Received 1999 August 16; accepted 2000 January 18

Transcription

1 THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 129:563È610, 2000 August ( The American Astronomical Society. All rights reserved. Printed in U.S.A. THE HUBBL E SPACE T EL ESCOPE QUASAR ABSORPTION LINE KEY PROJECT. XV. MILKY WAY ABSORPTION LINES1 BLAIR D. SAVAGE,2 BART WAKKER,2 BUELL T. JANNUZI,3 JOHN N. BAHCALL,4 JACQUELINE BERGERON,5 ALEC BOKSENBERG,6 GEORGE F. HARTIG,7 SOFIA KIRHAKOS,4 EDWARD M. MURPHY,8 W. L. W. SARGENT,9 DONALD P. SCHNEIDER,10 DAVID TURNSHEK,11 AND ARTHUR M. WOLFE12 Received 1999 August 16; accepted 2000 January 18 ABSTRACT This paper presents the results of an analysis of the Milky Way absorption lines found in the Hubble Space T elescope (HST ) Quasar Absorption Line Key Project database for 83 QSOs observed with the Faint Object Spectrograph G190H and G270H gratings, of which 16 QSOs are also observed with the G130H grating. The HST Key Project observations are supplemented with high-quality 21 cm H I emission-line observations mostly obtained with the NRAO 43 m radio telescope. The Milky Way halo gas exhibits mixed ionization ÏÏ absorption with high-ionization absorption from Si IV and C IV substantially weaker than the extremely strong intermediate- and low-ionization absorption from Si III, SiII, C II, MgII, and Fe II. For a sample of 16 QSOs observed in the far-uv, the median velocity equivalent widths of very strong lines of Si IV, SiIII, and Si II are 60, 180, and 180 km s~1, respectively. Velocity equivalent widths this large for Si III and Si II imply the existence of high velocity dispersion moderate- (Si III) and low-ionization (Si II) gas along many paths through the Galactic halo. Measures of the Galactic damped Lya line toward 14 QSOs permit the determination of N(H I) through the gaseous disk Lya and halo of the Galaxy. The values of N(H I) range from 0.64 ] 1020 to 3.37 ] 1020 cm~2 with Lya N(H I) o sin b o averaging (1.29 ^ 0.49) ] 1020 cm~2. A comparison of N(H I) with N(H I) Lya Lya 21 cm reveals that N(H I) /N(H I) for the 10 sight lines where the value of N(H I) is not signiðcantly Lya 21 cm Lya a ected by geocoronal emission ranges from 0.62 and This di erence is probably produced by a combination of systematic and random errors and contribution from the small angular scale structure in the H I distribution. Such structure can produce di erent column densities when sampling gas with an inðnitesimal beam in the UV (the angular size of the QSO) compared to the much larger 21@ beam of the NRAO 43 m radio telescope. The overall strength of the Mg II jj2796 and 2803 absorption appears to be correlated with the presence of high-velocity gas along the line to sight. Velocity-resolved Mg II absorption associated with highvelocity gas in the Magellanic Stream is detected toward eight QSOs, including PKS 0003]15, PG 0043]039, PKS 0637[75, 3C 454.3, PKS 2251]11, PG 2302]029, PKS 2340[36, and PKS 2344]09. Velocity-resolved Mg II absorption toward 15 QSOs is not accompanied by the existence of associated H I emission. Interesting objects in this category include PKS 0232[04 (l \ 174.5, b \[56.2), which has a high-velocity cloud (HVC) at v D ]270 km s~1 detected in Mg II, and PG 1116]215 (l \ 223.3, b \ 68.2) with a HVC at ]200 km s~1 detected in Mg II, CII, SiIV, and possibly C IV. The HVC toward PKS 0232[04 is interesting because all known H I HVCs in this general region of the sky have negative velocity rather than positive velocity. For 15 QSOs known to lie in the direction of H I HVCs, the Mg II lines have extremely strong principal absorption components, suggesting the detection of blended low- and high-velocity absorption. These lines of sight imply the detection of Mg II absorption by the high-velocity gas in HVC complexes C and A, in the outer Galaxy warp, and in the Magellanic Stream, as well as toward three smaller clouds. There are 11 QSO sight lines with very strong Mg II absorption for which there is no evidence for high-velocity H I emission. However, six of these sight lines lie near known H I HVCs. There are 38 QSOs with weak Mg II principal absorption and no known H I HVCs. These objects provide information about the H I absorption characteristics of disk and halo gas well away from H I HVCs. The sky covering factor of high-velocity Mg II is large, with 41 and 71 QSO 1 Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS Department of Astronomy, University of Wisconsin, 475 North Charter Street, Madison, WI 53706; savage=astro.wisc.edu, wakker=esquenar.astro.wisc.edu. 3 National Optical Astronomy Observatories, P.O. Box 26732, Tucson, AZ 85719; jannuzi=noao.edu. 4 Institute for Advanced Study, School of Natural Sciences, Princeton, NJ 