Radiatively Inefficient Accretion in Nearby Galaxies

We use new central stellar velocity dispersions and nuclear X-ray and Halpha luminosities for the Palomar survey of nearby galaxies to investigate the distribution of nuclear bolometric luminosities and Eddington ratios for their central black holes (BHs). This information helps to constrain the nature of their accretion flows and the physical drivers that control the spectral diversity of nearby active galactic nuclei. The characteristic values of the bolometric luminosities and Eddington ratios, which span over 7-8 orders of magnitude, from L_bol < 10^37 to 3 X 10^44 erg/s and L_bol/L_Edd ~ 10^-9 to 10^-1, vary systematically with nuclear spectral classification, increasing along the sequence absorption-line nuclei --> transition objects --> LINERs --> Seyferts. The Eddington ratio also increases from early-type to late-type galaxies. We show that the very modest accretion rates inferred from the nuclear luminosities can be readily supplied through local mass loss from evolved stars and Bondi accretion of hot gas, without appealing to additional fueling mechanisms such as angular momentum transport on larger scales. Indeed, we argue that the fuel reservoir generated by local processes should produce far more active nuclei than is actually observed. This generic luminosity-deficit problem suggests that the radiative efficiency in these systems is much less than the canonical value of 0.1 for traditional optically thick, geometrically thin accretion disks. The observed values of L_bol/L_Edd, all substantially below unity, further support the hypothesis that massive BHs in most nearby galaxies reside in a low or quiescent state, sustained by accretion through a radiatively inefficient mode.Comment: To appear in ApJ, 15 pages, 4 figure

The 2016 US Presidential Election on Facebook: An Exploratory Analysis of Sentiments

Social media platforms are valuable tools for political campaigns. In this study, we analyze a dataset representing over 22 thousand Facebook posts by candidates and over 48 million comments to understand the nature of online discourse. Specifically, we study the interaction between political candidates and the public during the 2016 presidential elections in the United States. We outline a novel method to classify commentators into four groups: strong supporters, supporters, dissenters, and strong dissenters. Comments by each group on policy-related topics are analyzed using sentiment analysis. Finally, we discuss avenues for future research to study the dynamics of social media platforms and political campaigns

Local-Group tests of dark-matter Concordance Cosmology: Towards a new paradigm for structure formation

(abridged) Predictions of the Concordance Cosmological Model (CCM) of the structures in the environment of large spiral galaxies are compared with observed properties of Local Group galaxies. Five new most probably irreconcilable problems are uncovered. However, the Local Group properties provide hints that may lead to a solution of the above problems The DoS and bulge--satellite correlation suggest that dissipational events forming bulges are related to the processes forming phase-space correlated satellite populations. Such events are well known to occur since in galaxy encounters energy and angular momentum are expelled in the form of tidal tails, which can fragment to form populations of tidal-dwarf galaxies (TDGs) and associated star clusters. If Local Group satellite galaxies are to be interpreted as TDGs then the sub-structure predictions of CCM are internally in conflict. All findings thus suggest that the CCM does not account for the Local Group observations and that therefore existing as well as new viable alternatives have to be further explored. These are discussed and natural solutions for the above problems emerge.Comment: A and A, in press, 25 pages, 9 figures; new version contains minor text adjustments for conformity with the published version and additional minor changes resulting from reader's feedback. The speculation on a dark force has been added. Also, the Fritz Zwicky Paradox is now included to agree with the published versio

