arXiv subjects

Mark L. Giroux

Publications and source records attributed to Mark L. Giroux.

29 records · Page 2Linked to original sources

arXiv2001

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

We present HST GHRS and STIS observations of five QSOs that probe the prominent high-velocity cloud (HVC) Complex C, covering 10% of the northern sky. Based upon a single sightline measurement (Mrk 290), a metallicity [S/H]=-1.05+/-0.12 has been associated with Complex C by Wakker et al. (1999a,b). When coupled with its inferred distance (5<d<30 kpc) and line-of-sight velocity (v=-100 to -200 km/s), 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 sightlines. We detect SII absorption in three targets (Mrk 290, Mrk 817, and Mrk 279); the resulting [SII/HI] values range from -0.36 (Mrk 279) to -0.48 (Mrk 817) to -1.10 (Mrk 290). Our preliminary OI FUSE analysis of the Mrk 817 sightline 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 HII and SIII cannot easily reconcile the higher apparent metallicities along the Mrk 817 and Mrk 279 sightlines with that seen toward Mrk 290, since H-alpha emission measures preclude the existence of sufficient HII. If gas along the other lines of sight has a similar pressure and temperature to that sampled toward Mrk 290, the predicted H-alpha 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.

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arXiv2001

The Ionization and Metallicity of the Intervening O VI Absorber at z = 0.1212 in the Spectrum of H1821+643

We use high-resolution UV spectra of the radio-quiet QSO H1821+643 (z_em = 0.297), obtained with STIS and FUSE, to study the ionization and metallicity of an intervening O VI absorption line system at z(abs) = 0.1212. This absorber has the following notable properties: (1) Several galaxies are close to the sight line at the absorber redshift, including an actively star-forming galaxy at a projected distance of 144 kpc. (2) There is a complex cluster of H I lya absorption lines near the O VI redshift, including at least five components spread over a velocity range of ~700 km/s . (3) The strongest lya line in the cluster appears to be composed of a mildly saturated component with a typical b-value blended with a broad component with b = 85 km/s . (4) The O VI absorption is not aligned with the strongest H I absorption, but instead is well-aligned with the broad component. (5) The only detected species (>4sigma) are O VI and H I despite coverage of strong transitions of other abundant elements. Based on these constraints, we find that the absorption line properties can be produced in collisionally ionized gas with roughly 5.3 < log T < 5.6 and -1.8 < [O/H] < -0.6. However, we find that photoionization is also viable if the pathlength l through the absorbing gas is long enough; simple photoionization models require 85 < l < 1900 kpc and -1.1 < [O/H] < -0.3. We briefly discuss how observations of X-ray absorption lines due to O VII and O VIII could be used, in principle, to break the ionization mechanism degeneracy.

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arXiv2001

Heating and Ionization of the Intergalactic Medium by an Early X-Ray Background

Observational studies indicate that the intergalactic medium (IGM) is highly ionized up to redshifts just over 6. A number of models have been developed to describe the process of reionization and the effects of the ionizing photons from the first luminous objects. In this paper, we study the impact of an X-ray background, such as high-energy photons from early quasars, on the temperature and ionization of the IGM prior to reionization, before the fully ionized bubbles associated with individual sources have overlapped. X-rays, which have large mean free paths relative to EUV photons, and their photoelectrons can have significant effects on the thermal and ionization balance. We find that hydrogen ionization is dominated by the X-ray photoionization of neutral helium and the resulting secondary electrons. Thus, the IGM may have been warm and weakly ionized prior to full reionization. We examine several related consequences, including the filtering of the baryonic Jeans mass scale, signatures in the cosmic microwave background, and the H$^{-}$-catalyzed production of molecular hydrogen.

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arXiv2000

Probing the First Stars with Hydrogen and Helium Recombination Emission

Unusual patterns of recombination emission from gas ionized by metal-free stars may distinguish early star-forming galaxies from their present-day counterparts. This pattern arises from the harder ionizing spectrum expected from metal-free stars, which strongly enhances the strength of He II recombination lines. Our calculations indicate that line fluxes of He II 1640 and 4686 are sufficiently large to be detected by narrowband and spectroscopic searches for high-redshift emission-line sources at redshift z ~ 5 using current instruments. An unknown fraction of Lya emitters may harbor low-metallicity or metal-free stars. As the predicted He II 1640 flux is comparable to and may exceed hydrogen Lya, searches for high-redshift galaxies should consider He II recombination lines as possible identifications for single emission lines in observed spectra. Spectra of metal-free stars may show both H I and He II emission lines, improving the constraints on their redshift and identification. We assess the considerable uncertainties that affect our expectations for the detection and identification of true first-generation stars with present search techniques, including the role of stellar mass loss in spectral evolution and the confusion of ionization by primordial stellar sources and AGN in the early universe.

