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The Local Lyman-Alpha Forest: Absorbers in Galaxy Voids

We have conducted pointed redshift surveys for galaxies in the direction of bright AGN whose HST far-UV spectra contain nearby (cz<~ 30,000 kms), low column density (12.5<= log N_{HI} (cm s^{-2})<= 14.5) Ly-alpha forest absorption systems. Here we present results for four lines-of-sight which contain nearby (cz<~ 3000 kms) Ly-alpha absorbers in galaxy voids. Although our data go quite deep (-13<= M_{B}(limit)<= -14) out to impact parameters of 100-250 h_{70}^{-1} kpc, these absorbers remain isolated and thus appear to be truly intergalactic, rather than part of galaxies or their halos. Since we and others have discovered no galaxies in voids, the only baryons detected in the voids are in the Ly-alpha ``clouds''. Using a photoionization model for these clouds, the total baryonic content of the voids is 4.5% +/- 1.5% of the mean baryon density.

Since the discovery three decades ago of foreground absorption systems in QSO spectra, their origin and nature has been a subject of ongoing interest, particularly the Lyα -only absorption systems. Bahcall & Spitzer (1969) suggested that the Lyα lines arose in the diffuse gas of extended galaxy halos, while Arons (1972) proposed that they might be primordial material falling into galaxy halos. More recently, computer simulations of structure formation (Bond et al. 1988;Hernquist et al. 1996;Miralda-Escudé et al. 1996;Davé et al. 1999) have supported the hypothesis that these absorbers are not directly related to galaxies but rather are gas associated with a densityvarying intergalactic medium (IGM).
Because the relationship between Lyα absorbers and galaxies can only be studied in detail at low-z, the Hubble Space Telescope (HST) is required to discover absorbers for this purpose. The HST QSO Absorption Line Key Project used the Faint Object Spectrograph (FOS) to detect higher column density absorbers (log N HI ( cm −2 ) 14) at z = 0 -1.6 Weymann et al. 1998), followed by more sensitive discovery programs with the Goddard High Resolution Spectrometer (GHRS) and Space Telescope Imaging Spectrograph (STIS) (Morris et al. 1991;Impey et al. 1999;Tripp et al. 1998) extending to somewhat lower equivalent width limits (W 50 − 100 mÅ at z 0.3). Finally, in a large (31 sightline) systematic study, Penton et al. (2000aPenton et al. ( ,b, 2002a have used GHRS and STIS at medium resolution (∼ 20 km s −1 ) to study the lowest column absorbers (12.3 log N HI ( cm −2 ) 14.5) at cz 30, 000 km s −1 . These very low-z studies are particularly useful for understanding the galaxy-absorber relationship, because at such low redshifts even faint dwarf galaxies can be detected in the vicinity of the absorption systems.
Attempts to find galaxies associated with the higher column density Lyα absorbers using the FOS Key Project data (Lanzetta et al. 1995, L95 hereafter) found that ∼ 30% of the FOS absorbers were associated with bright galaxies. Chen et al. (1998, C98 hereafter) extrapolated the L95 results to fainter luminosities to conclude that all W > 300 mÅ absorbers are associated with galaxies closer than impact parameters of ρ = 225 h −1 70 kpc. This extrapolation remains untested because the bulk of the FOS absorption systems are too far away for fainter galaxies (L < L * ) to be detected.
Using more sensitive HST/GHRS spectra, Morris et al. (1993) conducted a detailed galaxy survey of the 3C 273 sightline finding mixed evidence for galaxies near both high and low column density absorbers. Subsequent studies by Tripp et al. (1998) and Impey et al. (1999) also found mixed results, even for a subset of 11 absorbers within the Virgo Cluster where the luminosity function of galaxies is complete to M B = −16 (Impey et al. 1999).
In preliminary studies to the current work, Stocke et al. (1995) and Shull et al. (1996) found low column density Lyα absorbers in galaxy voids. Stocke et al. (1995) conducted optical and H I 21 cm searches for galaxies near one "void absorber", the cz = 7740 km s −1 absorber in the Mkn 501 sightline, finding no optical galaxies at ρ < 1 Using a sub-sample of our GHRS sightlines, there are 45 low column density Lyα absorbers in regions where the CfA redshift survey is complete to at least L * . Penton et al. (2002a) find that 29 ± 8% of Lyα absorbers lie in galaxy voids, and that, while there is some evidence that the remaining ∼71% of the absorbers are associated with the same large-scale structure filaments as the galaxies, there was no definitive association between any Lyα absorber and any individual galaxy. Penton et al. (2002a) find no correlation between impact parameter and equivalent width for W 200 mÅ, or for impact parameters greater than 200 h −1 70 kpc, contrary to the predictions of L95 and C98. However, Penton et al. (2002a) do not address the relationship between absorbers and fainter galaxies.
