8,189 research outputs found

    Intermediate-mass black holes in dwarf galaxies out to redshift \sim 2.4 in the Chandra COSMOS Legacy Survey

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    We present a sample of 40 AGN in dwarf galaxies at redshifts zz \lesssim 2.4. The galaxies are drawn from the \textit{Chandra} COSMOS-Legacy survey as having stellar masses 107M3×10910^{7}\leq M_{*}\leq3 \times 10^{9} M_{\odot}. Most of the dwarf galaxies are star-forming. After removing the contribution from star formation to the X-ray emission, the AGN luminosities of the 40 dwarf galaxies are in the range L0.510keV10391044L_\mathrm{0.5-10 keV} \sim10^{39} - 10^{44} erg s1^{-1}. With 12 sources at z>0.5z > 0.5, our sample constitutes the highest-redshift discovery of AGN in dwarf galaxies. The record-holder is cid\_1192, at z=2.39z = 2.39 and with L0.510keV1044L_\mathrm{0.5-10 keV} \sim 10^{44} erg s1^{-1}. One of the dwarf galaxies has M=6.6×107M_\mathrm{*} = 6.6 \times 10^{7} M_{\odot} and is the least massive galaxy found so far to host an AGN. All the AGN are of type 2 and consistent with hosting intermediate-mass black holes (BHs) with masses 104105\sim 10^{4} - 10^{5} M_{\odot} and typical Eddington ratios >1%> 1\%. We also study the evolution, corrected for completeness, of AGN fraction with stellar mass, X-ray luminosity, and redshift in dwarf galaxies out to zz = 0.7. We find that the AGN fraction for 109<M3×10910^{9}< M_{*}\leq3 \times 10^{9} M_{\odot} and LX10411042L_\mathrm{X} \sim 10^{41}-10^{42} erg s1^{-1} is \sim0.4\% for zz \leq 0.3 and that it decreases with X-ray luminosity and decreasing stellar mass. Unlike massive galaxies, the AGN fraction seems to decrease with redshift, suggesting that AGN in dwarf galaxies evolve differently than those in high-mass galaxies. Mindful of potential caveats, the results seem to favor a direct collapse formation mechanism for the seed BHs in the early Universe.Comment: 16 pages, 10 figures, accepted for publication in MNRA

    Mass-Radius Relation for Magnetic White Dwarfs

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    Recently, several white dwarfs with very strong surface magnetic fields have been observed. In this paper we explore the possibility that such stars could have sufficiently strong internal fields to alter their structure. We obtain a revised white dwarf mass-radius relation in the presence of strong internal magnetic fields. We first derive the equation of state for a fully degenerate ideal electron gas in a magnetic field using an Euler-MacLaurin expansion. We use this to obtain the mass-radius relation for magnetic 4^{4}He, 12^{12}C, and 56^{56}Fe white dwarfs of uniform composition.Comment: 7 pages, 7 figures and 1 table, To appear in Ap

    An extreme [OIII] emitter at z=3.2z=3.2: a low metallicity Lyman continuum source

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    [Abridged] We investigate the physical properties of a Lyman continuum emitter candidate at z=3.212z=3.212 with photometric coverage from UU to MIPS 24μ\mum band and VIMOS/VLT and MOSFIRE/Keck spectroscopy. Investigation of the UV spectrum confirms a direct spectroscopic detection of the Lyman continuum emission with S/N>5S/N>5. Non-zero Lyα\alpha flux at the systemic redshift and high Lyman-α\alpha escape fraction suggest a low HI column density. The weak C and Si low-ionization absorption lines are also consistent with a low covering fraction along the line of sight. The [OIII]λλ4959,5007+Hβ\lambda\lambda4959,5007+\mathrm{H}\beta equivalent width is one of the largest reported for a galaxy at z>3z>3 (EW([OIII]λλ4959,5007+Hβ)1600A˚\mathrm{EW}([\mathrm{OIII}]\lambda\lambda4959,5007+\mathrm{H}\beta) \simeq 1600\AA, rest-frame) and the NIR spectrum shows that this is mainly due to an extremely strong [OIII] emission. The large observed [OIII]/[OII] ratio (>10>10) and high ionization parameter are consistent with prediction from photoionization models in case of a density-bounded nebula scenario. Furthermore, the EW([OIII]λλ4959,5007+Hβ)\mathrm{EW}([\mathrm{OIII}]\lambda\lambda4959,5007+\mathrm{H}\beta) is comparable to recent measurements reported at z79z\sim7-9, in the reionization epoch. We also investigate the possibility of an AGN contribution to explain the ionizing emission but most of the AGN identification diagnostics suggest that stellar emission dominates instead. This source is currently the first high-zz example of a Lyman continuum emitter exhibiting indirect and direct evidences of a Lyman continuum leakage and having physical properties consistent with theoretical expectation from Lyman continuum emission from a density-bounded nebula.Comment: 14 pages, 11 figures, accepted for publication in A&A. Minor modifications, Figure 2 updated, Figure 9 adde

