From light to mass: accessing the initial and present-day Galactic globular cluster mass functions

The initial and present-day mass functions (ICMF and PDMF, respectively) of the Galactic globular clusters (GCs) are constructed based on their observed luminosities, the stellar evolution and dynamical mass-loss processes, and the mass-to-light ratio (MLR). Under these conditions, a Schechter-like ICMF is evolved for approximately a Hubble time and converted into the luminosity function (LF), which requires finding the values of 5 free parameters: the mean GC age (\tA), the dissolution timescale of a 10^5 \ms cluster ($\tau_5$), the exponential truncation mass (\mc) and 2 MLR parametrising constants. This is achieved by minimising the residuals between the evolved and observed LFs, with the minimum residuals and realistic parameters obtained with MLRs that increase with luminosity (or mass). The optimum PMDFs indicate a total stellar mass of $\sim4\times10^7$ \ms\ still bound to GCs, representing $\sim15$ of the mass in clusters at the beginning of the gas-free evolution. The corresponding ICMFs resemble the scale-free MFs of young clusters and molecular clouds observed in the local Universe, while the PDMFs follow closely a lognormal distribution with a turnover at \mto\sim7\times10^4\,\ms. For most of the GC mass range, we find an MLR lower than usually adopted, which explains the somewhat low \mto. Our results confirm that the MLR increases with cluster mass (or luminosity), and suggest that GCs and young clusters share a common origin in terms of physical processes related to formation.Comment: Accepted by MNRA

Mapping the differential reddening in globular clusters

We build differential-reddening maps for 66 Galactic globular clusters (GCs) with archival HST WFC/ACS F606W and F814W photometry. Because of the different GC sizes (characterised by the half-light radius $R_h$) and distances to the Sun, the WFC/ACS field of view (200\arcsec\times200\arcsec) coverage ($R_{obs}$) lies in the range 1\la R_{obs}/R_h\la15 for about 85% of the sample, with about 10% covering only the inner (R_{obs}\la R_h) parts. We divide the WFC/ACS field of view across each cluster in a regular cell grid, and extract the stellar-density Hess diagram from each cell, shifting it in colour and magnitude along the reddening vector until matching the mean diagram. Thus, the maps correspond to the internal dispersion of the reddening around the mean. Depending on the number of available stars (i.e. probable members with adequate photometric errors), the angular resolution of the maps range from \approx7\arcsec\times7\arcsec to \approx20\arcsec\times20\arcsec. We detect spatially-variable extinction in the 66 globular clusters studied, with mean values ranging from \mEBV\approx0.018 (NGC\,6981) up to \mEBV\approx0.16 (Palomar\,2). Differential-reddening correction decreases the observed foreground reddening and the apparent distance modulus but, since they are related to the same value of \EBV, the distance to the Sun is conserved. Fits to the mean-ridge lines of the highly-extincted and photometrically scattered globular cluster Palomar\,2 show that age and metallicity also remain unchanged after the differential-reddening correction, but measurement uncertainties decrease because of the reduced scatter. The lack of systematic variations of \mEBV\ with both the foreground reddening and the sampled cluster area indicates that the main source of differential reddening is interstellar.Comment: Accepted by MNRA

A correlation between the highest energy cosmic rays and nearby active galactic nuclei detected by Fermi

We analyze the correlation of the positions of gamma-ray sources in the Fermi Large Area Telescope First Source Catalog (1FGL) and the First LAT Active Galactic Nuclei (AGN) Catalog (1LAC) with the arrival directions of ultra-high-energy cosmic rays (UHECRs) observed with the Pierre Auger Observatory, in order to investigate the origin of UHECRs. We find that Galactic sources and blazars identified in the 1FGL are not significantly correlated with UHECRs, while the 1LAC sources display a mild correlation (2.6 sigma level) on a ~2.4 degree angular scale. When selecting only the 1LAC AGNs closer than 200 Mpc, we find a strong association (5.4 sigma) between their positions and the directions of UHECRs on a ~17 degree angular scale; the probability of the observed configuration being due to an isotropic flux of cosmic rays is 5x10^{-8}. There is also a 5 sigma correlation with nearby 1LAC sources on a 6.5 degree scale. We identify 7 "gamma-ray loud" AGNs which are associated with UHECRs within ~17 degree and are likely candidates for the production sites of UHECRs: Centaurus A, NGC 4945, ESO 323-G77, 4C+04.77, NGC 1218, RX J0008.0+1450 and NGC 253. We interpret these results as providing additional support to the hypothesis of the origin of UHECRs in nearby extragalactic objects. As the angular scales of the correlations are large, we discuss the possibility that intervening magnetic fields might be considerably deflecting the trajectories of the particles on their way to Earth.Comment: 23 pages, 7 figures, accepted for publication in Ap