Examinando por Autor "Nishiyama, S."

Mostrando 1 - 3 de 3

Acceso Abierto
A significant feature in the general relativistic time evolution of the redshift of photons coming from a star orbiting Sgr A
(Oxford Academics: Oxford University Press, 2019-10-22) Saida, H.; Nishiyama, S.; Ohgami, T.; Takamori, Y.; Takahashi, M.; Minowa, Y.; Najarro, F.; Hamano, S.; Omiya, M.; Iwamatsu, A.; Takahashi, M.; Gorin, H.; Kara, T.; Koyama, A.; Ohashi, Y.; Tamura, M.; Nagatomo, S.; Zenko, T.; Nagata, T.; Japan Society for the Promotion of Science (KAKENHI); Nishiyama, S. [https://orcid.org/0000-0002-9440-7172]; Unidad de Excelencia Científica María de Maeztu Centro de Astrobiología del Instituto Nacional de Técnica Aeroespacial y CSIC, MDM-2017-0737
The star S0-2, orbiting the Galactic central massive black hole candidate Sgr A*, passed its pericenter in 2018 May. This event is the first chance to detect the general relativistic (GR) effect of a massive black hole, free from non-gravitational physics. The observable GR evidence in the event is the difference between the GR redshift and the Newtonian redshift of photons coming from S0-2. Within the present observational precision, the first post-Newtonian (1PN) GR evidence is detectable. In this paper, we give a theoretical analysis of the time evolution of the 1PN GR evidence, under a presupposition that is different from used in previous papers. Our presupposition is that the GR/Newtonian redshift is always calculated with the parameter values (the mass of Sgr A*, the initial conditions of S0-2, and so on) determined by fitting the GR/Newtonian motion of S0-2 with the observational data. It is then revealed that the difference of the GR redshift and the Newtonian one shows two peaks before and after the pericenter passage. This double-peak appearance is due to our presupposition, and reduces to a single peak if the same parameter values are used in both GR and Newtonian redshifts as considered in previous papers. In addition to this theoretical discussion, we report our observational data obtained with the Subaru telescope by 2018. The quality and the number of Subaru data in 2018 are not sufficient to confirm the detection of the double-peak appearance.
Restringido
Early formation and recent starburst activity in the nuclear disk of the Milky Way.
(Nature Research Journals, 2020-04-01) Nogueras Lara, F.; Schödel, R.; Gallego Calvente, A. T.; Gallego Cano, E.; Shahzamanian, B.; Dong, H.; Neumayer, N.; Hilker, M.; Najarro, F.; Nishiyama, S.; Feldmeier Krause, A.; Girard, J. H. V.; Cassisi, S.; European Commission (EC); Ministerio de Educación, Cultura y Deporte (MECD); Agencia Estatal de Investigación (AEI); 0000-0001-5870-3735; 0000-0002-2363-5522; 0000-0002-7452-1496; 0000-0002-6922-2598; 0000-0002-6379-7593; 0000-0001-6437-6806; 0000-0002-0160-7221; 0000-0001-8627-0404; 0000-0002-6428-8045; 0000-0002-9440-7172; 0000-0001-5404-797X; Centros de Excelencia Severo Ochoa, INSTITUTO DE ASTROFISICA DE ANDALUCIA (IAA), SEV-2017-0709; Unidad de Excelencia Científica María de Maeztu Centro de Astrobiología del Instituto Nacional de Técnica Aeroespacial y CSIC, MDM-2017-0737
The nuclear disk is a dense stellar structure at the centre of the Milky Way, with a radius of ~150 pc (ref. (1)). It has been a place of intense star formation in the past several tens of millions of years(1-3), but its overall formation history has remained unknown(2). Here, we report that the bulk of its stars formed at least 8 Gyr ago. After a long period of quiescence, a starburst event followed about 1 Gyr ago that formed roughly 5% of its mass within ~100 Myr, in what may arguably have been one of the most energetic events in the history of the Milky Way. Star formation continued subsequently on a lower level, creating a few per cent of the stellar mass in the past ~500 Myr, with an increased rate up to ~30 Myr ago. Our findings contradict the previously accepted paradigm of quasi-continuous star formation at the Galactic Centre(4). The long quiescent phase agrees with the overall quiescent history of the Milky Way(2,5) and suggests that our Galaxy's bar may not have existed until recently, or that gas transport through the bar was extremely inefficient during a long stretch of the Milky Way's life. Consequently, the central black hole may have acquired most of its mass already in the early days of the Milky Way.
Acceso Abierto
GALACTICNUCLEUS: A high-angular-resolution JHKs imaging survey of the Galactic centre II. First data release of the catalogue and the most detailed CMDs of the GC
(EDP Sciences, 2019-10-15) Nogueras Lara, F.; Schödel, R.; Gallego Calvente, A. T.; Dong, H.; Gallego Cano, E.; Shahzamanian, B.; Girard, J. H. V.; Nishiyama, S.; Najarro, F.; Neumayer, N.; European Commission (EC); Agencia Estatal de Investigación (AEI); Ministerio de Economía y Competitividad (MINECO); Deutsche Forschungsgemeinschaft (DFG); Nogueras Lara, F. [0000-0002-6379-7593]; Schöedel, R. [0000-0001-5404-797X]; Gallego Calvente, A. T. [0000-0002-6428-8045]; Gallego Cano, E. [0000-0002-7452-1496]; Shahzamanian, B. [0000-0001-6437-6806]; Najarro, F. [0000-0002-9124-0039]; Neumayer, N. [0000-0002-6922-2598]; Unidad de Excelencia Científica María de Maeztu Centro de Astrobiología del Instituto Nacional de Técnica Aeroespacial y CSIC, MDM-2017-0737; Centros de Excelencia Severo Ochoa, INSTITUTO DE ASTROFISICA DE ANDALUCIA (IAA), SEV-2017-0709
Context. The high extinction and extreme source crowding of the central regions of the Milky Way are serious obstacles to the study of the structure and stellar population of the Galactic centre (GC). Existing surveys that cover the GC region (2MASS, UKIDSS, VVV, SIRIUS) do not have the necessary high angular resolution. Therefore, a high-angular-resolution survey in the near infrared is crucial to improve the state of the art. Aims. Here, we present the GALACTICNUCLEUS catalogue, a near infrared JHKs high-angular-resolution (0.2″) survey of the nuclear bulge of the Milky Way. Methods. We explain in detail the data reduction, data analysis, calibration, and uncertainty estimation of the GALACTICNUCLEUS survey. We assess the data quality comparing our results with previous surveys. Results. We obtained accurate JHKs photometry for ∼3.3 × 106 stars in the GC detecting around 20% in J, 65% in H, and 90% in Ks. The survey covers a total area of ∼0.3 deg2, which corresponds to ∼6000 pc2. The GALACTICNUCLEUS survey reaches 5σ detections for J ∼ 22 mag, H ∼ 21 mag, and Ks ∼ 21 mag. The uncertainties are below 0.05 mag at J ∼ 21 mag, H ∼ 19 mag, and Ks ∼ 18 mag. The zero point systematic uncertainty is ≲0.04 mag in all three bands. We present colour–magnitude diagrams for the different regions covered by the survey.