THE ASTROPHYSICAL JOURNAL, 490:L135L139, 1997 December 1
© 1997. The American Astronomical Society. All rights reserved. Printed in U.S.A.

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Detectability of High-Redshift Elliptical Galaxies in the Hubble Deep Field

DAN MAOZ

School of Physics and Astronomy and Wise Observatory, Tel-Aviv University, Tel-Aviv 69978, Israel; dani@wise.tau.ac.il

Received 1997 April 21; accepted 1997 October 2; published 1997 November 4

ABSTRACT

     Relatively few intensively star-forming galaxies at redshifts of z>2.5 have been found in the Hubble Deep Field (HDF). This has been interpreted to imply a low space density of elliptical galaxies at high z, possibly due to a late (z<2.5) epoch of formation or to dust obscuration of the ellipticals that are forming at z3. I use Hubble Space Telescope UV (2300 Å) images of 25 local early-type galaxies to investigate a third option, that ellipticals formed at z>4.5 and were fading passively by 2<z<4.5. Present-day early-type galaxies are faint and centrally concentrated in the UV. If elliptical galaxies formed their stars in a short burst at z>4.5 and have faded passively to their present brightnesses at UV wavelengths, they would generally be below the HDF detection limits in any of its bands at z>2.5. Quiescent z3 ellipticals, if they exist, should turn up in sufficiently deep IR images.

Subject headings: cosmology: observationsgalaxies: elliptical and lenticular, cDgalaxies: evolutiongalaxies: formationultraviolet: galaxies

CONTENTS

• 1.  INTRODUCTION
• 2.  DATA
• 3.  MOVING THE GALAXIES TO HIGH REDSHIFT
• ACKNOWLEDGMENTS
• REFERENCES
• FIGURES
• TABLES
• REFERENCES TO THIS ARTICLE

§1.  INTRODUCTION

     Elliptical galaxies are believed to be among the oldest galaxies, with most of their stars having formed at early cosmic epochs and little star-forming activity since. (See Dickinson 1997 for a review of the evidence for this.) Some arguments for a more recent formation of ellipticals remain, however (e.g., Kauffmann, Charlot, & White 1996).

     An important step toward understanding the star formation history of the universe was taken with the observation of the Hubble Deep Field (HDF; Williams et al. 1996). Based on earlier work by Guhathakurta, Tyson, & Majewski (1990), Steidel & Hamilton (1992), and Steidel, Pettini, & Hamilton (1995), Madau et al. (1996) have implemented a method for identifying galaxies in the HDF at redshifts of z2.5. The cumulative effect of H I in stellar atmospheres, in galaxies, and in absorption systems along the line of sight causes galaxies to "drop out" of the F300W band at z2.5 and out of the F450W band at z3.5, as the Lyman break passes through these bands. Steidel et al. (1996) have demonstrated, by means of Keck spectroscopy of the brighter of the HDF UV dropouts, the high efficiency of this method in identifying high-z galaxies.

     Madau et al. (1996) note that the high-z galaxies in the HDF are relatively faint. Their star formation rates are less than 20 M yr, while rates in excess of 50100 M yr would be expected from ellipticals that were observed to be forming in a short (1 Gyr) burst at high z. They estimate a deficit by a factor of 10 in the comoving density of star-forming ellipticals at =2.75, compared with the present-day density of L>L ellipticals. Assuming Madau et al.'s technique is efficient for the F450W dropouts as well, a similar deficit is implied for the 3.5<z<4.5 range. This raises the possibility that, contrary to most evidence, giant ellipticals (or at least the field population among them) formed at z<2.5.

     Alternatively, Meurer et al. (1997) have argued that dust could be dimming the light from actively forming z3 early-type galaxies. They find that the rest-frame UV colors of local starbursts and of the HDF dropouts are similar. Since local starbursts are moderately extinguished by dust, the same extinction could operate in the high-z galaxies. The extinction-corrected star formation rate would then be consistent with the formation of ellipticals at z3.

