arXiv subjects

Francesco Shankar

Publications and source records attributed to Francesco Shankar.

112 records · Page 7Linked to original sources

arXiv2007

The Optical-Radio Mapping: the kinetic efficiency of Radio-Loud AGNs

We constrain the mean kinetic efficiency of radio-loud active galactic nuclei by using an optically selected sample for which both the optical and the radio luminosity functions (LFs) have been determined; the former traces the bolometric luminosity L, while the latter traces the kinetic power L_k, empirically correlated to the radio emission. Thus in terms of the ratio g_k=L_k/L, we can convert the optical LF of the sample into a radio one. This computed LF is shown to match the directly observed LF for the same sample if g_k=0.10^{+0.05}_{-0.01} holds, with a scatter σ=0.38^{+0.04}_{-0.09} dex; with these values we also match a number of independent correlations between L_k, L and radio emission, that we derive through Monte Carlo simulations. We proceed to translate the value of g_k into a constraint on the kinetic efficiency for the production of radio jets or winds, namely, ε_k=L_k/(Mdot*c^2)~0.01 in terms of the rate Mdot of mass accretion onto the central black hole. Then, on assuming that on average the radio sources share the same kinetic efficiency, we compute a solid lower limit of about 25% on the contribution of radio sources to the local black hole mass density.

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arXiv2007

2MASS Reveals a Large Intrinsic Fraction of BALQSOs

The intrinsic fraction of broad absorption line quasars (BALQSOs) is important in constraining geometric and evolutionary models of quasars. We present the fraction of BALQSOs in 2MASS detected quasars within the SDSS DR3 sample in the redshift range of 1.7 < z < 4.38. The fraction of BALQSOs is 40.4^{+3.4}_{-3.3}% in the 2MASS 99% database K_s band completeness sample, and 38.5^{+1.7}_{-1.7}% in the larger 2MASS sample extending below the completeness limit. These fractions are significantly higher than the 26% reported in the optical bands for the same parent sample. We also present the fraction of BALQSOs as functions of apparent magnitudes, absolute magnitudes, and redshift in the 2MASS and SDSS bands. The 2MASS fractions are consistently higher than the SDSS fractions in every comparison, and the BALQSO fractions steadily increase with wavelength from the SDSS u to the 2MASS K_s bands. Furthermore, the i - K_s color distributions of BALQSOs and non-BALQSOs indicate that BALQSOs are redder than non-BALQSOs, with a K-S test probability of 2e-12. These results are consistent with the spectral difference between BALQSOs and non-BALQSOs including both the absorption troughs and dust extinction in BALQSOs, which leads to significant selection biases against BALQSOs in the optical bands. Using a simple simulation incorporating the luminosity function of quasars and the amount of obscuration for BALQSOs, we simultaneously fit the BALQSO fractions in the SDSS and 2MASS bands. We obtain a true BALQSO fraction of 43\pm2% for luminous quasars (M_{K_s} \lesssim -30.1 mag).

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arXiv2007

On the faint end of the high redshift AGN luminosity function

Using the results of recent optical surveys we conclude that the {\it non}-detection of quasars down to faint magnitudes implies a significant flattening of the high redshift (z~6) optical active galactic nuclei (AGN) luminosity function for M_{1450}>-24.7. We find that all the data are consistent with a faint-end slope for the optical AGN luminosity function of β=-2.2 and β=-2.8, at the 90% and 99% confidence level respectively, flatter than the bright-end slope of β'~ -3.2. We also show that X-ray deep surveys have probed even fainter magnitudes than the optical ones yielding more significant constraints on the shallow faint-end slope of the optical luminosity function. The inclusion of Type II AGN candidates, detected in the Chandra deep fields, hints towards an higher normalization for the total AGN luminosity function, if these sources lie at 5<z <6.5. We then discuss simple theoretical models of AGN formation and evolution in the context of cold dark matter cosmology. The comparison with the total AGN luminosity function favors a redshift-dependent relation between black hole and dark matter halo masses of the type M_BH~M_ halo^α, with 1.3< α< 1.7, compatible with independent studies from statistical analysis and rotation curve measurements. Finally we compute the quasar contribution to reionization to be <9% at z~6, up to ~30% when integrated within 5.5< z<6.5, significantly smaller than that from galaxies.

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arXiv2005

Measuring the Spin of Spiral Galaxies

We compute the angular momentum, the spin parameter and the related distribution function for Dark Matter halos hosting a spiral galaxy. We base on scaling laws, inferred from observations, that link the properties of the galaxy to those of the host halo; we further assume that the Dark Matter has the same total specific angular momentum of the baryons. Our main results are: (i) we find that the gas component of the disk significantly contributes to the total angular momentum of the system; (ii) by adopting for the Dark Matter the observationally supported Burkert profile, we compute the total angular momentum of the disk and its correlation with the rotation velocity; (iii) we find that the distribution function of the spin parameter $λ$ peaks at a value of about 0.03, consistent with a no-major-merger scenario for the late evolution of spiral galaxies.

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