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Publ. Astron. Soc. Aust., 1999, 16, 95­9

.

The Low-redshift Intergalactic Medium
J. Michael Shull1,2 , Steven V. Penton1 and John T. Stocke1
1 Center for Astrophysics and Space Astronomy, Department of Astrophysical and Planetary Sciences, CB-389, University of Colorado, Boulder, CO 80309, USA JILA, University of Colorado and NIST, Boulder, CO 80309, USA mshull@casa.colorado.edu

2

Received 1998 November 2, accepted 1999 January 20

Abstract: The low-redshift Ly forest of absorption lines provides a probe of large-scale baryonic structures in the intergalactic medium, some of which may be remnants of physical conditions set up during the epoch of galaxy formation. We discuss our recent Hubble Space Telescope (HST) observations and interpretation of low-z Ly clouds toward nearby Seyferts and QSOs, including their frequency, space density, estimated mass, association with galaxies, and contribution to b . Our HST/GHRS detections of 70 Ly absorbers with NHI 1012 · 6 cm-2 along 11 sightlines covering pathlength (cz ) = 114, 000 km s-1 show f (>N HI ) N HI -0 · 63±0 · 04 and a line frequency dN /dz = 200 ± 40 for NHI > 1012 · 6 cm-2 (one every 1500 km s-1 of redshift). A group of strong absorbers toward PKS 2155-304 may be associated with gas (400 - 800)h-1 kpc from four large galaxies, with low metallicity ( 0 · 003 75 solar) and D/H 2 â 10-4 . At low-z , we derive a metagalactic ionising radiation +0 field from AGN of J0 = 1 · 3-0 · 8 â 10-23 erg cm-2 s-1 Hz-1 sr-1 and a Ly-forest ·5 1 baryon density b = (0 · 008 ± 0 · 004)h-1 [J-23 N14 b100 ] 2 for clouds of characteristic 75 size b = (100 kpc)b100 . Keywords: intergalactic medium--quasars: absorption lines

1 Introduction Since the discovery of the high-redshift Ly forest over 25 years ago, these abundant absorption features in the spectra of QSOs have been used as evolutionary probes of the intergalactic medium (IGM), galactic halos, and now large-scale structure and chemical evolution. The rapid evolution in the distribution of lines per unit redshift, dN /dz (1 + z ) ( 2 · 5 for z 1 · 6), was consistent with a picture of these features as highly ionised `clouds' whose numbers and sizes were controlled by the evolution of the IGM pressure, the metagalactic ionising radiation field, and galaxy formation. Early observations also suggested that Ly clouds had characteristic sizes 10 kpc, were much more abundant than (L ) galaxies and showed little clustering in velocity space. They were interpreted as pristine, zerometallicity gas left over from the recombination era. One therefore expected low-redshift (z < 1) absorption clouds to show only traces of HI, due to photoionisation and evaporation in a lower pressure IGM. All these ideas have now changed with new data and new theoretical modeling. Absorption in the Ly forest of HI (and HeII) has long been considered an important tool for studying the high-redshift universe (Miralda-Escud´ e & Ostriker 1990; Shapiro, Giroux & Babul 1994; Fardal, Giroux & Shull 1998). A comparison of the HI
Astronomical Society of Australia 1999

