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L' and M' standard stars for the Mauna Kea Observatories Near-Infrared system
We present L' and M' photometry, obtained at the United Kingdom InfraredTelescope (UKIRT) using the Mauna Kea Observatories Near-Infrared(MKO-NIR) filter set, for 46 and 31 standard stars, respectively. The L'standards include 25 from the in-house `UKIRT Bright Standards' withmagnitudes deriving from Elias et al. and observations at the InfraredTelescope Facility in the early 1980s, and 21 fainter stars. The M'magnitudes derive from the results of Sinton and Tittemore. We estimatethe average external error to be 0.015 mag for the bright L' standardsand 0.025 mag for the fainter L' standards, and 0.026 mag for the M'standards. The new results provide a network of homogeneously observedstandards, and establish reference stars for the MKO system, in thesebands. They also extend the available standards to magnitudes whichshould be faint enough to be accessible for observations with moderndetectors on large and very large telescopes.
| High-Redshift Quasars and Star Formation in the Early Universe
In order to derive information on the star formation history in theearly universe, we observed six high-redshift (z~=3.4) quasars in thenear-infrared to measure the relative iron and Mg II emission strengths.A detailed comparison of the resulting spectra with those oflow-redshift quasars show essentially the same Fe II /Mg II emissionratios and very similar continuum and line spectral properties,indicating a lack of evolution of the relative iron to magnesiumabundance of the gas since z~=3.4 in bright quasars. On the basis ofcurrent chemical evolution scenarios of galaxies, where magnesium isproduced in massive stars ending in Type II Supernovae (SNe II), whileiron is formed predominantly in SNe Ia with a delay of ~1 Gyr andassuming as cosmological parameters H0=72 km s-1Mpc-1, ΩM=0.3, andΩΛ=0.7, we conclude that major star formationactivity in the host galaxies of our z~=3.4 quasars must have startedalready at an epoch corresponding to zf~=10, when the age ofthe universe was less than 0.5 Gyr. Based on observations collected atthe European Southern Observatory, La Silla, Chile.
| Sub-Arcsecond 2.2 micron Imaging of the Starburst Galaxy NGC 1808: Infrared Observations of Super Star Clusters
We present a high-resolution (0.6") 2.2 micron (K-band) image of thecentral (~750 Pc) circumnuclear region of the starburst galaxy NGC 1808.Contrary to previous lower resolution observations our image revealsdetailed structure in the distribution of near-infrared light that isproduced within the circumnuclear region. Our data reveal the presenceof many distinct and compact (<~ 0.7") near-infrared continuumsources spread throughout the circumnuclear region. These sources arelikely individual clusters containing a few to several hundred massive(~25-30 M_sun_) red supergiants. The inferred cluster masses range from10^5^ to 10^6^ M_sun_. Together with previous 2.17 micron Br? lineobservations we conclude that the circumnuclear clusters are young andthat their rates of star-formation are decaying rapidly. The clusterages are close to the "critical" age of 10 Myr at which clusters reachtheir maximum luminosities in the near-infrared. The individual stellarsystems we have detected are probably gravitationally bound. They appearto be the first unambiguous detections of the IR counterparts of youngclusters (possibly globular clusters) which have recently beenidentified in high- resolution optical and ultraviolet (HST) imaging ofseveral types of starburst galaxies. A 500 pc long infrared bar, andevidence of inner spiral or ring-like structures are present in ourimage. The infrared bar is well aligned with a more extended (~6 kpc)bar-structure visible in wide-field H? and 21 cm emission linemaps. A prominent, and just resolved, 2.2 micron continuum source ispresent at the galaxy nucleus. Most of the near-infrared light in thenuclear source is likely produced by massive red giants and supergiantstars. Both decaying and continuous modes of star formation areconsistent with the observations. For the decaying mode the inferred ageis 8 Myr with a burst mass of ~5 x 10^6^ M_sun_ within the central 50pc. For continuous star formation the inferred age is ~200 Myr with atotal accumulated mass of 3.2 x 10^7^ M_sun_. Continuous star formationmay be fueled by gas inflow along the bar at a rate of ~0.16 M_sun_yr^-1^.
