Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101
Charles C. Steidel
Palomar Observatory, California Institute of Technology, Mail Stop 105-24, Pasadena, CA 91125
Royal Greenwich Observatory, Madingley Road, Cambridge, CB3 0EZ, UK
Space Telescope Science Institute, 3700 San Martin Dr. Baltimore, MD 21218
Palomar Observatory, California Institute of Technology, Mail Stop 105-24, Pasadena, CA 91125
F. Duccio Macchetto
Space Telescope Science Institute, 3700 San Martin Dr. Baltimore, MD 21218
Based on observations obtained at the W. M. Keck Observatory, which is operated jointly by the California Institute of Technology and the University of California. Based also on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute which is operated by AURA under NASA contract NAS 5-26555. Hubble Fellow. Alfred P. Sloan Foundation Fellow. NSF Young Investigator. Affiliated with the Space Science Department, ESA.
Despite large observational efforts, the early evolutionary phases of the normal galaxies, i.e., the present-day bright elliptical and spirals that define the Hubble sequence, remain largely unconstrained from an empirical perspective. In the last few years, a number of deep redshift surveys (Lilly et al. 1995a,1995b, Steidel et al. 1995, Glazebrook et al. 1995a, Cowie et al. 1994, Cowie, Hu & Songaila 1995a) and deep post-refurbishment HST imaging (Driver et al. 1995a,1995b, Glazebrook et al. 1995b, Schade et al. 1995, Cowie Hu & Songaila 1995b) have extensively probed the evolutionary state of galaxies at or (60)% of the life of the Universe if km s Mpc and (). These works find that the later and less bright types of the luminosity function underwent a significant amount of evolution in number density and/or luminosity (star-formation rate) over the probed cosmic epochs, while during the same time span the bright and more massive galaxies have remained substantially unchanged.
Figure: The idea behind the photometric system. The transmittance curves of the filters are superimposed on the synthetic spectrum of an unreddened star-forming galaxy (Bruzual & Charlot 1993) redshifted at . The spectrum of Q2233+1310 is also plotted.
Several years ago we started a program to probe the evolutionary status of normal galaxies at redshifts z>3. To detect galaxies at similar distances, we adopted a custom photometric system, named , designed to reach very faint flux levels and provide accurate color photometry. As Figure 1 shows, the technique relies on the presence of the 912Å Lyman discontinuity, both the one intrinsic to the atmospheres of massive hot stars and that contributed by the intervening galaxian and extragalactic neutral hydrogen, in the otherwise very blue and flat (in units) rest-frame UV spectral energy distribution of relatively unreddened star formation (Steidel & Hamilton 1992,1993, Steidel, Pettini & Hamilton 1995). In practice, z>3 galaxy candidates are selected from a color-color plane by having very red colors (they must actually be undetected in , as this passband is always entirely blueward of the Lyman discontinuity) and blue colors.
The spectroscopic confirmation of a (radio quiet) Lyman emitter at (Macchetto et al. 1993, Giavalisco, Macchetto & Sparks 1994) which was also a z>3 candidate (Steidel & Hamilton 1993), provided empirical support of the validity of the adopted color criterion in isolating z>3 star-forming galaxies. Since then, with the system we observed a number of fields both around QSOs with known optically-thick absorption systems (known to be caused by normal, massive galaxies, see Steidel, Dickinson & Persson 1995), so that their identification could guide the search for further examples of bright galaxies at high redshifts, and general sky fields, building a sample of z>3 galaxy candidates. We also embarked on a systematic program of HST imaging of the z>3 candidates to study their morphological properties, finding that they are consistent with what expected from bright galaxies observed during an epoch of relatively intense star-formation activity (Giavalisco et al. 1995, Giavalisco, Steidel & Macchetto 1996). We have studied the spatial distribution of these candidates (Giavalisco, Steidel & Szalay 1994), finding that, similar to bright galaxies at the present cosmological epochs, they are characterized by relatively strong clustering. Finally, we embarked on a systematic program of HST imaging of the z>3 candidates to study their morphological properties, and showed that they are consistent with what expected from bright galaxies observed during an epoch of relatively intense star-formation activity (Giavalisco et al. 1995, Giavalisco, Steidel & Macchetto 1996).
