Norbert Pirzkal

We present observations of a luminous galaxy at z = 6.573—the end of the reionization epoch—which has been spectroscopically confirmed twice. The first spectroscopic confirmation comes from slitless Hubble Space Telescope Advanced Camera for Surveys grism spectra from the PEARS survey (Probing Evolution And Reionization Spectroscopically), which show a dramatic continuum break in the spectrum at rest frame 1216 Å. The second confirmation is done with Keck + DEIMOS. The continuum is not clearly detected with ground-based spectra, but high wavelength resolution enables the Lyα emission line profile to be determined. We compare the line profile to composite line profiles at z = 4.5. The Lyα line profile shows no signature of a damping wing attenuation, confirming that the intergalactic gas is ionized at z = 6.57. Spectra of Lyman breaks at yet higher redshifts will be possible using comparably deep observations with IR-sensitive grisms, even at redshifts where Lyα is too attenuated by the neutral intergalactic medium to be detectable using traditional spectroscopy from the ground.

We present grism spectra of emission-line galaxies (ELGs) from 0.6 to 1.6 μm from the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope. These new infrared grism data augment previous optical Advanced Camera for Surveys G800L 0.6-0.95 μm grism data in GOODS-South from the PEARS program, extending the wavelength coverage well past the G800L red cutoff. The Early Release Science (ERS) grism field was observed at a depth of two orbits per grism, yielding spectra of hundreds of faint objects, a subset of which is presented here. ELGs are studied via the Hα, [O III], and [O II] emission lines detected in the redshift ranges 0.2 z 1.4, 1.2 z 2.2, and 2.0 z 3.3, respectively, in the G102 (0.8-1.1 μm; R 210) and G141 (1.1-1.6 μm; R 130) grisms. The higher spectral resolution afforded by the WFC3 grisms also reveals emission lines not detectable with the G800L grism (e.g., [S II] and [S III] lines). From these relatively shallow observations, line luminosities, star formation rates, and grism spectroscopic redshifts are determined for a total of 48 ELGs to m _AB(F098M) 25 mag. Seventeen GOODS-South galaxies that previously only had photometric redshifts now have new grism-spectroscopic redshifts, in some cases with large corrections to the photometric redshifts (Δz 0.3-0.5). Additionally, one galaxy had no previously measured redshift but now has a secure grism-spectroscopic redshift, for a total of 18 new GOODS-South spectroscopic redshifts. The faintest source in our sample has a magnitude m _AB(F098M)= 26.9 mag. The ERS grism data also reflect the expected trend of lower specific star formation rates for the highest mass galaxies in the sample as a function of redshift, consistent with downsizing and discovered previously from large surveys. These results demonstrate the remarkable efficiency and capability of the WFC3 NIR grisms for measuring galaxy properties to faint magnitudes and redshifts to z 2.

The Advanced Camera for Surveys (ACS), installed in HST in SM3B, is fitted with a grism and three prisms for slitless spectrometry. The Wide Field Camera (WFC), with a field of 3.4x3.4' and 0.05" pixels, allows slitless grism spectrometry at a dispersion of 40Å per pixel over the range 0.55 to 1.05 μm. The High Resolution Camera (HRC) provides optimally sampled imaging above 0.6μm over a field of 26x29" (0.026" pixels), uses the same grism for slitless spectrometry at 24ÅA/pixel, and a prism covering the 1600-3300Å range with a 4maximum dispersion of 5Å/pixel. The Solar Blind Channel (SBC), with pixels of 0.030x0.034" (field size 31x35"), also has two prisms giving a maximum dispersion of about 2Å/pixel and covering the 1200-1800Å range; one prism excludes the geocoronal Lyman-α for lower background. The ground and in-flight calibration and astrophysical use of the grism and prism modes of ACS is described. Particular problems for slitless spectrometry include the dependence of spectral resolution on image size, determination of wavelength zero points and removal of flat field. The in-flight calibrations have been derived to extend and strengthen the ground calibrations taken using line and continuum lamps which will not be available in orbit. A software package is available to extract ACS spectra, based on a catalogue of images in a direct image, and has been extensively tested with a dedicated simulator. The wide range of astrophysical programs, for which the ACS spectral modes are well suited, will be outlined, with examples from accepted programs.