08540; jnb=ias.edu, soða=ias.edu. 5 Institut dïastrophysique de Paris, CNRS, 98 bis, boulevard Arago, F-75014, Paris, France; bergeron=iap.fr. 6 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK; boksy=ast.cam.ac.uk. 7 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218; hartig=stsci.edu. 8 Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218; emurphy=pha.jhu.edu. 9 Robinson Laboratory , California Institute of Technology, Pasadena, CA 91125; wws=deimos.caltech.edu. 10 Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802; dps=astro.psu.edu. 11 Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260; turnshek=vms.cis.pitt.edu. 12 Center for Astrophysics and Space Sciences, 0111, University of California, San Diego, La Jolla, CA 92093; art=ucsd.edu. 563

2 564 SAVAGE ET AL. lines of sight showing either resolved high-velocity Mg II absorption or principal absorption that is so strong that blended low- and high-velocity Mg II absorption is suggested. Subject headings: ISM: clouds È quasars: absorption lines È ultraviolet: interstellar È Galaxy: halo 1. INTRODUCTION The Hubble Space T elescope (HST ) Quasar Absorption Line Key Project has produced a homogeneous set of spectra of quasars at low and intermediate redshift in order to study gas in the nearby and distant universe. Each quasar spectrum contains strong absorption lines of abundant elements near zero redshift produced by gas in the Milky Way disk and halo. An analysis of the Galactic absorption provides an opportunity to study the absorbing properties of the Milky Way halo in many directions and to search for metal line analogs to the high-velocity H I clouds studied in the 21 cm emission line. The extreme sensitivity of the ultraviolet absorption lines to small amounts of gas makes a comparison of the ultraviolet absorption and 21 cm emission properties of gas in the Milky Way halo very interesting. The observations also permit a direct comparison of H I column densities obtained from Lya absorption and 21 cm emission for paths through disk and halo gas. Information about the Key Project observing program and results about the intergalactic medium (IGM) can be found in Bahcall et al. (1993 hereafter CAT1), Bahcall et al. (1996, hereafter CAT2), and Jannuzi et al. (1998, hereafter CAT3). The Key Project data reduction techniques are described by Schneider et al. (1993) and in CAT3. First results relating to Milky Way halo gas are found in Savage et al. (1993a, hereafter Paper I). The initial results for the Milky Way revealed high or very high velocity Mg II absorption toward seven of 15 quasars, yielded an improved estimate of the scale height of Galactic C IV, provided evidence for metal line absorption associated with high velocity cloud (HVC) complex C and the outer Galaxy, and provided important reference data for interpreting metal line absorption systems seen at higher redshift. These initial results for the Milky Way clearly demonstrated the importance of extending the observations and analysis to additional sight lines in order to understand better the nature of high-velocity gas in the halo and to determine the sky covering factor of high-velocity metal line absorption. This paper will present the results based on an analysis of the Milky Way absorption lines in the combined Key Project catalog (CAT1, CAT2, and CAT3 taken together; this group of papers will hereafter be referred to as the Catalog Papers ÏÏ). To aid in the interpretation of the Milky Way metal line data, high-quality H I 21 cm emission-line data were obtained with the NRAO 43 m radio telescope of all the Key Project quasars observable from Green Bank, West Virginia (Lockman & Savage 1995; see also Appendix A). Those data are important for studying the relationship between the high-velocity gas as observed in H I emission and in ultraviolet metal line absorption. The H I 21 cm data are also important for estimating the optical and ultraviolet extinction due to dust in the Milky Way toward each quasar. The organization of this paper is as follows: In 2we discuss the object sample, its distribution on the sky, and the H I observations for the directions to the Key Project QSOs. In 3 we discuss the HST Key Project QSO observations and the reductions. The observed properties of UV absorption for paths through the Milky Way halo are discussed in 4. In 14 cases the Galactic damped Lya line absorption permits a derivation of the Galactic H I column density. The column densities based on Lya line absorption are compared to 21 cm emission-line column densities in 5. Strong absorption by Mg II toward a sample of 83 quasars is studied in 6. Evidence for high velocity dispersion Galactic halo gas is discussed in 7. H I 21 cm radio observations obtained in support of the Key Project observations are discussed in Appendix A. Detailed comments about absorption along individual sight lines are contained in Appendix B. The results are summarized in THE OBJECT SAMPLE, DISTRIBUTION, AND H I OBSERVATIONS The 83 objects studied are listed in Table 1. They include all quasars observed in the higher resolution modes of the FOS (j/*j D 1300) that are found in the Catalog Papers. Sixty-six of the objects are from CAT3, with the remaining 17 from CAT1 and CAT2. The various entries to Table 1 include the object common name; R.A.