Semester: 2004A		Partner reference: Not Available		PI time requested: 3.0 hours
Gemini reference: Not Available		Partner ranking: Not Available		PI minimum time requested: 3.0 hours
Instruments(s): GMOS South		NTAC recommended time: 0.0 nights		PI future time requested: 0.0 nights
Observing mode: queue		NTAC minimum recommended: 0.0 nights		PI total from all partners: 3.0 hours (joint proposals)
Time awarded: Not Available		Proposal submitted to: Australia

SUMSS J1449-6047 is an extreme example of a class of rare radio sources which are being discovered by the new generation of deep radio surveys. Detailed studies of this source and others like it promise to reveal the properties of the intergalactic medium megaparsecs away from the host galaxies, as well as determining the physical or environmental reasons why some AGN can power radio sources which can grow to almost cosmological sizes.

Science Justification

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Giant radiogalaxies (GRGs) are one of the most extreme classes of AGN: the radio cocoons of these sources extend to Mpc distances from the host galaxy. Notably, GRGs provide a unique probe of the intergalactic medium (IGM); optical and radio observations allow us to determine the physical conditions in the lobes of the sources and the environment megaparsecs from the host galaxy (see, e.g., Subrahmanyan & Saripalli 1993).

Historically, GRGs were thought to be extremely rare. They were generally missed from radio surveys in the past because, despite having total fluxes greater than the nominal survey limit, their large angular sizes meant that they fell below the survey surface brightness limit. The largest known source at present, 3C236, was identified nearly 40 years ago, and then only because its compact core was sufficiently bright to be included in the original 3C catalogue. 3C236 is 4.5 Mpc across at z = 0.099 (we assume a WMAP cosmology with h=0.71, Omega_M = 0.27, Omega_Lambda = 0.73)

Recently, however, with the advent of new radio surveys especially sensitive to low surface brightness objects, we have demonstrated that GRGs are relatively common, even out to redshifts approaching z = 1 (Cotter et al. 1996). Extreme objects are always important for testing theories and models, but now it is clear that GRGs are a significant fraction of the AGN population, a detailed investigation into their properties is urgent, so that we may explain why they grow to such extreme size and how they relate to the rest of the population.

* Are they associated with particularly powerful central engines?

* Do they inhabit extremely under-dense environments?

* Are they in fact typical radiogalaxies, but seen at extreme age?

We have recently discovered a radiosource which may prove to be the largest example of a GRG. Our preliminary observations in the radio, optical and near-IR indicate that GMOS-S spectroscopy will allow us to address these questions directly.

In the 1990s, the Molonglo Observatory Synthesis Telescope (MOST) carried out a survey of a 3-degree strip of the southern Galactic Plane at 843 MHz with a resolution of 43'' (Green et al. 1999). This survey, the precursor to SUMSS (Sydney University Molonglo Sky Survey), a survey of the whole southern sky with the MOST, revealed a source with an appearance typical of a powerful radiogalaxy and an angular extent of 25'. We have now obtained a high-resolution radio image of the source with the Australia Telescope Compact Array (ATCA; see Fig. 1). However, SUMSS J1449-6047 lies only one degree from the Galactic plane, making conventional optical investigations difficult.

From our AT observations of J1449-6047, we have obtained an accurate position for the radio core. There is no identification on the Digitized Sky Survey, so we observed the field with CASPIR on the ANU 2.3 m in J and K_n. The radio core is coincident with an extended object (3'' FWHM cf 1.5'' for stars in the field) with integrated magnitude K_n = 15.0 +/- 0.3 (see Fig. 2). From the K-band magnitude-redshift relation, which has a narrow dispersion at redshifts out to z = 1 for all known types of radiosource host galaxy, including GRGs (McCarthy 1993, Eales et al. 1997), we estimate a redshift of 0.2 < z < 0.35. This is supported by the radio flux density and morphology: the source has typical FRII-class structure, so the well-defined FRI/FRII break in radio luminosity sets a lower limit on the redshift of the source of z = 0.24. (In FRI sources, the radio jets are disrupted close to the nucleus and flare to give the source a disturbed structure; in FRII sources the jets remain collimated to the extremities of the source and terminate in compact ``hotspots'' from which the jet plasma flows backward to inflate the radio lobes --- see, for example, Fanaroff & Riley 1974). If this redshift estimate is correct, J1449-6047 is the largest known single object in the universe, with a linear size of at least 5.6, and perhaps 7.4, Mpc, easily outclassing 3C236.

