On the arcmin structure of the X-ray Universe

We present the angular correlation function of the X-ray population of 1063 XMM-Newton observations at high Galactic latitudes, comprising up to ~30000 sources over a sky area of ~125 sq. degrees in the energy bands: soft (0.5-2 keV) and hard (2-10 keV). This is the largest sample of serendipitous X-ray sources ever used for clustering analysis purposes to date and the results have been determined with unprecedented accuracy. We detect significant clustering signals in the soft and hard bands (~10 sigma and ~5 sigma, respectively). We deproject the angular correlation function via Limber's equation and calculate the typical spatial lengths. We infer that AGN at redshifts ~1 are embedded in dark matter halos with typical masses of log M ~ 12.6/h Msol and lifetimes in the range ~3-5 x 10^8 years, which indicates that AGN activity is a transient phase in the life of galaxies.Comment: 4 pages, 1 figure. Proc. of the conference "X-ray Astronomy 2009: Present status, multiwavelength approach and future perspectives", September 2009, Bologna. To appear in AIP Conf. Proc. (editors: A. Comastri, M. Cappi, L. Angelini)

Mesozooplankton from Ría de Vigo (NW Spain) and its adjacent shelf between 1995 and 2003

The mesozooplankton (200 μm to 2 mm) community from monthly samples collected at two stations in Ría de Vigo (39 m) and its adjacent shelf (97 m) between 1995-2003 is described. The mesozooplankton was dominated by copepods. Inside the Ría, meroplankton (larvae of gasteropods, lamelibranchiae, echinodermate and cirripids nauplii) and other holoplanktonic organisms, such as apendicularia, cladocera, siphonophora and medusae, were also well represented. The copepod group inside the Ría was dominated by Acartia clausi, juveniles of Calanus helgolandicus, Paracalanus parvus, Pseudocalanus elongatus, Temora longicornis, Oithona plumifera, Oncaea media, Euterpina acutifrons and copepodites of Centropages spp. In the offshore station, dominant copepods species were the same than inshore together with other oceanic species, such as Paraeuchaeta hebes, Calanoides carinatus and Aetideus armatus. Biomass values within Ría de Vigo ranged between 2 and 151 mg dry weight /m3 and between 2 and 314 mg dry weight /m3 in the offshore station. Abundance ranged between 149 ind/m in January 1995 and 41814 ind/m3 in August of 2003 in the inner station, and between 22 ind/m3 in January 1996 and 93410 ind/m3 in May 1999 in the outer station. The most outstanding events were the high levels of the Mediterranean copepod Temora stylifera, usually rare in Galician waters recorded in the area in 1997 and 1998, and the increased levels of cirripid larvae observed inside the Ría

KIC 10449976: discovery of an extreme-helium subdwarf in the Kepler field

Optical spectroscopy of the blue star KIC 10449976 shows that it is an extremely helium-rich subdwarf with effective temperature T=40000+/-300 K and surface gravity log g=5.3+/-0.1. Radial-velocity measurements over a five-day timescale show an upper variability limit of ~50+/-20 km/s. Kepler photometry of KIC 10449976 in both long and short cadence modes shows evidence for a periodic modulation on a timescale of ~3.9 days. We have examined the possibility that this modulation is not astrophysical but conclude it is most likely real. We discuss whether the modulation could be caused by a low-mass companion, by stellar pulsations, or by spots. The identification of any one of these as cause has important consequences for understanding the origin of helium-rich subdwarfs.Comment: Accepted by MNRA

The Large-Scale Structure of the X-ray Background and its Cosmological Implications

A careful analysis of the HEAO1 A2 2-10 keV full-sky map of the X-ray background (XRB) reveals clustering on the scale of several degrees. After removing the contribution due to beam smearing, the intrinsic clustering of the background is found to be consistent with an auto-correlation function of the form (3.6 +- 0.9) x 10^{-4} theta^{-1} where theta is measured in degrees. If current AGN models of the hard XRB are reasonable and the cosmological constant-cold dark matter cosmology is correct, this clustering implies an X-ray bias factor of b_X ~ 2. Combined with the absence of a correlation between the XRB and the cosmic microwave background, this clustering can be used to limit the presence of an integrated Sachs-Wolfe (ISW) effect and thereby to constrain the value of the cosmological constant, Omega_Lambda < 0.60 (95 % C.L.). This constraint is inconsistent with much of the parameter space currently favored by other observations. Finally, we marginally detect the dipole moment of the diffuse XRB and find it to be consistent with the dipole due to our motion with respect to the mean rest frame of the XRB. The limit on the amplitude of any intrinsic dipole is delta I / I < 5 x 10^{-3} at the 95 % C.L. When compared to the local bulk velocity, this limit implies a constraint on the matter density of the universe of Omega_m^{0.6}/b_X(0) > 0.24.Comment: 15 pages, 8 postscript figures, to appear in the Astrophysical Journal. The postscript version appears not to print, so use the PDF versio