In order to determine what relationship exists between the low-column Lyα absorbers and the faintest dwarf galaxies, we have undertaken a project based on the absorber sample of Penton et al. (2000aPenton et al. ( , 2002b. This sample is ideal because it contains many systems closer than cz = 10, 000 km s −1 . At such distances, we are able to probe well down into the galaxy luminosity function with multi-object spectroscopy on modest aperture telescopes, in some cases reaching the faintest dwarf galaxies (M B ∼ −12). To achieve this aim we have undertaken pencil-beam galaxy redshift surveys in the direction of 15 of the 31 QSO sightlines studied with GHRS and STIS by Penton et al. (2000aPenton et al. ( , 2002b. In this paper we present results from a subset of our data, namely those sightlines in which absorption systems were found inside very nearby (cz ≤ 3000 km s −1 ) galaxy voids, defined by having no bright galaxy (M B −17.5) closer than 2 h −1 70 Mpc. The study of void absorbers is important because those that lie within bright galaxy filaments are very likely to have a bright galaxy nearby; fainter galaxies will lie slightly closer, given their higher space density (Impey et al. 1999;Penton et al. 2002a). In these cases, it is not clear whether the clouds are associated with individual galaxies or with the large scale structure in which both the absorbers and galaxies are embedded. Absorbers in galaxy voids are immune to this effect: there are no bright galaxies nearby, and fainter galaxies are not expected to be close merely by chance. Thus, these "void absorbers" are uniquely suited to help determine whether all absorbers are galaxy halos, or if some are truly intergalactic material, as is strongly suggested by simulations ( e.g., Davé et al. 1999 ).
We present our work as follows: In §2 we discuss our observational techniques and data reduction. In §3 we introduce the void absorber sightlines and discuss our results as they pertain to the voids. In §4 we summarize the conclusions from this work. We assume H 0 = 70 h 70 km s −1 Mpc −1 throughout.

observations and reductions
These observations were obtained over the past three years at the DuPont 2.5 m telescope of Las Campanas Observatory (LCO) in Chile and at the 3.5 m WIYN 6 telescope at Kitt Peak National Observatory (KPNO) in Arizona. The LCO observations were taken with the Wide Field CCD multi-object spectrometer designed and built by one of us (R.J.W.). The instrument employs metal slitmasks to obtain redshifts of up to sixty galaxies at a time over a field of view of approximately 12 ′ × 20 ′ . The DuPont spectra range from just shortward of 4000Å to 8000Å, with a resolution of λ/∆λ ≈ 2000. We typically used only the part of the spectrum shortward of 5500Å in order to avoid confusion caused by sky subtraction. The WIYN observations were taken with the HYDRA fiber-fed multi-object spectrometer using the blue fiber cables, the G3 filter and the KPC-007 600 lines/mm grating. The wavelength coverage for these spectra was from 3800Å to 5800Å with λ/∆λ ≈ 4000 at 4000Å. Typically, 60 -70 spectra were obtained in each fiber setup over a 40 ′ circular field of view. For both LCO and WIYN, we used a set of spectral templates, kindly provided by E. Ellingson, to measure galaxy redshifts using cross-correlation techniques. Velocities returned by this procedure have typical accuracies of ±150 km s −1 for the faintest galaxies with usable redshifts.
Astrometric images were taken either at the 0.9 m telescope of KPNO using the T2kA or MOSAIC detectors or at the Las Campanas 40 inch telescope. Gunn r or Johnson R images were obtained and galaxy magnitudes converted to Johnson B using the colors appropriate for an Scd galaxy, as reported in Fukugita et al. (1995). This is a conservative choice of color conversion since most galaxies have redder colors and thus fainter B magnitudes for their red magnitudes. Integration times ensured a depth well below our desired limiting magnitude of m r = 19 (m B ∼ 19.7). Object detection and galaxy/star separation were accomplished using PPP, provided for our use by H. Yee (Yee 1991), or SExtractor (Bertin & Arnouts 1996). Our surface brightness sensitivity varied somewhat from image to image, but typically we were able to automatically detect galaxies with µ r ≥ 23 mag arcsec −2 (µ B 23.7 mag arcsec −2 ). While these surface brightness limits do not extend to the very lowest values observed for galaxies in modern surveys, they do extend well into the low surface brightness (LSB) galaxy regime (McGaugh et al. 1995). A visual inspection of the images found a few objects of lower surface brightness which had been missed by the finding algorithm, and which were added to our spectroscopy lists. PPP and SExtractor showed adequate agreement in object classifications and magnitudes. Yee (1991) describes experiments which show that the PPP magnitudes are equivalent to total magnitudes for objects well above the magnitude limit.