Not only are the radio jets in J1449-6047 well collimated to a distance of ~4 Mpc from the AGN, they are almost exactly the same length in _both_ directions. This suggests that the environment of the source must have an extremely low density; indeed the lobes of the source must essentially be interacting with whatever comprises the IGM on the largest scales. The low-density hypothesis is further supported by the extreme axial ratio (lobe length/width) of the source, which is > 15 compared to the typical value of approximately 6. It seems plausible, therefore, that J1449-6047 may provide not only an example of a GRG which is unequivocally in a low-density environment compared with other classes of AGN, but, tantalizingly, the possibility of a massive galaxy inhabiting a void of size ~10 Mpc.

Scientific goals

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* We wish to obtain an accurate redshift for J1449-6047 using GMOS spectroscopy. We will obtain a spectrum of the host galaxy sensitive enough to obtain an absorption line redshift.

* We will use the pre-imaging of the field to determine if the host galaxy has any companions; we would expect it to lie in a galaxy-poor environement.

* An accurate redshift will allow us to determine the exact size and luminosity of the radiosource, and, since the near-IR light is dominated by emission from the old stellar population, we may use our existing near-IR imaging to measure the stellar mass of the host galaxy (see, e.g., Aragon-Salamanca et al. 1993, Best et al. 1997, Eales et al. 1997).

* An accurate redshift will also allow us to determine the physical size of the host galaxy, which in turn will provide useful information on the dynamical evolution, merger history, and environment of J1449-6047. Of particular interest is the characteristic radius of the host galaxy. Large characteristic sizes are indicative of ``cannibalistic'' histories (see e.g. Hausman & Ostriker 1978), and have been found for the most massive radiosource hosts (e.g. Best et al. 1997). Small characteristic sizes, however, indicate less dense environments, and are found for less massive host galaxies (Roche et al. 1997). By making an accurate measurement of the host galaxy's morphology we may infer whether the environment is indeed extremely rarefied, as suggested by our previous observations.

References

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Aragon-Salamanca, A., Ellis, R., Couch, W., Carter, D., 1993, MNRAS 262,764.

Best, P., Longair, M., Rottgering, H., 1997, MNRAS 295, 549.

Cotter, G., Rawlings, S., Saunders, R., 1996, MNRAS 281, 1081.

Eales, S., 1985, MNRAS 213, 899.

Eales, S., Rawlings, S., Law-Green, D., Cotter, G., Lacy, M., 1997, MNRAS 291, 593.

Fanaroff, B., Riley, J., 1974, MNRAS 167,31P.

Green, A., et al., 1999, ApJS 122, 207.

Hausman, M., Ostriker, J., 1978, ApJ 224, 320.

Laing, R., Jenkins, C., Wall, J., Unger, S., 1994, A.S.P. Conf. Series 54, 195.

Larkin, J., Armus, L., Knop, R., Soifer, B., Matthews, K., 1998, ApJS 114, 59.

McCarthy, P., 1993, ARA&A 31, 639.

Roche, N., Eales, S., Rawlings, S., 1998, MNRAS 297, 405.

Rawlings, S., Saunders, R., 1991, Nature 349, 138.

Subrahmanyan, R., Saripalli, L., 1993, MNRAS 260, 908.

Attachments:

Technical Justification

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Our request to use GMOS to obtain a redshift for this radiogalaxy is driven primarily by the low galactic latitude of the target (b = 1.2 degrees). Targetting the ID from our J- and Kn-band imaging, we obtained an 8000-s spectrum with the RGO spectrograph on the AAT. The spectral range was 5500---9500 Angstrom and the spectral resolution 12 Angstrom. This spectrum revealed that the target is indeed strongly reddened, with the continuum being too faint to obtain a redshift with a 4-m telescope.