and HeII absorption lines provides constraints on the photoionising background radiation, on the history of structure formation, and on internal conditions in the Ly clouds. In the past few years, these discrete Ly lines have been interpreted theoretically by N -body hydrodynamical models (Cen et al. 1994; Hernquist et al. 1996; Zhang et al. 1997) as arising from baryon density fluctuations associated with gravitational instability during the epoch of structure formation. The effects of hydrodynamic shocks, Hubble expansion, photoelectric heating by AGN, and galactic outflows and metal enrichment from early star formation must all be considered in understanding the IGM (Shull 1998). One of the delightful spectroscopic surprises from the Hubble Space Telescope (HST) was the discovery of Ly absorption lines toward the quasar 3C 273 at zem = 0 · 158 by both the Faint Ob ject Spectrograph (FOS, Bahcall et al. 1991) and the Goddard High Resolution Spectrograph (GHRS, Morris et al. 1991, 1995). In this review, we describe (Section 2) the current status of our group's long-term program with the HST and VLA to define the parameters and nature of the low-redshift Ly forest. In Section 3, we discuss related theoretical work on the metagalactic ionising background, J (z ), and the contribution of low-z Ly clouds to the baryon density, b .
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Figure 1--Pie-diagram distributions of recession velocity and RA of bright (CfA survey) galaxies and four Ly absorbers toward Mrk 501 and Mrk 421 (Shull, Stocke & Penton 1996). Two of these systems lie in voids; the nearest bright galaxies lie > 4h-1 Mpc from the 75 absorber.

2 HST Survey of Low-z Ly Absorbers A The frequency of low-z Ly lines with W 320 m° reported by the HST/FOS Key Pro ject, dN /dz = (24 · 3 ± 6 · 6)(1 + z )0 · 58±0 · 50 (Bahcall et al. 1996), was considerably higher than a simple extrapolation from the high-redshift forest. These higher-NHI absorbers exhibit associations with galaxies (D 200h-1 kpc) 75 about half the time (Lanzetta et al. 1995). In HST cycles 4­6, our group began GHRS studies of lower-NHI absorbers toward 15 bright targets (Stocke et al. 1995; Shull, Stocke & Penton 1996). These low-z targets were chosen because of their well-mapped distributions of foreground galaxies (superclusters and voids). Our studies were designed to measure the distribution of Ly absorbers in redshift (z 0 · 08) and column density (12 · 5 log NHI 16), to probe their association with galaxies, and to measure their clustering and large-scale structure. Toward 15 targets, we detected 70 Ly systems (plus a number of highvelocity clouds and associated Ly absorbers) over a cumulated pathlength cz 114, 000 km s-1 . In cycle 7, we will observe 14 more sightlines with the Space Telescope Imaging Spectrograph (STIS) to double our Ly sample. The locations of Ly absorbers toward two of our first sightlines are shown in Figure 1. In our first four sightlines, the frequency of absorbers with NHI 1013 cm-2 was dN /dz 90 ± 20, corresponding to a local space density, -2 0 = (0 · 7 Mpc-3 )R100 h75 for absorber radius (100 kpc)R100 . This space density is 40 times

that of bright (L ) galaxies, but similar to that of dwarf galaxies with L 0 · 01L . From a statistical, nearest-neighbour analysis, we found that the Ly clouds have some tendency to associate with large structures of galaxies and to `avoid the voids'. However, for the lower column systems, the nearest bright galaxies are often too far to be physically associated in hydrostatic halos or disks (Maloney 1993; Dove & Shull 1994). Of 10 absorption systems first analysed (Shull, Stocke & Penton 1996), three lie in voids, with the nearest bright galaxies several Mpc distant. In several cases, we identified dwarf HI galaxies within 100­300 kpc using the VLA (Van Gorkom et al. 1996). Figure 2 shows one system toward Mrk 335, where a dwarf galaxy with MHI (7 â 107 M )h-2 and 75 offset distance (100 kpc)h-1 is seen at heliocentric 75 velocity cz = 1955 km s-1 , remarkably near to that of the Ly absorber. Thus, some of the lower-NHI absorbers appear to be associated with dwarf galaxies, although much better statistics are needed. In HST cycle 6, we observed seven more sightlines with the GHRS/G160M. With better data, we were able to detect weaker Ly absorption lines, down to 20 m° (NHI = 1012 · 6 cm-2 ) in some cases. Many of A the new sightlines exhibit considerably more Ly absorbers; for these 15 sightlines, dN /dz = 200 ± 40 for NHI 1012 · 6 cm-2 or one line every 1500 km s-1 . Although there is wide variation, this frequency is over twice the value (one every 3400 km s-1 ) reported earlier (Shull, Stocke & Penton 1996) for NHI 1013


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cm-2 . For a curve of growth with b = 25 km s-1 , the 70 Ly absorbers with 12 · 6 log NHI 14 · 0 follow - a distribution f ( NHI ) NHI0 · 63±0 · 04 , remarkably close to the slope in the high-z Ly forest. These results have been corrected for incompleteness at low equivalent widths, for line blending, and for the GHRS sensitivity function versus wavelength (Penton, Stocke & Shull 1999).

