A Closer View of the Radio-FIR Correlation: Disentangling the Contributions of Star Formation and Active Galactic Nucleus Activity
Summary (3 min read)
1. INTRODUCTION
- The correspondence between the radiation in the (far-)infrared and that in the radio spans over nearly five orders of magnitude in various types of galaxies, ranging from dwarfs to ULIRGs.
- The AGN contribution to the radio–FIR correlation has been studied in the past to some extent.
- Data, Mauch & Sadler (2007) inferred a lower average FIR/radio ratio for AGN-bearing galaxies (Seyferts, LINERs, and quasars), but still within the scatter of the correlation for star-forming galaxies.
2.1. Unified Radio Catalog
- This “Unified Radio Catalog” has been generated in such a way that it allows a broad range of 20 cm based sample selections and source analysis (see Kimball & Ivezić 2008 for details).
- The 2.7 million entries are comprised of the closest three FIRST to NVSS matches (within 30′′) and vice versa, as well as unmatched sources from each survey.
- All entries have been supplemented by data from the other radio and optical surveys, where available.
- In the following section, the authors expand this catalog to IR wavelengths, and augment it with additional (spectroscopic and SED-based) information.
2.2.1. IRAS
- For the purpose of this paper, the authors have expanded the Unified Radio Catalog to IR wavelengths by cross-correlating it with the IRAS point-source and faint-source catalogs (hereafter PSC and FSC, respectively).
- The 60 and 100 μm magnitudes reported in the PSC and FSC are in agreement for the union of the two IR samples.
- The first column denotes the number of radio—IRAS (Point Source, PS, and Faint Source, FS) catalog with high quality IR photometry.
2.2.2. SDSS Quasar and Main Galaxy Sample Catalogs
- The authors have further matched the NVSS-selected sample from the Unified Radio Catalog with data drawn from (1) the SDSS DR5 quasar sample (Schneider et al. 2007), and (2) the DR4 “main” spectroscopic sample for which derivations of emissionline fluxes from the SDSS spectra are available (see Smolčić et al.
- The SED fitting was performed as described in detail in Smolčić et al. (2008).
- Furthermore, a small number (∼0.2%) of duplicate objects was present in both the SDSS “main” galaxy sample and the SDSS Quasar Catalog.
- A summary of the various radio–IR–optical samples is given in Table 1, and in Figure 3 and Figure 4 the authors show the radio (20 cm), optical (r band), and far-IR luminosities as a function of redshift for the final NVSS–SDSS and NVSS–SDSS–IRAS samples (see Equations (3) and (4)).
2.3.1. Star-forming and AGN Galaxy Subsamples
- The authors have used the optical spectroscopic information added to the NVSS selected sample to spectroscopically separate the galaxies present in the SDSS (DR4) “main” galaxy sample as absorption line, AGN (LINER/Seyfert), star-forming, or composite galaxies.
- The last two classes have been selected “unambiguously” by requiring combined criteria using three emission-line flux ratios .
- Note that the redshift distribution of 20 cm detected absorption line galaxies is biased toward higher redshifts, compared to all other galaxy types .
- The SDSS fiber aperture of 3′′ diameter collects such a fraction of light for galaxies of average size, type, and luminosity at z 0.04.
3.2. Radio–FIR Correlation for All Sources
- The radio–FIR correlation for the NVSS–SDSS–IRAS sample is summarized in Figure 7.
- In the middle panels the authors show the q parameter, that characterizes the slope of the radio–FIR correlation (see Equation (1)), as a function of FIR and radio luminosities.
- This is in very good agreement with previous findings (Condon 1992; Yun et al.
- The quasars in their sample comprise the high-luminosity end at both IR and radio wavelengths (they are also located at higher redshifts, compared to the IR- and radio-detected “main” galaxy sample).
3.3. Radio–FIR Correlation for Different Types of Galaxies
- In Figure 8, the authors present the radio–FIR correlation for the SDSS “main” galaxy sample subdivided into different, spectroscopically selected galaxy types .
- The authors find that the decrease of q with radio luminosity in the observed data is consistent with that in the simulated data, thus not requiring additional effects (such as increasing AGN contribution with increasing radio power) to explain this trend (at least in the radio luminosity range probed here).
