S. M. Pascarelle, R. A. Windhorst
Department of Physics & Astronomy, Arizona State University, Tempe, AZ 85287-1504, USA
W. C. Keel
Department of Physics & Astronomy, University of Alabama, Tuscaloosa, AL 35487-0324, USA
Keywords: galaxies: clusters: general---galaxies: distances and redshifts---galaxies: evolution---galaxies: formation---quasars: general
To explain the formation of the luminous elliptical and grand-design spiral galaxies is one of the most challenging problems in observational cosmology. Two basic scenarios exist at present: one based on the so-called `top-down' models, in which the largest structures in the universe formed first (Sunyaev & Zel'dovich 1975), and then subsequently fragmented into smaller and smaller sub-structures. The other is based on the so-called `bottom-up' models, in which small structures formed first, to eventually merge and coalesce as they build up to become the giant luminous galaxies that we see today (Navarro & White 1994). Several reviews on these topics, and of galaxy formation in general, exist in the literature (Cowie 1988, White 1989, Larson 1990, 1992, Silk & Wyse 1993).
Direct observational evidence of galaxy formation is critical to resolve this issue. The difficulty lies in finding large numbers of faint and presumably distant galaxies at or near their time of formation. Unfortunately, there exist only a few known cases of galaxy clusters or groups with spectroscopic confirmation at high redshift (e.g., Lowenthal et al. 1991, Steidel, Dickinson, & Sargent 1991, Dressler et al. 1993, Giavalisco, Steidel, & Szalay 1994, Hutchings 1995, Francis et al. 1996).
A piece of the puzzle of galaxy formation may come from recent findings on the subject of Faint Blue Galaxies (FBGs). The existence of this field galaxy population has been known for some time (Kron 1982, Broadhurst, Ellis, & Shanks 1988), but the true nature and evolution of the FBGs has remained a mystery. Recent results from deep HST images with the refurbished WFPC2 show that the FBG population is dominated by late-type or irregular galaxies (Driver et al. 1995) that have undergone substantial evolution since z 1. Many of these late-type/irregulars were found to be very compact galaxies with scale lengths, or half-light radii, 0.'' 3--0.'' 4 (Driver et al. 1995, Casertano et al. 1995, Odewahn et al. 1996). They have the steepest object counts, and clearly dominate the FBG counts at the faintest flux levels. Odewahn et al. (1996) suggest that a good fraction of these compact FBGs may play a role in the formation of giant galaxies at z1.
On the other hand, galaxy counts from deep HST images, along with the scale length--redshift (or --z) relation, of luminous early-type galaxies (E/S0's) and mid-type spiral galaxies (Sabc) indicate that they have been assembled largely before z1 and have undergone little or no evolution since z1 (Mutz et al. 1994, Driver et al. 1995, Odewahn et al. 1996). If the formation of luminous early- to mid-type galaxies occurred sometime before z1, then it is possible that they could have been assembled from the gradual merging of at least a sub-fraction of these compact, late-type FBGs. There is increasing evidence from both HST and ground-based work that the galaxy merger rate may have been larger at earlier cosmic epochs, roughly increasing with redshift as with m2--3 (Burkey et al. 1994, Carlberg et al. 1994, Yee & Ellingson 1995). It is thus possible that the FBG population provided the reservoir of building blocks out of which many of the luminous galaxies that we see today were formed through the process of repeated hierarchical merging (Navarro & White 1994). This is consistent with recent findings of a population of faint (B24) blue galaxies at 1z2 with rather unusual morphologies suggestive of dynamical formation processes or mergers (Cowie, Hu, & Songaila 1995).
Here we report our discovery of a cluster of 18 subgalactic-sized objects at . If these objects are typical of the building blocks from which the giant luminous galaxies were made, they would have eluded most ground-based searches in the past because of their small scale lengths, their low flux levels, and possibly their clustering properties (or rather any large-scale structure present at high redshifts).
The initial discovery of a possible cluster or group at was made from ground-based photometry with a medium-band (150Å-wide) filter centered at 4130Å (Lyman- at ) in the field around the weak radio galaxy 53W002 at z=2.390 (Windhorst et al. 1991, Windhorst & Keel 1996), resulting in two other candidates at the same redshift (Pascarelle et al. 1996a). These were later spectroscopically confirmed with the Multiple Mirror Telescope (MMT) , as shown in Fig. 1. Fortunately, the existence of a nearly identical medium-band filter on HST (F410M, centered at 4100Å) allowed the same observations to be conducted with higher sensitivity at much higher spatial resolution than could be achieved from the ground.
