The Effects of Dark Matter Decay and Annihilation on the High-Redshift 21 cm Background

The radiation background produced by the 21 cm spin-flip transition of neutral hydrogen at high redshifts can be a pristine probe of fundamental physics and cosmology. At z~30-300, the intergalactic medium (IGM) is visible in 21 cm absorption against the cosmic microwave background (CMB), with a strength that depends on the thermal (and ionization) history of the IGM. Here we examine the constraints this background can place on dark matter decay and annihilation, which could heat and ionize the IGM through the production of high-energy particles. Using a simple model for dark matter decay, we show that, if the decay energy is immediately injected into the IGM, the 21 cm background can detect energy injection rates >10^{-24} eV cm^{-3} sec^{-1}. If all the dark matter is subject to decay, this allows us to constrain dark matter lifetimes <10^{27} sec. Such energy injection rates are much smaller than those typically probed by the CMB power spectra. The expected brightness temperature fluctuations at z~50 are a fraction of a mK and can vary from the standard calculation by up to an order of magnitude, although the difference can be significantly smaller if some of the decay products free stream to lower redshifts. For self-annihilating dark matter, the fluctuation amplitude can differ by a factor <2 from the standard calculation at z~50. Note also that, in contrast to the CMB, the 21 cm probe is sensitive to both the ionization fraction and the IGM temperature, in principle allowing better constraints on the decay process and heating history. We also show that strong IGM heating and ionization can lead to an enhanced H_2 abundance, which may affect the earliest generations of stars and galaxies.Comment: submitted to Phys Rev D, 14 pages, 8 figure

The Effects of UV Continuum and Lyman alpha Radiation on the Chemical Equilibrium of T Tauri Disks

We show in this Letter that the spectral details of the FUV radiation fields have a large impact on the chemistry of protoplanetary disks surrounding T Tauri stars. We show that the strength of a realistic stellar FUV field is significantly lower than typically assumed in chemical calculations and that the radiation field is dominated by strong line emission, most notably Lyman alpha radiation. The effects of the strong Lyman alpha emission on the chemical equilibrium in protoplanetary disks has previously been unrecognized. We discuss the impact of this radiation on molecular observations in the context of a radiative transfer model that includes both direct attenuation and scattering. In particular, Lyman alpha radiation will directly dissociate water vapor and may contribute to the observed enhancements of CN/HCN in disks.Comment: 14 pages, 4 figures, accepted by ApJ Letter

Rate coefficients for rovibrational transitions in H_2 due to collisions with He

We present quantum mechanical and quasiclassical trajectory calculations of cross sections for rovibrational transitions in ortho- and para-H_2 induced by collisions with He atoms. Cross sections were obtained for kinetic energies between 10^-4 and 3 eV, and the corresponding rate coefficients were calculated for the temperature range 100<T<4000 K. Comparisons are made with previous calculations.Comment: 21 pages, 2 figures, AAS, eps

Astrochemistry in the Early Universe: Collisional Rates for H on H2

We present preliminary results of a full quantum calculation of state to state cross sections for H on H2. These cross sections are calculated for v=0,4 j=0,15 for energies up to 3.0 eV. The cross sections are calculated on the BKMP2 potential surface (Boothroyd et al. 1996) with the ABC scattering code (Skouteris et al. 2000)

A multi-transition HCN and HCO+ study of 12 nearby active galaxies: AGN versus SB environments