(1950.0); decl.(1950.0); Galactic longitude, l; Galactic latitude, b; visual magnitude, V ; redshift, z; and various measurements of H I from the sources listed at the end of the table. Table 1 of CAT3 provides J2000 coordinates and dates of observation. The H I entries in Table 1 include the LSR velocity limits of observable H I 21 cm emission on the negative- and positive-velocity sides of the peak H I emission, v and ; ~ v` the full velocity range of the detectable H I emission, *v \ [ v ; the average H I LSR emission velocity, SvT; the v` ~ integrated H I column density in the units 1019 cm~2 assuming a spin temperature of 200 K, N(H I); the estimated color excess, E(B[V ), using the value of N(H I) and the relation N(H I)/E(B[V ) \ 5.27 ] 1021 cm~2 mag~1 from Diplas & Savage (1994); and the source of the H I data. The H I observations are mostly taken from Lockman & Savage (1995) or Appendix A of this paper and were obtained with the NRAO 43 m radio telescope using a technique to remove the e ects of radio antenna sidelobe contamination. The positions of the Key Project QSOs on the sky are shown in Figure 1, where the Aito projection has the Galactic center at the right and Galactic longitude increasing to the left. Details about object selection are found in CAT1 and CAT3. Low-latitude ( o b o \ 20 ) directions were excluded. The objects are identiðed in Figure 1 with the Ðrst six and seven letters and/or numbers of their name as given in Table 1. The QSO distribution is compared to a map of the distribution of known high-velocity H I clouds in Figures 2a and 2b. The HVC map shown in these two Ðgures is adapted from that shown by Wakker & van Woerden (1997; see their Fig. 1). These Aito projections are centered on l \ 180 and b \ 0. The HVC map shows the 21 cm brightness temperature contour at 0.05 K and H I with o v o [ 90 km s~1. Various HVC complexes as discussed by LSR Wakker & van Woerden are identiðed. In Figures 2a and 2b the Ðlled circles show those QSOs where absorption by Milky Way Mg II jj2796, 2803 and Fe II j2600 have

3 TABLE 1 OBJECTS R.A. Decl. l b v ~ v` *v SvT N(200 K) E(B[V ) Source and QSO (1950) (1950) (deg) (deg) V z (km s~1) (km s~1) (km s~1) (km s~1) (1019 cm~29) (mag) Commenta (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) UM ] [ [ [ PKS 0003] ] [ [ [ , 3 NAB 0024] ] [ [ [ PG 0043] ] [ [ [ , 3 PKS 0044] ] [ [ [ PG 0117] ] [ [ [ PKS 0122[ [ [ [ [ C [ [ [ [ PKS 0232[ [ [ [ [ C [ [ [ [ PKS 0405[ [ [ [ C [ [ [ PKS 0439[ [ [ [ HS 0624] ] [ [ , 4 PKS 0637[ [ [ B2 0742] ] [ [ PKS 0743[ [ [ US ] [ OJ ] [ [ NGC 2841UB ] [ [ , 3 PG 0935] ] [ [ PG 0953] ] [ [ C ] [ [ Mrk ] [ [ , ]68W ] [ [ , 4 Ton ] [ [ , 3 4C ] [ [ , 3 PG 1008] ] [ [ , 5 Ton ] [ [ , 3 4C ] [ [ , 3 PG 1049[ [ [ [ C ] [ [ Q1101[ [ [ [ MC 1104] ] [ [ PG 1116] ] [ [ , 4 UM ] [ [ , ]106Y ] [ [ PKS 1136[ [ [ [ , 3 3C ] [ [ , ] ] [ [ PG 1206] ] [ [ , 5 PG 1216] ] [ [ Mrk ] [ [ , 3 3C ] [ [ PG 1241] ] [ [ PG 1248] ] [ [

4 TABLE 1ÈContinued R.A. Decl. l b v ~ v` *v SvT N(200 K) E(B[V ) Source and QSO (1950) (1950) (deg) (deg) V z (km s~1) (km s~1) (km s~1) (km s~1) (1019 cm~29) (mag) Commenta (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) PKS 1252] ] [ [ PG 1254] ] [ [ B ] ] [ [ PG 1259] ] [ [ , 4 PKS 1302[ [ [ [ Ton ] [ [ PG 1333] ] [ [ PG 1338] ] [ [ PG 1352] ] [ [ PKS 1354] ] [ [ PG 1407] ] [ [ PG 1411] ] [ PG 1415] ] [ [ PKS 1424[ [ [ [ , 4 S4 1435] ] [ [ , 4 PG 1444] ] [ [ B2 1512] ] [ [ PG 1538] ] [ [ , 3 3C ] [ [ PG 1634] ] [ [ , 3 PG 1700] ] [ [ , 3 3C ] [ [ , 3 PG 1715] ] [ [ , 5 PG 1718] ] [ [ , 3 H1821] ] [ [ , 4 4C ] [ [ , 4 PG 2112] ] [ [ PKS 2128[ [ [ [ PKS 2145] ] [ [ [ PKS 2243[ [ [ [ [ C ] [ [ [ , 3 PKS 2251] ] [ [ [ , 3 PKS 2300[ [ [ PG 2302] ] [ [ [ , 3 PKS 2340[ [ [ [ [ , 3 PKS 2344] ] [ [ [ , 3 PKS 2352[ [ [ [ [ , 3 a Source of H I data and comments: (1) Appendix A; (2) Lockman & Savage 1995; (3) values of and *v do not include contributions from a HVC; (4) values of and *v do include v ~, v`, v ~, v`, contributions from a HVC; (5) Murphy, Sembach, & Lockman 1999 (private communication), values of N(H I) for T \ 150 K.

