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Register a new userThe correlation of UHECRs with nearby galaxies in the Local Volume
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Added by
Antonio Jose Cuesta Vazquez
Publication date
2009
fields
Physics
and research's language is
English
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We explore the possibility of a local origin for ultra high energy cosmic rays (UHECRs). Using the catalogue of Karachentsev et al. including nearby galaxies with distances less than 10Mpc (Local Volume), we search for a correlation with the sample of UHECR events released so far by the Pierre Auger collaboration. The counterpart sample selection is performed with variable distance and luminosity cuts which extract the most likely sources in the catalogue. The probability of chance correlation after penalizing for scans is 0.96%, which corresponds to a correlation signal of 2.6sigma. We find that the parameters that maximize the signal are psi=3.0deg, D_{max}=4Mpc and M_B=-15 for the maximum angular separation between cosmic rays and galaxy sources, maximum distance to the source, and sources brighter than B-band absolute magnitude respectively. This implies a preference for the UHECRs arrival directions to be correlated with the nearest and most luminous galaxies in the Local Volume. We note that nearby galaxies with D<10Mpc show a similar correlation with UHECRs as compared to that found by The Pierre Auger Collaboration using active galactic nuclei (AGNs) within 70-100Mpc instead of local galaxies, although less than 20% of cosmic ray events are correlated to a source in our study. However, the observational evidence for mixed composition in the high-energy end of the cosmic ray spectrum supports the possibility of a local origin for UHECRs, as CNO nuclei can travel only few Mpc without strong attenuation by the GZK effect, whereas the observed suppression in the energy spectrum would require more distant sources in the case of pure proton composition interacting with the CMB.
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The arrival directions of cosmic rays detected by the Pierre Auger Observatory (Auger) with energies above 39~EeV were recently reported to correlate with the positions of 23 nearby starburst galaxies (SBGs): in their best-fit model, 9.7% of the cosmic-ray flux originates from these objects and undergoes angular diffusion on a $12.9^circ$~scale. On the other hand, some of the SBGs on their list, including the brightest one (M82), are at northern declinations outside the Auger field of view. Data from detectors in the northern hemisphere would be needed to look for cosmic-ray excesses near these objects. In this work, we tested the Auger best-fit model against data collected by the Telescope Array (TA) in a 9-year period, without trying to re-optimize the model parameters for our dataset in order not to introduce statistical penalties. The resulting test statistic (double log-likelihood ratio) was $-1.00$, corresponding to $1.1sigma$ significance among isotropically generated random datasets, and to $-1.4sigma$ significance among ones generated assuming the Auger best-fit model. In other words, our data is still insufficient to conclusively rule out either hypothesis. The ongoing fourfold expansion of TA will collect northern hemisphere data with much more statistics, improving our ability to discriminate between different flux models.
In this paper, we compare dwarf galaxies and galaxy clusters in order to elucidate which object class is the best target for gamma-ray DM searches with imaging atmospheric Cherenkov telescopes (IACTs). We have built a mixed dwarfs+clusters sample containing some of the most promising nearby dwarf galaxies (Draco, Ursa Minor, Wilman 1 and Segue 1) and local galaxy clusters (Perseus, Coma, Ophiuchus, Virgo, Fornax, NGC5813 and NGC5846), and then compute their DM annihilation flux profiles by making use of the latest modeling of their DM density profiles. We also include in our calculations the effect of DM substructure. Willman 1 appears as the best candidate in the sample. However, its mass modeling is still rather uncertain, so probably other candidates with less uncertainties and quite similar fluxes, namely Ursa Minor and Segue 1, might be better options. As for galaxy clusters, Virgo represents the one with the highest flux. However, its large spatial extension can be a serious handicap for IACT observations and posterior data analysis. Yet, other local galaxy cluster candidates with more moderate emission regions, such as Perseus, may represent good alternatives. After comparing dwarfs and clusters, we found that the former exhibit annihilation flux profiles that, at the center, are roughly one order of magnitude higher than those of clusters, although galaxy clusters can yield similar, or even higher, integrated fluxes for the whole object once substructure is taken into account. Even when any of these objects are strictly point-like according to the properties of their annihilation signals, we conclude that dwarf galaxies are best suited for observational strategies based on the search of point-like sources, while galaxy clusters represent best targets for analyses that can deal with rather extended emissions. Finally, we study the detection prospects for IACTs [ABRIDGED]
