Best P.N.,Institute for Astronomy |
Heckman T.M.,Johns Hopkins University
Monthly Notices of the Royal Astronomical Society | Year: 2012
A sample of 18286 radio-loud active galactic nuclei (AGN) is presented, constructed by combining the seventh data release of the Sloan Digital Sky Survey with the NRAO (National Radio Astronomy Observatory) VLA (Very Large Array) Sky Survey (NVSS) and the Faint Images of the Radio Sky at Twenty centimetres (FIRST) survey. Using this sample, the differences between radio galaxies of 'high-excitation' ('quasar-mode'; hereafer HERG) and 'low-excitation' ('radio-mode'; LERG) are investigated. A primary difference between the two radio source classes is the distinct nature of the Eddington-scaled accretion rate on to their central black holes: HERGs typically have accretion rates between one percent and 10 percent of their Eddington rate, whereas LERGs predominately accrete at a rate below one percent Eddington. This is consistent with models whereby the population dichotomy is caused by a switch between radiatively efficient and radiatively inefficient accretion modes at low accretion rates. Local radio luminosity functions are derived separately for the two populations, for the first time, showing that although LERGs dominate at low radio luminosity and HERGs begin to take over at L 1.4GHz~ 10 26WHz -1, examples of both classes are found at all radio luminosities. Using the V/V max test it is shown that the two populations show differential cosmic evolution at fixed radio luminosity: HERGs evolve strongly at all radio luminosities, while LERGs show weak or no evolution. This suggests that the luminosity dependence of the evolution previously seen in the radio luminosity function is driven, at least in part, by the changing relative contributions of these two populations with luminosity. The host galaxies of the radio sources are also distinct: HERGs are typically of lower stellar mass, with lower black hole masses, bluer colours, lower concentration indices and less pronounced 4000Å breaks indicating younger stellar populations. Even if samples are matched in radio luminosity and stellar and black hole masses, significant differences still remain between the accretion rates, stellar populations and structural properties of the host galaxies of the two radio source classes. These results offer strong support to the developing picture of radio-loud AGN in which HERGs are fuelled at high rates through radiatively efficient standard accretion discs by cold gas, perhaps brought in through mergers and interactions, while LERGs are fuelled via radiatively inefficient flows at low accretion rates. In this picture, the gas supplying the LERGs is frequently associated with the hot X-ray haloes surrounding massive galaxies, groups and clusters, as part of a radio-AGN feedback loop. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.
Bresolin F.,Institute for Astronomy
Astrophysical Journal | Year: 2011
Optical spectra of 25 H II regions in the inner 2kpc of the M33 disk have been obtained with the Gemini Multi-Object Spectrograph at the Gemini North telescope. The oxygen abundance gradient measured from the detection of the [O III]λ4363 auroral line displays a scatter of approximately 0.06dex, a much smaller value than recently reported by Rosolowsky & Simon in this galaxy. The analysis of the abundances for a large sample of H II regions derived from the R 23 strong-line indicator confirms that the scatter is small over the full disk of M33, consistent with the measuring uncertainties, and comparable to what is observed in other spiral galaxies. No evidence is therefore found for significant azimuthal variations in the present-day metallicity of the interstellar medium in this galaxy on spatial scales from 100pc to a fewkpc. A considerable fraction of M33 H II regions with auroral line detections show spectral features revealing sources of hard ionizing radiation (such as He II emission and large [Ne III], [O III] line fluxes). Since R 23 is shown to severely underestimate the oxygen abundances in such cases, care must be taken in chemical abundance studies of extragalactic H II regions based on this strong-line indicator. © 2011. The American Astronomical Society. All rights reserved.