Figure 2--Overlay of galaxy field around Mrk 335, showing a dwarf galaxy at 1955 km s-1 at nearly the same redshift as the 1970 km s-1 Ly absorber (NHI = 1013 · 5 cm-2 ). The offset distance is 95h-1 kpc. 75

We turn now to the extraordinary sightline toward PKS 2155­304 (Bruhweiler et al. 1993; Shull et al. 1998). This target exhibits numerous Ly absorbers (Figure 3), including a group of strong systems between cz = 15,700 and 17,500 km s-1 . The strong absorbers have an estimated combined column density NHI = (2 - 5) â 1016 cm-2 , based on Lymanlimit absorption seen by ORFEUS (Appenzeller et al. 1995). Using the VLA (Van Gorkom et al. 1996; Shull et al. 1998), we have identified these absorbers with the very extended halos or intragroup gas associated with four large galaxies at the same redshift (Figure 4). The offsets from the sightline to these galaxies are enormous. Despite the kinematic associations, it would be challenging to make a dynamical association with such galaxies. One must extrapolate from the 1020 cm-2 columns seen in galactic 21-cm emission to the range 1013-16 cm-2 probed by Ly absorption. Much of the strong Ly absorption may arise in gas of wide extent, 1 Mpc in diameter, spread throughout the group of galaxies at z = 0 · 057. Assuming that NHI 2 â 1016 cm-2 and applying corrections for ionisation (H /H 3 â 10-4 for J0 = 10-23 and 600 kpc cloud depth) and for helium mass (Y = 0 · 24), the gas mass could total 1012 M . These absorbers offer an excellent opportunity to set stringent limits on heavy-element abundances and D/H in low-metallicity gas in the far regions of such galaxies. For example, no Si III 1206 · 50 absorption is detected (rest equivalent width W 22 m° r NSiIII 1 · 0 â 1012 cm-2 at 4 ) at wavelengths Ao corresponding to the strong Ly absorbers near 1281 ° and 1285 ° Over a range of photoionisation models A A. for (H /H) and (Si+2 /Si), this limit corresponds to an abundance (Si/H) 0 · 003(Si/H) for an assumed NHI = 2 â 1016 cm-2 and 300­600 kpc cloud depth (Shull et al. 1998). The lack of observed C IV 1549 absorption leads to similar limits, [C/H] < 0 · 005 solar. A rudimentary analysis of the lack of observed D I (Ly) absorption in the blueward wings of the strong HI lines suggests that (D/H) 2 â 10-4 . These limits can be improved with more sophisticated profile fitting and future data from HST/STIS (cycle 8) and FUSE. The HI toward PKS 2155-304 appears to represent gas with the lowest detected metallicity. Was this gas was once inside the galaxies at cz = 17, 000 ± 1000 km s-1 , or is it pristine? We can perhaps answer this question by deeper spectral searches for traces of metals. The origin of the lower-column Ly systems would seem to be more diverse, possibly arising in extended halos or debris disks of dwarf galaxies, large galaxies, and small groups (Morris & van den Bergh 1994). 3 Theoretical Implications A primary theoretical issue is whether low-z clouds have any relation to the evolution of the baryons in

Figure 3--HST/GHRS (G160M) spectrum of PKS 2155­304 (Shull et al. 1998) shows multiple Ly absorption systems between 1281­1290 ° (cz = 15, 700 - 17, 500 km s-1 ). A Upper limits on Si III 1206 · 50 absorption at 1274 · 7 ° and A ° 1275 · 2 A correspond to [Si/H] 0 · 003 solar abundance.


