| 1. |
- Adamo, Angela, et al.
(författare)
-
Legacy ExtraGalactic UV Survey with The Hubble Space Telescope : Stellar Cluster Catalogs and First Insights Into Cluster Formation and Evolution in NGC 628
- 2017
-
Ingår i: Astrophysical Journal. - : IOP PUBLISHING LTD. - 0004-637X .- 1538-4357. ; 841:2
-
Tidskriftsartikel (refereegranskat)abstract
- We report the large effort that is producing comprehensive high-level young star cluster (YSC) catalogs for a significant fraction of galaxies observed with the Legacy ExtraGalactic UV Survey (LEGUS) Hubble treasury program. We present the methodology developed to extract cluster positions, verify their genuine nature, produce multiband photometry (from NUV to NIR), and derive their physical properties via spectral energy distribution fitting analyses. We use the nearby spiral galaxy NGC 628 as a test case for demonstrating the impact that LEGUS will have on our understanding of the formation and evolution of YSCs and compact stellar associations within their host galaxy. Our analysis of the cluster luminosity function from the UV to the NIR finds a steepening at the bright end and at all wavelengths suggesting a dearth of luminous clusters. The cluster mass function of NGC 628 is consistent with a power-law distribution of slopes similar to-2 and a truncation of a few times 10(5) M-circle dot. After their formation, YSCs and compact associations follow different evolutionary paths. YSCs survive for a longer time frame, confirming their being potentially bound systems. Associations disappear on timescales comparable to hierarchically organized star-forming regions, suggesting that they are expanding systems. We find massindependent cluster disruption in the inner region of NGC 628, while in the outer part of the galaxy there is little or no disruption. We observe faster disruption rates for low mass (<= 10(4) M-circle dot) clusters, suggesting that a massdependent component is necessary to fully describe the YSC disruption process in NGC 628.
|
|
| 2. |
- Dobbs, C. L., et al.
(författare)
-
The properties, origin and evolution of stellar clusters in galaxy simulations and observations
- 2017
-
Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 464:3, s. 3580-3596
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate the properties and evolution of star particles in two simulations of isolated spiral galaxies, and two galaxies from cosmological simulations. Unlike previous numerical work, where typically each star particle represents one 'cluster', for the isolated galaxies we are able to model features we term 'clusters' with groups of particles. We compute the spatial distribution of stars with different ages, and cluster mass distributions, comparing our findings with observations including the recent LEGUS survey. We find that spiral structure tends to be present in older (100s Myr) stars and clusters in the simulations compared to the observations. This likely reflects differences in the numbers of stars or clusters, the strength of spiral arms, and whether the clusters are allowed to evolve. Where we model clusters with multiple particles, we are able to study their evolution. The evolution of simulated clusters tends to follow that of their natal gas clouds. Massive, dense, long-lived clouds host massive clusters, whilst short-lived clouds host smaller clusters which readily disperse. Most clusters appear to disperse fairly quickly, in basic agreement with observational findings. We note that embedded clusters may be less inclined to disperse in simulations in a galactic environment with continuous accretion of gas on to the clouds than isolated clouds and correspondingly, massive young clusters which are no longer associated with gas tend not to occur in the simulations. Caveats of our models include that the cluster densities are lower than realistic clusters, and the simplistic implementation of stellar feedback.
|
|
| 3. |
- Grasha, K., et al.
(författare)
-
The Hierarchical Distribution of the Young Stellar Clusters in Six Local Star-forming Galaxies
- 2017
-
Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 840:2
-
Tidskriftsartikel (refereegranskat)abstract
- We present a study of the hierarchical clustering of the young stellar clusters in six local (3–15 Mpc) star-forminggalaxies using Hubble Space Telescope broadband WFC3/UVIS UV and optical images from the Treasury Program LEGUS (Legacy Extra Galactic UV Survey). We identified 3685 likely clusters and associations, each visually classified by their morphology, and we use the angular two-point correlation function to study the clustering of the sestellar systems. We find that the spatial distribution of the young clusters and associations are clustered with respect to each other, forming large, unbound hierarchical star-forming complexes that are in general very young. The strength of the clustering decreases with increasing age of the star clusters and stellar associations, becoming more homogeneously distributed after ∼ 40–60 Myr and on scales larger than a few hundred parsecs. In all galaxies, the associations exhibit a global behavior that is distinct and more strongly correlated from compact clusters. Thus, populations of clusters are more evolved than associations in terms of their spatial distribution, traveling significantly from their birth site within a few tens of Myr, whereas associations show evidence of disruption occurring very quickly after their formation. The clustering of the stellar systems resembles that of a turbulent interstellar medium that drives the star formation process, correlating the components in unbound star-forming complexes in a hierarchical manner, dispersing shortly after formation, suggestive of a single, continuous mode of star formation across all galaxies.
|
|
| 4. |
- Grasha, K., et al.
