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Register a new userIsotropic Luminosity Indicators in a Complete AGN Sample
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Aleksandar Diamond-Stanic
Publication date
2009
fields
Physics
and research's language is
English
Authors
Aleksandar M. Diamond-Stanic
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The [O IV] 25.89 micron line has been shown to be an accurate indicator of active galactic nucleus (AGN) intrinsic luminosity in that it correlates well with hard (10-200 keV) X-ray emission. We present measurements of [O IV] for 89 Seyfert galaxies from the unbiased Revised Shapley-Ames (RSA) sample. The [O IV] luminosity distributions of obscured and unobscured Seyferts are indistinguishable, indicating that their intrinsic AGN luminosities are quite similar and that the RSA sample is well suited for tests of the unified model. In addition, we analyze several commonly used proxies for AGN luminosity, including [O III] 5007 A, 6 cm radio, and 2-10 keV X-ray emission. We find that the radio luminosity distributions of obscured and unobscured AGNs show no significant difference, indicating that radio luminosity is a useful isotropic luminosity indicator. However, the observed [O III] and 2-10 keV luminosities are systematically smaller for obscured Seyferts, indicating that they are not emitted isotropically.
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We present a study of a large, statistically complete sample of star-forming dwarf galaxies using mid-infrared observations from the {it Spitzer Space Telescope}. The relationships between metallicity, star formation rate (SFR) and mid-infrared color in these systems show that the galaxies span a wide range of properties. However, the galaxies do show a deficit of 8.0 um polycyclic aromatic hydrocarbon emission as is apparent from the median 8.0 um luminosity which is only 0.004 lstarf while the median $B$-band luminosity is 0.05 lstarb. Despite many of the galaxies being 8.0 um deficient, there is about a factor of 4 more extremely red galaxies in the [3.6] $-$ [8.0] color than for a sample of normal galaxies with similar optical colors. We show correlations between the [3.6] $-$ [8.0] color and luminosity, metallicity, and to a lesser extent SFRs that were not evident in the original, smaller sample studied previously. The luminosity--metallicity relation has a flatter slope for dwarf galaxies as has been indicated by previous work. We also show a relationship between the 8.0 um luminosity and the metallicity of the galaxy which is not expected given the competing effects (stellar mass, stellar population age, and the hardness of the radiation field) that influence the 8.0 um emission. This larger sample plus a well-defined selection function also allows us to compute the 8.0 um luminosity function and compare it with the one for the local galaxy population. Our results show that below 10$^{9}$ $L$solar, nearly all the 8.0 um luminosity density of the local universe arises from dwarf galaxies that exhibit strong ha emission -- i.e., 8.0 um and ha selection identify similar galaxy populations despite the deficit of 8.0 um emission observed in these dwarfs.
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At low Eddington ratio (mdot), two effects make it harder to detect AGN given some selection criteria. First, even with fixed accretion physics, AGN are diluted/less luminous relative to their hosts; the magnitude of this depends on host properties and so on luminosity and redshift. Second, they may transition to a radiatively inefficient state, changing SED shape and dramatically decreasing in optical/IR luminosity. These effects lead to differences in observed AGN samples, even at fixed bolometric luminosity and after correction for obscuration. The true Eddington ratio distribution may depend strongly on luminosity, but this will be seen only in surveys robust to dilution and radiative inefficiency (X-ray or narrow-line samples); selection effects imply that AGN in optical samples will have uniformly high mdot. This also implies that different selection methods yield systems with different hosts: the clustering of faint optical/IR sources will be weaker than that of X-ray sources, and optical/IR Seyferts will reside in more disk-dominated galaxies while X-ray selected Seyferts will preferentially occupy early-type systems. If observed mdot distributions are correct, a large fraction of low-luminosity AGN currently classified as obscured are in fact diluted and/or radiatively inefficient, not obscured by gas or dust. This is equally true if X-ray hardness is used as a proxy for obscuration, since radiatively inefficient SEDs near mdot~0.01 are X-ray hard. These effects can explain most of the claimed luminosity/redshift dependence in the obscured AGN population, with the true obscured fraction as low as 20%.
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We present a carefully selected sub-sample of Swift Long Gamma-ray Bursts (GRBs), that is complete in redshift. The sample is constructed by considering only bursts with favorable observing conditions for ground-based follow-up searches, that are bright in the 15-150 keV Swift/BAT band, i.e. with 1-s peak photon fluxes in excess to 2.6 ph s^-1 cm^-2. The sample is composed by 58 bursts, 52 of them with redshift for a completeness level of 90%, while another two have a redshift constraint, reaching a completeness level of 95%. For only three bursts we have no constraint on the redshift. The high level of redshift completeness allows us for the first time to constrain the GRB luminosity function and its evolution with cosmic times in a unbiased way. We find that strong evolution in luminosity (d_l=2.3pm 0.6) or in density (d_d=1.7pm 0.5) is required in order to account for the observations. The derived redshift distribution in the two scenarios are consistent with each other, in spite of their different intrinsic redshift distribution. This calls for other indicators to distinguish among different evolution models. Complete samples are at the base of any population studies. In future works we will use this unique sample of Swift bright GRBs to study the properties of the population of long GRBs.
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