    Hubble imaging of the ionizing radiation from a star-forming galaxy at z=3.2 with fesc>50%

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    Star-forming galaxies are considered to be the leading candidate sources that dominate the cosmic reionization at z>7, and the search for analogs at moderate redshift showing Lyman continuum (LyC) leakage is currently a active line of research. We have observed a star-forming galaxy at z=3.2 with Hubble/WFC3 in the F336W filter, corresponding to the 730-890A rest-frame, and detect LyC emission. This galaxy is very compact and also has large Oxygen ratio [OIII]5007/[OII]3727 (>=10). No nuclear activity is revealed from optical/near-infrared spectroscopy and deep multi-band photometry (including the 6Ms X-ray, Chandra). The measured escape fraction of ionizing radiation spans the range 50-100\%, depending on the IGM attenuation. The LyC emission is detected at S/N=10 with m(F336W)=27.57+/-0.11 and it is spatially unresolved, with effective radius R_e<200pc. Predictions from photoionization and radiative transfer models are in line with the properties reported here, indicating that stellar winds and supernova explosions in a nucleated star-forming region can blow cavities generating density-bounded conditions compatible with optically thin media. Irrespective to the nature of the ionizing radiation, spectral signatures of these sources over the entire electromagnetic spectrum are of central importance for their identification during the epoch of reionization, when the LyC is unobservable. Intriguingly, the Spitzer/IRAC photometric signature of intense rest-frame optical emissions ([OIII]+Hbeta) observed recently at z~7.5-8.5 is similar to what is observed in this galaxy. Only the James Webb Space Telescope will measure optical line ratios at z>7 allowing a direct comparison with lower redshift LyC emitters, as reported here.Comment: 6 pages, 5 figures, ApJ submitted (comments welcome

    Analysis of White Dwarfs with Strange-Matter Cores

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    We summarize masses and radii for a number of white dwarfs as deduced from a combination of proper motion studies, Hipparcos parallax distances, effective temperatures, and binary or spectroscopic masses. A puzzling feature of these data is that some stars appear to have radii which are significantly smaller than that expected for a standard electron-degenerate white-dwarf equations of state. We construct a projection of white-dwarf radii for fixed effective mass and conclude that there is at least marginal evidence for bimodality in the radius distribution forwhite dwarfs. We argue that if such compact white dwarfs exist it is unlikely that they contain an iron core. We propose an alternative of strange-quark matter within the white-dwarf core. We also discuss the impact of the so-called color-flavor locked (CFL) state in strange-matter core associated with color superconductivity. We show that the data exhibit several features consistent with the expected mass-radius relation of strange dwarfs. We identify eight nearby white dwarfs which are possible candidates for strange matter cores and suggest observational tests of this hypothesis.Comment: 11 pages, 6 figures, accepted for publication in J. Phys. G: Nucl. Part. Phy

    Cold ideal equation of state for strongly magnetized neutron-star matter: effects on muon production and pion condensationn

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    Neutron stars with very strong surface magnetic fields have been suggested as the site for the origin of observed soft gamma repeaters (SGRs). In this paper we investigate the influence of such strong magnetic fields on the properties and internal structure of these magnetized neutron stars (magnetars). We study properties of a degenerate equilibrium ideal neutron-proton-electron (npe) gas with and without the effects of the anomalous nucleon magnetic moments in a magnetic field. The presence of a sufficiently strong magnetic field changes the ratio of protons to neutrons as well as the neutron drip density. We also study the appearance of muons as well as pion condensation in strong magnetic fields. We discuss the possibility that boson condensation in the interior of magnetars might be a source of SGRs.Comment: 10 pages included 9 figures, ApJ in pres
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