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4

6

8

10

12

-30 18 20 22

H0 = 75 km/s/Mpc

17,175 km/s

700 kpc

DECLINATION (B1950)

PKS 2155-304
24 26 28
800 kpc

30
400 kpc 560 kpc

32 34 36

16,788 km/s

17,100 km/s 16,350 km/s

21 57 00

56 45

30 15 00 RIGHT ASCENSION (B1950)

55 45

30

Figure 4--VLA field of 21-cm emission toward PKS 2155-304 at velocities 16,000­17,300 km s-1 near the Ly absorbers. Four large HI galaxies are detected at pro jected offsets of (400 - 800)h-1 kpc (Shull et al. 1998). At least two 75 galaxies, to the south and southwest, appear to be kinematically associated with Ly absorbers at 16,460 and 17,170 km s-1 .

the high-z forest. A quick estimate suggests that the low-z absorbers could contain a substantial (25%) fraction of the total baryons estimated from Big Bang nucleosynthesis, BBN = (0 · 0343 ± 0 · 0025)h-2 75 (Burles & Tytler 1998). Consider those Ly systems with NHI 1013 cm-2 , for which one can derive the space density 0 , dN 2c = 0 (R0 ) 100 . dz H0 (1)

which yields a cloud closure parameter in baryons, b 0 (b)Mcl (b)
2 2 = (0 · 008 ± 0 · 004)J-23 N14 b 1 1

-1 2 100 h75

1

.

(4)

The ma jor uncertainty in deriving absorber masses is the ionisation correction, which depends on the profile of gas density around the cloud centres. Assume, for simplicity, that nH (r) = n0 (r/r0 )-2 and adopt photoionisation equilibrium with photoionisation rate HI and a case-A hydrogen (A) recombination rate coefficient, H , at 20,000 K. The ionising radiation field is J = J0 (/0 )-s with s 1 · 8 and J0 = (10-23 erg cm-2 s-1 Hz-1 sr-1 )J-23 . The HI column density integrated through the cloud at impact parameter b is NHI (b) =
4 n2 r0 H (1+2nHe /nH ) 0 . 2HI b3 ( A)

(2)

2 We can solve for n0 r0 and find the total gas mass within b = (100 kpc)b100 for a fiducial column density NHI = (1014 cm-2 )N14 , 2 Mcl (b) = [4n0 r0 b(1 · 22mH )]
2 2 = (1 · 6 â 109 M )N14 J-23 b 1 1

100

5 2

,

(3)

For the spherical-cloud model, the radiation field, cloud size, and column-density distribution probably each contribute 30­40% to the uncertainty in b , while temperature Te and ionising spectral index s contribute 10%, for an overall uncertainty of 50%. However, as with the high-z forest, the greatest uncertainty in b lies in the cloud geometry and radial profile. These parameters can only be understood by building up statistics through many sightlines, particularly multiple targets that probe the same cloud structures. We have also increased our understanding of the metagalactic ionising background radiation and the `Gunn­Peterson' opacities, HI (z ) and HeII (z ). Using a new cosmological radiative transfer code and IGM opacity model, Fardal, Giroux & Shull (1998) modelled the ionisation fractions of HI and HeII in a fluctuating radiation field due to quasars and starburst galaxies. In this work, we have calculated the metagalactic ionising radiation field, J (z ), using QSO and stellar emissivities and including cloud diffuse emission and new (somewhat lower) IGM opacities derived from Keck Ly forest spectra. Figure 5 illustrates the evolution of J from z = 5 0, peaking at z 3. At z < 2, the absorption breaks at 1 Ryd (HI) and 4 Ryd (HeII) become much less prominent and J drops rapidly.


