genuine gas motion and because photons diffuse in wavelength to escape from optically thick regions. In what follows we assume that bulk motion is the dominant source of line broadening. Next Steps

These observations clearly demonstrate the ability of deep integral field spectroscopy to detect low surface brightness emission from distant galaxies in the early universe. They give us fascinating insight into the nature and structure of the ionised halo of SSA 22-1. It is interesting to now see how far this powerful new technique can be taken. On the one hand it is fundamental to establish whether the diversity of structure seen in SSA 22-1 is a generic property of other highly luminous sub-mm galaxies, or whether the deep potential well of the SSA 22 super-cluster is necessary to produce emission of this luminosity and extent. It will also be important to determine whether other Lyman break galaxies show mini-haloes similar to C15.

Our observations with the SAURON spectrograph also lay out a path for forthcoming integral fields units. For example, OASIS, an adaptive optics optimised integral field spectrograph, could be used to complement SAURON by studing the higher surface brightness emission line regions in greater detail. The MUSE spectrograph being designed for the VLT will offer the ideal combination of all these instruments providing a combination of wide-field coverage, good spatial resolution and optimal spectral resolution.


We thank the SAURON instrument team for their support of this program, and for creating an instrument with the superb sensitivity of SAURON. RGB acknowledges the support of the Leverhulme foundation.¤


Email contact: Richard Bower (

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