Demo Science

GOODS - see ESO Messenger article for excellent intro, at non-specialist astronomer level.

(topic created by AnitaRichards 23.05.2012@09:18:39)

Demo Science

GOODS - see ESO Messenger article for excellent intro. at non-specialist astronomer level.

Here is an attempt at an overview of how the demo might appeal to the audiences, firstly the SWG and other astronomers, secondly the media. It is very incomplete and needs input on:

1. Suitable images
2. Technical aspects (faster bigger ....)
3. Diversity/international nature of data
4. Science drivers from anyone who knows anything about galaxies, cosmology or wavelengths < 1cm.

Astronomers' view

• The HST observed the HDF(N) and HFF using WFPC2 in 1995. Since then observations have been made by
  • CFHT and other ground-based telescopes (wider fields and/or better spectral coverage)
  • VLA+MERLIN (and other radiotelescopes - EVN, WSRT)
  • CHANDRA
  • SCUBA
  • ISO
  • and more focussed observations of individual objects
  • etc. what have i missed, - elaborate above.
• Southern field....
  • ...
  • ...
  • ATCA and VLA data

In 6 yrs since the first data were released, although ?over 100 papers have been published, there are still data to be extracted from the northern field images, and scientific analysis of the results is far from complete. The full picture - understanding changes in the galaxy population with age, locating rare or very distant objects etc., can only be achieved by using data at all frequencies, but the sheer volume of data and the many different formats and conventions make this a very slow process.

There are ~3000 objects in the 4.7 arcmin² of the central WFPC2 triple HDF(N) field, and thousands more in the 8x3 HFF fields. There are nearly 200 ISO sources, and nearly 400 CHANDRA detections. There are ~100 secure detections of radio galaxies within a 10' square, some of which have no obvious optical counterparts, or counterparts too faint to have an obvious optical morphology or redshift.

The AVO will greatly speed up the interpretation of the images, comparison with catalogues and construction of new or extended data sets. This requires harmonising units, resolutions etc. as required as well as astrometric and photometric alignment, and implies that each catalogue and image is supplied with the necessary metadata.

This will achieve new scientific results in several ways including:

• Providing access to existing catalogues of sources and properties via a single interface - as by Vizier, but allowing more operations such as evaluating colours or giving errors on position cross-matches
• Plotting properties such as SEDs from catalogues, and allowing the original image data to be selected interactively
• Providing an interface to image analyise tools in order to
  • re-extract source list according to user criteria and meaure sizes, flux densities etc.
  • measure the flux density in other images at the positions and in apaertures corresponding to the sizes of sources from the first image (thus obtaining homogenous samples and upper limits, or allowing the statistical properties of faint objects to be deduced from stacking)
  • etc.

Example science projects which may be suitable for the demo:

• North: Does the nature of radio emission change with distance?
  • The VLA observations of a region 10' square centred on the HDF(N) are complete to 40 microJy at 2" resolution; the MERLIN+VLA data are complete to 27 microJy (7sigma) at 0".2 - 0".5 resolution. The latter have only been fully analysed to data within the inner 3'x4', but no new radio sources were found. Most of the existing sources were resolved. A few retained compact bright cores and at least one, also detected at 0".025 resolution by the EVN, is at z=4.4. This almost certainly contains an AGN. The local population of bright radio galaxies is dominated by AGN, as are the only three galaxies in the 10' field brighter than 1 mJy at 1.4 GHz. However about 2/3 of the radio galaxies in the HDF(N)/HFF are starbursts. A comparable trend is observed in CHANDRA data.

    Taking the ISO source list and measuring the MERLIN+VLA flux densities in the central 3' field in a suitable aperture at each position reveals a statistically significant excess of 3-7sigma sources. Comparison with optical data shows a similar phenomena at I<24. Downes et al. found that the properties of the SCUBA detections compared with other HDF(N) observations suggests that there is a population of dust-enshrouded starbursts at z= a few. Can we find these in the radio data?