Owing to our desire to obtain spectra of as many galaxies as possible within our survey regions, we were quite conservative with the use of the object classifiers. We thus obtained spectra of many objects classified as possible galaxies which are, in fact, stars. Since objects were selected for observation based primarily upon location, so as to maximize the number of objects per mask or fiber setup, we expect that the fraction of stars among our unobserved objects at each magnitude limit is similar to the fraction among our observed objects. Therefore, the galaxy completeness fraction in each field (see Table 1) takes into account the fraction of stars mis-classified as possible galax-ies in that field.

results
We have observed four sightlines from Penton et al. (2000aPenton et al. ( , 2002b) which contain 7 Lyα absorbers in voids (no galaxies at M B −17.5 within 2 h −1 70 Mpc), based upon bright galaxy data from the CfA catalog (Huchra et al. 1983). Table 1 gives a summary of our void absorber data, including by column: (1) target name; (2) AGN heliocentric recession velocity; (3) absorber heliocentric recession velocity; (4) rest equivalent width (W) of the Lyα absorption in mÅ; (5) apparent B magnitude limit of our spectroscopy; (6) radius of the field of view covered at the distance of the void absorber; (7) percentage of galaxies for which we have a redshift to m B (limit) listed in column (5) within the field of view listed in column (6); and (8) the absolute B magnitude limit of our spectroscopy at the distance of the absorber. The void absorber in the VII Zw118 sightline is actually a blend of two absorbers with comparable equivalent widths. The combined equivalent width (333 mÅ, as it would appear in an HST/FOS spectrum) of this system places it well within the equivalent width limits suggested by L95 to be galaxy halos. The Mkn 421 and VII Zw118 fields were observed at WIYN, the other two at LCO. At the bottom of Table 1 we have added data on the two "void absorbers" reported in Stocke et al. (1995) and Penton et al. (2002a). While farther away than the other four objects in Table 1, the results from these two absorbers are entirely consistent with the results presented herein.
We have detected no galaxy coincident in velocity (±300 km s −1 ) with an absorber out to substantial impact parameters (100 − 250 h −1 70 kpc, except for HE1029-140, where our galaxy survey work is 10% complete at large impact parameters). In fact, there are no galaxies detected within the voids containing them. This absence is quite striking: not only have we failed to find galaxies within several hundred km s −1 of any absorber, we have found no faint galaxies closer than the nearest bright galaxies that define the voids. Thus, even if we use the Lyα absorbers as markers for the presence of material in voids, a sensitive search for galaxies near these absorbers fails to find even very faint galaxies. These observations underscore previous negative searches for galaxies in voids (Popsescu et al. 1997;Szomoru et al. 1996a,b).
We have tested the sensitivity of our observational procedure by comparing the number of galaxies detected to that expected in 7 of our survey sightlines (five are from LCO and two are from WIYN) which are 80% complete to m B ≈ 19 out to impact parameters ρ 5 ′ . We assume a standard Schechter (1976) luminosity function and normalizations for this comparison, with faint-end slope of either α = −1.0 or −1.2. Out to cz = 40, 000 km s −1 , the number of galaxies detected by our survey agrees well with the number predicted (see Figure 1). Our success in measuring redshifts for faint galaxies gives us confidence that we are not missing objects simply because our program lacks the required sensitivity.