In October 1995 we followed-up suitable z>3 galaxy candidates from our list with the W. M. Keck 10-meter telescope and the LRIS spectrograph and confirmed that they are indeed high-redshift star-forming, but otherwise normal galaxies (Steidel et al. 1996). Using multi-slit masks we obtained spectra for 30 candidates in three fields, 25 of which were ``robust'' in the definition of Steidel et al. (1995), and the remaining five were ``marginal'' candidates, added to explore the parameter space and test the limits of the color selection criterion. Of the 25 robust candidates, 17 were proved to be in the redshift range , corresponding to an efficiency of 70% in finding high-redshift galaxies. Among the remaining candidates, three turned out to be QSOs in the redshift range (photometrically similar to star-forming galaxies), which brings the efficiency of the method of finding high-redshift objects to 80%, and five were inconclusive because of insufficient S/N. Of the five ``marginal'' candidates, one turned out to be a galaxy at , three were subdwarf stars, and one was inconclusive. It is important to note that all the inconclusive spectra are still consistent with being galaxies at , and no one of them shows any feature, such as a possible [OII] line emission, indicative of an interloper with redshift significantly lower than the targeted regime. Very likely, the efficiency of the color criterion in finding galaxies at z>3 is as high as 90%.
Figure 1 shows examples of the Keck spectra of z>3 galaxies, chosen to illustrate the variety of features encountered. In each case we have included for comparison a recent HST spectrum of the central starburst region in the Wolf-Rayet galaxy NGC 4214 (Leitherer et al. 1996). The similarity between the high-redshift galaxies and local examples of starbursts is striking. In each case the dominant characteristics of the far-UV spectrum are: (i) a flat continuum; (ii) weak or absent Lyman emission; (iii) prominent high-ionization stellar lines of He II, C IV, Si IV and N V; and (iv) strong interstellar absorption lines due to low-ionization stages of C, O, Si and Al. The continuum specific luminosity at 1500Å of a typical z >3 galaxy in our sample is erg sÅ (for ; 3 times greater for ); this is --1500 times higher (depending on the value of ) than the knot of star formation in NGC 4214 observed by Leitherer et al. (1996) and --100 times higher than the brightest such knots seen in nearby starburst galaxies. Thus, for a `normal' IMF, the far--UV continuum we see in the z > 3 objects is produced by the equivalent of -- O5 stars.
Modulo the uncertainties in models of young galaxies (see, e.g., Charlot 1996), we estimate that the maximum reddening allowed by the observed colors, in the range --, after accounting for the blanketing of the G band by the Lyman forest (see Madau 1995 and Steidel et al. 1995), is magnitudes. Taking this as an upper limit, and using the ``extragalactic'' reddening curve of Calzetti et al. (1994), the extinction at observed (rest Å) would be magnitudes, or a factor of < 5. The corresponding optical reddening in the rest-frame of the galaxies would be magnitudes.
Interestingly, despite the apparent lack of a large amount of dust, the Lyman emission is always much weaker (usually by factors of more than 10) than the ionization--bound, dust--free expectations, given the production of ionizing photons by massive stars which we measure directly from the UV continuum. Reasons for the preferential extinction of Lyman emission have been discussed by, e.g., Charlot & Fall (1993) and Chen & Neufeld (1994). Our observations are entirely consistent with the same low (but non--zero) dust content inferred to be present in the high redshift damped Lyman absorption systems (e.g., Pei et al. 1991, Pettini et al. 1994). In galaxies where the Lyman line is observed in emission, the typical rest--frame equivalent width is --20Å, but apparently most z>3 galaxies have Lyman \ emission lines weaker than these values, in striking similarity to nearby star--forming galaxies (e.g., Giavalisco, Koratkar, & Calzetti 1996). For example, C23 0000-263 in Figure 2 has among the largest implied star formation rates in our sample, and yet has no measurable Lyman emission.