Recent studies have indicated that the HCN-to-CO(J=1-0) and HCO+-to-HCN(J=1-0) ratios are significantly different between galaxies with AGN (active galactic nucleus) and SB (starburst) signatures. In order to study the molecular gas properties in active galaxies and search for differences between AGN and SB environments, we observed the HCN(J=1-0), (J=2-1), (J=3-2), HCO+(J=1-0) and HCO+(J=3-2), emission with the IRAM 30m in the centre of 12 nearby active galaxies which either exhibit nuclear SB and/or AGN signatures. Consistent with previous results, we find a significant difference of the HCN(J=2-1)-to-HCN(J=1-0), HCN(J=3-2)-to-HCN(J=1-0), HCO+(J=3-2)-to-HCO+(J=3-2), and HCO+-to-HCN intensity ratios between the sources dominated by an AGN and those with an additional or pure central SB: the HCN, HCO+ and HCO+-to-HCN intensity ratios tend to be higher in the galaxies of our sample with a central SB as opposed to the pure AGN cases which show rather low intensity ratios. Based on an LVG analysis of these data, i.e., assuming purely collisional excitation, the (average) molecular gas densities in the SB dominated sources of our sample seem to be systematically higher than in the AGN sources. The LVG analysis seems to further support systematically higher HCN and/or lower HCO+ abundances as well as similar or higher gas temperatures in AGN compared to the SB sources of our sample. Also, we find that the HCN-to-CO ratios decrease with increasing rotational number J for the AGN while they stay mostly constant for the SB sources.Comment: accepted for publication in ApJ; 20 pages, 7 figures; in emulateApJ forma

HNC, HCN and CN in Seyfert galaxies

Bright HNC 1--0 emission has been found towards several Seyfert galaxies. This is unexpected since traditionally HNC is a tracer of cold (10 K) gas, and the molecular gas of luminous galaxies like Seyferts is thought to have bulk kinetic temperatures surpassing 50 K. In this work we aim to distinguish the cause of the bright HNC and to model the physical conditions of the HNC and HCN emitting gas. We have used SEST, JCMT and IRAM 30m telescopes to observe HNC 3-2 and HCN 3-2 line emission in a selection of 5 HNC-luminous Seyfert galaxies. We estimate and discuss the excitation conditions of HCN and HNC in NGC 1068, NGC 3079, NGC 2623 and NGC 7469, based on the observed 3-2/1-0 line intensity ratios. We also observed CN 1-0 and 2-1 emission and discuss its role in photon and X-ray dominated regions. HNC 3-2 was detected in 3 galaxies (NGC 3079, NGC 1068 and NGC 2623). HCN 3-2 was detected in NGC 3079, NGC 1068 and NGC 1365. The HCN 3-2/1-0 ratio is lower than 0.3 only in NGC 3079, whereas the HNC 3-2/1-0 ratio is larger than 0.3 only in NGC 2623. The HCN/HNC 1-0 and 3-2 line ratios are larger than unity in all the galaxies. The HCN/HNC 3-2 line ratio is lower than unity only in NGC 2623, similar to Arp 220, Mrk 231 and NGC 4418. In three of the galaxies the HNC emissions emerge from gas of densities n<10^5 cm^3, where the chemistry is dominated by ion-neutral reactions. In NGC 1068 the emission of HNC emerges from lower (<10^5 cm^3) density gas than HCN (>10^5 cm^3). Instead, the emissions of HNC and HCN emerge from the same gas in NGC 3079. The observed HCN/HNC and CN/HCN line ratios favor a PDR scenario, rather than an XDR one. However, the N(HNC)/N(HCN) column density ratios obtained for NGC 3079 can be found only in XDR environments.Comment: Accepted for publication in A&A. A selection of this paper will be presented as a poster in the FIR workshop 2007, held at Bad Honnef, Germany. High resolution figures in original paper. 16 pages, 8 figure

Tracing star formation in galaxies with molecular line and continuum observations

We report our recent progress on extragalactic spectroscopic and continuum observations, including HCN(J=1-0), HCO$^+$(J=1-0), and CN(N=1-0) imaging surveys of local Seyfert and starburst galaxies using the Nobeyama Millimeter Array, high-J CO observations (J=3-2 observations using the Atacama Submillimeter Telescope Experiment (ASTE) and J=2-1 observations with the Submillimeter Array) of galaxies, and $\lambda$ 1.1 mm continuum observations of high-z violent starburst galaxies using the bolometer camera AzTEC mounted on ASTE.Comment: 6 pages, 5 figures, To appear in proceedings of "Far-Infrared and Submillimeter Emission of the Interstellar Medium", EAS Publication Series, Bad Honnef, November 2007, Eds. C. Kramer, S. Aalto, R. Simon. See http://www.nro.nao.ac.jp/~f0212kk/FIR07/kk-ver20.pdf for a version with high resolution figure

