Radio Galaxies at High Redshifts Bruce Partridge, Haverford College

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Radio Galaxies at High RedshiftsBruce Partridge,
Haverford College

• Outline
• 1. the problem of redshifts
• 2. source counts as an answer
• 3. evidence for evolution of n or L, or both
• 4. faint source counts a new population
• 5. S (21 cm) 10-2.34 S (100?) Why?
• 6. radio photometric redshifts
• other radio measures of redshift

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Measuring redshifts in radio

• Need a line 21 cm useful, but not in pure
  synchrotron sources
• and a technical remark most radio receivers
  have very narrow ? coverage
• Finding zs generally left to optical follow-up
  (e.g., 3C48, the first QSO)
• Without redshifts, cant use radio sources for
  standard ?-z or mag-z cosmological tests
• So Ryle suggests N(m) or N(S) (not N(z)) source
  counts

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Radio Source Counts

• Simply count number N brighter than S
• In static, Euclidean space, for n sources per
  Mpc3
• each of luminosity L, the number seen with S? gt
  S
• is N(gtS) ? S-3/2 since N ? d3 and S ? d -2
• In curved space V ? 4/3? d3 and A ? 4? d2
• so N(gtS) depends (slightly) on curvature
• especially at faint (large d) end

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• Observed Source Counts
• (e.g., Ryle, Ann. Rev. 6, 1968)
• Fit no cosmological model
• Unless evaluation assumed
• L or n or both higher in past
• So counts cant test cosmology
• but do require evolution

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• Normalized differential source counts
• Differential counts dont smooth over features
• Show dN/dS ?S-5/2 for uniform distribution in
  Euclidean space

Then normalize by dividing by S-5/2 So we see
departure from uniform distrib.
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(Normalized) Source Counts at 1.4 GHz
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Extending Counts to Fainter Sources

• Bulge dies away
• - suggests a concentrated era of
• radio source formation at z 2
• At faint end, return to Euclidean
• slope
• suggests a new, local,
• population of sources
• starbursts, not AGN
• (Windhorst, 1984)
• (8 GHz counts from VLA work)

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21 cm counts
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Spectral Properties

• If a new population appears, spectra may be
  different
• -- So check counts at increasing frequency

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Bulge decreases Note absence of counts at ?gt 5
GHz
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Predicting Counts at Even Higher Frequencies

• Crucial to estimates of foreground contamination
  of CMB measurements
• Not safe to extrapolate from low frequency
  measurements
• spectral shape changes
• GPS sources as one example
• also see concave spectra
• Cluster galaxies (work with
• Y.-T. Lin Khadija el Bouchefry)

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Partial Counts at 30 GHz
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Spectral Properties (1)
From Tucci et al.
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• High freq. (22-43 GHZ)
• vs low freq (5-8 GHZ)
• For cluster galaxies
• Work with Lin
• El Bouchefry

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Spectral Properties (2)

• Spectral index dis-
• tribution changes
• Cores (with flat spectra)
• start to dominate
• Sources with flat
• spectra at low ? begin
• to turn over
• Smoothes distribution
• of spectral indices
• Magnitude of effect in
• dispute AT20G will
• resolve

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Convergence of Counts?

• If N(gtS) ? S-3/2, then NxS (surface brightness)
  diverges as S ? 0
• So counts must converge (slope gt -1) at some
  point to avoid distorting CMB spectrum
• Haarsma and Partridge (1998) show convergence by
  2 ?Jy at 21 cm (corresponding to 3 x 108
  sources/ster)
• apply to convergence of FIR sources

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Radio/FIR Correlation (Helou et al., 1985)

• Tight correlation of 21 cm and 100? fluxes
• S(21 cm) 10-2.34S (100?) (Helou et al., 1985)
• Holds for ordinary spirals, starburst systems,
  ULIRGs etc.
• Why?
• S(21 cm) dominated by synchrotron (from SNR)
• S(100?) from warm dust heated by OB stars
• This correlation implies N(OB stars)/N(Mgt8Mostars)
  is constant

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Radio/FIR Correlation

• From VLA observations
• of ULIRGs (Crawford et al. 1996)

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• If correlation assumed,
• S(21 cm)/ S(100?) varies smoothly with z
• And S(21 cm)/S (850?) is a rough but
• useful redshift measure (Carilli and Yun, 1999)
• K correction for radio sharply negative (S drops
  with z )
• Opposite for submm
• For a source with S(21 cm) 10-2.34 S (100?),
  and S ?-.7 near 21 cm and ?3 at 300?, at what z
  does S(21 cm) S(850?)?
• This rough method less useful for high zboth 21
  cm and 850? fluxes have k correction of some
  sign.

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Carilli Yun (1999) Photometric Redshift
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• CO lines can give precise z
• if rough z is known.
• e.g., CO 1-2 rotational line
• ? 2.64 mm
• or CO 3-2, ? 0.87 mm
• Note precision of
• redshift measure
• Radio photometry CO lines can in principle
  pin down redshifts.

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Example (from Frayer et al., 1999)

• CO lines
• Freq.
• slices

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• To answer a question about the brightness of the
  radio sky

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?-ray to radio extragalactic background emission
CMB 960 nW m-2 sr-1