A fast version of the k-means classification algorithm for astronomical applications

Context. K-means is a clustering algorithm that has been used to classify large datasets in astronomical databases. It is an unsupervised method, able to cope very different types of problems. Aims. We check whether a variant of the algorithm called single-pass k-means can be used as a fast alternative to the traditional k-means. Methods. The execution time of the two algorithms are compared when classifying subsets drawn from the SDSS-DR7 catalog of galaxy spectra. Results. Single-pass k-means turn out to be between 20 % and 40 % faster than k-means and provide statistically equivalent classifications. This conclusion can be scaled up to other larger databases because the execution time of both algorithms increases linearly with the number of objects. Conclusions. Single-pass k-means can be safely used as a fast alternative to k-means

Astronomy & Astrophysics A fast version of the k -means classification algorithm for astronomical applications (Research Note)

ABSTRACT Context. K-means is a clustering algorithm that has been used to classify large datasets in astronomical databases. It is an unsupervised method, able to cope very different types of problems. Aims. We check whether a variant of the algorithm called single pass k-means can be used as a fast alternative to the traditional k-means. Methods. The execution time of the two algorithms are compared when classifying subsets drawn from the SDSS-DR7 catalog of galaxy spectra. Results. Single-pass k-means turn out to be between 20% and 40% faster than k-means and provide statistically equivalent classifications. This conclusion can be scaled up to other larger databases because the execution time of both algorithms increases linearly with the number of objects. Conclusions. Single-pass k-means can be safely used as a fast alternative to k-means

Chronological Age Interacts with the Circadian Melatonin Receptor 1B Gene Variation, Determining Fasting Glucose Concentrations in Mediterranean Populations. Additional Analyses on Type-2 Diabetes Risk

Gene-age interactions have not been systematically investigated on metabolic phenotypes and this modulation will be key for a better understanding of the temporal regulation in nutrigenomics. Taking into account that aging is typically associated with both impairment of the circadian system and a decrease in melatonin secretion, we focused on the melatonin receptor 1B (MTNR1B)-rs10830963 C>G variant that has been associated with fasting glucose concentrations, gestational diabetes, and type-2 diabetes. Therefore, our main aim was to investigate whether the association between the MTNR1B-rs10830963 polymorphism and fasting glucose is age dependent. Our secondary aims were to analyze the polymorphism association with type-2 diabetes and explore the gene-pregnancies interactions on the later type-2 diabetes risk. Three Mediterranean cohorts (n = 2823) were analyzed. First, a cross-sectional study in the discovery cohort consisting of 1378 participants (aged 18 to 80 years; mean age 41 years) from the general population was carried out. To validate and extend the results, two replication cohorts consisting of elderly individuals were studied. In the discovery cohort, we observed a strong gene-age interaction (p = 0.001), determining fasting glucose in such a way that the increasing effect of the risk G-allele was much greater in young (p = 5.9 × 10-10) than in elderly participants (p = 0.805). Consistently, the association of the MTNR1B-rs10830963 polymorphism with fasting glucose concentrations in the two replication cohorts (mean age over 65 years) did not reach statistical significance (p > 0.05 for both). However, in the elderly cohorts, significant associations between the polymorphism and type-2 diabetes at baseline were found. Moreover, in one of the cohorts, we obtained a statistically significant interaction between the MTNR1B polymorphism and the number of pregnancies, retrospectively assessed, on the type-2 diabetes risk. In conclusion, the association of the MTNR1B-rs10830963 polymorphism with fasting glucose is age-dependent, having a greater effect in younger people. However, in elderly subjects, associations of the polymorphism with type-2 diabetes were observed and our exploratory analysis suggested a modulatory effect of the number of past pregnancies on the future type-2 diabetes genetic risk

Genome-Wide Association Study for Serum Omega-3 and Omega-6 Polyunsaturated Fatty Acids: Exploratory Analysis of the Sex-Specific Effects and Dietary Modulation in Mediterranean Subjects with Metabolic Syndrome

