Ultraviolet Luminosity Density Research Articles

The Ultraviolet Luminosity Function at 0.6 < z < 1 from UVCANDELS

UVCANDELS is a Hubble Space Telescope Cycle-26 Treasury Program awarded 164 orbits of primary ultraviolet (UV) F275W imaging and coordinated parallel optical F435W imaging in four CANDELS fields—GOODS-N, GOODS-S, EGS, and COSMOS—covering a total area of ∼426 arcmin2. This is ∼2.7 times larger than the area covered by previous deep-field space UV data combined, reaching a depth of about 27 and 28 ABmag (5σ in 0.”2 apertures) for F275W and F435W, respectively. Along with new photometric catalogs, we present an analysis of the rest-frame UV luminosity function (LF), relying on our UV-optimized aperture photometry method, yielding a factor of 1.5 increase over H-isophot aperture photometry in the signal-to-noise ratios of galaxies in our F275W imaging. Using well-tested photometric redshift measurements, we identify 5810 galaxies at redshifts 0.6 < z < 1, down to an absolute magnitude of M UV = −14.2. In order to minimize the effect of uncertainties in estimating the completeness function, especially at the faint end, we restrict our analysis to sources above 30% completeness, which provides a final sample of 4726 galaxies at −21.5 < M UV < −15.5. We performed a maximum likelihood estimate to derive the best-fit parameters of the UV LF. We report a best-fit faint-end slope of α=−1.359−0.041+0.041 at z ∼ 0.8. Creating subsamples at z ∼ 0.7 and z ∼ 0.9, we observe a possible evolution of α with redshift. The unobscured UV luminosity density at M UV < −10 is derived as ρUV=1.339−0.030+0.027(×1026ergs−1Hz−1Mpc−3) using our best-fit LF parameters. The new F275W and F435 photometric catalogs from UVCANDELS have been made publicly available on the Barbara A. Mikulski Archive for Space Telescopes.

Galaxy luminosity functions at redshifts 0.6–1.2 in the Chandra Deep Field South

ABSTRACT We present the rest-frame ultraviolet (UV) galaxy luminosity function (LF) and luminosity density (LD) measurements in the far-UV (1500 Å) wavelength, in the redshift range $z$ = 0.6–1.2. The UV LF is derived using XMM-Newton Optical Monitor (XMM-OM), UV (1600–4000 Å) observations of the Chandra Deep Field South, over an area of 396 arcmin2. Using the deep UV imaging of the CDFS, we identified &amp;gt;2500 galaxies in our sample with UVW1AB ≤ 24.5 mag. This sample, along with various other catalogues containing redshift information, is used to calculate the binned representation of the galaxy UV LF in the two redshift bins 0.6 ≤ $z$ &amp;lt; 0.8 and 0.8 ≤ $z$ &amp;lt; 1.2, having a wide range of 1500 Å rest-frame UV magnitudes (ΔM1500 ≃ 3), reaching ≃ 1–1.5 magnitudes fainter than previous studies at similar redshifts. The binned LF is described well by the Schechter function form. Using maximum-likelihood, the Schechter function is fitted to the unbinned data to obtain the best-fitting values of the the UV galaxy LF parameters. We find that characteristic magnitude M* brightens by 0.8 mag from $z$ = 0.7 to $z$ = 1, implying an increase in the star formation activity between these redshifts, as reported by past studies. Our estimate of the faint-end slope $-1.10^{+0.19}_{-0.18}$ is on the shallower side compared with previous studies at $z$ = 0.7, whereas a value of $-1.56^{+0.19}_{-0.18}$ estimated for $z$ = 1.0, agrees with previous results given the uncertainties.

The z = 9-10 galaxy population in the Hubble Frontier Fields and CLASH surveys: the z = 9 luminosity function and further evidence for a smooth decline in ultraviolet luminosity density at z≥ 8

We present the results of a search for z=9-10 galaxies within the first 8 pointings of the Hubble Frontier Fields (HFF) (4 clusters plus 4 parallel fields) and 20 cluster fields from the CLASH survey. Combined with our previous analysis of the Hubble Ultra-Deep field (HUDF), we have now completed a search for z=9-10 galaxies over ~130 sq. arcmin, across 29 HST WFC3/IR pointings. As in our recent study of the first two HFF fields, we confine our primary search for high-redshift candidates in the HFF imaging to the uniformly deep (i.e. sigma_160>30 AB mag in 0.5-arcsec diameter apertures), relatively low magnification regions. In the CLASH fields our search was confined to uniformly deep regions where sigma_160>28.8 AB mag. Our SED fitting analysis unveils a sample of 33 galaxy candidates at z_phot>=8.4, five of which have primary photometric redshift solutions in the range 9.6<z_phot<11.2. By calculating a de-lensed effective volume for each candidate, the improved statistics and reduced cosmic variance provided by our new sample allows a more accurate determination of the UV-selected galaxy luminosity function (LF) at z~9. Our new results strengthen our previous conclusion that the LF appears to evolve smoothly from z=8 to z=9, an evolution which can be equally well modelled by a factor of ~2 drop in density, or a dimming of ~0.5 mag in M*. Moreover, based on our new sample, we are able to place initial constraints on the z=10 LF, finding that the number density at M_1500 ~ -19.7 is log(phi) = -4.1 (+0.2,-0.3), a factor of ~2 lower than at z=9. Finally, we use our new results to re-visit the issue of the decline in UV luminosity density at z>=8. We conclude that the data continue to support a smooth decline in rho_UV over the redshift interval 6<z<10, in agreement with simple models of early galaxy evolution driven by the growth in the underlying dark matter halo mass function.