     A third option, which I study here, is that ellipticals formed at even higher z and were already fading passively by z3. The selection criteria used by Madau et al. (1996) are based purely on colors in order to optimize the selection of high-z star-forming galaxies. Quiescent, passively evolving galaxies are faint in the rest-frame UV and, hence, are difficult to detect at high z. This has been realized since the first space-UV observations of local galaxies and their use for calculations of spectral k-corrections (e.g., Pence 1976; Coleman, Wu, & Weedman 1980; Bruzual 1983; King & Ellis 1985; Wyse 1985; Kinney et al. 1996). However, an accurate prediction of the appearance of high-z galaxies is complicated by the fact that galaxies are extended, structured, and diverse objects, while spectral k-corrections have always been based on a small number of galaxies whose UV spectra were integrated over a set aperture. In an attempt to obtain a "morphological k-correction," Giavalisco et al. (1996) have used UV images of seven (mostly spiral) nearby galaxies to simulate the appearance of galaxies in Hubble Space Telescope (HST) deep images. They show that galaxies change their morphologies radically when observed at high z, as a result of the shift to the rest-frame UV. (See also O'Connell & Marcum 1996 for a vivid demonstration of the effect.) Giavalisco et al. find that at z>1.4, most of their galaxies would be undetected by HST in a 4.3 hr exposure in the F606W band.

     The simulations of Giavalisco et al. (1996) were restricted to four normal spirals, one dusty starburst galaxy (M82), one Seyfert 2 galaxy (NGC 1068), and one dwarf elliptical (M32), which were imaged with the Ultraviolet Imaging Telescope (UIT). Furthermore, they simulated a particular HST exposure time and band and dealt with the issue of galaxy fading in an approximate way. Their work points to the need for a more extensive and quantitative examination of the expected appearance and brightness of quiescent galaxies at high z. Such an examination requires rest-frame UV images of local galaxies spanning a range in optical properties, particularly Hubble type and luminosity.

     Maoz et al. (1996b) carried out a UV (2300 Å) imaging survey with the Faint Object Camera (FOC) on HST of the central regions of 110 nearby galaxies. The 110 galaxies are an unbiased selection (about one-half) from a complete sample of all large (D>6) and nearby (V<2000 km s) galaxies. Since only the central 20×20 were observed, the UV images of most of the galaxies in the sample are not useful for predicting the appearance of high-z galaxies. The early-type galaxies in the sample, however, generally have very faint, diffuse, centrally concentrated UV emission, which becomes undetectable in the FOC exposures already at a radii less than 10. The projected physical radius probed by the FOC exposures at the distances of these galaxies is typically R1 kpc. (The effective radius, R, which includes half the light, is 4 kpc for an L elliptical.) Since the light profiles of ellipticals fall monotonically with radius in all bands, it is unlikely that the UV emission rises again outside the FOC field of view. The FOC images of early-type galaxies can therefore be used to estimate the appearance of the central and brightest regions of high-z galaxies of this kind.

     In this Letter I examine the detectability of quiescent high-z ellipticals in the HDF, based on HST UV observations of 25 nearby early-type galaxies. The observed galaxies are all the early-type galaxies in the UV survey of Maoz et al. (1996b). The 25 galaxies span a range of absolute luminosities and provide a representative collection of local galaxies of this type. Apart from the effects of redshift on their detectability, I also examine the effect of passive evolution.

