10,170 research outputs found

    Downsizing of Star-Forming Galaxies by Gravitational Processes

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    There is observed a trend that a lower mass galaxy forms stars at a later epoch. This downsizing of star-forming galaxies has been attributed to hydrodynamical or radiative feedback processes that regulate star formation. However, here we explain the downsizing by gravitational processes alone, in the bottom-up scenario where galaxies evolve from subgalactic-scale objects. Within a region of the initial density field that is to evolve into a lower mass galaxy, subgalactic-scale fluctuation is of a smaller amplitude. The formation of subgalactic-scale objects, i.e., gravitational collapse of the subgalactic-scale fluctuation, and the subsequent onset of star formation accordingly occur at a later epoch for a lower mass galaxy. As a function of galaxy mass, we calculate the peak epoch of formation of subgalactic-scale objects. The peak epoch is consistent with the peak epoch of star formation derived from observations. [abridged]Comment: 4 pages, to appear in Astronomy and Astrophysics (Research Note

    Runaway Merging of Black Holes: Analytical Constraint on the Timescale

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    Following the discovery of a black hole (BH) with a mass of 10^3-10^6 M(sun) in a starburst galaxy M82, we study formation of such a BH via successive merging of stellar-mass BHs within a star cluster. The merging has a runaway characteristic. This is because massive BHs sink into the cluster core and have a high number density, and because the merging probability is higher for more massive BHs. We use the Smoluchowski equation to study analytically the evolution of the BH mass distribution. Under favorable conditions, which are expected for some star clusters in starburst galaxies, the timescale of the runaway merging is at most of order 10^7 yr. This is short enough to account for the presence of a BH heavier than 10^3 M(sun) in an ongoing starburst region.Comment: 10 pages, no figures, to appear in The Astrophysical Journal (Letters

    Thermodynamic properties of quadrupolar states in the frustrated pyrochlore magnet Tb2_2Ti2_2O7_7

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    The low-temperature thermodynamic properties of the frustrated pyrochlore Tb2+x_{2+x}Ti2x_{2-x}O7+y_{7+y} have been studied using the single crystal of x=0.005x=0.005 sitting in a long range ordered phase in the xx-TT phase diagram. We observed that the specific heat exhibits a minimum around 2 K and slightly increases on cooling, similar to a Schottky-like anomaly for canonical spin ices. A clear specific-heat peak observed at Tc=0.53T_{\rm c} = 0.53 K is ascribable to the phase transition to a quadrupolar state, which contributes to a relatively large change in entropy, S2.7S \simeq 2.7 J K1^{-1}mol1^{-1}. However, it is still smaller than Rln2R\ln2 for the ground state doublet of the Tb ions. The entropy release persists to higher temperatures, suggesting strong fluctuations associated with spin ice correlations above TcT_{\rm c}. We discuss the field dependence of the entropy change for H[111]H||[111] and H[001]H||[001].Comment: 6 pages, 2 figure

    The Discovery of Two Lymanα\alpha Emitters Beyond Redshift 6 in the Subaru Deep Field

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    We have performed a deep optical imaging survey using a narrowband filter (NB921NB921) centered at λ=\lambda = 9196 \AA ~ together with ii^\prime and zz^\prime broadband filters covering an 814 arcmin2^2 area of the Subaru Deep Field. We obtained a sample of 73 strong NB921NB921-excess objects based on the following two color criteria; zNB921>1z^\prime - NB921 > 1 and iz>1.3i^\prime - z^\prime > 1.3. We then obtained optical spectroscopy of nine objects in our NB921NB921-excess sample, and identified at least two Lyα\alpha emitters atz=6.541±0.002z=6.541 \pm 0.002 and z=6.578±0.002z=6.578 \pm 0.002, each of which shows the characteristic sharp cutoff together with the continuum depression at wavelengths shortward of the line peak. The latter object is more distant than HCM-6A at z=6.56z=6.56 and thus this is the most distant known object found so far. These new data allow us to estimate the first meaningful lower limit of the star formation rate density beyond redshift 6; ρSFR5.2×104M\rho_{\rm SFR} \sim 5.2 \times 10^{-4} M_\odot yr1^{-1} Mpc3^{-3}. Since it is expected that the actual density is higher by a factor of several than this value, our new observation reveals that a moderately high level of star formation activity already occurred at zz \sim 6.6.Comment: 13 pages, 3 figures. PASJ (Letters), 55, vol.2, in pres

    Novel phase diagram for antiferromagnetism and superconductivity in pressure-induced heavy-fermion superconductor Ce2_2RhIn8_8 probed by In-NQR

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    We present a novel phase diagram for the antiferromagnetism and superconductivity in Ce2_2RhIn8_8 probed by In-NQR studies under pressure (PP). The quasi-2D character of antiferromagnetic spin fluctuations in the paramagnetic state at PP = 0 evolves into a 3D character because of the suppression of antiferromagnetic order for P>PQCPP > P_{\rm QCP}\sim 1.36 GPa (QCP: antiferromagnetic quantum critical point). Nuclear-spin-lattice-relaxation rate 1/T11/T_1 measurements revealed that the superconducting order occurs in the PP range 1.36 -- 1.84 GPa, with maximum TcT_c\sim 0.9 K around PQCPP_{\rm QCP}\sim 1.36 GPa.Comment: 5 pages, 5 figures, submitted to PR