We present the results of a comprehensive Spitzer survey of 70 radio galaxies across 1 < z < 5.2. Using IRAC, IRS and MIPS imaging we determine the rest-frame AGN contribution to the stellar emission peak at 1.6μm. The stellar luminosities are found to be consistent with that of a giant elliptical with a stellar mass of 10^(11−12)M_⊙. The mean stellar mass remains constant at 10^(11.5)M_⊙ up to z = 3 indicating that the upper end of the mass function is already in place at redshift 3. The mid-IR luminosities imply bolometric IR luminosities that would classify most sources as ULIRGs. The mid-IR to radio luminosity generally correlate implying a common origin for these emissions. The ratio is higher than that found for lower redshift, i.e. z < 1, radio galaxies

de Breuck, C.

De Breuck, F.

Dey, A.

Dickinson, M.

Eisenhardt, P.

Fosbury, R.

Lacy, M.

McCarthy, P.

Miley, G.

Rettura, A.

Rocca-Volmerange, B.

Röttgering, H.

Seymour, N.

Stern, D.

Teplitz, H.

van Breugel, W.

Vernet, J.

Vernet, L.

Zirm, A.

Caltech Authors - Main

The 2nd Spitzer Conference: Infrared Diagnostics of Galaxy Evolution
ASP Conference Series, Vol. 381, c©2008
Ranga-Ram Chary, Harry I. Teplitz, Kartik Sheth, eds.
Spitzer Observations of High Redshift Radio Galaxies
N. Seymour,1 D. Stern,2,1 C. De Breuck,3 J. Vernet,3 R. Fosbury,3 A.
Rettura,4,3 A. Zirm,5 B. Rocca-Volmerange,4 M. Lacy,1 H. Teplitz,1 A.
Dey,6 M. Dickinson,6 W. van Breugel,7 G. Miley,8 H. Ro¨ttgering,8 P.
Eisenhardt,2 P. McCarthy,9, F. De Breuck3 and L. Vernet3
1Spitzer Science Center, California Institute of Technology, Mail Code
220-6, 1200 East California Boulevard, Pasadena, CA 91125
2 Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, CA 91109
3 European Southern Observatory, Karl Schwarzschild Strasse, D-85748
Garching, Germany
4 Institute d’Astrophysique de Paris, 98bis Bd Arago, 75014 Paris,
France
5 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,
MD 21218
6 National Optical Astronomy Observatory, Tucson, AZ 85719
7 Institute of Geophysics and Planetary Physics, Lawrence Livermore
National Laboratory, Livermore, CA 94551
8 Leiden Observatory, University of Leiden, P.O. Box 9513, 2300 RA
Leiden, Netherlands
9 Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA
91101
Abstract. We present the results of a comprehensive Spitzer survey of 70
radio galaxies across 1 < z < 5.2. Using IRAC, IRS and MIPS imaging we de-
termine the rest-frame AGN contribution to the stellar emission peak at 1.6µm.
The stellar luminosities are found to be consistent with that of a giant elliptical
with a stellar mass of 1011−12M⊙. The mean stellar mass remains constant at
1011.5M⊙ up to z = 3 indicating that the upper end of the mass function is
already in place at redshift 3. The mid-IR luminosities imply bolometric IR
luminosities that would classify most sources as ULIRGs. The mid-IR to radio
luminosity generally correlate implying a common origin for these emissions.
The ratio is higher than that found for lower redshift, i.e. z < 1, radio galaxies.
1. Introduction
Classical radio galaxies are the quintessential type II active galactic nuclei (AGN):
accreting super-massive black holes that have their continuum emission in the
UV/optical/NIR absorbed by dust, thus primarily giving them the appearance
of normal star-forming galaxies at these wavelengths. The main evidence that
they host super-massive black holes comes from the high luminosities of their
radio ‘lobes’ that are fed by radio jets originating from the host galactic nuclei
(Rees 1978). The lobe spatial extents (tens of mega-parsecs) and the lumi-
398
High Redshift Radio Galaxies 399
nosities (L1.4GHz ≥ 1025WHz−1) rule out emission from star-formation. To
be more precise radio galaxies are Faranoff-Riley type II objects with edge-
