ABSTRACT: The Space Telescope Imaging Spectrograph (STIS) was successfully installed into the Hubble Space Telescope (HST) in 1997 February, during the second HST servicing mission, STS-82. STIS is a versatile spectrograph, covering the 115-1000 nm wavelength range in a variety of spectroscopic and imaging modes that take advantage of the angular resolution, unobstructed wavelength coverage, and dark sky offered by the HST. In the months since launch, a number of performance tests and calibrations have been carried out and are continuing. These tests demonstrate that the instrument is performing very well. We present here a synopsis of the results to date.
The Astrophysical Journal 01/2009; 492(2):L83. · 6.02 Impact Factor
ABSTRACT: We present optical long-slit spectroscopy of the nucleus of the nearby radio galaxy M84 (NGC 4374 = 3C 272.1) obtained with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. Our spectra reveal that the nuclear gas disk seen in the Wide Field Planetary Camera 2 imaging by Bower et al. is rotating rapidly. The velocity curve has an S-shape with a peak amplitude of 400 km s−1 at 01 = 8 pc from the nucleus. To model the observed gas kinematics, we construct a thin Keplerian disk model that fits the data well if the rotation axis of the gas disk is aligned with the radio jet axis. These models indicate that the gasdynamics are driven by a nuclear compact mass of 1.5 × 109 M☉ with an uncertainty range of (0.9-2.6) × 109 M☉, and that the inclination of the disk with respect to the plane of the sky is 75°-85°. Of this nuclear mass, only ≤2 × 107 M☉ can possibly be attributed to luminous mass. Thus, we conclude that a dark compact mass (most likely a supermassive black hole) resides in the nucleus of M84.
The Astrophysical Journal 01/2009; 492(2):L111. · 6.02 Impact Factor
ABSTRACT: We have obtained new Hubble Space Telescope observations of M84, a nearby massive elliptical galaxy whose nucleus contains a ≈1.5 × 109 M☉ dark compact object, which presumably is a supermassive black hole. Our Space Telescope Imaging Spectrograph spectrum provides the first clear detection of emission lines in the blue (e.g., [O II] λ3727, Hβ, and [O III] λλ4959, 5007), which arise from a compact region ≈028 across centered on the nucleus. Our Near-Infrared Camera and Multiobject Spectrometer images exhibit the best view through the prominent dust lanes evident at optical wavelengths and provide a more accurate correction for the internal extinction. The relative fluxes of the emission lines we have detected in the blue together with those recently detected in the wavelength range 6295-6867 Å by Bower et al. indicate that the gas at the nucleus is photoionized by a nonstellar process instead of hot stars. Stellar absorption features from cool stars at the nucleus are very weak. We update the spectral energy distribution of the nuclear point source and find that although it is roughly flat in most bands, the optical-to-UV continuum is very red, similar to the spectral energy distribution of BL Lac. Thus, the nuclear point source seen in high-resolution optical images by Bower et al. is not a star cluster but is instead a nonstellar source. Assuming isotropic emission from this source, we estimate that the ratio of bolometric luminosity to Eddington luminosity is ~5 × 10-7. However, this could be underestimated if this source is a misaligned BL Lac object, which is a possibility suggested by the spectral energy distribution and the evidence of optical variability we describe.
The Astrophysical Journal 12/2008; 534(1):189. · 6.02 Impact Factor
ABSTRACT: We have developed a model that allows us to defringe slitless 2-dimensional spectra taken with the Space Telescope Imaging Spectrograph (STIS). An IDL tool has been developed which allows the user to defringe any spectrum obtained with the G750L grating on STIS. This technique has been employed to model the fringing on Wide Field Camera 3 (WFC3) flight candidate CCDs.