When an old star smolders: On the detection of hydrocarbon emission from S-type AGB stars

Polycyclic aromatic hydrocarbons (PAHs) produce characteristic infrared emission bands that have been observed in a wide range of astrophysical environments, where carbonaceous material is subjected to ultraviolet (UV) radiation. Although PAHs are expected to form in carbon-rich AGB stars, they have up to now only been observed in binary systems where a hot companion provides a hard radiation field. In this letter, we present low-resolution infrared spectra of four S-type AGB stars, selected from a sample of 90 S-type AGB stars observed with the infrared spectrograph aboard the Spitzer satellite. The spectra of these four stars show the typical infrared features of PAH molecules. We confirm the correlation between the temperature of the central star and the centroid wavelength of the 7.9 {\mu}m feature, present in a wide variety of stars spanning a temperature range from 3 000 to 12 000 K. Three of four sources presented in this paper extend this relation towards lower temperatures. We argue that the mixture of hydrocarbons we see in these S-stars has a rich aliphatic component. The fourth star, BZ CMa, deviates from this correlation. Based on the similarity with the evolved binary TU Tau, we predict that BZ CMa has a hot companion as well.Comment: 5 pages, 2 figures, 2 table

HD/H2 Molecular Clouds in the Early Universe: The Problem of Primordial Deuterium

We have detected new HD absorption systems at high redshifts, z_abs=2.626 and z_abs=1.777, identified in the spectra of the quasars J0812+3208 and Q1331+170, respectively. Each of these systems consists of two subsystems. The HD column densities have been determined: log(N(HD),A)=15.70+/-0.07 for z_A=2.626443(2) and log(N(HD),B)=12.98+/-0.22 for z_B=2.626276(2) in the spectrum of J0812+3208 and log(N(HD),C)=14.83+/-0.15 for z_C=1.77637(2) and log(N(HD),D)=14.61+/-0.20 for z_D=1.77670(3) in the spectrum of Q1331+170. The measured HD/H2 ratio for three of these subsystems has been found to be considerably higher than its values typical of clouds in our Galaxy. We discuss the problem of determining the primordial deuterium abundance, which is most sensitive to the baryon density of the Universe \Omega_{b}. Using a well-known model for the chemistry of a molecular cloud, we have estimated the isotopic ratio D/H=HD/2H_2=(2.97+/-0.55)x10^{-5} and the corresponding baryon density \Omega_{b}h^2=0.0205^{+0.0025}_{-0.0020}. This value is in good agreement with \Omega_{b}h^2=0.0226^{+0.0006}_{-0.0006} obtained by analyzing the cosmic microwave background radiation anisotropy. However, in high-redshift clouds, under conditions of low metallicity and low dust content, hydrogen may be incompletely molecularized even in the case of self-shielding. In this situation, the HD/2H_2 ratio may not correspond to the actual D/H isotopic ratio. We have estimated the cloud molecularization dynamics and the influence of cosmological evolutionary effects on it

Enhanced cosmic-ray flux toward zeta Persei inferred from laboratory study of H3+ - e- recombination rate

The H3+ molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many interstellar molecules. In dense clouds, the H3+ abundance is understood using a simple chemical model, from which observations of H3+ yield valuable estimates of cloud path length, density, and temperature. On the other hand, observations of diffuse clouds have suggested that H3+ is considerably more abundant than expected from the chemical models. However, diffuse cloud models have been hampered by the uncertain values of three key parameters: the rate of H3+ destruction by electrons, the electron fraction, and the cosmic-ray ionisation rate. Here we report a direct experimental measurement of the H3+ destruction rate under nearly interstellar conditions. We also report the observation of H3+ in a diffuse cloud (towards zeta Persei) where the electron fraction is already known. Taken together, these results allow us to derive the value of the third uncertain model parameter: we find that the cosmic-ray ionisation rate in this sightline is forty times faster than previously assumed. If such a high cosmic-ray flux is indeed ubiquitous in diffuse clouds, the discrepancy between chemical models and the previous observations of H3+ can be resolved.Comment: 6 pages, Nature, in pres