brightened radio lobes. In terms of the orientation unification scheme for AGN
(Antonucci 1984) radio galaxies are analogous to radio loud quasars obscured in
the UV/optical/NIR.
Due to their large radio luminosities, radio galaxies were the predominant
way to probe the distant universe until the advent of ultra-deep optical surveys
in the last decade. In fact, radio galaxies were the first galaxies to be found
above redshifts 1, 2, 3 and 4 (e.g., Lacy et al. 1994). Since their first discovery
it has been known that the optical hosts of luminous radio sources are primarily
giant elliptical (gE and cD) galaxies (Matthews et al. 1964). In the more dis-
tant universe, indirect evidence that this association remains intact comes from
the detection of normal elliptical host galaxies with r1/4 law light profiles in
HST/NICMOS observations of high-redshift radio galaxies (HzRGs) at 1∼<z∼<2
(Pentericci et al. 2001; Zirm et al. 2003); the tendency for HzRGs to reside in
moderately rich (proto-)cluster environments (Venemans et al. 2002; Stern et
al. 2003); the spectacular (>100 kpc) luminous Lyα haloes seen around several
sources, implying large gas reservoirs (Reuland et al. 2003; Villar-Mart´ın et al.
2003); sub-mm detections of HzRGs, implying violent star formation activity up
to ∼100M⊙yr−1 (Archibald et al. 2001; Reuland et al. 2004); and a few direct
kinematic measurements of HzRGs (Dey & Spinrad 1996). The most compelling
evidence of this association of HzRGs with the most massive systems, however, is
the tight correlation of the observed near-infrared Hubble, or K− z diagram for
powerful radio sources (De Breuck et al. 2002, Rocca-Volmerange et al. 2004):
HzRGs form a narrow redshift sequence which traces the envelope of radio-quiet
galaxies and is well-modeled by the evolution of a stellar population formed at
high redshift from a reservoir of 1012M⊙. With the more recent discovery that
the stellar bulge and central black hole masses of galaxies are closely correlated,
it is no longer a surprise that the parent galaxies of the most powerful radio
sources occupy the upper end of the galaxy mass function (Maggorian 1998;
Tremaine 2002).
The peak of the stellar emission at 1.6µm of elliptical galaxies has been
found to be a reasonably robust measure of the stellar mass for old passively
evolving systems. The mass-to-light ratio at this wavelength does not vary
greatly for ages ∼> 1Gyr. The Spitzer Space Telescope now allows us to observe
this feature in distant sources in the rest frame. In particular the four bands
of the IRAC instrument and the IRS 16µm peak-up imager straddle the rest
frame 1.6µm flux density at 1 ≤z≤ 5.
1.1. Sample Selection
In order to investigate the formation and evolution of the most massive galaxies
we have performed a Spitzer survey of 70 HzRGs in GO cycle 1. These HzRGs
have also been carefully chosen to cover the full range of redshifts from z = 1
to the redshift of the highest known radio galaxy (z = 5.2) and two orders of
magnitude in radio luminosity, preferentially selecting targets with supporting
data from HST , Chandra and SCUBA/MAMBO. By covering this parameter
space, any trends with radio luminosity or redshift should be apparent. The
observations consist of photometry in eight bands from the three instruments
400 Seymour et al.
Figure 1. (Left) Postage stamp images of 4C23.56 at z=2.48: the 4 IRAC
bands (top row - wavelength increasing left to right) followed by the IRS 16µm
peak-up image and the MIPS bands (bottom row) showing a clear detection
in each waveband out to 70µm (Right) SED fitting of 4C23.56 Spitzer data
using an elliptical galaxy template and black-bodies of various temperatures.
aboard Spitzer, exercising the full complement of imaging capabilities (IRAC:
3.6, 4.5, 5.8 and 8µm; IRS: 16µm; MIPS: 24, 70 and 160µm). Due to uncertainty