Many early studies presented beneficial effects of polyunsaturated fatty acids (PUFA) on cardiovascular risk factors and disease. However, results from recent meta-analyses indicate that this effect would be very low or nil. One of the factors that may contribute to the inconsistency of the results is that, in most studies, genetic factors have not been taken into consideration. It is known that fatty acid desaturase (FADS) gene cluster in chromosome 11 is a very important determinant of plasma PUFA, and that the prevalence of the single nucleotide polymorphisms (SNPs) varies greatly between populations and may constitute a bias in meta-analyses. Previous genome-wide association studies (GWAS) have been carried out in other populations and none of them have investigated sex and Mediterranean dietary pattern interactions at the genome-wide level. Our aims were to undertake a GWAS to discover the genes most associated with serum PUFA concentrations (omega-3, omega-6, and some fatty acids) in a scarcely studied Mediterranean population with metabolic syndrome, and to explore sex and adherence to Mediterranean diet (MedDiet) interactions at the genome-wide level. Serum PUFA were determined by NMR spectroscopy. We found strong robust associations between various SNPs in the FADS cluster and omega-3 concentrations (top-ranked in the adjusted model: FADS1-rs174547, p = 3.34 × 10-14; FADS1-rs174550, p = 5.35 × 10-14; FADS2-rs1535, p = 5.85 × 10-14; FADS1-rs174546, p = 6.72 × 10-14; FADS2-rs174546, p = 9.75 × 10-14; FADS2- rs174576, p = 1.17 × 10-13; FADS2-rs174577, p = 1.12 × 10-12, among others). We also detected a genome-wide significant association with other genes in chromosome 11: MYRF (myelin regulatory factor)-rs174535, p = 1.49 × 10-12; TMEM258 (transmembrane protein 258)-rs102275, p = 2.43 × 10-12; FEN1 (flap structure-specific endonuclease 1)-rs174538, p = 1.96 × 10-11). Similar genome-wide statistically significant results were found for docosahexaenoic fatty acid (DHA). However, no such associations were detected for omega-6 PUFAs or linoleic acid (LA). For total PUFA, we observed a consistent gene*sex interaction with the DNTTIP2 (deoxynucleotidyl transferase terminal interacting protein 2)-rs3747965 p = 1.36 × 10-8. For adherence to MedDiet, we obtained a relevant interaction with the ME1 (malic enzyme 1) gene (a gene strongly regulated by fat) in determining serum omega-3. The top-ranked SNP for this interaction was ME1-rs3798890 (p = 2.15 × 10-7). In the regional-wide association study, specifically focused on the FADS1/FASD2/FADS3 and ELOVL (fatty acid elongase) 2/ELOVL 5 regions, we detected several statistically significant associations at p < 0.05. In conclusion, our results confirm a robust role of the FADS cluster on serum PUFA in this population, but the associations vary depending on the PUFA. Moreover, the detection of some sex and diet interactions underlines the need for these associations/interactions to be studied in all specific populations so as to better understand the complex metabolism of PUFA

Dust-to-gas ratio in a complete sample of AGN

Resumen del trabajo presentado al Spanish X-ray Astronomy 2017: the path towards Athena, celebrado en Granada del 23 al 25 de octubre de 2017.According to the Unified Model of Active Galactic Nuclei (AGN), an unobscured AGN based on the optical data (detection of rest-frame UV-optical broad emission lines, type1 AGN) should appear as an X-ray unabsorbed AGN. However, there is an important fraction (10-30 W) of AGN whose optical and X-ray classifications do not match. It is not well known whether the material that absorbs the X-ray emission in these apparently discordant objects is the same that the one that obscures the optical emission, or if it has intrinsically different properties than the main population. To provide insight into this topic, we have conducted a statistical analysis of the optical obscuration and X-ray absorption properties of the 125 optically type-1 AGN from the Bright Ultra-hard XMM-Newton Survey (BUXS) in the z = 0.05-1 range and with L2−10keV > 42 erg s−1. All of these AGNs have high-quality spectra from XMM-Newton and either Sloan Digital Sky Survey spectra or proprietary observations. As BUXS is a flux limited (f4,5−10KeV ≥ 6 × 10−14 erg s−1 cm−2) X-ray selected sample, it is complete for NH column densities up to the Compton-thick limit (∼1024 cm−2). In our works we have determined the impact of contamination from the AGN hosts galaxies in their optical classification. We derive the X-ray absorption by fitting their XMM-Newton spectra and the optical extinction using UV/optical spectral continuum fits. Most type-1 AGN in our sample show consistent optical and X-ray classification, but there is a large fraction (∼20 W) of objects with large NH column densities (NH > 4× 1021 cm−2). Our preliminary results shows that the majority (∼70 W) of the sources show dust-to-gas ratios compatible with the Galactic relation. Among the discordant dust-to-gas ratios we found both objects with more dust (∼13 W) than the Galactic relation and more gas (∼17 W) than the Galactic relation.Peer reviewe

Discordant optical and X-ray classification of AGN

Trabajo presentado al Spanish X-ray Astronomy, celebrado en Santander del 3 al 5 de junoio de 2015.To provide insight into the apparent mismatch between the optical and X-ray absorption properties observed in 10-30% of Active Galactic Nuclei (AGN), we have conducted a detailed study of two X-ray unabsorbed AGN with a type-2 optical spectroscopic classification. In addition to high quality X-ray spectroscopic observations, that we used to determine both the AGN luminosities and absorption, we have a VLT/XSHOOTER UV-to-near-IR high resolution spectrum for each object, that we used to determine the AGN intrinsic emission corrected for both contamination from the AGN hosts and extinction. Thanks to the high quality data available for these two intriguing sources, we unveil the origin of the mismatch. The most likely explanation is that both objects have an intrinsically high dust-to-gas ratio.Peer Reviewe

