6 research outputs found
Testing massive star evolution, star-formation history, and feedback at low metallicity: Photometric analysis of OB stars in the SMC Wing
The supergiant ionized shell SMC-SGS 1 (DEM 167), located in the outer Wing
of the Small Magellanic Cloud (SMC), resembles structures that originate from
an energetic star-formation event and later stimulate star formation as they
expand into the ambient medium. However, stellar populations within and
surrounding SMC-SGS 1 tell a different story. We present a photometric study of
the stellar population encompassed by SMC-SGS 1 in order to trace the history
of this structure and its potential influence on star formation within the
low-density, low-metallicity SMC Wing. For a stellar population that is
physically associated with SMC-SGS 1, we combined near-ultraviolet (NUV)
photometry from the Galaxy Evolution Explorer (GALEX) with archival optical
(V-band) photometry from the ESO Danish 1.54m Telescope. Given their colors and
luminosities, we estimated stellar ages and masses by matching observed
photometry to theoretical stellar isochrone models. We find that the
investigated region supports an active, extended star-formation event spanning
25 - 40 Myr ago, as well as continued star formation into the present.
Using a standard initial mass function (IMF), we infer a lower bound on the
stellar mass from this period of , corresponding
to a star-formation intensity of 6 10 M
kpc yr. The spatial and temporal distributions of young stars
encompassed by SMC-SGS 1 imply a slow, consistent progression of star formation
over millions of years. Ongoing star formation along the edge of and interior
to SMC-SGS 1 suggests a combined stimulated and stochastic mode of star
formation within the SMC Wing. A slow expansion of the shell within this
low-density environment may preserve molecular clouds within the volume of the
shell, leaving them to form stars even after nearby stellar feedback expels
local gas and dust.Comment: 9 pages, 6 figures, 3 table
NGC 5523: An isolated product of soft galaxy mergers?
Multi-band images of the very isolated spiral galaxy NGC 5523 show a number of unusual features consistent with NGC 5523 having experienced a significant merger. (1) Near-infrared images from the Spitzer Space Telescope (SST) and the WIYN 3.5-m telescope reveal a nucleated bulge-like structure embedded in a spiral disk; (2) the bulge is offset by ~1.8 kpc from a brightness minimum at the center of the optically bright inner disk; (3) a tidal stream, possibly associated with an ongoing satellite interaction, extends from the nucleated bulge along the disk. We interpret these properties as the results of one or more non-disruptive mergers between NGC 5523 and companion galaxies or satellites, raising the possibility that some galaxies become isolated because they have merged with former companions
Overview of the DESI Legacy Imaging Surveys
The DESI Legacy Imaging Surveys (http://legacysurvey.org/) are a combination of three public projects (the Dark Energy Camera Legacy Survey, the Beijing–Arizona Sky Survey, and the Mayall z-band Legacy Survey) that will jointly image ≈14,000 deg2 of the extragalactic sky visible from the northern hemisphere in three optical bands (g, r, and z) using telescopes at the Kitt Peak National Observatory and the Cerro Tololo Inter-American Observatory. The combined survey footprint is split into two contiguous areas by the Galactic plane. The optical imaging is conducted using a unique strategy of dynamically adjusting the exposure times and pointing selection during observing that results in a survey of nearly uniform depth. In addition to calibrated images, the project is delivering a catalog, constructed by using a probabilistic inference-based approach to estimate source shapes and brightnesses. The catalog includes photometry from the grz optical bands and from four mid-infrared bands (at 3.4, 4.6, 12, and 22 μm) observed by the Wide-field Infrared Survey Explorer satellite during its full operational lifetime. The project plans two public data releases each year. All the software used to generate the catalogs is also released with the data. This paper provides an overview of the Legacy Surveys project