ABSTRACT
The ultra-diffuse galaxies DF2 and DF4 in the NGC 1052 group share several unusual properties: they both have large sizes1, rich populations of overluminous and large globular clusters2-6, and very low velocity dispersions that indicate little or no dark matter7-10. It has been suggested that these galaxies were formed in the aftermath of high-velocity collisions of gas-rich galaxies11-13, events that resemble the collision that created the bullet cluster14 but on much smaller scales. The gas separates from the dark matter in the collision and subsequent star formation leads to the formation of one or more dark-matter-free galaxies12. Here we show that the present-day line-of-sight distances and radial velocities of DF2 and DF4 are consistent with their joint formation in the aftermath of a single bullet-dwarf collision, around eight billion years ago. Moreover, we find that DF2 and DF4 are part of an apparent linear substructure of seven to eleven large, low-luminosity objects. We propose that these all originated in the same event, forming a trail of dark-matter-free galaxies that is roughly more than two megaparsecs long and angled 7° ± 2° from the line of sight. We also tentatively identify the highly dark-matter-dominated remnants of the two progenitor galaxies that are expected11 at the leading edges of the trail.
ABSTRACT
Studies of galaxy surveys in the context of the cold dark matter paradigm have shown that the mass of the dark matter halo and the total stellar mass are coupled through a function that varies smoothly with mass. Their average ratio Mhalo/Mstars has a minimum of about 30 for galaxies with stellar masses near that of the Milky Way (approximately 5 × 1010 solar masses) and increases both towards lower masses and towards higher masses. The scatter in this relation is not well known; it is generally thought to be less than a factor of two for massive galaxies but much larger for dwarf galaxies. Here we report the radial velocities of ten luminous globular-cluster-like objects in the ultra-diffuse galaxy NGC1052-DF2, which has a stellar mass of approximately 2 × 108 solar masses. We infer that its velocity dispersion is less than 10.5 kilometres per second with 90 per cent confidence, and we determine from this that its total mass within a radius of 7.6 kiloparsecs is less than 3.4 × 108 solar masses. This implies that the ratio Mhalo/Mstars is of order unity (and consistent with zero), a factor of at least 400 lower than expected. NGC1052-DF2 demonstrates that dark matter is not always coupled with baryonic matter on galactic scales.
ABSTRACT
Observations of star formation and kinematics in early galaxies at high spatial and spectral resolution have shown that two-thirds are massive rotating disk galaxies, with the remainder being less massive non-rotating objects. The line-of-sight-averaged velocity dispersions are typically five times higher than in today's disk galaxies. This suggests that gravitationally unstable, gas-rich disks in the early Universe are fuelled by cold, dense accreting gas flowing along cosmic filaments and penetrating hot galactic gas halos. These accreting flows, however, have not been observed, and cosmic accretion cannot power the observed level of turbulence. Here we report observations of a sample of rare, high-velocity-dispersion disk galaxies in the nearby Universe where cold accretion is unlikely to drive their high star formation rates. We find that their velocity dispersions are correlated with their star formation rates, but not their masses or gas fractions, which suggests that star formation is the energetic driver of galaxy disk turbulence at all cosmic epochs.
ABSTRACT
Hierarchical galaxy formation is the model whereby massive galaxies form from an assembly of smaller units. The most massive objects therefore form last. The model succeeds in describing the clustering of galaxies, but the evolutionary history of massive galaxies, as revealed by their visible stars and gas, is not accurately predicted. Near-infrared observations (which allow us to measure the stellar masses of high-redshift galaxies) and deep multi-colour images indicate that a large fraction of the stars in massive galaxies form in the first 5 Gyr (refs 4-7), but uncertainties remain owing to the lack of spectra to confirm the redshifts (which are estimated from the colours) and the role of obscuration by dust. Here we report the results of a spectroscopic redshift survey that probes the most massive and quiescent galaxies back to an era only 3 Gyr after the Big Bang. We find that at least two-thirds of massive galaxies have appeared since this era, but also that a significant fraction of them are already in place in the early Universe.
ABSTRACT
Objetivo: Estimular en los estudiantes y odontólogos, un pensamiento más crítico y preventivo al momento de tomar una decisión en el tratamiento de una lesión cariosa, teniendo en cuenta la preservación de mayor estructura dentaria a través de la investigación documental sobre "Odontología Mínimamente Invasiva". Materiales y Métodos: La investigación se enmarca en un diseño de tipo documental. Su ejecución inició con una búsqueda de artículos y capítulos referentes al tema en revistas y libros especializados en Odontología Preventiva y Cariología; se revisaron bases de datos bibliográficos biomédicos en Internet y consulta directa en la Biblioteca de la Facultad de Odontología de la Universidad de El Salvador; concluida la recopilación de información, se procedió a seleccionarla en base a respaldo científico y grado de actualización, posteriormente se hicieron las traducciones respectivas, se analizó cada artículo y se procedió a la estructuración del trabajo final considerando un orden lógico y apegado a los objetivos propuestos. Las conclusiones se obtuvieron en base al análisis de la evidencia científica consultada y considerando los objetivos de la investigación. Resultados: Veinticinco libros referentes a prevención, cariología, anatomía dental, operatoria dental, adhesión en odontología restauradora, patología bucal, periodoncia y de historia de la odontología; treinta y dos artículos de revistas odontológicas; doce journals. Publicados por diferentes investigadores y dos direcciones de internet fueron consultados, analizados y considerados para elaborar la investigación.
Objective: To stimulate in students and dentists, a more critical and preventive thinking when making a decision in the treatment of a carious lesion, taking into account the preservation of greater dental structure through documentary research on "Minimally Invasive Dentistry" . Materials and Methods: The research is framed in a documentary-type design. Its execution began with a search for articles and chapters on the subject in specialized magazines and books on Preventive Dentistry and Cariology; Biomedical bibliographic databases were reviewed on the Internet and direct consultation in the Library of the Faculty of Dentistry of the University of El Salvador; Once the collection of information was concluded, it was selected based on scientific support and degree of updating, subsequently the respective translations were made, each article was analyzed and the final work was structured considering a logical order and attached to the proposed objectives . The conclusions were obtained based on the analysis of the scientific evidence consulted and considering the objectives of the research. Results: Twenty-five books referring to prevention, cariology, dental anatomy, dental surgery, adhesion in restorative dentistry, oral pathology, periodontics and the history of dentistry; thirty-two articles from dental journals; twelve journals. Published by different researchers and two internet addresses were consulted, analyzed and considered to develop the research.