ABSTRACT: We present Hubble Space Telescope (HST) and ground-based observations of a pair of galaxies at redshift 2.38, which are collectively known as 2142-4420 B1 (Francis et al. 1996). The two galaxies are both luminous extremely red objects (EROs), separated by 0.8 arcsec. They are embedded within a 100 kpc scale diffuse Ly-alpha nebula (or blob) of luminosity ~10^44 erg/s. The radial profiles and colors of both red objects are most naturally explained if they are young elliptical galaxies: the most distant yet found. It is not, however, possible to rule out a model in which they are abnormally compact, extremely dusty starbursting disk galaxies. If they are elliptical galaxies, their stellar populations have inferred masses of ~10^11 solar masses and ages of ~7x10^8 years. Both galaxies have color gradients: their centers are significantly bluer than their outer regions. The surface brightness of both galaxies is roughly an order of magnitude greater than would be predicted by the Kormendy relation. A chain of diffuse star formation extending 1 arcsec from the galaxies may be evidence that they are interacting or merging. The Ly-alpha nebula surrounding the galaxies shows apparent velocity substructure of amplitude ~ 700 km/s. We propose that the Ly-alpha emission from this nebula may be produced by fast shocks, powered either by a galactic superwind or by the release of gravitational potential energy. Comment: 33 pages, 9 figures, ApJ in press (to appear in Jun 10 issue)
ABSTRACT: We have developed a model that allows us to defringe slitless 2-dimensional spectra taken with the Space Telescope Imaging Spectrograph (STIS). Like all thin chip CCDs used in astronomy, the STIS CCD detector acts like an interferometer in the presence of monochromatic light, producing bright and dark fringes where there is constructive and destructive interference. These fringes are especially troublesome for spectra in the near-IR (> 7000A), reaching a peak amplitude of about 20% near 9500 A. Removing fringes from STIS spectra taken with a slit is simply a matter of dividing by an appropriate "fringe flat". A suitable flat is obtained by observing a continuum calibration source through the same slit. This however is not possible for slitless spectra, whose wavelength mapping onto the CCD detector varies with position of the object in the field. Beginning with an approximate knowledge of the STIS CCD structure, we have used 50 continuum source spectral flats taken at various central wavelengths to constrain the thickness of the detection layer at each pixel. Rom this result, we can compute a fringe flat for any astronomical source, no matter how it is positioned in the field. Flats made by this method reduce the fringing amplitude in astronomical spectra by a factor of about 5. The model fringe flat is a function of the index of refraction of each of four layers in the CCD, the thickness of each layer, and the wavelength of the light hitting each pixel. We show how the CCD structural parameters are incorporated into this "fitting function", and describe the procedure used to solve for the free parameters (the thickness of each layer at each pixel). The effectiveness of this technique will be demonstrated using slitless spectra taken as part of the STIS parallel program.
ABSTRACT: We discuss the utility of a low resolution prism as a component of a
Multi-Object Spectrometer (MOS) for NASA's proposed Next Generation
Space Telescope (NGST). Low resolution prism spectroscopy permits
simultaneous coverage of the 0.6-5 μ m regime at R<50. Such data
can take advantage of the modern techniques in spectral energy
distribution (SED) fitting to determine source redshifts, sometimes
called ``photometric redshifts''. We compare this approach with other
spectroscopic and photometric modes being considered for NGST. Low
resolution prism observations of galaxy SED's provide a significant
advantage over multi-filter observations for any realistic observing
strategy. For an ideal prism in background limited observing, the prism
has a signal-to-noise advantage of square root of the resolution over
serial observations by filters with similar resolution. This added depth
reaches at least a magnitude fainter in recovery of photometric
redshifts with the prism over filter observations at similar resolving
power. Extensive simulations suggest that in 105 seconds an
ideal prism will recover the redshift of 80% of measured objects
(subject to MOS selection) down to KAB=32 compared to 40% of
the objects with serial filter observations of equal total observing
time, while reducing the percent of outright failures from 15% to 1%.
The advantage is even greater for the subset of galaxies between
KAB=30 and KAB=32.
ABSTRACT: We present 1/2 square degree uBVRI imaging around the Hubble Deep Field
-- South. The images were obtained with the Big Throughput Camera at
CTIO in 1998 September. We will discuss the broad-band photometry of
this field and present the statistics of ~ 50000 galaxies, including
number-magnitude counts and clustering properties. The deep field was
chosen for the presence of the z=2.25 QSO J2233-606, which has multiple
absorption systems that may mark large scale structures. We also discuss
the color distribution of galaxies and u-band dropouts. We compare our
photometric measurements with the high resolution images available from
the HST observations. We will make the images and catalogs presented
here available over the WWW.