The majority of galaxies with current star-formation rates (SFRs), SFRo >= 10^-3 Msun/yr, in the Local Cosmological Volume where observations should be reliable, have the property that their observed SFRo is larger than their average star formation rate. This is in tension with the evolution of galaxies described by delayed-tau models, according to which the opposite would be expected. The tension is apparent in that local galaxies imply the star formation timescale tau approx 6.7 Gyr, much longer than the 3.5-4.5 Gyr obtained using an empirically determined main sequence at several redshifts. Using models where the SFR is a power law in time of the form propto (t - t1)^eta for t1 = 1.8 Gyr (with no stars forming prior to t1) implies that eta = 0.18 +- 0.03. This suggested near-constancy of a galaxys SFR over time raises non-trivial problems for the evolution and formation time of galaxies, but is broadly consistent with the observed decreasing main sequence with increasing age of the Universe.
Using a simple model of photodissociated atomic hydrogen on a galactic scale, it is possible to derive total hydrogen volume densities. These densities, obtained through a combination of atomic hydrogen, far-ultraviolet and metallicity data, provide an independent probe of the combined atomic and molecular hydrogen gas in galactic disks. We present a new, flexible and fully automated procedure using this simple model. This automated method will allow us to take full advantage of a host of available data on galaxies in order to calculate total hydrogen volume densities of giant molecular clouds surrounding sites of recent star formation. So far this was only possible on a galaxy-by-galaxy basis using by-eye analysis of candidate photodissociation regions. We test the automated method by adopting different models for the dust-to-gas ratio and comparing the resulting densities for M74, including a new metallicity map of M74 produced by integral field spectroscopy. We test the procedure against previously published M83 volume densities based on the same method and find no significant differences. The range of total hydrogen volume densities obtained for M74 is approximately 5-700 cm-3 . Different dust-to-gas ratio models do not result in measurably different densities. The cloud densities presented here add M74 to the list of galaxies analyzed using the assumption of photodissociated atomic hydrogen occurring near sites of recent star formation and further solidify the method. For the first time, full metallicity maps were included in the analysis as opposed to metallicity gradients. The results will need to be compared to other tracers of the interstellar medium and photodissociation regions, such as CO and CII, in order to test our basic assumptions, specifically, our assumption that the HI we detect originates in photodissociation regions.
The most distant AGN, within the allowed GZK cut-off radius, have been recently candidate by many authors as the best location for observed UHECR origination. Indeed, the apparent homogeneity and isotropy of recent UHECR signals seems to require a far cosmic isotropic and homogeneous scenario involving a proton UHECR courier: our galaxy or nearest local group or super galactic plane (ruled by Virgo cluster) are too much near and apparently too much anisotropic in disagreement with PAO and TA almost homogeneous sample data. However, the few and mild observed UHECR clustering, the North and South Hot Spots, are smeared in wide solid angles. Their consequent random walk flight from most far GZK UHECR sources, nearly at 100 Mpc, must be delayed (with respect to a straight AGN photon gamma flaring arrival trajectory) at least by a million years. During this time, the AGN jet blazing signal, its probable axis deflection (such as the helical jet in Mrk501), its miss alignment or even its almost certain exhaust activity may lead to a complete misleading correlation between present UHECR events and a much earlier active AGN ejection. UHECR maps maybe anyway related to galactic or nearest (Cen A, M82) AGN extragalactic UHECR sources shining in twin Hot Spot. Therefore we defend our (quite different) scenarios where UHECR are mostly made by lightest UHECR nuclei originated by nearby AGN sources, or few galactic sources, whose delayed signals reach us within few thousand years in the observed smeared sky areas.
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