Spectral energy distributions and multiwavelength selection of type 1 quasars
Summary (2 min read)
3. MIR/OPTICAL COLORS OF TYPE 1 QUASARS
- For the Spitzer color, the authors chose the two highest S/N bands (S3:6 and S4:5); this choice happens to produce the greatest separation of classes and has the added attraction that it does not rely on the longer wavelength bands that will be lost when Spitzer’s coolant runs out.
- Judicious rotation of the axes in Figure 6 may allow for relatively clean AGN selection without having to rely on morphology information.
- Quasars with z > 2:2 have redder optical colors even if they are not dust-reddened, and a large fraction of this population will still be identified by the SDSS quasar-selection algorithm.
- A multidimensional MIR + optical Bayesian color-selection approach (Richards et al. 2004) that avoids any morphology bias may yield optimal completeness and efficiency for all AGN subclasses and will be the subject of future work.
4. THE OBSCURED QUASAR FRACTION
- SinceMIR emission fromAGNs comes from larger scales and is thought to bemore isotropic than optical/UVemission, theMIR is an ideal part of the spectrum to constrain the fraction of quasars that are obscured (within the context of the so-called unifiedmodel; Antonucci 1993).
- E.g., Polletta et al. 2000; Kuraszkiewicz et al. 2003; Risaliti & Elvis 2004), complete SEDs have been compiled for only a small number (P100) of quasars and the mean SED from Elvis et al. (1994) is arguably still the best description of the SED of quasars and is certainly the most commonly used.
- To assess the importance of the host galaxy correction where it matters most, the authors determine the ratio of host galaxy to total luminosity at 1.6 m in the rest frame, where the elliptical template spectrum has its peak.
- The standard deviation of the overall mean and the luminosity- and color-subdivided mean SEDs give the reader an idea of the range of SED shapes.
- There are significant differences between the most and least optically luminous quasars in their sample.
6. BOLOMETRIC LUMINOSITIES AND ACCRETION RATES
- The determinations of quasar physical parameters such as bolometric luminosity, black hole mass, and accretion rate have been revolutionized by two bodies of work from the past decade or so.
- As discussed above, the biases inherent to the sample of objects used by Elvis et al. (1994) in addition to these authors’ warnings of the diversity of individual SEDs, coupled with the use of their mean SED as a single universal template, is what motivates this investigation.
- It seems likely that the minimum in this region results from this region being a relative minimum in the combination of host galaxy contamination in the near-IR and dust extinction in the UV.
- Figures 12 and 13 demonstrate that the smallest bolometric corrections and errors are found at optical wavelengths.
- Clearly, if the authors are ever to understand the accretion rate distribution of quasars, they must either measure the bolometric luminosity directly or determine bolometric corrections to an accuracy better than that which is afforded by assuming the mean SED.
7. CONCLUSIONS
- The authors have compiled a sample of 259 SDSS type 1 quasars with four-band Spitzer IRAC detections.
- Figure 14 presents the individual SEDs of each of the 259 quasars in their sample.
- The SDSS spectra are shown as solid black lines (smoothed by a 19 pixel boxcar).
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...As a relatively short phase, such objects constitute only∼ 20− 40% of the quasar population, similar to that observed (Gregg et al. 2002; White et al. 2003; Richards et al. 2003, 2006a; Hopkins et al. 2004)....
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...…luminosity density (and especially the number density of bright quasars corresponding to& 108M⊙ BHs at high Eddington ratio; see Fan et al. 2004; Richards et al. 2006b) declines rapidly at z & 2 − 3 (roughly as∼ (1+ z)4−6), compared to the global star formation rate density of the Universe,…...
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...…it produces good agreement with the distribution of Compton-thick column densities subsequently reported by Treister et al. (2004), Mainieri et al. (2005), and Tozzi et al. (2006) and is consistent with upper limits to the obscured fraction from the mid-IR observations of Richards et al. (2006c)....
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...…fitted luminosity function outside of its measured luminosity and redshift range can be inaccurate by orders of magnitude (see, e.g. Figure 19 of Richards et al. (2006b)), and we have demonstrated the importance of accounting for the detailed luminosity dependence of quasar SEDs and obscuration....
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...Figure 1 shows these corrections as a function of luminosity, which agree broadly with the values in e.g. Richards et al. (2006c) over the luminosity range they consider....
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...…the brightend slope of the QLF appears to become shallower towards higher redshifts, from both direct measurements (Fan et al. 2001b, 2003; Richards et al. 2006b) and (albeit weaker) constraints from gravitational lensing (Comerford et al. 200 ; Wyithe & Loeb 2002; Wyithe 2004; Richards…...
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...…brightend slope of the QLF appears to become shallower towards higher redshifts, from both direct measurements (Fan et al. 2001b, 2003; Richards et al. 2006b) and (albeit weaker) constraints from gravitational lensing (Comerford et al. 200 ; Wyithe & Loeb 2002; Wyithe 2004; Richards et al. 2006a)....
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898 citations
Cites methods from "Spectral energy distributions and m..."
...Nonetheless, a number of different techniques based on near- and mid-IR color selection were developed to find AGN using Spitzer data (e.g. Lacy et al. 2004, Stern et al. 2005, Richards et al. 2006, Donley et al. 2008)....
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857 citations
References
101 citations
"Spectral energy distributions and m..." refers background or methods in this paper
...In the XFLS field, 24 m sources are cataloged by Fadda et al. (2006) and we include matches from that catalog as well....
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...…dependent on a lack of evolution in the distribution of the ratio of radio to optical luminosities of quasars from low to high redshifts, as the majority of high-redshift quasars are expected to have much lower radio luminosities than probed by the radio survey of the XFLS by Condon et al. (2003)....
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...We include matches toMIPS 70 m sources in the XFLS (FLS70_sn7_ jul05.txt; Frayer et al. 2006) and in the SWIRE (SWIRE2_EN1_70um_23nov05.tbl, SWIRE2_EN2_70um_ 23nov05.tbl, SWIRE3_Lockman_70um_23nov05.tbl; Surace et al. 2005) areas....
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...Within a matching radius of 1B0 there are 44 SDSS-DR3 quasar matches in the XFLS area, 29 in the ELAIS-N1 area, 44 in the ELAIS-N2 area, and 142 in the Lockman Hole area....
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...We expand on these results by determining a number of different ‘‘mean’’ SEDs as a function of color and luminosity for 259SDSSquasars in the SpitzerExtragalactic First Look Survey14 (XFLS), SWIRE15ELAIS-N1/N2, andSWIRELockman Hole areas....
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98 citations
95 citations
90 citations
"Spectral energy distributions and m..." refers background in this paper
...%) photometric errors means that IRAC colors alone may not be useful for accurate quasar photometric redshift estimation (e.g., Richards et al. 2001;Weinstein et al. 2004), but MIR color information in addition to optical colors will be extremely useful in breaking redshift degeneracies in the…...
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88 citations