HCN versus HCO+ as dense molecular gas mass tracer in Luminous Infrared Galaxies

It has been recently argued that the HCN J=1--0 line emission may not be an unbiased tracer of dense molecular gas (\rm n\ga 10^4 cm^{-3}) in Luminous Infrared Galaxies (LIRGs: $\rm L_{FIR}> 10^{11} L_{\odot}$) and HCO$^+$ J=1--0 may constitute a better tracer instead (Graci\'a-Carpio et al. 2006), casting doubt into earlier claims supporting the former as a good tracer of such gas (Gao & Solomon 2004; Wu et al. 2006). In this paper new sensitive HCN J=4--3 observations of four such galaxies are presented, revealing a surprisingly wide excitation range for their dense gas phase that may render the J=1--0 transition from either species a poor proxy of its mass. Moreover the well-known sensitivity of the HCO$^+$ abundance on the ionization degree of the molecular gas (an important issue omitted from the ongoing discussion about the relative merits of HCN and HCO$^+$ as dense gas tracers) may severely reduce the HCO$^+$ abundance in the star-forming and highly turbulent molecular gas found in LIRGs, while HCN remains abundant. This may result to the decreasing HCO$^+$/HCN J=1--0 line ratio with increasing IR luminosity found in LIRGs, and casts doubts on the HCO$^+$ rather than the HCN as a good dense molecular gas tracer. Multi-transition observations of both molecules are needed to identify the best such tracer, its relation to ongoing star formation, and constrain what may be a considerable range of dense gas properties in such galaxies.Comment: 16 pages, 4 figures, Accepted for publication in the Astrophysical Journa

CO Emission in Low Luminosity, HI Rich Galaxies

We present 12CO 1-0 observations of eleven low luminosity M_B > -18), HI--rich dwarf galaxies. Only the three most metal-rich galaxies, with 12+log(O/H) ~ 8.2, are detected. Very deep CO spectra of six extremely metal-poor systems (12+log(O/H) < 7.5) yield only low upper limits on the CO surface brightness, I_CO < 0.1 K km/s. Three of these six have never before been observed in a CO line, while the others now have much more stringent upper limits. For the very low metallicity galaxy Leo A, we do not confirm a previously reported detection in CO, and the limits are consistent with another recent nondetection. We combine these new observations with data from the literature to form a sample of dwarf galaxies which all have CO observations and measured oxygen abundances. No known galaxies with 12+log(O/H) < 7.9 (Z < 0.1 solar) have been detected in CO. Most of the star-forming galaxies with higher (12+log(O/H) > 8.1) metallicities are detected at similar or higher I_CO surface brightnesses. The data are consistent with a strong dependence of the I_CO/M_H_2 = X_CO conversion factor on ambient metallicity. The strikingly low upper limits on some metal-poor galaxies lead us to predict that the conversion factor is non-linear, increasing sharply below approximately 1/10 of the solar metallicity (12+log(O/H) < 7.9).Comment: 25 pages, 4 figures, 3 tables. Accepted for publication in AJ Tables replaced -- now formated for landscape orientatio

High Excitation Molecular Gas in the Magellanic Clouds

We present the first survey of submillimeter CO 4-3 emission in the Magellanic Clouds. The survey is comprised of 15 6'x6' maps obtained using the AST/RO telescope toward the molecular peaks of the Large and Small Magellanic Clouds. We have used these data to constrain the physical conditions in these objects, in particular their molecular gas density and temperature. We find that there are significant amounts of molecular gas associated with most of these molecular peaks, and that high molecular gas temperatures are pervasive throughout our sample. We discuss whether this may be due to the low metallicities and the associated dearth of gas coolants in the Clouds, and conclude that the present sample is insufficient to assert this effect.Comment: 18 pages, 3 figures, 5 tables. To appear in Ap

Charged Rotating Black Holes on a 3-Brane

We study exact stationary and axisymmetric solutions describing charged rotating black holes localized on a 3-brane in the Randall-Sundrum braneworld. The charges of the black holes are considered to be of two types, the first being an induced tidal charge that appears as an imprint of nonlocal gravitational effects from the bulk space and the second is a usual electric charge arising due to a Maxwell field trapped on the brane. We assume a special ansatz for the metric on the brane taking it to be of the Kerr-Schild form and show that the Kerr-Newman solution of ordinary general relativity in which the electric charge is superceded by a tidal charge satisfies a closed system of the effective gravitational field equations on the brane. It turns out that the negative tidal charge may provide a mechanism for spinning up the black hole so that its rotation parameter exceeds its mass. This is not allowed in the framework of general relativity. We also find a new solution that represents a rotating black hole on the brane carrying both charges. We show that for a rapid enough rotation the combined influence of the rotational dynamics and the local bulk effects of the "squared" energy momentum tensor on the brane distort the horizon structure of the black hole in such a way that it can be thought of as composed of non-uniformly rotating null circles with growing radii from the equatorial plane to the poles. We finally study the geodesic motion of test particles in the equatorial plane of a rotating black hole with tidal charge. We show that the effects of negative tidal charge tend to increase the horizon radius, as well as the radii of the limiting photon orbit, the innermost bound and the innermost stable circular orbits for both direct and retrograde motions of the particles.Comment: RevTeX 4, 33 pages, 4 figures, new references adde

The Origin of the Dust Arch in the Halo of NGC 4631: An Expanding Superbubble?