§2.  DATA

     Table 1 lists the 25 galaxies from Maoz et al. (1996b) that have de Vaucouleurs T-type classifications T0 (i.e., ellipticals and S0s). Also listed for each galaxy are its heliocentric velocity, its optical major and minor axis as listed in the ESO and UGC catalogs (in tenths of arcminutes), its integrated B magnitude (from de Vaucouleurs et al. 1991), its absolute B magnitude (see below), its Hubble type (from Sandage & Tammann 1987), and the total f(2300 Å) in units of 10 ergs s cm Å, integrated above the background over the entire 20×20 area of the image, and 1 uncertainty (from Maoz et al. 1996b). Although among the galaxies classified as early-type galaxies by de Vaucouleurs (1991), four are classified by Sandage & Tammann (1987) as Sa and one as Sb, such differences of one or two Hubble classes are common when classifying galaxy types by eye (see Naim et al. 1995) and are not very meaningful. Distances to these galaxies are generally not available and are difficult to estimate from their recession velocity alone because they are so nearby. To get an idea of the distribution in absolute luminosity of the sample, I have taken the distances estimated by Tully (1988) based mostly on recession velocity corrected for local flows. For NGC 1023 and NGC 4649 I have used the distances measured by Ciardullo, Jacoby, & Tonry (1991) using planetary nebulae and for NGC 4486 (M87) the surface brightness fluctuation distance found by Tonry et al. (1997). From the absolute magnitude entries in Table 1, the early-type galaxies in the sample span a range from about 0.01L to 1.5L, with about half of the galaxies being approximately L (M=-20.7).

     Figure 1 (Plate L9) shows, for a selection of 12 of the galaxies, the FOC UV image from Maoz et al. (1996b) and an optical image from the Palomar Sky Survey. Note how, despite the fact that most of the galaxies are optically bright, they are generally faint and centrally concentrated in the UV. However, UV emission is detected at the nucleus and measured with typically 5 accuracy, when integrated over the field of view (see Table 1). The flux from the central region of such a galaxy, when observed at high z, can therefore be reliably calculated. Since the light distribution in early-type galaxies falls radially at all bands (including the UV, as evidenced by these images), all the surrounding regions of the galaxy will necessarily have lower surface brightness than the nuclear region probed by the FOC.

Fig. 1

     The FOC field of view is comparable to the International Ultraviolet Explorer (IUE) spectral aperture. Meurer et al. (1995) and Maoz et al. (1996b) have shown that there is generally an excellent agreement between nuclear UV fluxes measured with the FOC and with IUE. One may wonder, then, what advantage the present data have over existing spectra of ellipticals for characterizing the broadband UV flux of their central regions. First, the morphological information in the FOC images allow one to see what is in the aperture. For example, the FOC image of NGC 5253 (see Fig. 1) immediately reveals that it is an atypical galaxy with a nuclear starburst, whereas someone simply collecting all IUE spectra of ellipticals would include this galaxy as well. Second, atlases and averages of IUE spectra (e.g., Kinney et al. 1996) are collections of the objects that happened to be observed by IUE. These collections are not complete samples in any sense and are dominated by the objects with the highest signal-to-noise ratio, i.e., the brightest ones. The average IUE elliptical spectrum may therefore be biased by unrepresentatively blue ellipticals. The galaxies in the FOC sample are an unbiased selection from a complete sample of galaxies. This difference is especially important in the present context, where we are trying to study the UV detectability of typical galaxies.

§3.  MOVING THE GALAXIES TO HIGH REDSHIFT

     I now calculate the appearance of the FOC early-type galaxies if they were seen at various redshifts in the HDF bands. The wide field (WF) CCDs on WFPC2 have a scale of 01 pixel, versus 00225 pixel in the FOC in the zoomed f/96 format used by Maoz et al. (1996b), i.e., a factor of 4.4. A given part of the galaxy, when viewed at redshift z, would subtend an angle on the sky given by its size divided by the angular diameter distance (see, e.g., Giavalisco et al. 1996). Assuming a Virgo-like distance of 15 Mpc for the typical galaxy in the UV sample, and H=70 km s Mpc, at redshift z=2.5 the same region of the galaxy will subtend an angular extent 90140 times smaller, for q=0.5 and q=0.05, respectively. The entire 1024×1024 pixel FOC field of view therefore corresponds to about 2×2 WF pixels at z>2.5. Fortuitously, this area equals the minimum source detection area used by Williams et al. (1996) to create the HDF source catalog. Since their object detection algorithm searches for peaks in such segments of the image and then "grows" them, an elliptical galaxy whose nuclear region is undetected will remain undetected. Therefore, despite the limited angular extent of the FOC images of these large galaxies, the images can constrain the detectability of such galaxies at high z.