While there is a great deal of variety in the strengths of the lines which are predominantly of stellar origin (C IV , Si IV ,1402 and He II ), we find these features to be generally weaker than in the spectra of present-day starbursts. This is probably an abundance effect; these lines are formed predominantly in the winds of massive stars, where both mass-loss rates and wind terminal velocities are known to depend sensitively on metallicity (e.g., Walborn et al. 1995).
Figure: Examples of Keck spectra of z>3 galaxies. Below each observed spectrum, we have plotted the spectrum of a star forming ``knot'' in NGC 4214, a nearby starburst galaxy (Leitherer et al. 1996), after shifting the spectrum to the measured redshift of the high-z galaxy. Some of the spectral features seen in local star--forming regions (both stellar and interstellar) have been labeled, for comparison with the high redshift objects (see text for discussion).
Figure: Same as Fig. 2.
The strongest interstellar lines indicated in Figures 2 and 3 have typical rest-frame equivalent widths --3.5Å. While the interstellar medium of these galaxies has obviously undergone some chemical enrichment, it is not possible to deduce metallicities from our spectra, since the absorption lines in question are undoubtedly heavily saturated. Under these circumstances, values of metallicity anywhere between 1/1000 of solar and solar are compatible with the line strengths and higher-resolution observations of intrinsically weaker lines are required to measure element abundances (Pettini & Lipman 1995). The equivalent widths of saturated absorption lines are much more sensitive to the velocity dispersion of the gas than to its column density. The observed --Å correspond to FWHM --700 which in turn imply approximate velocity dispersions --320 (slightly higher values apply if rotation is the dominant effect). In any case, if these velocity spreads reflect primarily gravitationally induced motions in the large-scale interstellar medium of the galaxies, the masses implied are comparable to those of present-day luminous galaxies. Smaller masses would result if interstellar shocks, local to the star-forming regions, contribute to the line widths. Spectra of higher S/N and resolution are required to resolve this question.
More recent observations of blank-sky fields with the COSMIC camera at the prime focus of the Palomar 5 m telescope, covering a field of view of 9 9, have allowed an accurate measurement of the surface density of the z>3 galaxies. These new images allow us to be complete in our identification of z>3 galaxies down to , and we find a total of 31 robust candidates satisfying the ``robust'' selection criterion out of a total of 2340 objects detected to the same apparent magnitude level. We, therefore, deduce a surface density of Lyman break candidates of arcmin (or per square degree). Thus, Lyman break objects in the redshift range represent about 1.3% of all objects to , and 2.0% of all objects in the magnitude range .
In 1995 October we also obtained deep band images of a small subset of the z>3 candidates using the Near Infrared Camera on the W. M. Keck telescope. A total of five candidates was observed, with typical integration times of 6000s, four of which with confirmed redshift. The measured K magnitudes (typically measuring rest--frame B or V of the galaxies) range from --22.1, with . These colors are consistent with models of continuous star formation which could have begun as early as Gyr prior to the epoch at which we observe them and are redder than would be expected if we were seeing instantaneous ``bursts'' of star formation (Bruzual & Charlot 1993). A consequence of adopting the maximum amount of reddening allowed by the UV colors of the galaxies is a reddening in of magnitudes; this would lower the ages significantly and formally allow single burst models younger than a few times years. We regard such short lifetimes as unlikely as they would imply that we are seeing large numbers of galaxies all bursting simultaneously.
