Deep Hubble Space Telescope/ACS Observations of I Zw 18: a Young Galaxy in Formation

We present V and I photometry of the resolved stars in the most metal-deficient blue compact dwarf galaxy known, I Zw 18 (Zsun/50), using Hubble Space Telescope/Advanced Camera for Surveys (ACS) images, the deepest ones ever obtained for this galaxy. The resulting I vs. V-I color-magnitude diagram (CMD) reaches limiting magnitudes V=I=29 mag. It reveals a young stellar population of blue main-sequence (MS) stars (age <30 Myr) and blue and red supergiants (10 Myr<age<100 Myr), but also an older evolved population of asymptotic giant branch (AGB) stars (100 Myr<age<500 Myr). We derive a distance to I Zw 18 in the range 12.6 Mpc - 15 Mpc from the brightness of its AGB stars, with preferred values in the higher range. The red giant branch (RGB) stars are conspicuous by their absence, although, for a distance of I Zw 18 <15 Mpc, our imaging data go ~ 1-2 mag below the tip of the RGB. Thus, the most evolved stars in the galaxy are not older than 500 Myr and I Zw 18 is a bona fide young galaxy. Several star formation episodes can be inferred from the CMDs of the main body and the C component. There have been respectively three and two episodes in these two parts, separated by periods of ~ 100-200 Myr. In the main body, the younger MS and massive post-MS stars are distributed over a larger area than the older AGB stars, suggesting that I Zw 18 is still forming from the inside out. In the C component, different star formation episodes are spatially distinct, with stellar population ages decreasing from the northwest to the southeast, also suggesting the ongoing build-up of a young galaxy.Comment: 29 pages, 13 Postscript figures, accepted for publication in the Astrophysical Journa

Statistics of the electromagnetic response of a chaotic reverberation chamber

This article presents a study of the electromagnetic response of a chaotic reverberation chamber (RC) in the presence of losses. By means of simulations and of experiments, the fluctuations in the maxima of the field obtained in a conventional mode-stirred RC are compared with those in a chaotic RC in the neighborhood of the Lowest Useable Frequency (LUF). The present work illustrates that the universal spectral and spatial statistical properties of chaotic RCs allow to meet more adequately the criteria required by the Standard IEC 61000-4-21 to perform tests of electromagnetic compatibility.Comment: 6 pages, 9 figure

A spectroscopic study of component C and the extended emission around I Zw 18

Long-slit Keck II, 4m Kitt Peak, and 4.5m MMT spectrophotometric data are used to investigate the stellar population and the evolutionary status of I Zw 18C, the faint C component of the nearby blue compact dwarf galaxy I Zw 18. Hydrogen H$\alpha$ and H$\beta$ emission lines are detected in the spectra of I Zw 18C, implying that ionizing massive stars are present. High signal-to-noise Keck II spectra of different regions in I Zw 18C reveal H$\gamma$, H$\delta$ and higher order hydrogen lines in absorption. Several techniques are used to constrain the age of the stellar population in I Zw 18C. Ages derived from two different methods, one based on the equivalent widths of the H$\alpha$, H$\beta$ emission lines and the other on H$\gamma$, H$\delta$ absorption lines are consistent with a 15 Myr instantaneous burst model. We find that a small extinction in the range $A_V$ = 0.20 -- 0.65 mag is needed to fit the observed spectral energy distribution of I Zw 18C with that model. In the case of constant star formation, all observed properties are consistent with stars forming continuously between ~ 10 Myr and < 100 Myr ago. We use all available observational constraints for I Zw 18C, including those obtained from Hubble Space Telescope color-magnitude diagrams, to argue that the distance to I Zw 18 should be as high as ~ 15 Mpc. The deep spectra also reveal extended ionized gas emission around I Zw 18. H$\alpha$ emission is detected as far as 30" from it. To a B surface brightness limit of ~ 27 mag arcsec$^{-2}$ we find no observational evidence for extended stellar emission in the outermost regions, at distances > 15" from I Zw 18.Comment: 38 pages, 11 Postscript figures, accepted for publication in the Astrophysical Journa

