Steven V. Penton

THE COSMIC ORIGINS SPECTROGRAPH

The Cosmic Origins Spectrograph (COS) is a moderate-resolution spectrograph with unprecedented sensitivity that was installed into the Hubble Space Telescope (HST) in 2009 May, during HST Servicing Mission 4 (STS-125). We present the design philosophy and summarize the key characteristics of the instrument that will be of interest to potential observers. For faint targets, with flux F ? 1.0 ? 10?14?erg?cm?2?s?1 ??1, COS can achieve comparable signal to noise (when compared to Space Telescope Imaging Spectrograph echelle modes) in 1%-2% of the observing time. This has led to a significant increase in the total data volume and data quality available to the community. For example, in the first 20 months of science operation (2009 September-2011 June) the cumulative redshift pathlength of extragalactic sight lines sampled by COS is nine times than sampled at moderate resolution in 19 previous years of Hubble observations. COS programs have observed 214 distinct lines of sight suitable for study of the intergalactic medium as of 2011 June. COS has measured, for the first time with high reliability, broad Ly? absorbers and Ne VIII in the intergalactic medium, and observed the He II reionization epoch along multiple sightlines. COS has detected the first CO emission and absorption in the UV spectra of low-mass circumstellar disks at the epoch of giant planet formation, and detected multiple ionization states of metals in extra-solar planetary atmospheres. In the coming years, COS will continue its census of intergalactic gas, probe galactic and cosmic structure, and explore physics in our solar system and Galaxy.

Multiwavelength Observations of a Dramatic High-Energy Flare in the Blazar 3C 279

The blazar 3C 279, one of the brightest identified extragalactic objects in the γ-ray sky, underwent a large (factor of ~10 in amplitude) flare in γ-rays toward the end of a 3 week pointing by Compton Gamma Ray Observatory (CGRO), in 1996 January-February. The flare peak represents the highest γ-ray intensity ever recorded for this object. During the high state, extremely rapid γ-ray variability was seen, including an increase of a factor of 2.6 in ~8 hr, which strengthens the case for relativistic beaming. Coordinated multifrequency observations were carried out with Rossi X-Ray Timing Explorer (RXTE), Advanced Satellite for Cosmology and Astrophysics (ASCA; or, Astro-D), Roentgen Satellite (ROSAT), and International Ultraviolet Explorer (IUE) and from many ground-based observatories, covering most accessible wavelengths. The well-sampled, simultaneous RXTE light curve shows an outburst of lower amplitude (factor of 3) well correlated with the γ-ray flare without any lag larger than the temporal resolution of ~1 day. The optical-UV light curves, which are not well sampled during the high-energy flare, exhibit more modest variations (factor of ~2) and a lower degree of correlation. The flux at millimetric wavelengths was near a historical maximum during the γ-ray flare peak, and there is a suggestion of a correlated decay. We present simultaneous spectral energy distributions of 3C 279 prior to and near to the flare peak. The γ-rays vary by more than the square of the observed IR-optical flux change, which poses some problems for specific blazar emission models. The synchrotron self-Compton (SSC) model would require that the largest synchrotron variability occurred in the mostly unobserved submillimeter/far-infrared region. Alternatively, a large variation in the external photon field could occur over a timescale of a few days. This occurs naturally in the mirror model, wherein the flaring region in the jet photoionizes nearby broad emission line clouds, which, in turn, provide soft external photons that are Comptonized to γ-ray energies.

Multiwavelength Observations of Short-Timescale Variability in NGC 4151. IV. Analysis of Multiwavelength Continuum Variability