5 HST QUASAR ABSORPTION LINE KEY PROJECT. XV. 567 FIG. 1.ÈGalactic distribution of the Key Project QSOs included in this study. In this Aito projection, the Galactic center is at the right and Galactic longitude increases to the left. To avoid overlapping identiðcations the Key Project QSOs are identiðed in this plot with the Ðrst six or seven letters and/or numbers of their name as given in Table 1. been reliably measured. In these Ðgures the symbol size is a measure of the strength of the absorption as discussed in 6. An extra circle indicates the existence of a second component that is seen in both ( full circle) or one (half-circle) of the Mg II lines. Flags attached to symbols indicate observations which appear to have been contaminated by IGM Lya absorption (i.e., z [ 1.3). In Figures 3a and 3b the Ðlled squares illustrate QSOs for which Key Project Galactic C IV j1548 measurements exist, and the other symbols represent observations of C IV j1548 taken from the literature (see 4). The contours are the H I HVC maps from Wakker & van Woerden (1997), while the gray-scale display shows the ROSAT 0.75 kev (Fig. 3a) and 0.25 kev (Fig 3b) X-ray background (Snowden et al. 1995). The all-sky displays shown in Figures 3a and 3b are centered on l \ 0 and b \ 0. The implications of Figures 2 and 3 for the detection of high-velocity metal line absorption associated with the known H I HVCs and the X-ray background are discussed in 4 and 6. Table 2 contains summary information about the properties of known H I HVCs in the direction of each QSO. H I 21 cm radio observations of most of the Key Project QSOs were obtained by Lockman & Savage (1995), Murphy, Lockman, & Savage (1995), and E. M. Murphy, K. R. Sembach, & F. J. Lockman (1999, private communication). Appendix A provides technical information about these H I observations and updates the database to include a number of QSOs not included in the three referenced papers. The H I observations were obtained with the NRAO 43 m radio telescope at Green Bank, West Virginia, and, as discussed in Appendix A, involve special observing techniques to reveal low column density HVCs. The entries in Table 2 include the object, l, b, N(H I), T (mk), FWHM(v), and v of HVCs detected by the observations listed in the source LSR column. Here N(H I) is the H I column density of the HVC assuming optically thin emission, T (mk) is the peak 21 cm brightness temperature, and FWHM(v) is the full width at half-maximum of Gaussian component Ðts to the observed HVCs having the indicated LSR velocity. The comment code in the table provides summary information about HVCs in the direction to each QSO and in nearby directions as revealed through the NRAO observations. The comment column in Table 2 gives the common name for detected HVCs if they are associated with known HVC complexes. The comment column also indicates the angular distances to the nearest HVC when no HVC is detected. These nearest distances were determined using the original HVC data of Hulsbosch & Wakker (1988); see Figures 2a and 2b. There are 83 Key Project QSOs listed in Tables 1 and 2. For 35 objects H I HVCs or 21 cm H I emission wings extending beyond o v o \ 100 km s~1 have been detected. LSR For three of the 35 objects, including HS 0624]6907, H1821]643, and 4C 73.18, the high-velocity H I emission is associated with the outer arm of the Galaxy. For 43 QSOs there is no evidence for a HVC closer than 1.4. For 31 of these cases the nearest HVC lies more than 3 away. 3. HST FOS OBSERVATIONS AND REDUCTIONS The ultraviolet observations we discuss were made with the HST Faint Object Spectrograph (FOS). The observing and data processing procedures are described in Schneider et al. (1993) and in CAT3. A full description of the FOS and its performance characteristics are found in Ford & Hartig (1990). Most of the observations were obtained with the FOS G190H and G270H gratings, which operate from 1600 to 2310 A and from 2230 to 3270 A, respectively. For 16 objects measurements were also obtained with the G130H grating, which provides spectral coverage from 1150 to 1606 A. Most of the FOS observations were obtained through the 0A.25] 2A.0 slit, with some through the 0A.3 circular aperture. The data include pre- and post-costar

