Red Supergiant Population Research Articles

The Star Clusters in the Starburst Irregular Galaxy NGC 1569

We examine star clusters in the irregular, starburst galaxy NGC 1569 from HST images. In addition to the two known super star clusters, we identify 45 other clusters that are compact but resolved. Integrated UVI colors of the clusters span a large range, and suggest that ages range from 3 Myrs to 1 Gyr. However, most of the clusters were formed at the tail end of the recent starburst. Numerous clusters in addition to the know super star clusters are similar in luminosity to a small globular cluster. We examined the radial surface brightness of four of the clusters. Their half-light radii and core radii are in the range observed in present-day globular clusters. Therefore, conditions that produced the recent starburst have also been those necessary for producing compact, bright star clusters. We examine resolved stars in the outer parts of the two super star clusters. Cluster A is dominated by bright blue stars with a small population of red supergiants. Sub-components A1 and A2 have similar colors and a two-dimensional color map does not offer evidence that one component is dominated by red supergiants and the other not. The contradiction of the presence of red super- giants with Wolf-Rayet stars may instead not be a contradiction at all since there coexistence in a coeval population is not inconsistent with the evolution of massive stars. Cluster B is dominated by red supergiants, and this is confirmed by the presence of the stellar CO absorption feature in an integrated spectrum. The various age indicators are consistent with a picture in which cluster B is of order 10--20 Myrs old, and cluster A is >4-5 Myrs old.

Hubble Space Telescope/NICMOS Observations of I Zw 18: A Population of Old Asymptotic Giant Branch Stars Revealed.

I present the first results from a Hubble Space Telescope/NICMOS imaging study of the most metal-poor blue compact dwarf galaxy, I Zw 18. The near-infrared color-magnitude diagram (CMD) is dominated by two populations, one 10-20 Myr population of red supergiants and one 0.1-5 Gyr population of asymptotic giant branch stars. Stars older than 1 Gyr are required to explain the observed CMD at the adopted distance of 12.6 Mpc, showing that I Zw 18 is not a young galaxy. The results hold also if the distance to I Zw 18 is significantly larger. This rules out the possibility that I Zw 18 is a truly young galaxy formed recently in the local universe.

Infrared Photometry of Red Supergiants in Young Clusters in the Magellanic Clouds

We present broad-band infrared photometry for 52 late-type supergiants in the young Magellanic Clouds clusters NGC 330, NGC 1818, NGC 2004 and NGC 2100. Standard models are seen to differ in the temperature they predict for the red supergiant population on the order of 300K. It appears that these differences most probably due to the calibration of the mixing-length parameter, $\alpha_{P}$, in the outermost layers of the stellar envelope. Due to the apparent model dependent nature of $\alpha_{P}$ we do not quantitatively compare $\alpha_{P}$ between models. Qualitatively, we find that $\alpha_{P}$ decreases with increased stellar mass within standard models. We do not find evidence for a metallicity dependence of $\alpha_{P}$.

Use of Red Supergiant Spectral Features as Age Indicators in Starburst Regions

We investigate techniques that can be used to determine ages of starburst regions containing populations beyond their early nebular phase. In particular, we study the strength of the Ca II triplet (λλ8498, 8542, 8662) and the CO index (2.31-2.40 μm band) using synthetic models as the starburst evolves. For an instantaneous burst of star formation, both of these absorption features remain strongest between 7 and 14 Myr, which corresponds to the red supergiant population. The detailed evolutionary behavior of the starburst is strongly metallicity dependent. Low-metallicity starburst models successfully reproduce the distribution of equivalent widths of the Ca II triplet with age in Large Magellanic Cloud clusters. The clusters in the red supergiant phase strongly favor the stellar evolutionary models incorporating mass-loss rates higher than the standard values. We suggest using of diagrams involving Ca II triplet equivalent width, CO index, and nebular recombination lines to infer the history as well as the age of starburst regions.

2.2 micron image of 3C 368 at Z = 1.13, a galaxy with aligned radio and stellar axes

view Abstract Citations (97) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS 2.2 Micron Image of 3C 368 at z=1.13, a Galaxy with Aligned Radio and Stellar Axes Chambers, K. C. ; Miley, G. K. ; Joyce, R. R. Abstract We present a K-band infrared image of the z = 1.13 radio galaxy 3C 368. This galaxy is one of the brightest examples of the recently discovered phenomenon of alignment between the optical and radio axes of powerful distant radio galaxies. Our observations show that the infrared morphology is also elongated and aligned along the optical and radio axes. However, it is not coincident with the radio emission. We discuss various mechanisms for producing the infrared and optical flux, and the resultant constraints on the origin of the alignment effect in high-redshift radio galaxies. The most likely explanation is that the emission is produced mainly by young stars formed by interaction of the radio source with the interstellar medium. The infrared flux is then interpreted as dominated by a population of red supergiants. Independent of the origin of the emission, the observed alignment implies that powerful radio galaxies at high redshifts are distinct from giant ellipticals, even in the infrared. Hence, attempts to derive a cosmological "standard candle" using studies which combine these two types of galaxies are likely to be invalid. Publication: The Astrophysical Journal Pub Date: June 1988 DOI: 10.1086/185180 Bibcode: 1988ApJ...329L..75C Keywords: Cosmology; Galactic Evolution; Infrared Imagery; Interstellar Matter; Radio Galaxies; Red Shift; Morphology; Radio Emission; Radio Sources (Astronomy); Star Formation; Astrophysics; COSMOLOGY; GALAXIES: EVOLUTION; GALAXIES: REDSHIFTS; GALAXIES: STELLAR CONTENT; INFRARED: GENERAL; RADIO SOURCES: GALAXIES full text sources ADS | data products SIMBAD (5) NED (3)

10 to the 12th solar luminosity starbursts and shocked molecular hydrogen in the colliding galaxies ARP 220 (= IC 4553) and NGC 6240

Infrared spectroscopy and photometry and optical spectroscopy of the exceedingly luminous interacting galaxies Arp 220 (= IC 4553) and NGC 6240 are presented. These galaxies are the sites of exceedingly powerful bursts of star formation, as shown by (1) the luminosity and extent of the stellar component seen near 2 microns; (2) the large population of red supergiants required to account for the depth of the stellar CO absorption; (3) the absorption and emission features in the spectrum longward of 3 microns; and (4) the extended luminosity source indicated by the spatial extent of the galaxies at 10 microns. Scaled-up models similar to those developed for less luminous starburst galaxies are consistent with this hypothesis. Arp 220 also appears to have an active nucleus, possibly of Seyfert type 2. Both galaxies also show strong lines from shocked molecular hydrogen, which may arise in the collisions of their interstellar clouds in an ongoing interaction.