For pt.III see ibid., vol.470, no.1, p.349-63 (1996). Combines data from the three preceding papers in order to analyze the multi wave-band variability and spectral energy distribution of the Seyfert 1 galaxy NGC 4151 during the 1993 December monitoring campaign. The source, which was near its peak historical brightness, showed strong, correlated variability at X-ray, ultraviolet, and optical wavelengths. The strongest variations were seen in medium-energy (~1.5 keV) X-rays, with a normalized variability amplitude (NVA) of 24%. Weaker (NVA=6%) variations (uncorrelated with those at lower energies) were seen at soft gamma-ray energies of ~100 keV. No significant variability was seen in softer (0.1-1 keV) X-ray bands. In the ultraviolet/optical regime, the NVA decreased from 9% to 1% as the wavelength increased from 1275 to 6900 Aring. These data do not probe extreme ultraviolet (1200 Aring to 0.1 keV) or hard X-ray (250 keV) variability. The phase differences between variations in different bands were consistent with zero lag, with upper limits of lsim0.15 day between 1275 Aring and the other ultraviolet bands, lsim0.3 day between 1275 Aring and 1.5 keV, and lsim1 day between 1275 and 5125 Aring. These tight limits represent more than an order of magnitude improvement over those determined in previous multi-wave-band AGN monitoring campaigns. The ultraviolet fluctuation power spectra showed no evidence for periodicity, but were instead well fitted with a very steep, red power law (ales-2.5)

Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. XI. Intensive monitoring of the ultraviolet spectrum of NGC 7469

From 1996 June 10 to July 29, the International Ultraviolet Explorer monitored the Seyfert 1 galaxy NGC 7469 continuously in an attempt to measure time delays between the continuum and emission-line fluxes. From the time delays, one can estimate the size of the region dominating the production of the UV emission lines in this source. We find the strong UV emission lines to respond to continuum variations with time delays of about 23-31 for Lyα, 27 for C IV λ1549, 19-24 for N V λ1240, 17-18 for Si IV λ1400, and 07-10 for He II λ1640. The most remarkable result, however, is the detection of apparent time delays between the different UV continuum bands. With respect to the UV continuum flux at 1315 A, the flux at 1485 A, 1740 A, and 1825 A lags with time delays of 021, 035, and 028, respectively. Determination of the significance of this detection is somewhat problematic since it depends on accurate estimation of the uncertainties in the lag measurements, which are difficult to assess. We attempt to estimate the uncertainties in the time delays through Monte Carlo simulations, and these yield estimates of ~007 for the 1 σ uncertainties in the interband continuum time delays. Possible explanations for the delays include the existence of a continuum-flux reprocessing region close to the central source and/or a contamination of the continuum flux with a very broad time-delayed emission feature such as the Balmer continuum or merged Fe II multiplets.

The Galaxy Environment of O VI Absorption Systems

We combine a FUSE sample of O VI absorbers (z < 0.15) with a database of 1.07 million galaxy redshifts to explore the relationship between absorbers and galaxy environments. All 37 absorbers with N ≥ 1013.2 cm-2 lie within 800 h kpc of the nearest galaxy, with no compelling evidence for O VI absorbers in voids. The O VI absorbers often appear to be associated with environments of individual galaxies. Gas with 10% ± 5% solar metallicity (O VI and C III) has a median spread in distance of 350-500 h kpc around L galaxies and 200-270 h kpc around 0.1L galaxies (ranges reflect uncertain metallicities of gas undetected in Lyα absorption). In order to match the O VI line frequency, (d/dz) ≈ 20 for N ≥ 1013.2 cm-2, galaxies with L ≤ 0.1L must contribute to the cross section. Lyα absorbers with N ≥ 1013.2 cm-2 cover ~50% of the surface area of typical galaxy filaments. Two-thirds of these show O VI and/or C III absorption, corresponding to a 33%-50% covering factor at 0.1 Z☉ and suggesting that metals are spread to a maximum distance of 800 h kpc, within typical galaxy supercluster filaments. Approximately 50% of the O VI absorbers have associated Lyα line pairs with separations (Δv)Lyα = 50-200 km s-1. These pairs could represent shocks at the speeds necessary to create copious O VI, located within 100 h kpc of the nearest galaxy and accounting for much of the two-point correlation function of low-z Lyα forest absorbers.

The Local Lyα Forest. II. Distribution of H I Absorbers,Doppler Widths, and Baryon Content

