• Scott Chapman
  • (University Reader)
  • Caitlin Casey
  • (PhD student)
  • Matt Bothwell
  • (PhD student)

    Redshifts measured for over 100 SMGs

  • Caltech Press Release

    A comparable number of radio sources have been identified at z~2, which are similar to Submm Galaxies, except they lack strong submm detections: Are they are hotter `cousins' to the SMG population, doubling the volume density of star forming ULIRGs?

  • ... details on OFRGs


    Distances measured for the most luminous and enigmatic galaxies in the Universe, suggest the epoch of massive galaxy building was at redshift of around 2.5.

      Our team of astronomers based in the US and the UK has for the first time measured the redshifts of a significant sample of puzzling "submillimetre galaxies" (pictured below in Hubble Space Telescope images), discovered by the team in 1997. These are remote galaxies with high redshifts, and are likely to contain huge numbers of young stars heavily enshrouded by dust. They appear as large, distorted, and merging galaxies in Hubble Space Telescope images below.  Because of the time it takes light to travel, they are seen how they were when the universe was only one fifth its present age. Until now the nature of submillimetre galaxies has remained an enigma. Astronomers will detect them in large numbers with the upcoming Submillimetre Common User Bolometer Array 2 (SCUBA) on the 15-m James Clerk Maxwell Telescope (JCMT) located on the 4,000-metre-high volcano Mauna Kea on Hawaii. To date, a few hundred have been identified from facilities like JCMT/SCUBA and AzTEC, IRAM30m/MAMBO, APEX/LABOCA. They appear very bright at millimetre and submillimetre wavelengths but their extreme faintness in the optical and near-infrared parts of the spectrum means that studying them in detail has been difficult. One possibility was that these are galaxies of relatively modest luminosity at similar distances to the optically-bright galaxies that dominate pictures of the extragalactic sky, such as the Hubble Deep Field. Alternatively, they could lie at far greater distances and be intrinsically much more luminous. It has even been suggested that some of the objects might not be galaxies at all but very cold, very faint structures within our own galaxy.

    Hubble Space Telescope images of submm galaxies

    RadioSMG9   radioSMG11   radioSMG13

    To measure the redshifts of submillimetre galaxies, astronomers needed to obtain spectra of their visible light but until recently they had been deterred by the extreme faintness of these objects and the difficulty of pinning down their exact positions. Chapman et al. (2003, 2005) and Swinbank et al. (2004) measured redshifts for a large sample of submillimetre galaxies by using the LRIS-B and NIRSPEC spectrographs on the Keck-I/II 10-m telescopes on Mauna Kea. The extreme blue end of the visible spectrum has ironically proved useful to identify strong emission lines in the spectra of many submillimetre galaxies (which were previously thought to be completely dust obscured). This made it possible to secure accurate redshifts for a statistically significant sample of submillimetre galaxies for the first time and increased more than tenfold the number of submillimetre galaxies with known redshifts. On the basis of this data, it is clear that a typical submillimetre galaxy lies at a high redshift, with a look-back time equivalent to 80% of the age of the universe (their redshift distribution is shown below in red, with models of the radio, submm and quasars overlaid for comparison). That puts them at a much earlier epoch in the history of the universe than optically-bright galaxies seen in deep images of the sky, and their high luminosities suggest that they contain vast numbers of young stars concealed by dust. The total number of stars formed in this population of submillimetre galaxies is comparable to or greater than the numbers of stars in optically-bright galaxies at these epochs. Many of the old stars we see in the universe around us today were probably formed in such galaxies long ago.

    The Nature letter anouncing the first 10 redshift measurements can be found HERE.
    A conference proceeding from an international meeting on star formation in Granada, Spain (Oct.02) can be found HERE.

    Redshift distribution of submm galaxies

    The redshift distribution (68 galaxies) is shown in red, with models of the radio, submm and quasars overlaid for comparison.


    Facilities used to study the SMGs:
    We start from the VLA (Very Large Array) radio telescope in New Mexico to pinpoint the submm galaxies uncovered by the JCMT telescope equipped with the SCUBA camera (located atop Mauna Kea in Hawaii). With precise positions in hand, we go to the Keck observatory and the LRIS-B spectrograph to align the radio positions in the spectrograph slits. The redshifts are measured!!! We proceed to millimeter arrays, such as IRAM Plateau de Bure (France) or the OVRO (Owens Valley Radio Observatory - pictured below) to confirm that molecular gas (as traced by the CO molecule) lies at the optical redshift obtained at Keck.

    VLA The image “http://www.nrao.edu/imagegallery/images/VLASouth_lo.jpg” cannot be displayed, because it contains errors. The image “http://www.jach.hawaii.edu/JACpublic/JCMT/Home_pics/jcmt_outside.jpg” cannot be displayed, because it contains errors.JCMT
    KECK The image “http://www.astro.caltech.edu/mirror/keck/twilightpic.jpg” cannot be displayed, because it contains errors.The image “http://www.ovro.caltech.edu/cmarray.gif” cannot be displayed, because it contains errors. OVRO

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    Last revised: 1st of Oct 2008