  • The MERLIN+VLA data have been imaged in a region 8'.5 square for the first time for the demo (5x the area previously mapped). It is unlikely there will be many, if any, new >7sigma detections (which would be very compact objects). However we can use catalogues derived from other images to investigate the distribution of faint radio flux (binned by query catalogue properties):
    • At the position of ISO sources;
    • At the position of CHANDRA sources;
    • At the position of optical sources, or subsets chosen for colours indicative of starburst or other nature or by z.
    • At the position of sources with a reasonably secure classification, e.g. by optical type, so that data can be stacked (as done e.g. for CHANDRA data ...), to obtain the radio end of the SED for faint sources of various types.
    • Compare flux density at a single wavelength on different scales (by using different apertures and/or comparing different telescopes e.g. VLA, MERLIN, EVN) is it more compact for objects of a particular type or at high redshift?
    NB
    • In each case it is desirable to select a resolution comparable to the query catalogue, to perform a control measurement with positions offset by ~10".
    • At present we only have the HST and MERLIN+VLA data 'SExtractor ready' so source positions and flux densities from other telescopes e.g. CHANDRA, ISO will have to be taken from catalogues.
    This will reveal whether there is a statistically significant excess of radio flux associated with other emission processes and/or with particular types of source (Starburst etc.), and if so, out to what distance. This can be compared with e.g. X-ray/optical comparisons to see if radio flux is associated with candidate dust-enshrouded starbursts.

  We need to clarify what can be wrapped as part of the demo and what might be done by SWG members etc. exporting demo results and using other packages for further processing.

• South: What are the properties of faint high-z galaxies? (based on a conversation with Richard McMahon^? who can expand/correct misconceptions....)
  • To date, most high-z galaxies (?>4) detected are the brightest or in some other way special (e.g. lensed).
  • The Southern field data can be used to find and classify a homogenous, less flux-limited sample.
    • Use existing SED catalogues to select potential high-z sources (from optical data?) - outliers
    • Use Aladin-based tool to inspect outliers and refine list
    • Re-extract SED from all available data (SExtractor) (at several resolutions, e.g. at full resolution for optical data, repeat at slightly lower resolution to also include CHANDRA, lowest to also include ATCA)
    • Interface with hyperZ or export SEDS to feed to hyperZ to classify galaxies and measure photometric redshift (possibly use low-res, widest wavelength coverage for classification, and then higher-res optical only for z?)
    • Compare results with catalogues for any known classifications/spectroscopic z as checks.
    • Also compare flux at given wavelength at different scales as in northern project
    NB - some considerations, probably off-line from the demo itself:
    • Correction for Malmquist bias needed (drop out of fainter objects at greater distances)? (for both projects?) And K-correction (e.g. the colour ratio indicating a starburst is itself redshifted at high z).
    • Although the limiting flux density is similar in each particular image set (in some cases some loss of sensitivity at large angular separations from pointing centre, or between fields), it is different at different wavebands, so compensation should be attempted based on model SEDs or something.

    We need to clarify what can be wrapped as part of the demo and what might be done by SWG members etc. exporting demo results and using other packages for further processing.

Popular science: Formation and evolution of galaxies

• In the universe today, stars are formed when clouds of gas and dust collapse. A typical galaxy like the Milky Way contains (check) 10^10 stars and ten times that mass or gas and dust (or have I got that the wrong way round).
  • The Milky Way is a spiral galaxy
  • Others are bigger, smaller, elliptical, irregular - does one sort evolve into another?
  • Mergers and interactions
  • Some form stars steadily (like Milky Way - ?1Msun/yr), others have exhausted the raw materials (usually ellipticals) and others (Starbursts) have orgies of star formation and supernovae explosions.
  • Some active galaxies contain more exotic processes - collisions; matter plummeting into a central black hole; gigantic jets
  Optical telescopes show the stars in all their colours, or the average starlight from distant galaxies, and indicate the age of stars. However some Starbursts are so dusty they can only be seen in Infra-red, radio or X-ray emission. Jets of rarified plasma from active galaxies also have characteristic signatures at extremely long (radio) and short (X-ray) wavelengths. The violent processes around black holes produces radiation at every wavelength but the exact proportions give clues to the mass, environment and duration.

• How have galaxies changed with time?
  • The further away a galaxy, the more redshifted its emission, and so multi-colour observations tell us the distance of a galaxy and hence the time since its light left it. Ultra-sensitive observations let us look back in time to when the universe was only a half or even a tenth of its age today.
  • Thus we can see if, when the universe was younger, there were more starburst galaxies? More mergers - were galaxies closer together when the whole universe was smaller? ...
• In addition, there may be unusual galaxies of unknown type...

The GOODS data were taken by ... different ground-based and satellite instruments run by ... countries. Converting the data to formats suitable for comparison and lining everything up is a major operation.

Data come in two forms: the images, and catalogues of galaxies and their properties extraacted from images. But galaxies look different at different wavelengths, and you can miss a very faint object in, say, blue light but it shows up in red or in radio. So you need to compare images and catalogues and plot their properties rapidly.

(Size of data sets... summary of Demo processes...)

So will we find the most distant galaxies? Will we find exotic objects? Starburst galaxies hidden in shrouds of dust? Or will we 'just' get a clearer picture of how different the universe was when it was younger...

(feed in science projects as above..)

Back to AVO Demo details page

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