ABSTRACT: The in-orbit performance of the Goddard High Resolution Spectrograph
onboard the Hubble Space Telescope (HST) is presented. This report
covers the pre-COSTAR period, when instrument performance was limited by
the effects of spherical aberration of the telescope's primary mirror.
The digicon detectors provide a linear response to count rates spanning
over six orders of magnitude, ranging from the normal background flux of
0.01 counts diode ^-1 s^-1 to values larger than 10^4 counts diode^-1
s^-1. Scattered light from the first-order gratings is small and can be
removed by standard background subtraction techniques. Scattered light
in the echelle mode is more complex in origin, but it also can be
accurately removed. Data have been obtained over a wavelength range from
below 1100 A to 3300 A, at spectral resolutions as high as R =
lambda/delta-lambda = 90,000. The wavelength scale is influenced by
spectrograph temperature, outgassing of the optical bench, and
interaction of the magnetic field within the detector with the earth's
magnetic field. Models of these effects lead to a default wavelength
scale with an accuracy better than 1 diode, corresponding to 3 km s^-1
in the echelle mode. With care, the wavelength scale can be determined
to an accuracy of 0.2 diodes. Calibration of the instrument sensitivity
functions is tied into the HST flux calibration through observations of
spectrophotometric standard stars. The measurements of vignetting and
the echelle blaze function provide relative photometric precision to
about 5% or better. The effects of fixed-pattern noise have been
investigated, and techniques have been devised for recognizing and
removing it from the data. The ultimate signal-to-noise ratio achievable
with the spectrograph is essentially limited only by counting
statistics, and values approaching 1000:1 have been obtained. (SECTION:
Publications of the Astronomical Society of the Pacific 08/1995; 107:871. · 3.58 Impact Factor
ABSTRACT: A previously described Monte Carlo code has been improved and used to
investigate the evolution of clusters of galaxies of different richness.
With one important exception, results of the previous study are
confirmed. Under a specific set of assumptions about cluster conditions
at virialization, the simulations do account for a number of observed
properties of clusters. In particular, starting with 85 percent of the
cluster mass in the intracluster medium, roughly 25 percent of the
simulations produced cD galaxies. This percentage is roughly independent
of initial or final cluster richness. The dependence of the magnitude of
the second-ranked galaxy (M2) on M2-M1 predicted by these simulations
may disagree with observations. Although the frequency of formation of a
massive galaxy by mergers is roughly independent of richness, the
luminosity stripped from the galaxies varies roughly as N-squared (where
N is the total number of galaxies) in the simulations. The relative
importance of the cD halo (or intracluster light) is therefore a strong
function of richness, consistent with observational claims and with the
proposal that the central region of the cD is produced by mergers while
the halo is produced by stripping.
The Astrophysical Journal 12/1983; 276:413-422. · 6.02 Impact Factor
ABSTRACT: A multiple one-body Monte Carlo code is used to investigate the evolution of galaxies in a steady cluster potential under the influence of dynamical friction, two-body relaxation, tidal stripping, and galaxy mergers. The basic assumptions, estimated time scales, method and computer program, and effects of uncertainties in galaxy encounter physics are addressed. Numerical experiments in which the mass initially carried by galaxies and the mass function are varied are reported. It is shown that the formation of a very massive cluster galaxy depends critically on the number of galaxies, the initial division of cluster material between galaxies and a smooth intracluster medium, the mass spectrum of the galaxies, and chance. By the end of all simulations, less than half of the mass in the central regions of the cluster is bound to galaxies. It appears possible to produce any of the Bautz-Morgan classes from very similar initial conditions.
ABSTRACT: Revealing faint circumstellar nebulosity and faint stellar or substellar
companions to bright stars typically requires use of techniques for
rejecting the direct, scattered, and diffracted light of the star. One
such technique is Lyot coronagraphy. We summarize the performance of the
white-light coronagraphic capability of the Space Telescope Imaging
Spectrograph, on board the Hubble Space Telescope.