6 FIG.2a FIG.2b FIG. 2.È(a) Galactic distribution of HVCs adapted from Wakker & van Woerden (1997). In this Aito projection, the Galactic center is at the right and Galactic longitude increases to the left as in Fig. 1. The HVC map shows the 21 cm H I brightness temperature contour at 0.05 K for H I with o v o [ 90 km LSR s~1. Various HVC complexes are indicated along with the positions of Local Group galaxies. The Ðlled circles illustrate the average velocity equivalent width for the Mg II jj2796, 2803 doublet lines with the circle size coded according to the legend. An extra circle is for cases where a second component is seen in both ( full circle) or one (half-circle) of the Mg II lines. Flags are attached to symbols likely contaminated by IGM absorption. The Mg II absorption of greatest strength is clearly associated with the presence of H I HVCs along the line of sight. In the case of the Magellanic Stream directions, the HVCs are resolved by the FOS. (b) Same as (a), except that the symbols indicate the value of the velocity equivalent width for the Fe II j2600 absorption line. 568

7 FIG.3a FIG.3b FIG. 3.ÈContours illustrate the Galactic distribution of H I HVCs from Wakker & van Woerden (1997) in an Aito projection with the Galactic center at the center of the display and longitude increasing to the left. The symbols indicate the value of the velocity equivalent width for the C IV j1548 absorption line. The squares are for 13 Key Project objects, including four nondetections (see Table 3). The Ðlled triangles are for Ðve objects observed with FOS reported by Burks et al. (1994). The Ðlled circles are for 11 extragalactic objects and seven LMC stars observed with the GHRS, as well as three Magellanic Cloud stars observed with the IUE. When GHRS and FOS data exist for the same object, the GHRS result, which was obtained at higher resolution, is plotted. The open squares for PKS 0003]15, PKS 0624]6907, PG 1411]442, and PKS 2251]11 represent nondetections, with the symbol size representing the 3 p limit. The outer circles surrounding some of the GHRS Ðlled circles indicate the total C IV velocity equivalent width when the measurement includes the contributions from the highly ionized HVCs that are detected toward four objects. In (a) the gray scale displays the ROSAT 0.75 kev background, and in (b) the ROSAT 0.25 kev background from Snowden et al. (1995). 569

8 TABLE 2 HIGH-VELOCITY CLOUDS l b N(H I) T FWHM(v) v LSR QSO (deg) (deg) (1018 cm~2) (mk) (km s~1) (km s~1) Sourcea Codeb HVC Complex and Commentc (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) UM [ MS ([285 km s~1) at1.4 PKS 0003] [ [ MS NAB 0024] [ W483 ([364 km s~1) at2.0 PG 0043] [ [ MS PKS 0044] [ MS ([386 km s~1) at1.6 PG 0117] [ W503 ([110 km s~1) at2.7 PKS 0122[ [ W538 ([114 km s~1) at1.9 3C [ HVC 169[68[164 at 1.7 PKS 0232[ [ W525 ([232 km s~1) at2.6 3C [ Nearest HVC [ 3 PKS 0405[ [ Nearest HVC [ 3 3C [ Nearest HVC [ 3 PKS 0439[ [ Nearest HVC [ 3 HS 0624] [ OA PKS 0637[ [ MS (245 km s~1; 0.18 K) B2 0742] Nearest HVC [ 3 PKS 0743[ [ W339 (104 km s~1) at2.7 US Nearest HVC [ 3 OJ W142 (89 km s~1) at1.9 NGC 2841UB [ A PG 0935] Nearest HVC [ 3 PG 0953] M ([91 km s~1) at1.7 3C M ([124 km s~1) at1.4 Mrk [ A 0959]68W [ C Ton HVC 200]53]120 4C [ M PG 1008] HVC 225]50]120 Ton W21 4C W34 PG 1049[ W35 (103 km s~1) at2.1 3C C ([148 km s~1) at2.8 Q1101[ Nearest HVC [ 3 MC 1104] Nearest HVC [ 3 PG 1116] HVC 223]68]100 UM W ]106Y , 8 W7 (76 km s~1) at2.8 PKS 1136[ W62 3C [ C 1202] Nearest HVC [ 3 PG 1206] [ M PG 1216] Nearest HVC [ 3 Mrk [ W84 3C Nearest HVC [ 3 PG 1241] Nearest HVC [ 3 PG 1248] Nearest HVC [ 3 PKS 1252] Nearest HVC [ 3 PG 1254] Nearest HVC [ 3 B ] Nearest HVC [ 3 PG 1259] [ C PKS 1302[ Nearest HVC [ 3 Ton Nearest HVC [ 3 PG 1333] Nearest HVC [ 3 PG 1338] Nearest HVC [ 3 PG 1352] , 8 Nearest HVC [ 3 PKS 1354] Nearest HVC [ 3 PG 1407] Nearest HVC [ 3 PG 1411] Nearest HVC [ 3 PG 1415] Nearest HVC [ 3 PKS 1424[ [ HVC 337]44[120 S4 1435] [ C [ C PG 1444] W13 ([83 km s~1) at1.4 B2 1512] Nearest HVC [ 3 PG 1538] C ([169 km s~1) at1.1

9 HST QUASAR ABSORPTION LINE KEY PROJECT. XV. 571 TABLE 2ÈContinued l b N(H I) T FWHM(v) v LSR QSO (deg) (deg) (1018 cm~2) (mk) (km s~1) (km s~1) Sourcea Codeb HVC Complex and Commentc (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) 3C Nearest HVC [ 3 PG 1634] [ C PG 1700] [ C 3C [ C [ C PG 1715] [ C PG 1718] [ C H 1821] [ OA 4C [ OA PG 2112] [ Nearest HVC [ 3 PKS 2128[ [ W482 ([223 and [171 km s~1) at1.7 PKS 2145] [ Nearest HVC [ 3 PKS 2243[ [ , 8 Nearest HVC [ 3 3C [ [ MS [ HVC 86[38[140 PKS 2251] [ [ MS PKS 2300[ [ MS (136 km s~1; 0.06 K) PG 2302] [ [ MS PKS 2340[ [ [ MS [ MS PKS 2344] [ [ MS [ MS PKS 2352[ [ MS MS a The HVC information is from (1) Appendix A; (2) Lockman & Savage 1995; (3) Murphy, Sembach & Lockman 1999, private communication; (4) For three southern objects the HVC information is from Bajaja et al b The HVC information code is (1) No HVC; (2) HVC in direction of source; (3) HVC in west position; (4) HVC in east position; (5) HVC in direction of source and east or west position; (6) Galaxy warp to listed velocity; (7) HV tail to listed velocity; (8) Object observed only in frequency-shifting mode. c The HVC complex name or number from Wakker & van Woerden (1997) is listed along with values for velocity and brightness temperature. The angular distance to the nearest HVC is determined from Fig. 2. observations. COSTAR refers to the Corrective Optics Space Telescope Axial Replacement package installed to correct for the HST primary-mirror spherical aberration. For additional details see the Catalog Papers. Except where noted, the red Digicon recorded the G190H and G270H spectra while the blue Digicon recorded the G130H spectra. The spectral integrations provided four spectrum samples per Digicon diode width. The signal-to-noise ratios generally range from 15 to 40 per resolution element, which corresponds approximately to one diode width. For several of the brighter objects (3C 273, PG 1116]215, H1821]643 and PG 1634]706) the data have signal-to-noise ratios of 40È70 per resolution element. The pre-costar spectral spread functions of the FOS are well described by Gaussian proðles with FWHMs of 1.1, 1.5, and 2.0 A for gratings G130H, G190H, and G270H, respectively. In velocity units these resolutions correspond to FWHM \ 240, 230, and 220 km s~1, respectively. The post-costar FWHMs are 0.9, 1.4, and 1.9 A for the three gratings. The basic absorption-line lists are found in the Catalog Papers. These published lists include all observed lines strong enough to be in the complete sample,ïï which implies that the measured equivalent width exceeds by a factor of 4.5 the 1 p equivalent width error for an unresolved line (see CAT3). Residual errors from the Ñat Ðelding correction process dominate the errors in some portions of the spectra (see Schneider et al and Jannuzi & Hartig 1994). The measurements were made from continuum normalized spectra by Ðtting the observed absorption-line pro- Ðles with variable width Gaussian absorption proðles. Multiple Gaussian proðles were used to separate obvious blends of lines. The line lists in the Catalog Papers include the observed wavelength, j (A ), the observed equivalent obs width, W (A ), the 1 p error in the equivalent width, p(w ), j j and the full width at half-maximum of the Gaussian Ðt to the line, FWHM (A ). In preparing these lists, measurements were also obtained for weaker lines which have equivalent widths between 3 and 4.5 p. This additional list of lines is referred to as the incomplete list ÏÏ in Key Project papers. The incomplete list was not published, to avoid introducing a large number of spurious detections into the literature. Many of these lines are mentioned or discussed in the comment sections on individual QSOs presented in the Catalog Papers. However, for particularly important interstellar medium (ISM) absorption lines it is valuable to have information on these weaker features. Table 3 lists the incomplete sample results for weak absorption features within 1000 km s~1 of the important Galactic ISM lines of Si III j , Si IV jj , , C IV jj , , and Mg II jj , Hereafter, wavelengths listed for ISM ions will be rounded down and listed with four digits. Thus C IV jj , will be listed as C IV jj1548, For more accurate values of the wavelengths see Table 4. Table 3 has the same form as the observed line lists in the Catalog Papers. The entries include the name of the object; the observed wavelength in angstroms; j(obs); the wavelength error, p(j), the observed equivalent width, W (A ), the error in the equivalent width, j p(w ), the statistical signiðcance of the feature, SL, and the