| Near-infrared spectral imaging of NGC 1808: Probing the starburst
We report 1.8 sec resolution imaging of 2.17 micrometers H I Br(gamma)and 2.12 micrometers H2 1-0 S(1) line emission and 2.15 micrometerscontinuum emission toward the nuclear region of the hot spot spiralgalaxy NGC 1808. The data, taken with a high-resolution imagingspectrometer, show that the Br(gamma) emission originates from severaldistinct emission knots which are distributed throughout thecircumnuclear region extending approximately 14 sec (750 pc) along themajor axis of the galaxy. Most of the H2 emission is confined to aregion within approximately 4 sec (200 pc) of the nucleus. The 2.15micrometers continuum emission is distributed smoothly (at 1.8 secresolution) throughout the circumstellar region, is not produced indiscrete hot spots, and peaks sharply at the galactic nucleus. TheBr(gamma) emission knots are spatially well correlated with the set ofcompact radio sources that are present in the circumnuclear region. OurBr(gamma) data imply that between 10% and 50% of the compactcircumnuclear 5 GHz radio emission is thermal, free-free emission. TheBr(gamma) emission knots do not coincide with the locations of optical'hot-spots' observed in NGC 1808. From a comparison of the Br(gamma) andH-alpha emission knots we conclude that the Br(gamma) knots trace theactual sites of star-formation activity, while the optical hot spots aremainly directions of low foreground extinction. We use ourhigh-resolution near-infrared observations together with radio andfar-infrared continuum observations and theoretical models of evolvingstar clusters to constrain the properties of the distinct star-formationcomplexes in NGC 1808. The data suggest that star-formation has beenproceeding, at rates of approximately 1 solar mass/yr, for approximatelygreater than 5 x 107 yr in the nuclear cluster, and forapproximately less than to 107 yr in the circumnuclear starclusters. The data are consistent with stellar initial mass functions ofthe form M-alpha with alpha approximately 2.5 which aretruncated at upper mass cutoffs of approximately 30 solar mass. The IMFsmay extend to small (approximately 0.1 solar mass) lower mass limits ifthe star-formation activity in the star-forming complexes issufficiently efficient. Much of the extranuclear 2.15 micrometerscontinuum emission is probably produced by evolved bulge stars which aremuch older than the lifetimes of the circumnuclear star-formingclusters. However, these clusters also contribute significantly to theobserved 2.15 micrometers continuum. The intense 2.15 micrometerscontinuum emission at the nucleus is very likely dominated by red giantsand supergiants recently produced in the nuclear star-forming cluster.We argue that most of the bolometric luminosity generated in the centerof NGC 1808 is produced by the star-forming clusters. There is nonear-infrared evidence for an embedded active galactic nucleus in NGC1808.
| JHKLM standard stars in the ESO system
A list of 199 standard stars suitable for the ESO standard photometricsystem at JHKLM is given. Faint stars (although brighter than K = 7.7)to be used on larger telescopes are included. This list is based on ananalysis of all infrared photometric observations carried out at LaSilla from 1979 until 1989 inclusive. The accuracy of the data (about0.02 mag. at J, H, K, L, and M) is similar to the one achieved at SAAOand CTIO. Comparisons with these systems, as well as with the AAO andMSSO systems, are made: it is shown that the ESO system is very close tothe other ones, with the exception of CTIOs.
| The luminosity function in NGC 2023
Sellgren (1983) has shown that the young stellar cluster within themolecular cloud associated with the reflection nebula NGC 2023 has aluminosity function consistent with the Salpeter IMF for stars brighterthan K = 12 mag. Extrapolation predicts that approximately 50 additionalstars brighter than K = 15 mag, arguably of spectral types G through K,should be present. However, images of the cluster using a sensitivetwo-dimensional IR array reveal no additional cluster members, whichsuggests that either no low-mass stars have formed or that they arestill heavily shrouded in molecular 'cocoons' left over from the starformation process. A straightforward experimental resolution of thealternatives is proposed.
| Southern JHKL standards
The basis for the current SAAO standard photometric system at JHKL isgiven. This depends on an extensive investigation involving 230 starsdistributed around the sky. The accuracy is estimated at + or - 0.02 magfor J, H and K and + or - 0.05 mag for L.
| Infrared standard stars
The results of an observational program aimed at setting up a network offaint near-infrared standards of sufficient accuracy are reported. Thenetwork covers both northern and southern hemispheres and includesstandards red enough to provide at least a limited check on colortransformations. The standards are set up at J (1.2 micron), H (1.6micron), K (2.2 microns), and L (3.5 microns), and their H2O and COmolecular absorption indices are determined. The problem of colortransformations between observatories is discussed briefly. Allmagnitudes presented are transformed to the natural system defined bythe CIT observations.
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