One of the advantages of searching for high-z galaxies in the optical is that one observes directly the far--UV continuum produced by early-type stars (our bandpass samples the rest-frame continuum at Å). Consequently, in the absence of dust (see above), relatively minor assumptions are necessary to deduce the formation rate of massive stars and the accompanying production of ionizing photons; this is not the case for measurements of [OII] line luminosities which are subject to uncertainties as large as a factor of (see Gallagher, Bushouse, & Hunter 1989). To estimate star formation rates from the observed UV luminosities, we have made use of the calculations by Leitherer, Robert, & Heckman (1995) which are based on ultraviolet libraries of massive star spectra coupled with an evolutionary synthesis code. We have used the ``continuous star formation'', rather than ``single burst'', models, as we consider the former case more likely, for reasons given above. For a Salpeter initial mass function with an upper mass cut-off of 80 M, a SFR = yr produces a luminosity at 1500Å (in the rest-frame) erg sÅ. At (in the middle of the redshift range to which we are sensitive to the detection of Lyman break galaxies), an apparent (on the AB system) corresponds to erg sÅ ( km s Mpc) for , and erg sÅ for . The population of Lyman break galaxies we detect is in the range ; therefore, the implied star formation rates range from -- Myr () to -- Myr (), with the weighted average being () Myr for (0.05).
Assuming that we have uniformly probed the redshift range in our surveys (an assumption that is supported by the results of the spectroscopy), the co-moving density of the star--forming galaxies is at least () Mpc for (0.05), or about 1/2 (1/10) of the space density of present--day galaxies with for (Ellis et al. 1996). The total star formation rate per co--moving volume produced by the observed population at z>3 is then () Myr Mpc for (0.05). These numbers must be viewed as strict lower limits on the total star formation rate at z > 3---the fraction arising in only the most actively star--forming objects; taken at face value, the star formation density that we observe is only of the total star formation rate seen at the present epoch (Gallego et al. 1995). For comparison, the star formation rates (per object) and star formation densities recently reported by Cowie, Hu, & Songaila (1995b) at are approximately the same as we have observed at .
In parallel with the ground-based imaging and spectroscopy observations, we have been investigating the morphological properties of the z>3 galaxies with HST and WFPC2 (Giavalisco, Steidel & Macchetto 1996). To improve the overall observing efficiency, we have also followed the strategy of performing our ground-based photometric searches for z>3 galaxies in existing deep HST fields, and we now have images of galaxies from several fields, both our own and archival.
The images typically probe the rest-frame UV spectrum in the range 1400--1900Å and have high enough resolution that we can attempt a quantitative discussion of their sizes and light distribution.
Figure: A mosaic of HST images of the selected z>3 galaxies and galaxy candidates. The compact morphology of these galaxies is clearly evident from the images, which show the relatively more regular cores surrounded by diffuse nebulosities.
Figure 4 shows a mosaic of z>3 galaxies from the 0000-263, 0347-383 and SSA22 (2217-003) fields. Even from a visual inspection of the images it is clear that the majority of the z>3 galaxies have compact morphologies, typically characterized by a bright ``core'', often surrounded by more diffuse nebulosities with significantly lower surface brightness. The core is typically arcsec in radius, with a characteristic central surface brightness mag arcsec. The nebulosities extend on larger areas and are more irregularly distributed. However, there are four cases of more diffuse galaxies in the sample, C13, C27, and C28 in the 0000-263 field (we note that C28 is a candidate damped Lyman absorber in the spectrum of Q0000-263, see Steidel & Hamilton 1992), and C24 in the SSA22-L3 field. The diffuse galaxies have larger sizes than the compact galaxies, do not possess an obvious ``core'' and have lower central surface brightness. There are also two cases of ``double-core'' galaxies, SSA22-CW-C12 and 0000-263-C12. Finally, the galaxy 0000-263-C14, seems to be an interacting system.
We have determined quantitatively the morphology of the galaxies in two different ways. We have analyzed their radial light profile and found that many of them can be reasonably well described in terms of the traditional or exponential profiles. However, we caution that these fits are intended to broadly classify the light profiles and are not meant to imply that a particular galaxy rigorously follows a given model. In addition, we are fitting functions designed to model the light distribution of present-day galaxies at 4500Å rest-frame, and characterized by a very modest amount of star formation, to galaxies at z>3 observed at 1600Å rest-frame and with a star-formation rate one order of magnitude (or more) higher. Clearly, we do not yet know whether the same physical interpretation relating morphology to the dynamical state of the galaxies would hold.