Dynamical effects of stellar mass loss on a Kuiper-like belt

A quarter of DA white dwarfs are metal polluted, yet elements heavier than helium sink down through the stellar atmosphere on timescales of days. Hence, these white dwarfs must be currently accreting material containing heavy elements. Here, we consider whether the scattering of comets or asteroids from an outer planetary system, following stellar mass loss on the asymptotic giant branch, can reproduce these observations. We use N-body simulations to investigate the effects of stellar mass loss on a simple system consisting of a planetesimal belt whose inner edge is truncated by a planet. Our simulations find that, starting with a planetesimal belt population fitted to the observed main sequence evolution, sufficient mass is scattered into the inner planetary system to explain the inferred heavy element accretion rates. This assumes that some fraction of the mass scattered into the inner planetary system ends up on star-grazing orbits, is tidally disrupted and accreted onto the white dwarf. The simulations also reproduce the observed decrease in accretion rate with cooling age and predict accretion rates in old (>1Gyr) white dwarfs, in line with observations. The efficiency we assumed for material scattered into the inner planetary system to end up on star-grazing orbits is based on a Solar-like planetary system, since the simulations show that a single planet is not sufficient. Although the correct level of accretion is reproduced, the simulations predict a higher fraction of accreting white dwarfs than observed. This could indicate that evolved planetary systems are less efficient at scattering bodies onto star-grazing orbits or that dynamical instabilities post-stellar mass loss cause rapid planetesimal belt depletion for a significant fraction of systems

The Solar System's Post-Main Sequence Escape Boundary

The Sun will eventually lose about half of its current mass nonlinearly over several phases of post-main sequence evolution. This mass loss will cause any surviving orbiting body to increase its semimajor axis and perhaps vary its eccentricity. Here, we use a range of Solar models spanning plausible evolutionary sequences and assume isotropic mass loss to assess the possibility of escape from the Solar System. We find that the critical semimajor axis in the Solar System within which an orbiting body is guaranteed to remain bound to the dying Sun due to perturbations from stellar mass loss alone is approximately 1,000 AU - 10,000 AU. The fate of objects near or beyond this critical semimajor axis, such as the Oort Cloud, outer scattered disc and specific bodies such as Sedna, will significantly depend on their locations along their orbits when the Sun turns off of the main sequence. These results are applicable to any exoplanetary system containing a single star with a mass, metallicity and age which are approximately equal to the Sun's, and suggest that few extrasolar Oort Clouds could survive post-main sequence evolution intact.Comment: 15 pages, 11 figures, accepted for publication in MNRA

The Great Escape: How Exoplanets and Smaller Bodies Desert Dying Stars

Mounting discoveries of extrasolar planets orbiting post-main sequence stars motivate studies aimed at understanding the fate of these planets. In the traditional "adiabatic" approximation, a secondary's eccentricity remains constant during stellar mass loss. Here, we remove this approximation, investigate the full two-body point-mass problem with isotropic mass loss, and illustrate the resulting dynamical evolution. The magnitude and duration of a star's mass loss combined with a secondary's initial orbital characteristics might provoke ejection, modest eccentricity pumping, or even circularisation of the orbit. We conclude that Oort clouds and wide-separation planets may be dynamically ejected from 1-7 Solar-mass parent stars during AGB evolution. The vast majority of planetary material which survives a supernova from a 7-20 Solar-mass progenitor will be dynamically ejected from the system, placing limits on the existence of first-generation pulsar planets. Planets around >20 Solar-mass black hole progenitors may easily survive or readily be ejected depending on the core collapse and superwind models applied. Material ejected during stellar evolution might contribute significantly to the free-floating planetary population.Comment: 23 pages, 16 figures, accepted for publication in MNRA

Hot gas flows on global and nuclear galactic scales

Since its discovery as an X-ray source with the Einstein Observatory, the hot X-ray emitting interstellar medium of early-type galaxies has been studied intensively, with observations of improving quality, and with extensive modeling by means of numerical simulations. The main features of the hot gas evolution are outlined here, focussing on the mass and energy input rates, the relationship between the hot gas flow and the main properties characterizing its host galaxy, the flow behavior on the nuclear and global galactic scales, and the sensitivity of the flow to the shape of the stellar mass distribution and the mean rotation velocity of the stars.Comment: 22 pages. Abbreviated version of chapter 2 of the book "Hot Interstellar Matter in Elliptical Galaxies", Springer 201

Effectiveness of community-based condom distribution interventions to prevent HIV in the United States: A systematic review and meta-analysis.