Hypercalcémie majeure révélatrice d’une sarcoïdose induite par étanercept

Introduction The principal secondary effects of anti-TNF alpha therapy are now well understood, particularly the risk of opportunistic infections. Other paradoxical effects have been described much more occasionally such as the developement of sarcoid-like granulomatous reactions. Case report We report here the case of a woman of 39 years treated for severe rheumatoid arthritis for five years with etanercept. She was admitted to hospital as an emergency with vomiting and diffuse abdominal pain. Investigations revealed severe hypercalcaemia and acute renal failure. After correction of the metabolic disturbances with rehydration and biphosphonates, CT scanning of the abdomen, pelvis and thorax showed bilateral interstitial infiltration and splenomegaly. The diagnosis of sarcoidosis was confirmed by endoscopic bronchial biopsies. Progress was satisfactory following withdrawal of the etanercept and corticosteroid therapy in reducing dosage. Conclusion The risk of induced sarcoidosis should be understood in patients receiving anti-TNF therapy and should be considered in cases of hypercalcaemia and/or splenomegaly

Small grain variety testing and demonstration program, 1966-67

The Oklahoma Cooperative Extension Service periodically issues revisions to its publications. The most current edition is made available. For access to an earlier edition, if available for this title, please contact the Oklahoma State University Library Archives by email at [email protected] or by phone at 405-744-6311

Spring freeze injury to winter wheat

The Oklahoma Cooperative Extension Service periodically issues revisions to its publications. The most current edition is made available. For access to an earlier edition, if available for this title, please contact the Oklahoma State University Library Archives by email at [email protected] or by phone at 405-744-6311

Small grain variety testing and demonstration program, 1965-66

The Oklahoma Cooperative Extension Service periodically issues revisions to its publications. The most current edition is made available. For access to an earlier edition, if available for this title, please contact the Oklahoma State University Library Archives by email at [email protected] or by phone at 405-744-6311

Cold injury to winter wheat

The Oklahoma Cooperative Extension Service periodically issues revisions to its publications. The most current edition is made available. For access to an earlier edition, if available for this title, please contact the Oklahoma State University Library Archives by email at [email protected] or by phone at 405-744-6311

A Serendipitous Search for High-Redshift Lyman alpha Emission: Two Primeval Galaxy Candidates at z~3

In the course of our ongoing search for serendipitous high-redshift Lyman alpha (LyA) Emissionin deep archival Keck spectra, we discovered two very high equivalent width (W_{obs} ~ 450A, 2-sigma) LyA emission line candidates at z ~3 in a moderate dispersion (R~1200) spectrogram. Both lines have low velocity dispersions (sigma_v ~ 60 km/s) and deconvolved radii r ~ 1 kpc (h = 0.5). We argue that the lines are LyA, and are powered by stellar ionization. The surface density of robust, high equivalent width LyA candidates is estimated to be ~3 \pm 2 per arcmin^2 per unit redshift at z ~ 3, consistent with the estimate of Cowie etal (1998). The LyA emission line source characteristics are consistent with the galaxies undergoing their first burst of star formation, ie, with being primeval. Source sizes and velocity dispersions are comparable to the theoretical primeval galaxy model of Lin and Murray (1992) based on the inside-out, self-similar collapse of an isothermal sphere. In this model, star formation among field galaxies is a protracted process. Galaxies are thought to be able to display high equivalent widths for only the first few x 10 Myr. This time is short in relation to the difference in look back times between z=3 and z=4, and implies that a substantial fraction of strong line-emitting galaxies at z=3 were formed at redshifts z < 4. We discuss the significance of high-equivalent width LyA-emitting galaxies in terms of the emerging picture of the environment, and the specific characteristics of primeval galaxy formation at high redshift.Comment: 17 pages, 3 figures, one table. To appear in the Astrophysical Journa