10 572 SAVAGE ET AL. TABLE 3 EQUIVALENT WIDTHS FOR ADDITIONAL ISM LINES IN THE INCOMPLETE SAMPLE j obs p(j) W obs p(w ) FWHM j lab *j Object (A ) (A ) (A ) (A ) SL (A ) Ion (A ) (A ) (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) PKS 0003] Si III Si IV Mg II [2.90 PG 0043] Mg II [3.62 HS 0624] Si IV [ Si IV PG 0953] Mg II C Mg II [ ]68W Mg II Ton C IV Si IV C Si IV Mg II MC 1104] Mg II [2.20 UM PG 0953] Si IV [0.01 PG 1216] Si IV [0.16 PG 1259] C IV PKS 1302[ C IV PG 1411] C IV PG 1415] Mg II PG 1700] C C IV H 1821] Mg II [2.39 PG 2112] Mg II [2.40 PKS 2251] C IV? [ Mg II [2.84 PKS 2344] Mg II [2.89 Subsequent Splitting of Wide Mg II Componentsa PKS 2344] Mg II [ Mg II [0.01 NOTE.ÈAdditional lines from the incomplete sample are listed for the wavelength regions near Si III j1206, Si IV jj1393, 1403; C IV jj1548, 1550; and Mg II jj2796, Additional incomplete sample lines for PG 1216]069 are found in CAT3 Table 7. For the complete sample of detected lines see CAT3. a These results were obtained as part of the preparation of this paper. full width at half-intensity FWHM (A ) of the Gaussian component Ðtted to the absorption. The last three columns provide our identiðcations (ion and laboratory wavelength j ) for features that likely have a Galactic ISM origin and the 0 value of j [ j. In the case of the results for Mg II, the entries mostly OBS provide 0 information about weak additional absorbing components near the principal Mg II Galactic absorption lines at jj2796 and Blending of the Milky Way ISM lines and H I Lyman forest lines and QSO metal lines is occasionally a problem. See the Catalog Papers and Schneider et al. (1993) for discussions of the Key Project automated line identiðcation procedures and how they operate in situations where there are ambiguous identiðcations. In the case of species with multiple absorption lines (for example, Mg II, FeII, and Si II), blending of the Galactic ISM lines can sometimes be recognized by intercomparing the measurements of the different multiplet lines available. Comments about individual lines of sight including discussions of possible blending problems are given in Appendix B. In some cases the blended IGM line is partially resolved from the ISM absorption. In this situation the uncertainty associated with the separation of the two absorptions will a ect the reliability of the equivalent width of the ISM line. In other cases we have noted that the ISM line width (FWHM) and/or equivalent width is unusually wide or strong compared to that found for other lines of the same ion. In fact, the blending problem is obvious in Mg II for all QSOs with z [ 1.3 (except for Ton 34). For such high-redshift QSOs, Lya forest lines are usually blended with the Mg II jj2796, 2803 doublet. Blending between ISM lines is also a problem particularly in the far-uv, where the ISM line density is high. The combined catalog contains a total of 3238 absorption lines, of which 1446, or 45%, are identiðed as Milky Way ISM absorption lines. The remaining 1792 lines are attributed to extragalactic absorption with 1129 identiðed as Lya lines. See CAT3 for a summary and Weymann et al. (1998) for a discussion of the intergalactic Lya lines. 4. UV ABSORPTION THROUGH THE MILKY WAY HALO Table 4 lists the Milky Way absorption lines that are detected in the Key Project observations obtained with all three gratings including G130H, G190H, and G270H. This

11 TABLE 4 MILKY WAY ABSORPTION[LINE OBSERVATIONS FOR QSOs OBSERVED WITH THE G130H, G190H, AND G270H GRATINGS PKS 0003]15 PKS 0405[123 HS 0624]6907 PG 0953]415 TON 28 3C PG 1116]215 PG 1216]069 j W W W W W W W W obs obs obs obs obs obs obs obs ION (A ) log ( fj) (A ) p(a ) (A ) p(a ) (A ) p(a ) (A ) p(a ) (A ) p(a ) (A ) p(a ) (A ) p(a ) (A ) p(a ) (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) Mg I Mg II Mg II Mn II Fe II Mn II Fe II Mn II Fe II Fe II Fe II Fe II Fe II Zn IIa Zn IIb Al III Al III Si II Al II C IV C IV Si II Si IV Si IV C II* C IIc Si II O Id Si IIe S II S II Si III N If Si II Si IIg

12 TABLE 4ÈContinued 3C 273 PG 1259]593 PKS 1302[102 PG 1411]442 3C 351 H1821]643 PKS 2251]11 j W W W W W W W obs obs obs obs obs obs obs ION (A ) log ( fj) (A ) p(a ) (A ) p(a ) (A ) p(a ) (A ) p(a ) (A ) p(a ) (A ) p(a ) (A ) p(a ) (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) Mg I Mg II Mg II Mn II Fe II Mn II Fe II Mn II Fe II Fe II Fe II Fe II Fe II Zn IIa Zn IIb Al III Al III Si II Al II C IV C IV Si II Si IV Si IV C II* C IIc Si II O Id Si IIe S II S II Si III N If Si II Si IIg a Zn II j blends with Cr II j b Zn II j blends with Mg I j c For most objects C II* j and C II j are blended; the equivalent width listed refers to the blend. d The interstellar O I j measurement is contaminated by Earth atmospheric O I emission. e The Si II j absorption is blended with S II jj absorption unless a separate equivalent width is reported of S II. f The N I absorption is from a closely spaced triplet of blended lines at jj , , and A. The value of log (jf ) listed is for the multiplet. g The Si II j line is blended with weak S III j absorption.