In the blue, where strong features such as the Ca H & K lines are expected to lie, we estimate magnitudes of V_AB = 21, B_AB = 21.4 through a 2-arcsecond slit in 1.5--2 arcsecond seeing. We detect no features in the faint continuum. The target is somewhat brighter in the red---perhaps by up to 3 AB-mags by 9000 Angstrom---but again in our AAT observations we have found it impossible to obtain a redshift. This region of the spectrum is close to featureless for E gals; moreover, we have found that the slit is almost completely filled with foreground objects in this crowded field and so subtraction of the many OH and H2O skylines in the red is extremely inaccurate (see Fig. 3). We have found that any absorption feature with an equivalent width of less than about 10 Angstrom is undetectable. Similarly, the crowded field effectively rules out AAT spectroscopy in the IR, where nodding along the slit---onto foreground stars or HII-region emission rather than blank sky---would be necessary for sky subtraction.

We therefore believe that a spectrum taken in the blue with an 8-m class telescope will be essential to obtain a spectroscopic redshift for SUMSS J1449-6047; moreover, as outlined below, this is an observation which could be carried out successfully in less than ideal conditions of seeing and transparency.

Calculation of required observations

------------------------------------

We propose to obtain short images in g and r: first, to determine the optical core position of SUMSS J1449-6047 to sub-arcsecond accuracy and to confirm our spectrophotometric measurement of the galaxy's magnitude, and second, to determine if the galaxy has any companions. These filters straddle the 4000-A break at the estimated source redshift and so companions can be selected by their g - r colour. We propose to image 2 magnitudes fainter than the host galaxy, that is roughly to g = 24 and r=22. This may be done in grey time: we find using the GMOS ITC that a 900-s exposure in g and a 600-s exposure in r are sufficient to obtain S/N of 10 in 50-percentile sky brightness. We do not expect that overheads for acquisition and readout should be more than ten minutes so our total request for pre-imaging is 30 minutes.

We then propose to obtain a single spectrum deep enough to ensure that we can secure an absorption-line redshift. We require a deep spectrum in the blue, so we request dark time.

The low radio luminosity of the source means that forbidden emission lines may be very weak, and perhaps not detectable in a reasonable integration time (see, e.g., Rawlings & Saunders 1991). Therefore our spectrum must detect the absorption features of the rest-frame blue and near-UV, such as the Ca H & K features, the 4000-Angstrom break, and metal absorption bands such as G-band and MgI (for the old stellar populations typically found in z <1 radiogalaxies we do not expect strong Balmer absorption features). We expect equivalent widths of a few Angstrom and velocity widths ~ 100 to 300 km/s, in common with other radiogalaxies at these redshifts. To guarantee an absorption-line redshift from such features we aim for a spectrum with a S/N of 10 per pixel using the 600 line/mm grating and 2x binning in the spectral direction.

From our AAT observations, we estimate a B_AB-band magnitude of 22.5 through a 1-arcsec slit in 0.8-arcsec seeing. Next, we take an E-gal model in the GMOS integration calculator to determine the optimum configuration and exposure time. We find that in 50-percentile cloud, a 7200-s exposure will allow a S/N > 10 at all wavelengths. Estimating a total of 25 mins overheads for acquisition and readouts between individual frames, our total request is 145 minutes.

Observation Details

Resources

• Gemini South
  • g image (GMOS South)
    • Filter
      • g_G0325
  • r image (GMOS South)
    • Filter
      • r_G0326
  • spectroscopy (GMOS South)
    • Focal Plane Unit
      • Longslit 1.00 arcsec
    • Disperser
      • B600_G5323

Observing Conditions

Name	Image Quality	Sky Background	Water Vapor	Cloud Cover
imaging	70%	50%	Any	50%
spectroscopy	70%	20%	Any	50%

Scheduling Information:

Synchronous dates:

Optimal dates:

Impossible dates:

Additional Information

Keywords: Diffuse intergalactic medium, Host galaxies, Jets, Radio galaxies

Publications:

• Mauch, T., et al, 2003MNRAS.342.1117M
• Venturi, T., et al, 2003A&A...402..913V
• Subrahmanyan, R., et al, 2003AJ....125.1095S
• Cotter, G., et al, 2002MNRAS.334..283C
• Haynes, T., et al., 2002MNRAS.334..262H
• Cotter, G., et al., 2002MNRAS.331....1C
• Inskip, K., et al., 2002MNRAS.337.1407I
• Inskip, K., et al., 2002MNRAS.337.1381I
• Bolton, R., et al., 2003astro.ph..2356B
• Buttery, H., et al., 2003astro.ph..2390B

Proposal Contents