- The spectroscopic selection of pure star-forming galaxies allows us to quantify the radio–IR correlation in a rather unbiased manner.
- Interestingly, the scatter is the highest for Seyfert types of galaxies, for which the authors also find the lowest average q-value, 〈q〉 = 2.14 ± 0.05.
3.4. Radio–FIR Correlation for Quasars
- In Figure 10, the authors quantify the radio–FIR correlation for the 21 IR-detected quasars in their sample.
- The distribution of the FIR/radio ratio cannot be well fit with a Gaussian distribution.
- The median q-value of the sample is 2.04, comparable to the average q value the authors have found for Seyfert galaxies (2.14), and lower than that for star-forming galaxies .
- It is worth noting that the higher redshift quasars (0.2 z 0.4) appear to be biased toward more radio-loud AGNs.
4. AN INDEPENDENT VIEW OF THE RADIO–FIR CORRELATION: A LINK TO STAR FORMATION
- It is generally taken that recent star formation drives both the radio and FIR emission of galaxies that lie on the radio–FIR correlation (Condon 1992; Mauch & Sadler 2007).
- To shed light on the source of radio/FIR emission in their galaxies, in this section the authors investigate the correlation between their radio/FIR luminosities and SFRs, independently determined based on fitting stellar population synthesis models to the NUV-NIR SED).
- The most obvious examples of this are the LINER and absorption galaxies from both the NVSS–SDSS and NVSS–SDSS–IRAS samples.
- The rms scatter is 0.35, and 0.32 for the FIR and radio distributions, respectively.
5.1. Comparison with Previous Results
- Extensive studies of the radio–FIR correlation (e.g., Helou et al.
- A lower average q-value is generally inferred when using radio-selected samples, and reaching fainter in the IR (see Sargent et al. 2010 for a detailed discussion of selection effects).
- This is in very good agreement with the results from Mauch & Sadler (2007).
- The average FIR/radio ratio for the 21 quasars in their sample is q = 2.04, comparable to that inferred for Seyferts and lower than that for star-forming galaxies.
5.2. AGN Contribution to the Radio–FIR Correlation
- A low q-value is often used to discriminate between starforming galaxies and AGNs.
- The median Δ log L values and the fractional star formation/ AGN contributions are summarized in Table 2.
- Further, the FIR emission from Seyfert galaxies arises predominantly from star formation (∼75%), while the AGN contribution to radio luminosity in Seyfert galaxies is about a factor of 2 higher in the radio than in the FIR (see Table 2).
- They demonstrate that the four galaxies having the largest mid-IR AGN fractions (>60%) in their sample have q-values consistent with the canonical value.
6. SUMMARY AND CONCLUSIONS
- Based on a new radio–optical–IR catalog the authors have separated their radio- (NVSS) and IR- (IRAS) detected SDSS galaxies (0.04 < z 0.2) into star-forming, composite, Seyfert, LINER, absorption line galaxies, and quasars, and they have performed an in-depth analysis of the radio–FIR correlation for various types of star-forming and AGN-bearing galaxies.
- In summary, their results imply that most AGN-containing galaxies in their sample have FIR/radio flux ratios indistinguishable from those of the star-forming galaxies.
- I.M. thanks California Institute of Technology for generous hospitality.
- A.K. and Z.I. acknowledge NSF grant AST-0507259 to the University of Washington.
- Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S. Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England.
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References
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...…in the low (Helou et al. 1985; Condon 1992; Garrett 2002; Mauch & Sadler 2007; Yun et al. 2001; Bell 2003) and high redshift universe (Sargent et al. 2010; Kovacs et al. 2006; Sajina et al. 2008; Murphy et al. 2009; Appleton et al. 2004; Vlahakis et al. 2008; Ibar et al. 2008; Chapman et al. 2005)....
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"A Closer View of the Radio-FIR Corr..." refers background in this paper
...…rising with redshift (especially at z & 3) due to the increase of the cosmic microwave background (CMB) energy density (UCMB) with redshift, UCMB ∝ (1 + z) 4, which surpresses the non-thermal component of a galaxy’s radio continuum via inverse-Compton (IC) scattering (see Murphy 2009 for details)....
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