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RSAA News of the Month: May 2004

A Galactic Wind at the Edge of the Visible Universe
RSAA astronomers identify a very active young galaxy at extreme distance

 

Two RSAA astronomers , Dr Heath Jones and Mr Eduard Westra, and former RSAA astronomer Dr Chris Lidman, have shared in the discovery of one the most distant galaxies yet identified. The galaxy is 55,000 Mega-parsecs (Mpc), or 179,390,000,000 light-years (ly), distant in the constellation of Leo.  Astronomers express this large distance as redshift 5.7. The limit of the observable universe is believed to be around redshift 6. According to current cosmology, at redshift 5.7 the Universe was only 1 billion years (1 Gyr) old, less than 10% the age it is now (13.5 Gyr).

The galaxy, provisionally catalogued as S11_13510, is less than 3.2 kpc (10,500 ly) in diameter. It is a much more compact system than our own Milky Way Galaxy which is around 10 times bigger. It also is undergoing a huge burst of star formation, which is producing a large-scale outflow of material from the galaxy, a "galactic wind".

Near Infra-red image of S11-13510
taken with the FORS2 instrument on the ESO VLT at Paranal Observatory, Chile.
The image was taken as part of the pre-imaging for the spectroscopy run.

S11-13510 was found as part of the "Wide Field Imager Lyman-Alpha Survey" (WFILAS), a massive effort involving 65 nights of observations spread over 3 years, and people in four different continents. The observations were conducted with the European Southern Observatory's (ESO) Wide Field Imager camera on the ESO/MPI 2.2 m telescope at La Silla Observatory, Chile, and followed-up with the FORS2 spectrograph on the Very Large Telescope (VLT) at ESO's Paranal Observatory in Chile. Spectroscopic follow-up confirms that the target is a distant galaxy and gives its redshift.

The galaxy was found through imaging using three specially-designed narrowband colour filters and a medium band filter which covers all of the bands covered by the narrow band filters. ANU PhD student Eduard Westra examined deep narrowband images (over 40000 sec exposure per image), each containing many tens of thousands of galaxies. He searched for galaxies that peak in brightness in one of the colours. He then checked to see if the galaxies were absent on broadband B and R images. Selected objects were then followed-up spectroscopically.

Discovery images of S11_13510
Note that the galaxy is visible on the medium and narrow band #2 images,
but does not show on the blue and red images.

Spectra of the galaxy gave two important results. First, having positively identified the Lyman-alpha line in the spectrum, the group was able to calculate the redshift. Lyman-alpha is the brightest line of Hydrogen in the ultra-violet (UV) region of the spectrum, but at these redshifts it is moved way up into the near Infra-red. Secondly, the line was very broad and asymmetric. This indicates the presence of a large-scale outflow.

S11_13510 is the most-distant clear example of a large-scale galaxy outflow to date. The Lyman-alpha line indicates an outflow of around 250 km/sec away from the central galaxy. Galactic outflows of this kind are expected to be common in the early universe. They are driven by the starbursts observed in actively star-forming galaxies. UV radiation from newly-born hot young stars causes the spectral line emission. The starburst wind drives an expanding shell of neutral hydrogen around the galaxy which absorbs the Lyman-alpha photons on the blue (approaching) side, while backscattering those on the red (receding) side. The result is the sharp cut-off on the blue side of the line profile and a long redward tail, giving the asymmetric line profile we observe.

Plots of the Lyman-alpha line of S11_13510
The broad and asymmetric nature of the line is evident.
The top plot shows the data from the FORS2 instrument, the bottom one is a theoretical model for a 250km/sec outflow.
The agreement between the observed and theoretical lines is extremely good.

The study of galaxies and QSOs at this redshift is very important because it is thought that these redshifts are where the first stars and galaxies formed. Discovering sources at these redshifts allows us to directly observe this important phase in the history of the Universe.

The WFILAS team have surveyed a large area of 0.8 square degrees, tripling the volume of comparable surveys to date. They plan to:

1. confirm the high-redshift nature of the sources,
2. measure line fluxes and hence star-formation rates in these galaxies, and
3. determine whether it is many small, actively-star-forming galaxies (like this one) that are responsible for reionizing the universe at redshift 6, or if it was other more powerful sources like QSOs and/or AGN.

The WFILAS team are:

• Heath Jones (RSAA, ANU)
• Eduard Westra (RSAA, ANU)
• Chris Lidman (former Stromlo student, now at ESO Chile)
• Emanuela Pompei (ESO Chile)
• Leo Vanzi (ESO Chile)
• Thomas Szeifert (ESO Chile)
• Ramana Athreya (National Centre for Radiophysics, Pune, India)
• Klaus Meisenheimer (MPIA Heidelberg)

Westra is working on aspects of the WFILAS project as part of his PhD thesis at the ANU.

For previous Monthly News items, click here.