     Table 1 lists the total observed flux density at 2300 Å, f(2300 Å), for each of the early-type galaxies, as measured by Maoz et al. (1996b). The flux density of a galaxy at redshift z is proportional to D(1+z), where D is the luminosity distance. The central wavelengths of the HDF bands, F300W, F450W, F606W, and F814W, correspond to 2300 Å at redshifts of about 0.3, 1, 1.6, and 2.5, respectively. The flux density of a galaxy in Virgo will decrease by a factor of 4×10 (1×10) when it is moved to z=2.5 for q= (q=0). Since the F300W dropouts in the HDF are at z2.5, they are seen in their rest-frame UV even in the longest wavelength (F814W) band.

     From Table 1 we see that the three early-type galaxies that are brightest in the UV are NGC 3077, 5102, and 5253. From Figure 1, however, we see that these galaxies are morphologically unusual, in that they display knots of bright UV emission (NGC 3077, not shown in Fig. 1, is similar in appearance to NGC 5253). The UV emission probably extends outside the small FOC field of view, as well. These knots of "super star clusters" turn up in large numbers in HST UV images of galaxies where intense star formation is occurring (e.g., Conti & Vacca 1994; Meurer et al. 1995; Maoz et al. 1996a). In fact, NGC 5102 and 5253 are well-known starburst galaxies. All the other early-type galaxies in the sample, however, are an order of magnitude fainter in their UV images. They generally show only diffuse, centrally concentrated emission and sometimes some other faint features, such as a central compact source (e.g., NGC 404, NGC 4976), a jet (NGC 4486 = M87), or a circumnuclear ring (NGC 1079). The brightest one among these more typical, quiescent, galaxies, NGC 4649, has f(2300 Å) = 4.5×10 ergs s cm Å, integrated over the FOC field of view. At z=2.5, it would have f(2300 Å) = (410)×10 ergs s cm Å, depending on q. In the AB magnitude system (m=-48.6-2.5f, with f in units of ergs s cm Hz), this corresponds to an F814W magnitude of 30.631.6 mag. From examination of the Williams et al. (1996) object catalog, the 5 limiting magnitude of the HDF in this band for a 0.04 square arcsecond aperture (2×2 WF pixels) is about 29.2 mag, i.e., 1.52.5 mag brighter. At shorter wavelengths, the UV flux from early-type galaxies is similar or less (e.g., Kinney et al. 1996), and the HDF detection limits in the shorter wavelength bands are similar. The central 2×2 WF pixels would therefore not be detected. Since this is the brightest piece of the galaxy, the entire galaxy would remain undetected. Hence, unevolving high-z quiescent early-type galaxies are undetectable in the HDF.

     However, even if ellipticals formed all their stars in a short burst at high z, some passive evolution in their luminosity may be expected as a result of the aging of the stellar population. (See, however, Driver et al. 1996, who argue that the fading caused by the aging is, on average, balanced by the brightening caused by ongoing merging, so that "no evolution" is a good approximation to the expected behavior.) This evolution can be modeled by stellar population synthesis calculations. Maoz et al. (1996a) showed that the 2300 Å luminosity of an exponentially decaying starburst, after a few e-folding times, can be parametrized with respect to time as Lt. Spinrad et al. (1997) show that at 2300 Å most of the light from a coeval stellar population is produced by stars at the main-sequence turnoff. Since much of the uncertainty in population synthesis models arises from the different treatments of evolved stars, the time dependence of the broadband UV luminosity should be a robust prediction of the models. The age of the universe depends on redshift as t=1/H(1+z), with =1 for q=0 to = for q=. For H=70 km s Mpc, the universe at z=2.5 is 24 Gyr old, for q= to 0. If, for example, ellipticals formed their stars in an early burst lasting 500 Myr, by z=2.5 active star formation would have mostly ceased, and they would be fading passively with time. Quiescent ellipticals at redshift z will therefore be more luminous than their present-day counterparts at rest-wavelength 2300 Å by (1+z). At z=2.5, this is a factor of 6 (q=0) to 16 (q=), or 23 mag.