Bresolin F.,Institute for Astronomy |
Kennicutt R.C.,University of Cambridge |
Ryan-Weber E.,Swinburne University of Technology
Astrophysical Journal | Year: 2012
We have obtained spectra of 135 H II regions located in the inner and extended disks of the spiral galaxies NGC1512 and NGC3621, spanning the range of galactocentric distances 0.2-2 × R 25 (from ∼2-3kpc to ∼18-25kpc). We find that the excitation properties of nebulae in the outer (R > R 25) disks are similar to those of the inner disks, but on average younger H II regions tend to be selected in the bright inner disks. Reddening by dust is not negligible in the outer disks and subject to significant large-scale spatial variations. For both galaxies, the radial abundance gradient flattens to a constant value outside of the isophotal radius. The outer disk O/H abundance ratio is highly homogeneous, with a scatter of only ∼0.06dex. In the case of the interacting galaxy NGC1512 we find a number of H II regions with peculiar metallicity for their radius, a result which can be interpreted by gas flows activated by the gravitational encounter with NGC1510. Based on the excitation and chemical (N/O ratio) analysis, we find no compelling evidence for variations in the upper initial mass function of ionizing clusters of extended disks. The O/H abundance in the outer disks of the target galaxies corresponds to ∼35% of the solar value (or higher, depending on the metallicity diagnostic). This agrees with our earlier measurements in M83 and NGC4625, and conflicts with the notion that metallicities in extended disks of spiral galaxies are low and on the order of ∼0.1 × Z ⊙. We show that, in general, the observed metal enrichment cannot be produced with the current level of star formation, even if the latter extends over a Hubble time. We discuss the possibility that metal transport mechanisms from the inner disks lead to metal pollution of the outer disks. Gas accretion from the intergalactic medium, enriched by outflows, offers an alternative solution, justified within the framework of hydrodynamic simulations of galaxy evolution. Specific model predictions of the chemical enrichment and the flat gradients in extended disks of nearby galaxies will be valuable to discriminate between these different scenarios. © 2012. The American Astronomical Society. All rights reserved.
Wolfgang A.,University of California at Santa Cruz |
Lopez E.,University of California at Santa Cruz |
Lopez E.,Institute for Astronomy
Astrophysical Journal | Year: 2015
The Kepler Mission has found thousands of planetary candidates with radii between 1 and 4 R⊕. These planets have no analogues in our own solar system, providing an unprecedented opportunity to understand the range and distribution of planetary compositions allowed by planet formation and evolution. A precise mass measurement is usually required to constrain the possible composition of an individual super-Earth-sized planet, but these measurements are difficult and expensive to make for the majority of Kepler planet candidates (PCs). Fortunately, adopting a statistical approach helps us to address this question without them. In particular, we apply hierarchical Bayesian modeling to a subsample of Kepler PCs that is complete for P < 25 days and Rp1 > 1.2 R⊕ and draw upon interior structure models that yield radii largely independent of mass by accounting for the thermal evolution of a gaseous envelope around a rocky core. Assuming the envelope is dominated by hydrogen and helium, we present the current-day composition distribution of the sub-Neptune-sized planet population and find that H+He envelopes are most likely to be ∼1% of these planets' total masses with an intrinsic scatter of ±0.5 dex. We address the gaseous/rocky transition and illustrate how our results do not result in a one-to-one relationship between mass and radius for this sub-Neptune population; accordingly, dynamical studies that wish to use Kepler data must adopt a probabilistic approach to accurately represent the range of possible masses at a given radius. © 2015. The American Astronomical Society. All rights reserved.
Heckman T.M.,Johns Hopkins University |
Best P.N.,Institute for Astronomy
Annual Review of Astronomy and Astrophysics | Year: 2014
We summarize what large surveys of the contemporary Universe have taught us about the physics and phenomenology of the processes that link the formation and evolution of galaxies with their central supermassive black holes. We present a picture in which the population of active galactic nuclei (AGNs) can be divided into two distinct populations. The radiative-mode AGNs are associated with black holes (BHs) that produce radiant energy powered by accretion at rates in excess of ∼1% of the Eddington limit. They are primarily associated with less massive BHs growing in high-density pseudobulges at a rate sufficient to produce the total mass budget in these BHs in ∼10 Gyr. The circumnuclear environment contains high-density cold gas and associated star formation. Major mergers are not the primary mechanism for transporting this gas inward; secular processes appear dominant. Stellar feedback is generic in these objects, and strong AGN feedback is seen only in the most powerful AGNs. In jet-mode AGNs the bulk of energetic output takes the form of collimated outflows (jets). These AGNs are associated with the more massive BHs in more massive (classical) bulges and elliptical galaxies. Neither the accretion onto these BHs nor star formation in their host bulge is significant today. These AGNs are probably fueled by the accretion of slowly cooling hot gas that is limited by the feedback/heating provided by AGN radio sources. Surveys of the high-redshift Universe paint a similar picture. Noting that the volume-averaged ratio of star formation to BH growth has remained broadly constant over the past 10 Gyrs, we argue that the processes that linked the cosmic evolution of galaxies and BHs are still at play today. Copyright © 2014 by Annual Reviews.