Figure: MMT spectra of four of the cluster candidates. The spectra for 53W002 (object 6) and object 19 were taken with the Blue Spectrograph and the 300 gpm grating, and have been smoothed to 20Å resolution (Windhorst et al. 1991). The spectra for objects 18 and 12 were taken with the Red Spectrograph and the 150 gpm grating, and have been slightly smoothed to 50Å (Pascarelle et al. 1996a).
The new HST data are presented in the color-color diagram of Fig. 2, which shows photometry for 115 objects detected simultaneously in the broad-band WFPC2 filters F606W (V) and F450W (B), and in the medium-band filter F410M (Lyman- at ). As with the ground-based data, the Lyman- passband is contained entirely inside the filter, so that proper continuum subtraction is possible. At , these bands sample well shortward of the redshifted 4000Å break, where young galaxy spectral energy distributions are relatively featureless (Windhorst et al. 1991).
Figure: The (F410M--) vs. (--) color-color diagram from 51 orbits in Cycles 4--5 on the cluster in the 53W002 field. The plot contains 115 objects which were detected in all three filters. The best signal-to-noise colors were determined from sufficiently large object apertures selected to be the same in all three filters. The sub-pixelated images were registered to well within 1 pixel. The sky was interpolated underneath the object aperture by fitting a sloped plane to the pixels unaffected by faint neighbors, hence correcting for any remaining small gradients in the flat-fielded images. Compared to automated object finding in the same field (Odewahn et al. 1996), the sky estimates are consistent to within 0.07% and the fluxes on average within 0.05 mag. Including the WFPC2 zeropoints of Holtzman et al. (1995), photometry presented here is accurate to 0.05--0.10 mag.
Another 250 objects were detected in and (down to 26.0 mag) but not in the F410M image, which was unavoidably underexposed. They have 2 upper limits of (F410M--) -0.2 mag and are not plotted here. For 26.0 27.5 mag the -band sample is 90% complete (Odewahn et al. 1996), but the underexposed F410M image does not provide useful Lyman- upper limits for such faint objects.
The solid line in Figure 2 shows the expected relation for field objects with a featureless power-law (F) across the three adjacent filters, labeled with values of , around which most of the general field objects (open circles) are distributed. Approximate 2 error boundaries for their WFPC2 photometry are indicated by the dotted lines. Error bars are plotted for 18 objects that are at least 2 away from the power-law line, which we believe are significant cluster candidates. The four large triangles have a spectroscopic confirmation at , out of five objects for which spectra were obtained, indicating that the reliability of this method to find compact candidates is about 80%. Apart from the three brighter objects, which were easily seen in ground-based photometry (Pascarelle et al. 1996a) and contain a weak active galactic nucleus, the rest of the sample has Lyman- emission with equivalent widths (estimated from the F410M photometry) more typical of ionization arising from star formation.
Although some of these cluster candidates could be foreground galaxies with O II] (at 3727Å) redshifted into the F410M filter (at ), such intrinsically faint and compact objects would be quite unusual at such a low redshift. In addition, the differential volume element at is 18--86 times bigger than at for q=0--0.5, and no other strong emission lines exist for star-forming objects between 1216Å and 3727Å, implying that most of the 18 significant candidates are likely at .
The restframe ultraviolet reddening vector (Seaton 1979) expected at is indicated in Figure 2, and suggests that the few reddest cluster candidates may have a visual absorption of 2--3 mag. These reddest cluster candidates should be viewed with some caution, however, because of the slight dependence of the magnitudes on (--) color, which increases towards redder (--) (Holtzmann et al. 1995). We corrected for this to first order, but higher order terms may affect the reliability of the few candidates with (--) 1.5 mag.
The reliability of the 18 cluster candidates in Fig. 2 is further strengthened by the fact that two of the spectroscopically confirmed members are actually at the lower boundary in the (F410M--) color of the general field population. It is of course possible that there exist additional cluster members that are not in Fig. 2 because they are significantly obscured by dust.