     The brightest quiescent elliptical in the sample would therefore just pass the HDF F814W magnitude limit, assuming a q= cosmology and ignoring the opposite effect of merging on the luminosity of the typical elliptical. Since these are extreme assumptions, and most of the early-type galaxies in the sample are about 1 mag fainter, I conclude that most high-z ellipticals that formed their stars in an early burst, ending before z>4.5, are undetectable in the HDF. An early epoch of elliptical galaxy formation is therefore consistent with the HDF results and is a viable alternative to obscured star formation at z3 (Meurer et al. 1997) or late formation at z<2.5 (Madau et al. 1996).

     It is instructive to compare this result, based on the FOC data, with what one obtains using traditional k-corrections. The elliptical galaxy template of Kinney et al. (1996), which is based on four ellipticals observed by IUE, has an f(8140 Å)/f(2300 Å) ratio of 22. An L elliptical with M=-20.7 has an F814W AB magnitude of -22.4. At z=2.5, the integrated F814W magnitude would be 28.3 (q=,H=70 km s Mpc, as above). If taking only the central 2 kpc diameter (corresponding to the FOC field of view of the local galaxies) of a de Vaucouleurs profile with R=4 kpc, the observed magnitude would be 30.2, versus 30.6 derived from the FOC data. The agreement is reasonable, but the spectral k-correction overpredicts the brightness of high-z ellipticals. Furthermore, recall that the galaxy used in this calculation, NGC 4649, is the brightest one, rather than a representative one, in the UV among the quiescent L early-type galaxies in the FOC sample.

     Part of the UV flux in the FOC images may be emission from some residual star formation, or "UV-upturn" emission, generally thought to originate in evolved stellar populations (e.g., Dorman, O'Connell, & Rood 1995). In that case, however, only a fraction of the present-day UV emission is from the main-sequence population; i.e., the main-sequence UV emission is even smaller than observed. This will only strengthen the conclusion on the undetectability of the high-z ellipticals that are viewed in their rest-frame UV.

     It may be argued that the distances of the galaxies in the sample are uncertain, compromising the accuracy of the luminosities and the predicted HDF magnitudes. However, seven of the FOC early-type galaxies, all fairly bright, are in Virgo and/or have accurate distances, so their luminosities are no more uncertain than those of most galaxies. Among the 10 most luminous (M>-20) galaxies, on which the argument hinges, three are in Virgo (two with accurate distances), and four others have velocities greater than 1500 km s, so their Hubble distances are probably already not that bad an approximation. It is unlikely that all of these galaxies are anomalously closer than it seems, and hence substantially sub-L, or that their distances are all much beyond 15 Mpc, and that this is the cause of their UV faintness. By setting an upper limit on the expected brightness of high-z ellipticals that is based on the brightest quiescent galaxy in the local sample, the conclusion is robust to the uncertainty in distances.

     Can the detectability of ellipticals in the HDF be improved by using a larger detection aperture? Assuming a background-noiselimited detection and that the UV light follows a de Vaucouleurs surface brightness profile, the signal-to-noise ratio as a function of the aperture radius R is



where x=R/R. The optimal aperture for detection is at the maximum of this function, which occurs at x0.14. Since the FOC images typically extend to x=0.25, using a larger aperture in the HDF will not change the conclusions, unless the UV light distribution falls off much less steeply than a de Vaucouleurs profile. This can be examined in the future using UV images of local galaxies having a larger field of view than the FOC.