We study the nature and the origin of the dust arch in the halo of the edge-on galaxy NGC 4631 detected by Neininger & Dumke (1999). We present CO observations made using the new On-The-Fly mapping mode with the FCRAO 14m telescope, and find no evidence for CO emission associated with the dust arch. Our examination of previously published HI data shows that if previous assumptions about the dust temperature and gas/dust ratio are correct, then there must be molecular gas associated with the arch, below our detection threshold. If this is true, then the molecular mass associated with the dust arch is between 1.5 x 10^8 M(sun)and 9.7 x 10^8 M(sun), and likely towards the low end of the range. A consequence of this is that the maximum allowed value for the CO-to-H_2 conversion factor is 6.5 times the Galactic value, but most likely closer to the Galactic value. The kinematics of the HI apparently associated with the dust arch reveal that the gas here is not part of an expanding shell or outflow, but is instead two separate features (a tidal arm and a plume of HI sticking out into the halo) which are seen projected together and appear as a shell. Thus there is no connection between the dust "arch" and the hot X-ray emitting gas that appears to surround the galaxy Wang et al. (2001).Comment: 14 pages, including 4 figures. Accepted by A.J. for March 200

Exactly solvable model of wormhole supported by phantom energy

We have found a simple exact solution of spherically-symmetrical Einstein equations describing a wormhole for an inhomogeneous distribution of the phantom energy. The equation of state is linear but highly anisotropic: while the radial pressure is negative, the transversal one is positive. At infinity the spacetime is not asymptotically flat and possesses on each side of the bridge a regular cosmological Killing horizon with an infinite area, impenetrable for any particles. This horizon does not arise if the wormhole region is glued to the Schwarzschild region. In doing so, the wormhole can enclose an arbitrary amount of the phantom energy. The configuration under discussion has a limit in which the phantom energy turns into the string dust, the areal radius tends to the constant. In this limit, the strong gravitational mass defect is realized in that the gravitational active mass is finite and constant while the proper mass integrated over the total manifold is infinite.Comment: 6 pages. Two references added, typos corrected. Accepted for publication in Phys. Rev. D as Rapid Communicatio

Quantum information processing based on P-31 nuclear spin qubits in a quasi-one-dimensional Si-28 nanowire

We suggest a new method of quantum information processing based on the precise placing of P-31 isotope atoms in a quasi-one-dimensional Si-28 nanowire using isotope engineering and neutron-transmutation doping of the grown structures. In our structure, interqubit entanglement is based on the indirect interaction of P-31 nuclear spins with electrons localized in a nanowire. This allows one to control the coupling between distant qubits and between qubits separated by non-qubit neighboring nodes. The suggested method enables one to fabricate structures using present-day nanolithography. Numerical estimates show the feasibility of the proposed device and method of operation.Comment: 7 pages, 4 figure

Reply to "On scaling solutions with a dissipative fluid"

In this paper we show that the claims in [Class. Quantum Grav. 19 (2002) 3067, gr-qc/0203081] related to our analysis in [Phys. Rev. D 62, 063508 (2000), astro-ph/0005070] are wrong.Comment: 4 pages, uses RevTeX. v2: To appear in Class. Quantum Gra

Planck Observations of M33

We have performed a comprehensive investigation of the global integrated flux density of M33 from radio to ultraviolet wavelengths, finding that the data between $\sim$100 GHz and 3 THz are accurately described by a single modified blackbody curve with a dust temperature of $T_\mathrm{dust}$ = 21.67$\pm$0.30 K and an effective dust emissivity index of $\beta_\mathrm{eff}$ = 1.35$\pm$0.10, with no indication of an excess of emission at millimeter/sub-millimeter wavelengths. However, sub-dividing M33 into three radial annuli, we found that the global emission curve is highly degenerate with the constituent curves representing the sub-regions of M33. We also found gradients in $T_\mathrm{dust}$ and $\beta_\mathrm{eff}$ across the disk of M33, with both quantities decreasing with increasing radius. Comparing the M33 dust emissivity with that of other Local Group members, we find that M33 resembles the Magellanic Clouds rather than the larger galaxies, i.e., the Milky Way and M31. In the Local Group sample, we find a clear correlation between global dust emissivity and metallicity, with dust emissivity increasing with metallicity. A major aspect of this analysis is the investigation into the impact of fluctuations in the Cosmic Microwave Background (CMB) on the integrated flux density spectrum of M33. We found that failing to account for these CMB fluctuations would result in a significant over-estimate of $T_\mathrm{dust}$ by $\sim$5 K and an under-estimate of $\beta_\mathrm{eff}$ by $\sim$0.4.Comment: Accepted for publication in MNRA

Anisotropic domain walls

We find an anisotropic, non-supersymmetric generalization of the extreme supersymmetric domain walls of simple non-dilatonic supergravity theory. As opposed to the isotropic non- and ultra-extreme domain walls, the anisotropic non-extreme wall has the \emph{same} spatial topology as the extreme wall. The solution has naked singularities which vanish in the extreme limit. Since the Hawking temperature on the two sides is different, the generic solution is unstable to Hawking decay.Comment: 11 pages, LaTeX, 3 PostScript figures, uses amstex and epsfi