We report on the discovery of recently ionized hydrogen-deficient gas in the immediate circumstellar environment of the final helium shell flash star V4334 Sgr (Sakurai's object). On spectra obtained with FORS2 multi-object spectroscopy we have found spatially extended (about 2") emission from [N II], [O I], [O II] and very faint Halpha and [S II]. In the [N II] (ll6548,83) lines we have identified two components located at velocities -350 +/-50 and +200 +/-50 km/s, relative to V4334 Sgr itself. The full width of the [N II] l6583 feature at zero intensity corresponds to a velocity spread of about 1500 km/s. Based on the available data it is not possible to conclusively determine the mechanism of ionization. Both photo-ionization, from a rapidly evolving central star, and shock excitation, as the result of the collision of the fast ouflows with slower circumstellar matter, could account for the observed lines. The central star is still hidden behind strong dust absorption, since only a faint highly reddened continuum is apparent in the spectra. Theory states that it will become hotter and will retrace its post-asymptotic giant branch evolution towards the planetary nebula domain. Our detection of the ionized ejecta from the very late helium shell flash marks the beginning of a new phase in this star's amazingly rapid evolution.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Following the second HST servicing mission in 1997 when the STIS instrument was installed and the capability for parallel observations was enhanced, a substantial archive of non-proprietary parallel data has been accumulating. In this paper, we discuss the use of unfiltered STIS imaging data for a project that requires deep observations along as many independent lines-of-sight as possible. We have developed a technique to determine which datasets in the archive can safely be co-added together and have developed an iterative co-addition technique which enabled us to produce 498 high-quality, deep images. The principal motivation for this work is to measure the Cosmic Shear on small angular scales and a value derived from these data will be presented in a subsequent paper. A valuable by-product of this work is a set of high quality combined fields which can be used for other projects. The data are publicly available at http://www.stecf.org/projects/shear/

IR and radio-band observations of heavily extinguished regions in starburst galaxies suggest a high supernova (SN) rate associated with such regions. Optically measured SN rates may therefore underestimate the total SN rate by factors of up to 10, as a result of the very high extinction (A_B ~ 10–20 mag) to core-collapse SNe in starburst regions. The IR/radio SN rates come from a variety of indirect means, however, which suffer from model dependence and other problems. We describe a direct measurement of the SN rate from a regular patrol of starburst galaxies done with K'-band imaging to minimize the effects of extinction. A collection of K'-band measurements of core-collapse SNe near maximum light is presented. Such measurements (excluding 1987A) are not well reported in the literature. Results of a preliminary K'-band search, using the MIRC camera at the Wyoming Infrared Observatory and an improved search strategy using the new ORCA optics, are described. A monthly patrol of a sample of IRAS bright (mostly starburst) galaxies within 25 Mpc should yield 1–6 SNe yr^-1, corresponding to the range of estimated SN rates. Our initial MIRC search with low resolution (22 pixels) failed to find extinguished SNe in the IRAS galaxies, limiting the SN rate outside the nucleus (at greater than 15'' radius) to less than 3.8 far-IR SN rate units (SNe per century per 10¹⁰ L_⊙ measured at 60 and 100 μm, or FIRSRU) at 90% confidence. The MIRC camera had insufficient resolution to search nuclear starburst regions, where starburst and SN activity is concentrated; therefore, we were unable to rigorously test the hypothesis of high SN rates in heavily obscured star-forming regions. We conclude that high-resolution nuclear SN searches in starburst galaxies with small fields are more productive than low-resolution, large-field searches, even for our sample of large (often several arcminutes) galaxies. With our ORCA high-resolution optics, we could limit the total SN rate to less than 1.3 FIRSRU at 90% confidence in 3 years of observations, lower than most estimates.

The installation of the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) allows for the first time two-dimensional optical and ultraviolet slitless spectroscopy of faint objects from space. The STIS Parallel Survey (SPS) routinely obtains broadband images and slitless spectra of random fields in parallel with HST observations using other instruments. The SPS is designed to study a wide variety of astrophysical phenomena, including the rate of star formation in galaxies at intermediate to high redshift through the detection of emission-line galaxies. We present the first results of the SPS, which demonstrate the capability of STIS slitless spectroscopy to detect and identify high-redshift galaxies.