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arXiv2000

The Local Ly-alpha Forest: HI in Nearby Intergalactic Space

Detecting HI using redshifted Ly-alpha absorption lines is 1e6 times more sensitive than using the 21cm emission line. We review recent discoveries of HI Ly-alpha absorbers made with the Hubble Space Telescope (HST) which have allowed us a first glimpse at gas in local intergalactic space between us and the ``Great Wall''. Despite its mere 2.4m aperture, HST can detect absorbers with column densities as low as those found using Keck at high-z (log N(HI)=12.5 1/cm**2). New results that will be discussed include: the evolution of absorbers with redshift, the location of absorbers relative to galaxies (including the two-point correlation function for absorbers), the metallicity of absorbers far from galaxies, and the discovery of hot 1e5-1e6 K (shock-heated?) absorbers. The unique ability of VLA HI observations in discovering the nearest galaxies to these absorbers is stressed.

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arXiv2000

Do High-Velocity Clouds Really Fuel Galactic Star Formation?

Tantalizing evidence has been presented supporting the suggestion that a large population of extragalactic gas clouds permeates the Local Group, a population which has been associated with the Galactic High-Velocity Clouds (HVCs). We comment on both the strengths and weaknesses of this suggestion, informally referred to as the Blitz/Spergel picture. Theoretical predictions for the spatial and kinematic distributions, metallicities, distances, and emission properties of Blitz/Spergel HVCs will be confronted with extant observational data.

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arXiv2000

Metal Abundances in the Magellanic Stream

We report on the first metallicity determination for gas in the Magellanic Stream, using archival 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 MSI HI component of the Stream (near its head) in SII1250,1253 yielding a metallicity of [SII/H]=-0.55+/-0.06(r)+/-0.2(s), consistent with either an SMC or LMC origin and with the earlier upper limit set by Lu et al. (1994). We also detect the saturated SiII1260 line, but set only a lower limit of [SiII/H]>-1.5. We present serendipitous detections of the Stream, seen in MgII2796,2803 absorption with column densities of (0.5-1)x10^13 cm^-2 toward the Seyfert galaxies III Zw 2 and NGC 7469. These latter sightlines probe gas near the tip of the Stream (80 deg down-Stream of Fairall 9). For III Zw 2, the lack of an accurate HI column density and the uncertain MgIII ionization correction limits the degree to which we can constrain [Mg/H]; a lower limit of [MgII/HI]>-1.3 was found. For NGC 7469, an accurate HI column density determination exists, but the extant FOS spectrum limits the quality of the MgII column density determination, and we conclude that [MgII/HI]>-1.5. Ionization corrections associated with MgIII and HII 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 the Magellanic Clouds. Remaining uncertainties do not allow us to differentiate between an LMC versus SMC origin to the Stream gas.

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arXiv1999

New HST Observations of the Halo Gas of NGC 3067: Limits on the Extragalactic Ionizing Background at Low Redshift and the Lyman Continuum Escape Fraction

We present UV spectroscopy from HST/GHRS and reanalyze existing H_alpha images of the quasar/galaxy pair 3C 232/NGC 3067 and of the halo gas associated with NGC 3067. The spectra permit measurement of, or limits on, the column densities of Fe I, Fe II, Mg I, and Mg II in the absorbing cloud. Two distinct models of the extragalactic radiation field are considered: (1) the ionizing spectrum is dominated by a power-law extragalactic continuum, and (2) the power-law spectrum contains a Lyman break, implying enhanced flux longward of 912 A relative to the hydrogen-ionizing flux. The H_alpha images constrain the escape fraction of Lyman continuum photons from the galaxy to f_esc <= 0.02. With the assumption that the cloud is shielded from all galactic contributions, we can constrain the intensity and shape of the extragalactic continuum. For an AGN-dominated power-law extragalactic spectrum, we derive a limit on the extragalactic ionizing flux Phi_ion >= 2600 photons cm^-2 s^-1, or I_0 >= 10^-23 erg cm^-2 s^-1 Hz^-1 sr^-1 for an ionizing spectrum with power-law index of 1.8 and a cloud of constant density. When combined with previous upper limits from the absence of H_alpha recombination emission from intergalactic clouds, our observations require 2600 <= Phi_ion <= 10000 photons cm^-2 s^-1. We show that if galactic contributions to the incident radiation are important, it is difficult to constrain Phi_ion. These results demonstrate that galactic halo opacities and their wavelength dependence are crucial to understanding the abundance of low-ionization metals in the IGM.