While we cannot rule out the possibility that very LSB galaxies (µ B 24 mag arcsec −2 ) might be responsible for the void absorbers (Linder 1998), our surface brightness limits are low enough to require that any undetected galax-ies in these regions are extreme LSBs (McGaugh et al. 1995). Rauch et al. (1996) did not find any LSB galaxy associated with two very nearby absorbers in the 3C 273 sightline, nor did Impey et al. (1999) find any LSB galaxies in the Virgo Cluster associated with absorbers. Recent observations by Bowen et al. (2001) find Lyα absorption in the PKS1004+130 sightline 200 h −1 70 kpc from an LSB galaxy. These authors find a high surface brightness galaxy closer to the sightline and interpret the complex Lyα absorption as arising in an intra-group medium, not from a combination of galaxy halos. H I observations could detect gas-rich LSB galaxies that might be missed in optical studies (Szomoru et al. 1996b,a;Spitzak & Schneider 1998;Zwaan et al. 1997;Rosenberg & Schneider 2000). Van Gorkom et al. (1993 used Westerbork and the VLA to search for H I toward nine Lyα absorbers in four sightlines, detecting galaxies close to four of them. However, in only one case, that of a dwarf (M B = −16) toward Mkn 335, is the projected distance from the sightline to the galaxy 100 h −1 70 kpc. But this object has a high central surface brightness (m B ≈ 19.5 mag sec −2 ) and would not have been missed by this survey. For the other five absorbers, the nearest galaxy neighbors are several hundred kpc away. At these distances other galaxies become comparably close, making association with a single galaxy halo problematic (Shull et al. , 1998Penton et al. 2002a).
Van Gorkom et al. (1996) find that the rate of H I detections is not affected by the presence of Lyα absorbers, evidence that the matches they have found between absorbers and galaxies are likely to be chance associations rather than actual individual physical connections. New VLA H I observations have targeted three of the void absorbers in Table 1, with sufficient sensitivity to address the H I-rich LSB galaxy possibility definitively (Hibbard et al. 2002). Thus, while we cannot exclude categorically the presence of extreme LSB galaxies near these void absorbers, there is no positive evidence for association between any Lyα forest absorber and an LSB galaxy to date.

summary and conclusions
We have presented a subset of the results from our pencil-beam redshift survey toward 15 AGN containing nearby Lyα absorbers: namely, four sightlines that pass through very nearby (cz 3000 km s −1 ) galaxy voids. The absorbers in these voids are all sufficiently near that we could have detected faint dwarf galaxies to limits of M B −14.3 or fainter within 100 − 250 kpc of the sightline. In one sightline, HE1029-140, our survey is too incomplete to be definitive. The remaining fields are more than 90% complete for M B −12.8 to −14.5, or to µ B 23.5 mag arcsec −2 . While our survey puts strong constraints on the optical luminosity of any normal surface brightness galaxy that could be associated with the void absorbers, the possibility remains that extreme LSB galaxies (µ B 24 mag arcsec −2 ) could be present. A 21 cm search has been made for H I near three void absorbers (Hibbard et al. 2002) and will address the presence of hydrogen-rich LSBs near the void absorbers.
The results of this optical survey strongly suggest that the void absorbers are a primordial population of objects. This is in keeping with results from cosmic structure for-mation simulations (e.g., Davé et al. 1999) and is contrary to the hypothesis of C98 that all Lyα absorbers are associated with galaxies. The primordial nature of these void absorbers can be definitively addressed by searching for metals in them.
The current observations allow a first determination of the total baryon content in voids. Since no galaxies have been found by us or others in voids, the only baryons detected inside the voids are the Lyα absorbing clouds. Based upon a standard photoionization model, Penton et al. (2002a) have determined that the local Lyα forest accounts for ∼ 20% of the total baryons predicted by big bang nucleosynthesis. By comparing their measured value of dN/dz outside of voids to that in the voids, and by assuming the same values for the local ionizing radiation flux and the sizes of Lyα clouds in both void and non-void absorbers, Penton et al. (2002b) show that the void absorbers account for ∼ 22 ± 8 % of local absorption per unit pathlength (i.e., the 29% quoted earlier, corrected for the relative amount of pathlength through voids in the survey). This implies that 4.5 ± 1.5 % of the total baryons reside in the voids. This calculation assumes that all baryons in voids are detectable either as galaxies or Lyα absorbers; i.e., if a warm-hot IGM component is present without detectable Lyα absorption, we have not accounted for it. Support at Colorado for work on the local Lyα forest comes from grants GO-06593.01 and GO-08182.01 from the Hubble Space Telescope Science Institute.  -The cumulative number of galaxies expected compared to the number observed as a function of velocity (cz) for our survey. We have used the functional form and normalizations from Schechter (1976) with different faint-end slope, α, to compute the expected number of galaxies. The expected values are plotted in dashed-dots on the upper bound of the shaded region, and dots on the lower bound, for α = −1.2 and −1.0, respectively. Both curves assume a limiting magnitude of m B = 19. The top plot is for two WIYN fields, and the bottom for five fields taken at Las Campanas. The points and error bars mark the number of galaxies we have found in our sample, along with √ n uncertainties.