(författare)
-
Hierarchical Star Formation in Turbulent Media : Evidence from Young Star Clusters
- 2017
-
Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 842:1
-
Tidskriftsartikel (refereegranskat)abstract
- We present an analysis of the positions and ages of young star clusters in eight local galaxies to investigate the connection between the age difference and separation of cluster pairs. We find that star clusters do not form uniformly but instead are distributed so that the age difference increases with the cluster pair separation to the 0.25-0.6 power, and that the maximum size over which star formation is physically correlated ranges from similar to 200. pc to similar to 1 kpc. The observed trends between age difference and separation suggest that cluster formation is hierarchical both in space and time: clusters that are close to each other are more similar in age than clusters born further apart. The temporal correlations between stellar aggregates have slopes that are consistent with predictions of turbulence acting as the primary driver of star formation. The velocity associated with the maximum size is proportional to the galaxy's shear, suggesting that the galactic environment influences the maximum size of the star-forming structures.
|
|
| 5. |
- Ryon, J. E., et al.
(författare)
-
Effective Radii of Young, Massive Star Clusters in Two LEGUS Galaxies
- 2017
-
Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 841:2
-
Tidskriftsartikel (refereegranskat)abstract
- We present a study of the effective (half-light) radii and other structural properties of a systematically selected sample of young, massive star clusters (>= 5. x. 10(3) M-circle dot and <= 200 Myr) in two nearby spiral galaxies, NGC. 628 and NGC. 1313. We use Hubble Space Telescope (HST) WFC3/UVIS and archival ACS/WFC data obtained by the Legacy Extragalactic UV Survey (LEGUS), an HST Treasury Program. We measure effective radii with GALFIT, a two-dimensional image-fitting package, and with a new technique to estimate effective radii from the concentration index of observed clusters. The distribution of effective radii from both techniques spans similar to 0.5-10. pc and peaks at 2-3. pc for both galaxies. We find slight positive correlations between effective radius and cluster age in both galaxies, but no significant relationship between effective radius and galactocentric distance. Clusters in NGC. 1313 display a mild increase in effective radius with cluster mass, but the trend disappears when the sample is divided into age bins. We show that the vast majority of the clusters in both galaxies are much older than their dynamical times, suggesting they are gravitationally bound objects. We find that about half of the clusters in NGC. 628 are underfilling their Roche lobes, based on their Jacobi radii. Our results suggest that the young, massive clusters in NGC. 628 and NGC. 1313 are expanding, due to stellar mass loss or two-body relaxation, and are not significantly influenced by the tidal fields of their host galaxies.
|
|
| 6. |
- Díaz, Martín D., et al.
(författare)
-
Self-reactivity and autoimmunity [Autorreactividad y autoinmunidad]
- 2017
-
Ingår i: Medicine (Spain). - : Ediciones Doyma, S.L.. - 0304-5412. ; 12:24, s. 1418-1427
-
Tidskriftsartikel (refereegranskat)abstract
- Definición de autoinmunidad y enfermedad autoinmuneLa autoinmunidad es una respuesta inmune contra uno o varios antígenos propios. Las enfermedades autoinmunes son el resultado del daño o la pérdida de función fisiológica en órganos y tejidos debido a una respuesta autoinmune. Este reconocimiento específico está mediado por las células del sistema inmune adaptativo, es decir, linfocitos T y B, si bien en los mecanismos de daño participan también células y moléculas del sistema inmune innato.Características de las enfermedades autoinmunesLas enfermedades autoinmunes son crónicas y suelen ser progresivas. La persistencia del antígeno, de las células T y B memoria específicas de los mismos y los poderosos mecanismos de amplificación inflamatorios son los mecanismos patogénicos que perpetúan la enfermedad.Tolerancia inmunitariaLos mecanismos de tolerancia son esenciales para el control de la autorreactivad, principalmente, los mediados en periferia por los linfocitos T. Estos mecanismos se dividen habitualmente en tolerancia central, tolerancia periférica y tolerancia mediada por células T reguladoras (Treg).Etiología de las enfermedades autoinmunesLa etiología de las enfermedades autoinmunes no está esclarecida, aunque parece claro que es multifactorial. La propensión genética es un factor importante y determinados conjuntos de alelos de genes pueden predisponer a la enfermedad. Además, los factores ambientales como la infección y la lesión tisular parecen fundamentales en su desarrollo.
|
|