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arXiv1998

A Cluster of Low-Redshift Lyman-alpha Clouds toward PKS 2155-304. I. Limits on Metals and D/H

We report observations from the Hubble Space Telescope (HST) and the VLA on the galactic environment, metallicity, and D/H in strong low-redshift Lya absorption systems toward the bright BL Lac object PKS 2155-304. GHRS/G160M spectra at 20 km/s resolution show 14 Lya absorbers, 6 clustered at cz = 16,100-18,500 km/s. ORFEUS claimed LyC absorption at z = 0.056 with N(HI) = (2-5)x10^16 cm^-2, while our Lya data suggest N(HI) = (3-10)x10^14 cm^-2. Higher columns are possible if the Lya line core at 17,000 +/- 50 km/s contains narrow HI components. We identify the Lya cluster with a group of five HI galaxies offset by 400-800 kpc from the sightline. The two strongest absorption features cover the same velocity range as the HI emission in the two galaxies closest to the line of sight. If the Lya is associated with these galaxies, they must have huge halos of highly turbulent, mostly ionized gas. The Lya absorption could also arise from an extended sheet of intragroup gas, or from smaller primordial clouds and halos of dwarf galaxies. We see no absorption from SiIII 1206, CIV 1548, or DI Lya. Photoionization models yield limits of (Si/H) < 0.003 solar, (C/H) < 0.005 solar, (D/H) < 2.8x10^-4 (4 sigma) if N(HI) = 2x10^16 cm^-2. The limits increase to 0.023 solar and D/H < 2.8x10^-3 if N(HI) = 2x10^15 cm^-2. The data suggest that the IGM in this group has not been enriched to the levels suggested by X-ray studies of intracluster gas and that these absorbers could be primordial gas clouds.

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arXiv1998

The High-Redshift He II Gunn-Peterson Effect: Implications and Future Prospects

Absorption due to HeII Ly-alpha has been detected at low resolution in the spectra of four quasars between redshifts z = 2.74-3.29. We assess these observations, giving particular attention to the radiative transfer of the ionizing background radiation, cloud diffuse emission and ionization physics, and statistical fluctuations. We use high-resolution observations of HI absorption towards quasars to derive improved models for the opacity of the intergalactic medium (IGM) and use these models to calculate the HI and HeII photoionization history in both optically-thin and self-shielded clouds, and the average line-blanketing contribution to HeII absorption. The derived ionization rate at z = 2-4, Gamma(HI) = (1-3)x10^-12 s^-1, is consistent with the ionizing background intensity inferred from the proximity effect, but it remains larger than that inferred by N-body hydrodynamical simulations of the Lya absorber distribution. The HeII observations are con- sistent with line blanketing from clouds with N(HI) > 10^12 cm^-2, although a contribution from a more diffuse IGM would help to explain the observed opacity. We compute the expected HeII optical depth and examine the sizable fluc- tuations that arise from variations in the cloud numbers and ionizing radiation field. We assess how HeII absorption constrains the intensity and spectrum of the ionizing radiation and the fractional contributions of the dominant sources (quasars and starburst galaxies). Finally, we demonstrate how high-resolution UV observations can distinguish between absorption from the diffuse IGM and the Lya forest clouds and determine the source of the ionizing background.

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arXiv1997

The Influence of the Photoionizing Radiation Spectrum on Metal-Line Ratios in Ly alpha Forest Clouds

Recent measurements of Si IV/C IV ratios in the high-redshift Ly-alpha forest (Songaila & Cowie, AJ, 112, 335 [1996a]; Savaglio et al., A&A, in press [1997]) have opened a new window on chemical enrichment and the first generations of stars. However, the derivation of accurate Si/C abundances requires reliable ionization corrections, which are strongly dependent on the spectral shape of the metagalactic ionizing background and on the ``local effects'' of hot stars in nearby galaxies. Recent models have assumed power-law quasar ionizing backgrounds plus a decrement at 4 Ryd to account for He II attenuation in intervening clouds. However, we show that realistic ionizing backgrounds based on cosmological radiative transfer models produce more complex ionizing spectra between 1-5 Ryd that are critical to interpreting ions of Si and C. We also make a preliminary investigation of the effects of He II ionization front non-overlap. Because the attenuation and re-emission by intervening clouds enhance Si IV relative to C IV, the observed high Si IV/C IV ratios do not require an unrealistic Si overproduction [Si/C $\geq 3 (Si/C)_{\odot}$]. If the ionizing spectrum is dominated by ``local effects'' from massive stars, even larger Si IV/C IV ratios are possible. However, unless stellar radiation dominates quasars by more than a factor of 10, we confirm the evidence for some Si overproduction by massive stars; values Si/C $\approx 2 (Si/C)_{\odot}$ fit the measurements better than solar abundances. Ultimately, an adequate interpretation of the ratios of C IV, Si IV, and C II may require hot, collisionally ionized gas in a multiphase medium.

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