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The Spectral Energy Distribution of Galaxies Proceedings IAU Symposium No. 284, 2011 R.J. Tuffs & C.C.Popescu, eds.

c 2011 International Astronomical Union DOI: 00.0000/X000000000000000X

No Evidence for Evolution in the Far-Infrared-Radio Correlation out to z 2 i n t h e E C DF S
Minnie Y. Mao1,2,3,4 , Minh T. Huynh2,5 , Ray P. Norris3 , Mark Dickinson6 , Dave Frayer7 , George Helou2 and Jacqueline A. Monkiewicz8
School of Mathematics and Physics, University of Tasmania, Private Bag 37, Hobart, 7001, A u s t r a lia 2 Infrared Processing and Analysis Center, California Institute of Technology, Pasadena CA 91125 3 CSIRO Astronomy and Space Science, PO Box 76, Epping, NSW, 1710, Australia 4 Australian Astronomical Observatory, PO Box 296, Epping, NSW, 1710, Australia 5 International Centre for Radio Astronomy Research, M468, University of Western Australia, Crawley, WA 6009, Australia 6 National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719, USA 7 National Radio Astronomy Observatory, PO Box 2, Green Bank, WV 24944, USA 8 School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA email: minnie.mao@csiro.au Abstract. The Far-Infrared Radio Correlation (FRC) is the tightest and most universal correlation known among global parameters of galaxies. Here we present the results of our investigation of the 70 µm FRC of starforming galaxies in the Extended Chandra Deep Field South (ECDFS) out to z > 2. In order to quantify the evolution of the FRC we used both survival analysis and stacking techniques, which gave similar results. We also calculated the FRC using total infrared luminosity and rest-frame radio luminosity, qTIR, and find that qTIR is constant (within 0.22) over the redshift range 0 - 2. We see no evidence for evolution in the FRC at 70 µm, which is surprising given the many factors that are expected to change this ratio at high redshifts. Keywords. galaxies: evolution, galaxies: formation, infrared: galaxies, radio continuum: galaxies
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1. Intro duction
The correlation between the far-infrared (FIR) and radio emission for star-forming galaxies in the local Universe was first observed by van der Kruit (1973). The correlation is linear, spans five orders of magnitude of bolometric luminosity and has been shown to hold for a wide-range of Hubble types (e.g. Helou, Soifer & Rowan-Robinson 1985). The far-reaching nature of the FRC has made it a valuable diagnostic and astronomers have used it to identify radio-loud AGN, define the radio luminosity/SFR relation and, at higher redshifts, estimate distances of sub-mm galaxies with no optical counterparts. Consequently it is of great importance to determine whether the FRC holds at high r e d s h ift s . The FRC may fail at higher redshifts for a number of reasons. Electrons are expected to lose energy by inverse Compton (IC) interactions with the cosmic microwave background (CMB), whose energy density scales as (1+z)4 , implying a lower level of radio emission at higher redshifts. Moreover, synchrotron emission is proportional to the magnetic field strength squared, so evolution of magnetic field strength should affect the FRC at higher redshifts. Changes in the spectral energy distributions may also be expected due to evo1


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Mao et al.

lution in dust properties and metallicity. However, current studies show no firm evidence for evolution in the FRC (e.g., Garrett 2002; Appleton et al. 2004; Seymour et al. 2009; Bourne et al. 2010; Ivison et al. 2010a,b; Sargent et al. 2010a,b; Huynh et al. 2010).

2. Data
Our pro ject studies the FRC's dependence on redshift using deep 70 µm data from the Far-Infrared Deep Extragalactic Survey (FIDEL, PI: Dickinson), which uses the Spitzer Space Telescope, and 1.4 GHz data from the Very Large Array (VLA) in the Extended Chandra Deep Field South (ECDFS). This work is the first to use such deep 70 µm data to study the evolution of the FRC based on individual sources. We focus on 70 µm data because it is an excellent tracer of star-formation as it probes closer to the 100 µm star-formation peak in the IR SED. Our final catalogue comprises 617 70 µm sources, 91% of which have redshift information, over a third of which are spectroscopically determined. 353 of these sources have radio counterparts from Miller et al. (2008). In order to account for the radio nondetections we used both a stacking analysis and Survival Analysis. Our sample detects LIRGs to z 1.25 and ULIRGs to z 3 (Mao et al. 2011).

3. The FRC Shows No Evidence for Evolution with z
We computed q
T IR

for all sources that had redshift information using: q
T IR

= log(L

IR

/L

1. 4 GH z

),

(3.1)

where LI R is the total infrared luminosity, and L1.4GH z is the rest-frame 1.4 GHz luminosity. Figure 1 shows the median qT I R for only the detected sources (black circles), and for all sources by using survival analysis (red triangles). Our qT I R values are all within 0.22 of each other. Our values agree, within the errors, with the work of Sargent et al. (2010b), and also agree with the work of Bourne et al. (2010), with the exception of the redshift bins 0.75 < z <1 and z > 1.5, where our values differ by <2 compared to the values given by Bourne et al. (2010) for similar redshift ranges.

4. Summary and Conclusions
We have used the deepest 70 µm data to date to study the FRC out to z > 2 of ULIRGs in ECDFS. To quantify the evolution of the FRC we binned our data in redshift and calculate the FRC using luminosities and find that evolution in qT I R is constrained within 0.22. A more detailed discussion of these results and their implications may be found in Mao et al. (2011) The fact that we see no evidence for evolution is very surprising. Perhaps IC cooling and other effects such as evolution of the magnetic field strength, or evolution of dust properties are insignificant to z 2, or perhaps there is a complex interplay between these factors conspiring in the preservation of the FRC at higher redshifts. In the near future Herschel will measure the FIR properties of normal galaxies to z 1, and ULIRGs to z 4. This will allow us to study the FRC to higher redshifts and hence gain a better understanding of the evolution of star-forming galaxies.


The FRC Should Evolve!

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Figure 1. Median qT I R values for different redshift bins. The black filled circles show the median qT I R values for sources with both 70 µm and 1.4 GHz detections and the red triangles show the median qT I R value for all sources taking into account the lower limits using survival analysis. The grey upside-down open triangles show the median qT I R values derived by Bourne et al. (2010) and the grey open squares show the median qT I R values derived by Sargent et al. (2010b) for star-forming galaxies. Vertical error bars are standard errors for both our dataset and Bourne's data, but the vertical error bars for Sargent's data are upper and lower 95% confidence levels. The blue star at (1+z ) = 1 represents the median qT I R = 2.64 ± 0.02 from Bell (2003).

References
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