in the ability of MIPS to image against the Galactic infrared background at the
time of submission of GO Cycle 1, we chose only to image 26/70 HzRGs with
MIPS. The other 54 sources have been applied to be observed in Spitzer Cycle 3.
The IRS images are only for the 46 objects above z = 2 as below this redshift the
8µm IRAC channel adequately covers the longward side of the 1.6µm bump.
The ‘SHizRaG’ team keep track of this project through a private webpage
which we intend to make public eventually. Currently a restricted version of the
webpage is available here:
http://spider.ipac.caltech.edu/staff/seymour/SHizRaGs.html
2. Stellar Luminosities and Masses
2.1. SED Fitting
In order to determine the 1.6µm stellar luminosity, the contribution at this wave-
length from hot, AGN heated dust needs to be ascertained. We have performed
this analysis for the sample of 17 MIPS HzRGs which have 24µm detections as
well IRAC observations. For the modeling presented here we have chosen to use
just the Spitzer data. The principle reason is that shorter wavelengths, i.e. the
rest-frame optical, are more likely to have significant contributions from young
stellar populations, emission lines, and AGN continuum.
We model the observed 3.6 to 24µm data using an elliptical stellar template
and three black-bodies. The elliptical stellar SED is taken from the PE´GASE
(Fioc & Rocca-Volmerange 1997) software and corresponds to a passively evolv-
ing 1012M⊙ stellar population formed at z = 10; i.e. for each redshift a template
of the correct age was chosen although the SED does not evolve significantly af-
ter 1Gyr (i.e. z ∼ 4.4). The three black-bodies where chosen to be at 60K
(analogous to the temperature of cold dust found from sub-mm observations),
250K and 600-1200K. This last value was allowed to vary and the best tem-
perature from the fitting applied. Full details of this modeling are presented in
Seymour et al. (in prep.).
High Redshift Radio Galaxies 401
Figure 1 illustrates the Spitzer observations and the SED modeling for one
representative source, 4C23.56 at z = 2.48. The best fit hot dust temperature
is 750K and one can see that the 1.6µm stellar peak has considerable AGN
contamination. A more detailed analysis of the broad-band, X-ray to radio SED
of 4C23.56 is presented in De Breuck et al. (in prep.).
The other 53 HzRGs with no MIPS detections can at least provide upper
limits to the stellar luminosities and, in some cases only the upper limits come
from detections only in IRAC channels 1 and 2. So we fit a maximum elliptical
template SED to the IRAC data. In some case the elliptical template fits quite
well to IRAC channels 1-3 and in others the SED is steeply rising at the longer
wavelengths and the fit is restricted only by channel 1. In the former case the
we calculate a ’nominal’ stellar mass from the fit, but in the later we may only
derive upper limits.
2.2. Results
The derived rest-frame, AGN-subtracted, 1.6µm stellar luminosities are shown
against redshift in Fig. 3 (top) for all our HzRGs. Also laid on the plot by
a solid line is the stellar luminosity of a 1012M⊙ elliptical galaxy. The stellar
luminosities imply stellar masses in the range 1011− 1012M⊙ with a mean mass
of ∼ 1011.5M⊙ (Fig. 3 bottom). This mean mass remains consistent out to z = 3
(beyond which the parameter space becomes less well sampled) suggesting that
the upper end of the mass function is already in place by at least z = 3.
3. Mid-IR luminosities
Figure 3 shows the rest 6.75µm luminosity against the rest 3GHz luminosity
for the 18 radio galaxies with MIPS observations. The wavelength of 6.75µm
was chosen as a fiducial mid-IR wavelength as it is the mean rest wavelength of
the observed MIPS 24µm band for our sample and also the wavelength of the
LW2 filter from ISOCAM, allowing a direct comparison of derived luminosities.