Large Scale Fluctuations in the X-Ray Background

We present an attempt to measure the large angular scale fluctuations in the X-Ray Background (XRB) from the HEAO1-A2 data, expressed in terms of spherical harmonics. We model the harmonic coefficients assuming a power spectrum and an epoch-dependent bias parameter, and using a phenomenological scenario describing the evolution of the X-ray sources. From the few low-order multipoles detected above shot noise, we estimate the power-spectrum normalization on scales intermediate between those explored by local galaxy redshift surveys (~ 100 Mpc) and by the COBE Microwave Background measurements (~ 1000 Mpc). We find that the HEAO1 harmonics are consistent with present epoch rms fluctuations of the X-ray sources bx(0)sigma8 ~ 1-2 in 8 Mpc spheres. Therefore the observed fluctuations in the XRB are roughly as expected from interpolating between the local galaxy surveys and the COBE CMB experiment. We predict that an X-ray all-sky surface brightness survey resolving sources a factor of 10 fainter than HEAO1, may reveal fluctuations to significantly larger angular scales and therefore more strongly constrain the large scale structure of the Universe on scales of hundreds of Mpcs.Comment: 14 pages, 3 Postscript figures, uses aaspp4.sty and psfig. Revised following referee's report. Accepted for publication in Ap

Remnants of Sagittarius Dwarf Spheroidal Galaxy around the young globular cluster Palomar 12

Photometry of a large field around the young globular cluster Palomar 12 has revealed the main-sequence of a low surface-brightness stellar system. This main-sequence is indicative of a stellar population that varies significantly in metallicity and/or age, but in the mean is more metal poor than Pal 12. Under different assumptions for the properties of this population, we find distances from the Sun in the range 17-24 kpc, which encompasses the distance to Pal 12, $19.0\pm0.9$ kpc. The stellar system is also detected in a field 2\arcdeg North of Pal 12, which indicates it has a minimum diameter of $\sim0.9$ kpc. The orbit of Pal 12 (Dinescu et al. 2000), the color-magnitude diagram of the stellar system, their positions on the sky, and their distances suggest that they are debris from the tidal disruption of the Sgr dSph galaxy. We discuss briefly the implications for the evolution of Sgr and the Galactic halo.Comment: 16 pages, 2 figures, accepted for ApJ Letters. Some importante changes after revision, including a new figur

The Infrared Ca II triplet as metallicity indicator

From observations of almost 500 RGB stars in 29 Galactic open and globular clusters, we have investigated the behaviour of the infrared Ca II triplet (8498, 8542 and 8662 \AA) in the age range 13$\leq$Age/Gyr$\leq$0.25 and the metallicity range $-2.2\leq$ [Fe/H] $\leq$+0.47. These are the widest ranges of ages and metallicities in which the behaviour of the Ca II triplet lines has been investigated in a homogeneous way. We report the first empirical study of the variation of the CaII triplet lines strength, for given metallicities, with respect to luminosity. We find that the sequence defined by each cluster in the Luminosity-$\Sigma$Ca plane is not exactly linear. However, when only stars in a small magnitude interval are observed, the sequences can be considered as linear. We have studied the the Ca II triplet lines on three metallicities scales. While a linear correlation between the reduced equivalent width ($W'_V$ or $W'_I$) versus metallicity is found in the \citet{cg97} and \citet{ki03} scales, a second order term needs to be added when the \citet{zw84} scale is adopted. We investigate the role of age from the wide range of ages covered by our sample. We find that age has a weak influence on the final relationship. Finally, the relationship derived here is used to estimate the metallicities of three poorly studied open clusters: Berkeley 39, Trumpler 5 and Collinder 110. For the latter, the metallicity derived here is the first spectroscopic estimate available.Comment: 52 pages, 16 figures, accepted for publication in Astronomical Journa