We have also derived measures of the light concentration, but because of the approximate nature of the analysis above, we have not computed them from the parameters of the fit, as deviations from the adopted model and inaccurate modelling of the wings of the light distribution would have resulted in large errors. Rather, we have measured the isophotal magnitudes (using FOCAS) and a set of concentric aperture magnitudes, centered at the peak of the light distribution (the centering box had a size of 5 pixels), to produce a growth curve and derive the half-light radius.
Several interesting results have emerged from the analysis of the HST images:
1) Most of the the z>3 galaxies are characterized by a compact morphology, generally having a ``core'' arcsec in diameter, which at redshift (the mean value for our survey) corresponds to () kpc. The core typically contains about 90--95% of the total luminosity of the galaxy, and has a half-light radius in the range 0.2--0.3 arcsec, corresponding to -- (--) kpc. Since at the observed rest-frame far--UV wavelengths, the emission is directly proportional to the formation rate of massive stars, one can conclude that --95% of the stars that are being formed in these galaxies are concentrated in a region whose size is that of a present-day luminous galaxy. Moreover, the characteristic central concentration of the star formation has a scale size similar to a present--day spheroid. If the morphology of the massive stars is a good tracer of the overall stellar distribution, then the light distribution of the core is consistent with a dynamically relaxed structure.
2) The core is very often surrounded by low surface brightness nebulosities, generally distributed asymmetrically, and which may extend for a few arcsecs (see, e.g., 0000-263-C09 and 0347-383-N05). These nebulosities are clearly seen in Figures 2 and 3 as deviations from the more regular profile of the core. We note that the presence of such halos is consistent with the intense star-formation activity observed in the z>3 galaxies, even if the morphology of the underlying stellar distribution is relatively regular. For instance, the presence of extended gaseous components is expected from the intense supernovae rate that must be taking place in these systems (Ikeuchi & Norman 1991). Also, halos with irregular morphology and lower SFR are consistent with dissipative collapse (Baron & White 1987) of the cores. In such a scenario a core-halo segregation is actually expected due to the increased cloud collision rate and SFR in the denser, more collapsed regions (Silk & Norman 1981).
3) The central SB of the compact z>3 galaxies is consistently close to 23 mag arcsec. At the observed rest-frame wavelengths, the surface brightness is proportional to the star-formation efficiency. Given that these galaxies have all comparable UV extinction, we can conclude that they have also comparable star-formation efficiency, indicative of a similarity of physical processes in the core regions.
4) There are four cases of diffuse galaxies, namely 0000-263-C13, 0000-263-C27, 0000-263-C28 and SSA22-L3-C24, whose light profiles are very well fitted by exponential laws. The central surface brightness of these galaxies is significantly lower than that of their more compact counterparts, showing an overall reduced star-formation efficiency. Interestingly, galaxy 0000-263-C28 is a candidate to be one of the two damped Lyman systems in the spectrum of Q0000-263, either at or .
5) In three cases out of 19, SSA22-CW-C12, 0000-263-C12, and 0000-263-C14, we observe galaxies with multiple morphology, where two major light concentrations with similar apparent luminosity, two ``cores'' in our terminology, are separated by about 1 arcsec or less, corresponding to () kpc. The two individual sub-components are spatially resolved in the first case, where one is much more concentrated than the other, barely resolved in the second case, and too small to conclude anything in the third case. In all cases, the galaxies clearly show extended diffuse nebulosity around or extending from them, which is suggestive of systems in interaction. This could be interpreted as evidence of hierarchical merging of sub-units into more massive systems taking place with time scales about an order of magnitude shorter than the time stretch of the probed cosmic epoch, or yr. We note that in all cases the ``merging'' units have smaller luminosity than the other systems, but comparable central surface brightness. This is fully consistent with the interpretation of these galaxies as young spheroidal systems. Interestingly, both parents and daughters of this possible merging scenario have comparable morphologies.