It is clear to first approximation that the two luminosities correlate and by
implication have a common origin. This correlation makes sense if the radio
luminosity comes from lobes induced by a jet from the AGN and the mid-IR
comes from hot AGN-heated dust.
The mid-IR luminosities are also all greater than 1011L⊙ implying bolomet-
ric luminosities on ULIRG scales assuming local relations hold for these objects.
These radio galaxies tend to have a higher mean mid-IR to radio luminosity ra-
tio then those selected at lower redshift, e.g., Ogle et al. (2006) finds that z < 1
radio galaxies tend to have Lmid−IR/Lradio ∼ 10− 100.
4. Conclusions and Future Work
We have presented a stellar-luminosity/redshift relation of HzRGs, a more phys-
ical representation of the K−z diagram. This distribution seems to confirm the
long held paradigm that radio galaxies are hosted by massive ellipticals out to
high redshifts, and that the most massive galaxies are already in place by red-
shift 4 and possibly earlier. We also observe a correlation between the infrared
and radio luminosities which is unsurprising if they are both fueled by the AGN.
402 Seymour et al.
Figure 2. (Top) HzRG Stellar luminosities plotted against redshift of each
HzRG. The solid line indicates a of 1012M⊙ elliptical galaxy. (Bottom)
Stellar masses from the SED fitting plotted against redshift. Most stellar
luminosities indicate with stellar masses of 1011−1012M⊙. Stellar luminosities
and masses derived from sources with MIPS 24µm detections are circled, the
dots with out circles are ’nominal’ masses from just IRAC data. Downward
arrows indicate upper limits.
High Redshift Radio Galaxies 403
Figure 3. Rest mid-IR luminosity against rest radio luminosity which ap-
proximately correlate implying a common origin. This correlation may be
explained by the AGN producing the radio jets and also heating the hot dust
radiating in the mid-IR. The mid-IR/radio ratio increases at higher redshifts
compared to low redshift samples (e.g., Ogle et al. 2006) most likely due to
the hotter temperatures of the AGN-heated dust.
404 Seymour et al.
Current on-going work includes mm/sub-mm observations with SCUBA,
MAMBO and the CSO to constrain the cold dust component at longer wave-
lengths and hence estimate the mass of this cold dust. Over-densities of sources
around radio galaxies are being investigated to look for evidence of cluster for-
mation (Zirm et al. in prep.). Over half the sources with 24µm images have
over-densities of factors of 2-5 greater than that expected from 24µm source
counts. These radio galaxies mainly lie at 1.5 < z < 2.5 where the strong
6 − 8µm PAH feature passes though the 24µm MIPS band, enhancing the
24µm flux density.
Acknowledgments. We thank the LOC for organising a great conference
and were particularly impressed by the nifty design of the webpage. We also
liked Harry’s conference haircut.
References
Antonucci, R. 1993 ARA&A, 31, 473A
Archibald, E. et al. 2001, MNRAS, 323, 417
Blandford, R. & Payne, D. 1982, MNRAS, 199, 883
De Breuck, C. et al. 2002, AJ, 123, 637
De Breuck, C. et al. in prep.
Dey, A. & Spinrad, H. 1996, AJ, 459, 133
Lacy, M. et al. 1994, MNRAS, 271, 504
Matthews, T, et al. 1964, ApJ, 140, 35
Pentericci, L. et al. 2001, ApJS, 135, 63
Rees, M. 1978, Nature, 275, 516
Reuland, M. et al. 2003, ApJ, 592, 755
Reuland, M. et al. 2004, MNRAS, 353, 377
Rocca-Volmerange, B. et al. A&A, 415, 931
Seymour, N. et al. AJ, in prep.
Stern, D. et al. 2003, AJ, 125, 2759
Venemans et al. 2002, ApJ, 596, L11
Villar-Mart´ın, M. et al. 2003, MNRAS, 346, 273
Zirm, A., Dickinson, M. & Dey, A, 2003, ApJ, 585, 90


Spitzer Observations of High Redshift Radio Galaxies

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Spitzer Observations of High Redshift Radio Galaxies

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