Dust-to-gas ratio in a complete sample of type-1 AGN

Resumen del póster presentado al XIII Scientific Meeting of the Spanish Astronomical Society, celebrado en la Universidad de Salamanca del 16 al 20 de julio de 2018.According to the Unified Model of Active Galactic Nuclei (AGN), unobscured AGN based on its optical spectrum (detection of rest-frame UV-optical broad emission lines, type-1 AGN) should appear as X-ray unabsorbed AGN. However, there is an important fraction (10-30%) of AGN whose optical and X-ray classifications do not match, and the origin of the discrepancy is not clear. To provide insight into this topic, we have conducted a statistical analysis of the optical obscuration and X-ray absorption properties of the optically type1 AGN from the Bright Ultra-hard XMM-Newton Survey (BUXS) with L2−10keV >1042 erg s−1 and z=0.05-1. We have high-quality spectra from XMM-Newton and either SDSS spectra or proprietary observations for the selected sample. In order to provide the most complete sample as possible, we have conducted a detailed analysis of the emission lines to provide a reliable classification of the AGNs. We derive the X-ray absorption by fitting their XMM-Newton spectra and the optical extinction using UV/optical spectral continuum fits. As BUXS is a flux limited X-ray selected sample at hard energies (f4.510keV ≤6×10−14 ergs−1cm−2), it is complete for NH column densities up to the Compton-thick limit (∼1024cm−2). Our preliminary results show that most type-1 AGN in our sample show consistent optical and X-ray classification, but there is a large fraction ( 20%) of objects with large NH column densities (NH >4×1021 cm−2)

Searching for luminous absorbed sources in the WISE AGN catalogue

Mid-infrared (IR) colour selection techniques have proved to be very efficient in finding active galactic nuclei (AGN). This is because the AGN heats the surrounding dust producing warm mid-IR colours. Using the Wide-field Infrared Survey Explorer 3.6, 4.5 and 12 μm colours, the largest sample of IR selected AGN has already been produced containing 1.4 million AGN over the whole sky. Here, we explore the X-ray properties of this AGN sample by cross-correlating it with the subsample of the 3XMM X-ray catalogue that has available X-ray spectra and at the same time optical spectroscopy from Sloan Digital Sky Survey (SDSS). Our goal is to find rare luminous obscured AGN. Our final sample contains 65 QSOs with log νLν ≥ 46.2 erg s−1. This IR luminosity cut corresponds to log  LX ≈ 45 erg s−1, at the median redshift of our sample (z = 2.3), that lies at the bright end of the X-ray luminosity function at z > 2. The X-ray spectroscopic analysis reveals seven obscured AGN having a column density NH > 1022 cm−2. Six of them show evidence for broad [C IV] absorption lines and five are classified as broad absorption line QSOs. We fit the optical spectra of our X-ray absorbed sources to estimate the optical reddening. We find that none of these show any obscuration according to the optical continuum. These sources add to the growing evidence for populations of luminous QSOs with evidence for substantial absorption by outflowing ionized material, similar to those expected to be emerging from their absorbing cocoons in the framework of AGN/galaxy co-evolution.The research leading to these results has received funding from the European Union’s Horizon 2020 Programme under the AHEAD project (grant agreement no. 654215). GM acknowledges financial support from the AHEAD project that is funded by the European Union as Research and Innovation Action under Grant no. 654215. FJC and IO-P acknowledge financial support through grant AYA2015-64346-C2-1-P (MINECO/FEDER). SM acknowledges financial support by the Spanish Ministry of Economy and Competitiveness through Grant no. AYA2016-76730-P. Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the US Department of Energy Office of Science.Peer Reviewe

Searching for luminous absorbed sources in the WISE AGN catalogue

Mid-infrared (IR) colour selection techniques have proved to be very efficient in finding active galactic nuclei (AGN). This is because the AGN heats the surrounding dust producing warm mid-IR colours. Using the Wide-field Infrared Survey Explorer 3.6, 4.5 and 12 μm colours, the largest sample of IR selected AGN has already been produced containing 1.4 million AGN over the whole sky. Here, we explore the X-ray properties of this AGN sample by cross-correlating it with the subsample of the 3XMM X-ray catalogue that has available X-ray spectra and at the same time optical spectroscopy from Sloan Digital Sky Survey (SDSS). Our goal is to find rare luminous obscured AGN. Our final sample contains 65 QSOs with log vLv ≥ 46.2 erg s-1. This IR luminosity cut corresponds to log LX ≈ 45 erg s-1, at the median redshift of our sample (z = 2.3), that lies at the bright end of the X-ray luminosity function at z &amp;gt; 2. The X-ray spectroscopic analysis reveals seven obscured AGN having a column density NH &amp;gt; 1022 cm-2. Six of them show evidence for broad [C IV] absorption lines and five are classified as broad absorption line QSOs. We fit the optical spectra of our X-ray absorbed sources to estimate the optical reddening.We find that none of these show any obscuration according to the optical continuum. These sources add to the growing evidence for populations of luminous QSOs with evidence for substantial absorption by outflowing ionized material, similar to those expected to be emerging from their absorbing cocoons in the framework of AGN/galaxy co-evolution. © 2017 The Authors