6) Very interestingly, in the sample available to us, the geometry of the cores has a high degree of spherical symmetry, and there are no cases of highly elongated structures, such as the ``chain galaxies'' discussed by Cowie et al. (1995b). These chain galaxies are apparently formed by strings of star-forming regions of similar surface brightness. Given their knotty structure and the fact that they seem to have comparable star-formation rates to those of the z>3 galaxies, if placed at z>3, we would expect that the ``morphological k-correction'' (namely the change in the apparent morphology of a galaxy due to shifting the observed rest-frame light towards blue wavelengths coupled with surface brightness selection effects) would make them appear even more elongated because of surface-brightness dimming of the more diffuse regions that surround the knots. None among the 19 galaxies observed so far has, even approximately, such a morphology. We have quantified this by studying the axial ratios of the isophotes, which provide an upper limit to the axial ratios of the galaxy core regions. These values have a mean of , are larger than --2 only for the markedly exponential galaxies and are never larger than . In comparison, Cowie et al. report unconvolved axial ratios as high as 9.5, with a mean value of 4.7 for the chain galaxies.
7) The z>3 galaxies are often intrinsically relatively ``round'' and centrally concentrated, and seem to show a small dispersion of morphologies, in contrast with the larger variety observed in the galaxies harboring most of the star formation at later epochs (cf. Cowie et al. 1995a,1995b, Driver et al. 1995a,1995b, Glazebrook et al. 1995b). In interpreting the far--UV morphologies of galaxies at substantial redshifts, one must bear in mind the strong surface brightness selection effects (. On the other hand, the SED of unreddened star-forming galaxies is essentially flat from Å to Å rest-frame, and even further if the galaxy is young. At the wavelength of a typical WFPC2 filter (e.g., the F702W with Å), this corresponds to observing galaxies in the redshift interval with minimal ``morphological k-corrections'' (see Giavalisco et al. 1996). This means that it is not unfair to compare UV morphologies of the z >3 galaxies with that of the systems.
In summary, we have discovered a population of star--forming galaxies with redshifts using a color--selection technique whose efficiency is very high, allowing a first systematic study of the nature of galaxies at such large redshifts. The fact that we detect such a substantial population using a flat--UV spectrum selection criterion suggests that dust obscuration may not be an important limiting factor in searches for high-redshift galaxies.
The space density, star formation rates, morphologies and physical sizes, masses, and early epoch of the galaxy population that we have isolated all support the idea that the progenitors of the present-day bright galaxies have finally been observed during a phase characterized by intense star formation. The observed volume density of the z>3 galaxies is within a factor of a few that of present--day galaxies with , and their integrated star-formation rate is at least 25% of what is observed in the local universe. Rough dynamical estimates suggest that these galaxies are massive systems. In view of the fact that galactic spheroids must have formed relatively early to attain a state of quiescent evolution by , we find the centrally concentrated star formation that characterizes the z>3 population, together with all the other established properties, compelling evidence that we are observing directly, and for the first time, the ongoing formation of the spheroid components of what would become the luminous galaxies of the present epoch. At we are probably seeing an epoch where the star formation was concentrated primarily in the central regions of massive galaxies (the ``spheroid epoch''). This star formation appears to have ``migrated'' over time to more morphologically peculiar objects, and finally to the present where the bulk of the star formation is distributed in spiral disks.
Our results demonstrate beyond doubt that by z>3 massive galaxy formation was well under way, and the galaxies that we have identified are the sites of the most active star formation at that epoch, therefore, representing an important phase in the early history of galaxy formation. The properties of these objects must be reproduced by any theory attempting to explain the formation of normal galaxies.
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