Figure: ( top) Histogram of WFPC2 continuum scale lengths for the significant narrow-band redshifted Lyman- emitting candidates in the 53W002 field from Figure 2. Scale lengths were determined from the average of their half-light radii in , , and . Most candidates have compact cores with 0.'' 1--0.'' 2 or 0.5--1.5 kpc, which is why they are relatively easy to detect with HST. ( bottom) Histogram of WFPC2 luminosities for the significant Lyman- emitting candidates. The upper axis indicates their absolute magnitude distribution, assuming K-corrections for young stellar populations at from Windhorst et al. (1991).
Figure 3 ( top) shows the size distribution for the cluster candidates and indicates that most are very compact, with 0.'' 1--0.'' 2, or 0.5--1.5 kpc at (all sizes in the text are quoted using H=80 Mpc and the range q=0--0.5). Each object is typically seen at least 3--4 scale lengths out in the WFPC2 images (see Fig. 4). Most candidates are much smaller than the median scale length of WFPC2 field galaxies at the same flux levels, which is about 0.'' 3--0.'' 4 (Casertano et al. 1995).
Average light profiles were generated for the cluster candidates, assuming that they are all to first order similar in shape and size. These profiles are shown in Fig. 4, and are in each filter better fit by an bulge-like profile than by a disk-like exponential. To derive a mean intensity profile with the greatest dynamic range, average intensity-weighted composite images of the 14 compact and isolated candidates were produced in each filter, so that the total effective exposure times of the image stacks become 145.7 hours. To measure the scale length of the mean observed profile, model profiles were convolved with an empirical point-spread function taken from a star in the images (for the undersampled and images an additional `pyramid' function was used to account for subpixel shifts among the objects, since shifting by interpolation introduces artifacts in undersampled images). The continuum scale lengths of the cluster candidates are very similar in and , showing no dependency on restframe wavelength below the 4000Å break (Odewahn et al. 1996). Their half-light radii were measured to be 0.'' 11--0.'' 14 in and 0.'' 10--0.'' 12 in , consistent with a mean value of 0.'' 12 ( 0.5--1.0 kpc at z).
Figure: Light profiles of the intensity-weighted composite images of 14 compact and isolated cluster candidates in the and filters. The profiles are reliable for radii between the two vertical dotted lines, but are affected by point spread function and pixel-interpolation problems at smaller radii, and by errors in the sky-determination at larger radii. In each filter, the profiles are better-fit by an -law than by a disk-like exponential. The allowed range of effective radii in WFC pixels (=0.'' 0996) is: 1.1--1.4 for (=1.30) and 1.0--1.2 for (=1.12), corresponding to 0.5--1.0 kpc at (H=80, q=0--0.5).
The question arises as to whether or not we are seeing the full extent of these objects. The K-correction for young spectral energy distributions at could have compensated for at least some of the cosmological surface brightness dimming (Windhorst et al. 1991). With the exception of 53W002 itself, which already has at a well-developed -law profile with a large scale length (0.'' 72) indicating a massive early-type galaxy (Windhorst, Mathis, & Keel 1992, Windhorst et al. 1994, Windhorst & Keel 1996), and was by selection included in this WFPC2 field, it appears that none of the other cluster candidates are yet fully assembled massive ellipticals or grand-design spirals. However, their scale lengths are quite comparable to those of bulges in local spirals (Simien & de Vaucouleurs 1987), which range from 0.2--4 kpc with a type-dependent median that is close to 1 kpc for types S0--Sbc. Therefore, despite their small sizes, these objects are not unusually small for the bulges of early- to mid-type disk galaxies. Given that the ratio of bulge-to-disk scale lengths of nearby late-type galaxies is (Courteau, deJong, & Broeils 1996), these objects may be subgalactic-sized (compact) and young (blue) spheroids, possibly representing the bulges of young galaxies that have not (yet) developed a significant disk around them at , or a disk that is depressed in the HST images by the severe cosmological surface brightness dimming.
The implied luminosities at for all cluster candidates are shown in Fig. 3 ( bottom), and typically range from M -23 to -18 mag (based on the stellar population models, age estimates, and K-corrections as described in Windhorst et al. 1991). With multicolor BVI photometry, the K-corrections are straightforward, provided that their redshifts are known to be at either from spectroscopy or their (F410M-) colors). Subtraction of any contributions from active galactic nuclei were also performed following the point-source subtraction method of Windhorst, Mathis, & Keel (1992). Some of the faintest candidates have 25--26 mag (the figure becomes incomplete for 25.5 mag due to the lower sensitivity in F410M than in F450W), so that it will require the concentrated efforts of the world's largest telescopes to confirm the redshifts of all 18 candidates spectroscopically. Given the apparent completeness limit, the initial (luminous) mass spectrum of this cluster could thus be quite steep.