BackgroundDespite significant public health implications, the extent to which community-based condom distribution interventions (CDI) prevent HIV infection in the United States is not well understood.MethodsWe systematically reviewed research evidence applying Cochrane Collaboration methods. We used a comprehensive search strategy to search multiple bibliographic databases for relevant randomized controlled trials (RCTs) and non-RCTs published from 1986-2017. We focused on CDI that made condoms widely available or accessible in community settings. Eligible outcomes were HIV infection (primary), sexually transmitted infections, condom use, and multiple sexual partnership. Two reviewers independently screened citations to assess their eligibility, extracted study data, and assessed risk of bias. We calculated risk ratios (RR) with 95% confidence intervals (CI) and pooled them using random-effects models. We assessed evidence quality using GRADE.ResultsWe reviewed 5,110 unique records. Nine studies (including one RCT) met eligibility criteria. Studies were conducted in 10 US states between 1989 and 2011. All studies were at high risk of bias. Interventions were categorized into three groups: "Ongoing" (unlimited access to condoms), "Ongoing-plus" (unlimited access to condoms, with co-interventions), and "Coupon-based" (coupons redeemed for condoms). No studies reported incident HIV. Ongoing CDI (four non-RCTs) modestly reduced condomless sex (RR 0.88, 95% CI 0.78 to 0.99). Ongoing-plus CDI (two non-RCTs) significantly reduced multiple sexual partnership (RR 0.37, 95% CI 0.16 to 0.87). Of two coupon-based studies, one (non-RCT) showed reduction in condomless sex in female participants (Odds Ratio 0.67, 95% CI 0.47 to 0.96), while the other one (RCT) showed no effect on STI incidence (RR 0.91, 95% CI 0.63 to 1.31). Evidence quality was "very low" for all outcomes.ConclusionsCDI may reduce some risky sexual behaviors, but the evidence for any reduction is limited and of low-quality. Lack of biological outcomes precludes assessing the link between CDI and HIV incidence

Effectiveness of community-based condom distribution interventions to prevent HIV in the United States: A systematic review and meta-analysis

BackgroundDespite significant public health implications, the extent to which community-based condom distribution interventions (CDI) prevent HIV infection in the United States is not well understood.MethodsWe systematically reviewed research evidence applying Cochrane Collaboration methods. We used a comprehensive search strategy to search multiple bibliographic databases for relevant randomized controlled trials (RCTs) and non-RCTs published from 1986-2017. We focused on CDI that made condoms widely available or accessible in community settings. Eligible outcomes were HIV infection (primary), sexually transmitted infections, condom use, and multiple sexual partnership. Two reviewers independently screened citations to assess their eligibility, extracted study data, and assessed risk of bias. We calculated risk ratios (RR) with 95% confidence intervals (CI) and pooled them using random-effects models. We assessed evidence quality using GRADE.ResultsWe reviewed 5,110 unique records. Nine studies (including one RCT) met eligibility criteria. Studies were conducted in 10 US states between 1989 and 2011. All studies were at high risk of bias. Interventions were categorized into three groups: "Ongoing" (unlimited access to condoms), "Ongoing-plus" (unlimited access to condoms, with co-interventions), and "Coupon-based" (coupons redeemed for condoms). No studies reported incident HIV. Ongoing CDI (four non-RCTs) modestly reduced condomless sex (RR 0.88, 95% CI 0.78 to 0.99). Ongoing-plus CDI (two non-RCTs) significantly reduced multiple sexual partnership (RR 0.37, 95% CI 0.16 to 0.87). Of two coupon-based studies, one (non-RCT) showed reduction in condomless sex in female participants (Odds Ratio 0.67, 95% CI 0.47 to 0.96), while the other one (RCT) showed no effect on STI incidence (RR 0.91, 95% CI 0.63 to 1.31). Evidence quality was "very low" for all outcomes.ConclusionsCDI may reduce some risky sexual behaviors, but the evidence for any reduction is limited and of low-quality. Lack of biological outcomes precludes assessing the link between CDI and HIV incidence