13 HST QUASAR ABSORPTION LINE KEY PROJECT. XV. 575 list also includes results for four QSOs presented in Paper I. The highest quality data exist for 3C 273 and H1821]643. These two objects have line strengths representative of sight lines with relatively weak low-ionization absorption (3C 273) and very strong low-ionization absorption (H1821]643). The impact of the relatively low resolution of the FOS on interstellar studies is considered in Paper I. In the case of 3C 273, it has been possible to compare high-quality FOS measurements and Goddard High Resolution Spectrograph Measurements (GHRS) having a resolution of approximately 20 km s~1 (FWHM) but broad wings on the spread function. Although the FOS resolution is low, the ultraviolet resonance lines are extremely strong and permit a study of high and very high velocity metal line absorption. The very strong lines are saturated over a large velocity interval, and the basic equivalent width measurement itself can provide information about high-velocity gas even if the absorption by that gas is not resolved. For example, for H1821]643 the Mg II equivalent widths listed in Table 4 of 1.73 and 1.71 A for the jj2796 and 2803 lines, respectively, imply strongly saturated absorption with velocity equivalent widths, W \ cw /j \ 185 and 183 km s~1, respec- v j tively. (To avoid confusion in the text we indicate equivalent widths as W or W to distinguish between equivalent v j widths in velocity or wavelength units.) The values of W for v the same two lines toward 3C 273 are 115 and 108 km s~1. The strong and wide lines toward H1821]643 are related to the fact that the path to this quasar passes through the warped outer Galaxy, which exhibits absorption extending to [150 km s~1 (see Paper I and Savage, Sembach, & Lu 1995). Absorption-line proðles for selected Milky Way ISM lines are illustrated in Figures 4a, 4b, 4c, and 4d for PG 0953]415, PG 1116]215, 3C 273, and H1821]643. For each object Ñux versus velocity is shown, where zero velocity is referenced to the ISM absorption indicated on the individual panel plots. When more than one ISM line is situated in a given panel, the zero of velocity is referenced to the Ðrst line listed. The actual observed Ñux is shown as the histogram and the Key Project Ðtted continuum as the smooth solid line. The Gaussian proðle Ðts to the ISM lines are also displayed, while Ðts to identiðed IGM lines are not displayed. The lines just above the zero Ñux level in each panel show the calculated 1 p error vector for each observation. Below the zero level in each panel we indicate for the ISM lines with the horizontal line the observed velocity equivalent width of the measurement along with a numerical value for the velocity equivalent width. The horizontal lines are centered on the observed velocity of the absorption. When more than one absorption line appears on a panel the horizontal lines can be used to locate those absorptions. The vertical tick marks below the zero level denote the velocity extent of H I 21 cm emission in the direction of each object. These values are taken from Table 1. A vertical arrow denotes the velocity of a known H I HVC. If only one line and one arrow appear, the H I velocity extent includes the HVC because the latter is not separated from the main H I emission component. Intergalactic lines appearing in the various panels are indicated with the IGM identiðcation. Reference to the catalog paper identiðcation lists are required to determine the actual Key Project identiðcations of these features. An inspection of Figures 4aÈ4d will allow the reader to assess the quality level of the Key Project component Ðts to the ISM absorptions for data of high quality (PG 1116]215, 3C 273, and H1821]643) and moderate quality (PG 0953]415). The observations listed in Table 4 and displayed in Figure 4 for four QSOs are useful for determining the typical values of the ISM absorption-line strengths for paths through the Galactic halo. An alternate view of these results is shown in Figure 5, where we illustrate as a function of ion type the observed velocity equivalent widths for all the 83 sight lines observed. This Ðgure includes all identi- Ðed ISM features. The lines are sorted into two groups. The group of six at the top of the Ðgure includes the high- and intermediate-ionization lines. This is followed by the lowionization lines. Within each group the lines are sorted by decreasing expected velocity equivalent width based on the depleted gas-phase abundances found in the warm neutral medium of the Galactic disk (Savage & Sembach 1996). In several cases the velocity equivalent widths shown in Figure 5 include blended contributions from adjacent features. The display of Figure 5 resembles a similar display shown in Paper I that included a comparison with velocity equivalent widths for these same ions observed in highredshift damped Lya systems. The increase in sample size from six to 83 has not changed the basic conclusion of Paper I that many damped Lya absorption-line systems have mixed-ionization absorption-line strengths roughly similar to that found along the Milky Way disk-halo sight lines included in our study. Those systems that are most similar have velocity equivalent widths for the strong lowionization metal lines ranging from D 50 to D 250 km s~1. Such systems represent an appreciable fraction of damped Lya systems. The increased sample size of objects for which we have G130H observations allow us to intercompare the relative line strengths (velocity equivalent widths) for a wide range of ion types. From Figure 5 we see that the strongest Milky Way lines not a ected by ISM blending are produced by Si III j1206, Si II j1193, and Mg II j2796. These absorption lines have median velocity equivalent widths ranging from 125 to 180 km s~1. In the case of silicon, measurements exist for three ion states, including Si II, SiIII and Si IV. The values of log jf for strong lines of each of these ions are Si II j1193, log jf \ 2.775; Si III j1206, log jf \ 3.304; and Si IV j1393, log jf \ The median velocity equivalent widths for these absorption lines are W (Si II j1193) \ 180 km s~1, W (Si III j1206) \ 180 km s~1, v and W (Si IV j1393) \ 60 km v s~1. The roughly comparable and very v large velocity equivalent widths for the strong lines of Si II and Si III reveal the existence of highly turbulent low-ionization (Si II) and moderate-ionization (Si III) gas along many paths through the halo. We have chosen to report results for Si II j1193 rather than Si II j1260 because j1260 usually includes blended absorption from S II j1259. The various possible causes of metal line absorption with these large velocity equivalent widths are discussed more fully in 7. Although the high-ionization lines of Si IV and C IV are considerably weaker than the strong low-ionization lines, these species trace an important aspect of Galactic halo gas. In the FOS spectra the C IV lines are more often detected than the Si IV lines and allow the study of the distribution of halo gas in the temperature range from 3 ] 104 to 3 ] 105 K. In Figure 6 we show the C IV proðles for 16 QSOs