     As is well known (see, e.g., Ellis 1997), near-infrared k-corrections are much smaller than optical ones, so high-z ellipticals may be more readily detected in the IR. In the context of the present discussion, the B and V spectral regions are shifted into the infrared H (1.65 m) and K (2.2 m) bands, respectively, at z=2.62.8. The integrated brightness of an approximately L_* elliptical is V11.0 mag at Virgo, and the brightness of the fraction of the galaxy within an aperture of radius 0.25R (a 025 diameter aperture at z>2.5) is V12.8. The galaxy flux within this aperture will be ×10 ergs s cm Å in the K band at z=2.7, depending on q. In f units, this is ×10 ergs s cm Hz, or 0.030.08 Jy, or a Johnson K magnitude of 2526 mag. As at UV wavelengths, passive evolution can potentially make the galaxy several magnitudes brighter at high z. Such surface brightnesses are challenging but detectable in long exposures with NICMOS on HST or on large ground-based telescopes. If passively fading ellipticals existed at these redshifts, they will turn up in deep enough IR images. Objects of these brightnesses that are undetected in optical images of the same field (e.g., the HDF) would be prime candidates for passively evolving ellipticals that formed at high z. Apart from revealing the formation history of ellipticals, the age estimation of such galaxies, if detected, can set powerful constraints on q (as in Spinrad et al. 1997). Alternatively, the nondetection of such a population in the IR would indicate that field ellipticals formed at z3 but were obscured or have formed more recently than z=2.5.

ACKNOWLEDGMENTS

     I thank Piero Madau, Gerhardt Meurer, Amiel Sternberg, and the anonymous referee for valuable comments. This work was supported by grant 94-00300 from the U.S.-Israel Binational Science Foundation and by a grant from the Israel Science Foundation.

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FIGURES

Full image (180kb)

TABLES

TABLE 1
EARLY-TYPE GALAXIES IN THE FOC UV SURVEY

NGC (1)	V (2)	D (3)	D (4)	B (5)	M (6)	T (7)	Classification (8)	f (9)	(10)
185...	-227	144	120	11.00	-13.2	-5	dE3pec	1.5	0.4
404...	-36	60	60	11.30	-15.7	-3	S03	3.5	0.2
1023...	648	85	40	10.50	-19.5	-3	SB01	4.0	1.1
1079...	1465	70	50	12.30	-18.8	0	Sa	2.8	0.2
1291...	839	130	130	9.42	-20.3	-3	SBa	2.9	0.3
1332...	1469	60	20	11.20	-20.0	-3	S01	2.9	0.2
1543...	1088	70	70	11.57	-19.1	-2	RSB0/a	1.7	0.2
2768...	1363	65	30	11.10	-20.8	-5	S01/2	1.2	0.4
2784...	708	90	50	11.25	-18.0	-2	S01	1.5	0.2
3077...	7	60	45	10.70	-15.9	0		21.9	5.0
4438...	86	97	39	12.00	-19.1	0	Sb	1.5	0.3
4486...	1292	70	70	10.40	-20.6	-6	E0	1.6	0.2
4636...	937	70	50	11.80	-19.3	-5	E/S01	1.8	0.3
4649...	1095	70	60	10.30	-20.6	-5	S01	4.5	0.4
4762...	1006	90	20	11.10	-20.0	-2	S01	2.0	0.4
4866...	1980	60	13	11.90	-19.1	-1	Sa	1.2	0.2
4976...	1503	60	35	11.17	-20.2	-5	S01	2.5	0.6
5023...	400	75	8	13.20	-15.7	-5		2.2	0.4
5084...	1739	190	28	12.02	-20.1	-2	S01	1.2	0.3
5101...	1864	70	70	11.52	-20.7	0	SBa	1.5	0.4
5102...	420	120	50	10.35	-17.5	-3	S01	20.2	1.3
5195...	558	70	50	10.60	-19.3	0	SB01pec	1.5	0.4
5253...	417	60	20	10.99	-16.5	0	Amorphous	95.4	6.2
5322...	1804	60	40	11.30	-21.2	-5	E4	3.2	0.8
5866...	672	65	30	11.10	-19.8	-1	S03	1.5	0.4

     NOTES. Col. (2): Heliocentric velocity, in km s. Cols. (3), (4): Optical major and minor axis as listed in the ESO and UGC catalogs (in tenths of arcminutes). Col. (5): Integrated B magnitude (from