In Paper I of this series we described observations of 15 extragalactic targets taken with the Hubble Space Telescope+GHRS+G160M grating for studies of the low-z Lyα forest. We reported the detection of 111 Lyα absorbers at significance level (SL) ≥3 σ, 81 with SL ≥ 4 σ, in the redshift range 0.002 1.6. Similar to the high equivalent width ( > 240 mA) absorbers, the number density of low- absorbers at z = 0 is well above the extrapolation of d/dz from z > 2, but we observe no difference in the mean evolution of d/dz between absorbers of high ( > 240 mA) and low ( ≤ 100 mA) equivalent width. While previous work has suggested slower evolution in number density of lower absorbers, our new data do not support this conclusion. A consistent evolutionary pattern is that the slowing in the evolution of the low column density clouds occurs at lower redshift than for the higher column density clouds. A 4-5 σ signal in the two-point correlation function of Lyα absorbers for velocity separations Δv ≤ 150 km s-1 is consistent with results at high z, but with somewhat greater amplitude. Applying a photoionization correction, we find that the low-z Lyα forest may contain ~20% of the total number of baryons, with closure parameter ΩLyα = (0.008 ± 0.001)h, for a standard absorber size and ionizing radiation field. Some of these clouds appear to be primordial matter, owing to the lack of detected metals (Si III) in a composite spectrum, although current limits on composite metallicity are not strong.

The Metagalactic Ionizing Radiation Field at Low Redshift

We compute the ionizing radiation field at low redshift, arising from Seyferts, QSOs, and starburst galaxies. This calculation combines recent Seyfert luminosity functions, extrapolated ultraviolet fluxes from our IUE-AGN database, and a new intergalactic opacity model based on Hubble Space Telescope and Keck Lyα absorber surveys. At z = 0 for AGNs only, our best estimate for the specific intensity at 1 ryd is I0 = 1.3 × 10-23 ergs cm-2 s-1 Hz-1 sr-1 , independent of H0, Ω0, and Λ. The one-sided ionizing photon flux is Φion ≈ 3400 photons cm-2 s-1, and the H I photoionization rate is Γ = 3.2 × 10-14 s-1, for αs = 1.8. We also derive ΓH I for z = 0–4. These error ranges reflect uncertainties in the spectral indexes for the ionizing EUV (αs = 1.8 ± 0.3) and the optical/UV (αUV = 0.86 ± 0.05), the IGM opacity model, the range of Seyfert luminosities (0.001L*–100L*), and the completeness of the luminosity functions. Our estimate is a factor of 3 lower than the most stringent upper limits on the ionizing background (Φion < 104 photons cm-2 s-1) obtained from Hα observations in external clouds, and it lies within the range implied by other indirect measures. Starburst galaxies with a sufficiently large Lyman continuum escape fraction, fesc ≥ 0.05, may provide a comparable background to AGNs, I0(z = 0) = 1.1 × 10-23 ergs cm-2 s-1 Hz-1 sr-1 . An additional component of the ionizing background of this magnitude would violate neither upper limits from Hα observations nor the acceptable range from other measurements.

Metal Abundances in the Magellanic Stream

We report on the first metallicity determination for gas in the Magellanic Stream, using archival Hubble Space Telescope (HST) GHRS data for the background targets Fairall 9, III Zw 2, and NGC 7469. For Fairall 9, using two subsequent HST revisits and new Parkes multibeam narrowband observations, we have unequivocally detected the MS I H I component of the Stream (near its head) in S II λλ1250, 1253, yielding a metallicity of [S II/H] = -0.55 ± 0.06(r)(s), where r and s are the associated random and systematic uncertainties, consistent with either an SMC or LMC origin and with the earlier upper limit set recently by Lu, Savage, & Sembach. We also detect the saturated Si II λ1260 line, but set a lower limit of only [Si II/H] -1.5. We present two HST serendipitous detections of the Stream, seen in Mg II λλ2796, 2803 absorption with column densities of (0.5–1) × 1013 cm-2 toward the Seyfert galaxies III Zw 2 and NGC 7469. These latter sight lines probe gas near the tip of the Stream (~15° from the peak of the MS V H I component and ~80° down-Stream of Fairall 9). For III Zw 2, the lack of an accurate H I column density determination and the uncertain Mg III ionization correction severely limit the degree to which we can constrain [Mg/H]; we found a lower limit of [Mg II/H I] -1.3 for this sight line. For NGC 7469, an accurate H I column density determination exists, but the extant Faint Object Spectrograph (FOS) spectrum limits our ability to constrain the Mg II column density, and we conclude that [Mg II/H I] -1.5 for this sight line. Ionization corrections associated with Mg III and H II suggest that the corresponding [Mg/H] may range lower by ~0.3–1.0 dex. However, an upward revision of ~0.5–1.0 dex would be expected under the assumption that the Stream exhibits a dust depletion pattern similar to that seen in both the Large and Small Magellanic Clouds. While our abundance analysis allows us to rule out a primordial origin for the Stream, the remaining systematic uncertainties in the H I column density along the lines of sight makes it difficult to differentiate between an LMC and an SMC origin.