The (evolving) absolute magnitude of an L galaxy at (M mag) was estimated by assuming that there would have been 2 mag of stellar evolution since for a typical starburst 0.3--0.510 years earlier (as the unreddened colors suggest, see Fig. 2; Windhorst et al. 1991,1994, Bruzual & Charlot 1993). Therefore, if indeed their stellar populations are young, most of the cluster candidates have luminosities of 0.1--1 L, and so possibly had only 10--10 M processed into stars at . For these parameters, the free-fall time expected for these clumps is 20--4010 years, long enough that the short-lived O and B stars are gone, but much shorter than the age of the dominant stellar population (A stars). Hence, there was indeed enough time for their mass distributions to settle into regular -like light profiles.
Most of the objects that were seen in and were also detected in , limiting the maximum redshift sampled in this 2.' 42.' 4 field to typically z3.5--4, or else the expected Lyman limit would have significantly damped out the light in the images (Guhathakurta, Tyson, & Majewski 1990). We suggest that these subgalactic-sized objects exist throughout the entire redshift range --4, and could have grown into the luminous giant galaxies (ellipticals and early-type spirals, with disks subsequently growing through accretion) seen today through the process of repeated hierarchical merging (Navarro & White 1994). The epoch-dependent merger rate, mentioned earlier and roughly , would result in a time integral of 10--20 mergers of compact objects from to z=0, which could yield a few L galaxies today. Since the HST counts of early-type galaxies show little evolution since z1, this process of repeated merging would have to be largely complete by . Our 18 cluster candidates could thus have merged to produce a few L galaxies today. The total luminosity for the 18 objects is M -24.7 to -25.8 at , which agrees quite well with the combined luminosities of a few typical L galaxies today of M -22.3 (including the expected -2 mag from their K-corrections plus evolution; Windhorst et al. 1991).
We note that the substantial number of luminous galaxies found at z1 by recent redshift surveys (Cowie, Hu, & Songaila 1995, Koo et al. 1996) is not inconsistent with our finding of small subgalactic-sized objects at . Our detection of this substantial, previously unrecognized population derives from their weak Lyman- fluxes, near the limit of ground-based narrow-band imaging surveys, and their characteristically small sizes, so that the HST images realize nearly the full point-source sensitivity gain over ground-based data. The medium-band HST images are more efficient in selecting samples of faint compact Lyman- emitting candidates, but ground-based spectroscopic confirmation is still necessary for unambiguous redshift determinations.
We find that only about 5% of the faint blue objects in our field can be classified as the `chain galaxies' of Cowie, Hu, & Songaila (1995), which is lower than the 20%--50% estimated in their sample. We believe that such objects are likely the short-lived ( 310yr out of a total of 610yr available for q=0 and z1) merger events among the many faint blue subgalactic clumps, in which the gas is drawn out of the merging objects during the encounter (Navarro & White 1994, Mihos 1995).
In conclusion, we believe that a non-negligible fraction of the compact FBGs observed in modern deep galaxy surveys may be such high-redshift subgalactic-sized clumps. The merger rate would have to have been much higher in the past in order to produce the little evolving early-type populations observed out to . It is thus possible that these subgalactic clumps may be hiding as many of these compact FBGs, and have escaped proper recognition from the ground until now because they are so small. Future work to test the universality of our findings in the 53W002 field is necessary. Random WFPC2 observations through the F450W and F410M filters will tell whether our cluster is unique in the early universe, or is typical of the general redshift distribution at 2.4 (c.f., Francis et al. 1996), which may be arranged in sheets on scales up to 125--156 Mpc as seen at lower redshifts (Broadhurst, Ellis, & Shanks 1988, Landy et al. 1996). The results of the work presented here are presented in detail in Pascarelle et al. (1996b).
All image data were obtained with the NASA/ESA Hubble Space Telescope through the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under contract to NASA. The spectroscopic observations were obtained at the Multiple Mirror Telescope Observatory, a joint facility of the University of Arizona and the Smithsonian Institution. We thank Simon Driver for providing his models for the late-type galaxies and for helpful discussions. We acknowledge support from HST grants GO.5308.0*.93A and GO.5985.0*.94A (to RAW & WCK).
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