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Institute. Theoretical work was supported by NSF grant AST96­17073. Our colleagues in the VLA studies are Jacqueline Van Gorkom (Columbia University) and Chris Carilli (NRAO). Theoretical work was done in collaboration with Mark Giroux and Mark Fardal (Colorado) and undergraduate research student David Roberts (Cornell). References
Appenzeller, I., Mandel, H., Krautter, J., Bowyer, S., Hurwitz, M., Grewing, M., Kramer, G., & Kappelmann, N. 1995, ApJ, 439, L33 Bahcall, J. N., Januzzi, B. T., Schneider, D. P., Hartig, G. F., Bohlin, R., & Junkkarinen, V. 1991, ApJ, 377, L5 Bahcall, J. N., et al. 1996, ApJ, 457, 19 Bruhweiler, F. C., Boggess, A., Norman, D. J., Grady, C. A., Urry, C. M., & Kondo, Y. 1993, ApJ, 409, 199 Burles, S., & Tytler, D. 1998, ApJ, 499, 699 Cen, R., Miralda-Escud´ J., Ostriker, J. P., & Rauch, M. e, 1994, ApJ, 437, L9 Dove, J. B., & Shull, J. M. 1994, ApJ, 423, 196 Fardal, M. A., Giroux, M. L., & Shull, J. M. 1998, AJ, 115, 2206 Hernquist, L., Katz, N., Weinberg, D. H., & Miralda-Escud´ e, J. 1996, ApJ, 457, L51 Lanzetta, K. M., Bowen, D. V., Tytler, D., & Webb, J. K. 1995, ApJ, 442, 538 Maloney, P. 1993, ApJ, 414, 41 Miralda-Escud´ J., & Ostriker, J. P. 1990, ApJ, 350, 1 e, Morris, S. L., & van den Bergh, S. 1994, ApJ, 427, 696 Morris, S. L., Weymann, R. J., Savage, B. D., & Gilliland, R. L. 1991, ApJ, 377, L21 Morris, S. L., et al. 1995, ApJ, 419, 524 Penton, S., Stocke, J. T., & Shull, J. M. 1999, in preparation Shapiro, P., Giroux, M., & Babul, A. 1994, ApJ, 427, 25 Shull, J. M. 1998, Nature, 394, 17 Shull, J. M., Penton, S., Stocke, J. T., Giroux, M. L., Van Gorkom, J. H., & Carilli, C. 1998, AJ, 116, 2094 Shull, J. M., Roberts, D., Giroux, M. L., Penton, S., & Fardal, M. A. 1999, AJ, submitted Shull, J. M., Stocke, J. T., & Penton, S. 1996, AJ, 111, 72 Stocke, J. T., Shull, J. M., Penton, S., Donahue, M., & Carilli, C. 1995, ApJ, 451, 24 Van Gorkom, J. H., Carilli, C. L., Stocke, J. T., Perlman, E. S., & Shull, J. M. 1996, AJ, 112, 1397 Zhang, Y., Meiksin, A., Anninos, P., & Norman, M. 1997, ApJ, 495, 63

Figure 5--Spectrum J (z ) of ionising background from redshift z = 5 0 from new opacity and radiative transfer models (Fardal, Giroux, & Shull 1998; Shull et al. 1999).

At low redshift (z < 0 · 5), J depends both on the local (Seyfert) luminosity function and on the opacity model. We have recomputed the ionising radiation field at z 0 (Shull et al. 1999) using a new opacity model from HST absorption data and extrapolated EUV emissivities of QSOs and low-redshift Seyferts from our IUE-AGN database (Penton & Shull, unpublished). We find J0 = (1 · 3+0 · 8 ) â 10-23 erg -0 · 5 cm-2 s-1 Hz-1 sr-1 at z = 0, very close to our adopted scaling parameter, J-23 = 1. We clearly still have an enormous amount to learn about the nature and distribution of the low-redshift Ly clouds. It seems likely that future studies may uncover valuable information about their connection to large-scale structure and to the processes of galaxy formation and evolution. Acknowledgments Our Ly observations were made with the NASA/ESA Hubble Space Telescope supported by grant GO06586 · 01­95A through the Space Telescope Science