AN HST/COS SURVEY OF THE LOW-REDSHIFT INTERGALACTIC MEDIUM. I. SURVEY, METHODOLOGY, AND OVERALL RESULTS*

We use high-quality, medium-resolution Hubble Space Telescope/Cosmic Origins Spectrograph (HST/COS) observations of 82 UV-bright active galactic nuclei (AGNs) at redshifts z(AGN) \textless 0.85 to construct the largest survey of the low-redshift intergalactic medium (IGM) to date: 5138 individual extragalactic absorption lines in H I and 25 different metal-ion species grouped into 2611 distinct redshift systems at z(abs) \textless 0.75 covering total redshift pathlengths Delta z(HI) = 21.7 and Delta z(O VI) = 14.5. Our semi-automated line-finding and measurement technique renders the catalog as objectively defined as possible. The cumulative column density distribution of H I systems can be parametrized dN (\textgreater N)/dz = C-14 (N/10(14) cm(-2))(-(beta-1)), with C-14 = 25 +/- 1 and beta = 1.65 +/- 0.02. This distribution is seen to evolve both in amplitude, C-14 infinity (1+z)(2.3 +/- 0.1), and slope beta(z) = 1.75-0.31 z for z \textless= 0.47. We observe metal lines in 418 systems, and find that the fraction of IGM absorbers detected in metals is strongly dependent on N-H I. The distribution of O VI absorbers appears to evolve in the same sense as the Ly alpha forest. We calculate contributions to Omega(b) from different components of the low-z IGM and determine the Ly alpha decrement as a function of redshift. IGM absorbers are analyzed via a two-point correlation function in velocity space. We find substantial clustering of H I absorbers on scales of Delta v = 50-300 km s(-1) with no significant clustering at Delta(v) greater than or similar to 1000 km s(-1). Splitting the sample into strong and weak absorbers, we see that most of the clustering occurs in strong, N-H I greater than or similar to 10(13.5) cm(-2), metal-bearing IGM systems. The full catalog of absorption lines and fully reduced spectra is available via the Mikulski Archive for Space Telescopes (MAST) as a high-level science product at http://archive.stsci.edu/prepds/igm/.

High-Velocity Cloud Complex C: Galactic Fuel or Galactic Waste?

We present HST Goddard High Resolution Spectrograph and Space Telescope Imaging Spectrograph observations of five quasi stellar objects that probe the prominent high-velocity cloud (HVC) Complex C, covering ~10% of the northern sky. Based upon a single sight-line measurement (Mrk 290), a metallicity [S/H] = -1.05 ? 0.12 has been associated with Complex C by Wakker et al. When coupled with its inferred distance (5 d 30 kpc) and line-of-sight velocity (v ~ -100 to -200 km s-1), Complex C appeared to represent the first direct evidence for infalling low-metallicity gas onto the Milky Way, which could provide the bulk of the fuel for star formation in the Galaxy. We have extended the abundance analysis of Complex C to encompass five sight lines. We detect S II absorption in three targets (Mrk 290, 817, and 279); the resulting [S II/H I] values range from -0.36 (Mrk 279) to -0.48 (Mrk 817) to -1.10 (Mrk 290). Our preliminary O I FUSE analysis of the Mrk 817 sight line also supports the conclusion that metallicities as high as 0.3 times solar are encountered within Complex C. These results complicate an interpretation of Complex C as infalling low-metallicity Galactic fuel. Ionization corrections for H II and S III cannot easily reconcile the higher apparent metallicities along the Mrk 817 and Mrk 279 sight lines with that seen toward Mrk 290, since H? emission measures preclude the existence of sufficient H II. If gas along the other lines of sight has a similar pressure and temperature to that sampled toward Mrk 290, the predicted H? emission measures would be ~900 mR. It may be necessary to reclassify Complex C as mildly enriched Galactic waste from the Milky Way or processed gas torn from a disrupted neighboring dwarf, as opposed to low-metallicity Galactic fuel.