Ultraviolet luminosity density of the universe during the epoch of reionization.

The spatial fluctuations of the extragalactic background light trace the total emission from all stars and galaxies in the Universe. A multiwavelength study can be used to measure the integrated emission from first galaxies during reionization when the Universe was about 500 million years old. Here we report arcmin-scale spatial fluctuations in one of the deepest sky surveys with the Hubble Space Telescope in five wavebands between 0.6 and 1.6 μm. We model-fit the angular power spectra of intensity fluctuation measurements to find the ultraviolet luminosity density of galaxies at redshifts greater than 8 to be . This level of integrated light emission allows for a significant surface density of fainter primeval galaxies that are below the point-source detection level in current surveys.

CANDELS: THE CONTRIBUTION OF THE OBSERVED GALAXY POPULATION TO COSMIC REIONIZATION

We present measurements of the specific ultraviolet luminosity density from a sample of 483 galaxies at 6<z<8. These galaxies were selected from new deep near-infrared HST imaging from the CANDELS, HUDF09 and ERS programs. In contrast to the majority of previous analyses, which assume that the distribution of galaxy ultraviolet (UV) luminosities follows a Schechter distribution, and that the distribution continues to luminosities far below our observable limit, we investigate the contribution to reionization from galaxies which we can observe, free from these assumptions. We find that the observable population of galaxies can sustain a fully reionized IGM at z=6, if the average ionizing photon escape fraction (f_esc) is ~30%. A number of previous studies have measured UV luminosity densities at these redshifts that vary by 5X, with many concluding that galaxies could not complete reionization by z=6 unless a large population of galaxies fainter than the detection limit were invoked, or extremely high values of f_esc were present. The observed UV luminosity density from our observed galaxy samples at z=7-8 is not sufficient to maintain a fully reionized IGM unless f_esc>50%. Combining our observations with constraints on the emission rate of ionizing photons from Ly-alpha forest observations at z=6, we can constrain f_esc<34% (2-sigma) if the observed galaxies are the only contributors to reionization, or <13% (2-sigma) if the luminosity function extends to M_UV = -13. These escape fractions are sufficient to complete reionization by z=6. These constraints imply that the volume ionized fraction of the IGM becomes less than unity at z>7, consistent with a number of complementary reionization probes. If faint galaxies dominate reionization, future JWST observations will probe deep enough to see them, providing an indirect constraint on the ionizing photon escape fraction [abridged].

A Flat Photoionization Rate at 2 ≤ z ≤ 4.2: Evidence for a Stellar-Dominated UV Background and against a Decline of Cosmic Star Formation beyond z ~ 3

We investigate the implications of our measurement of the Lyα forest opacity at redshifts -->2 ≤ z ≤ 4.2 from a sample of 86 high-resolution quasar spectra for the evolution of the cosmic ultraviolet luminosity density and its sources. The derived hydrogen photoionization rate Γ is remarkably flat over this redshift range, implying an increasing comoving ionizing emissivity with redshift. Because the quasar luminosity function is strongly peaked near -->z ~ 2, star-forming galaxies likely dominate the ionizing emissivity at -->z 3. Our measurement argues against a star formation rate density declining beyond -->z ~ 3, in contrast to existing state-of-the-art determinations of the cosmic star formation history from direct galaxy counts. Stellar emission from galaxies therefore likely reionized the universe.

The Stellar Initial Mass Function at the Epoch of Reionization

I provide estimates of the ultraviolet and visible light luminosity density at z~6 after accounting for the contribution from faint galaxies below the detection limit of deep Hubble and Spitzer surveys. I find the rest-frame V-band luminosity density is a factor of ~2-3 below the ultraviolet luminosity density at z~6. This implies that the maximal age of the stellar population at z~6, for a Salpeter initial mass function, and a single, passively evolving burst, must be 7 is 1.3+/-0.4\times10^{7} Msun/Mpc^3.

Where are the cosmic metals at z∼ 3?

The global temperature distribution of the cosmic gas-phase oxygen at z∼ 3 is determined by combining high-resolution cosmological simulations of individual protogalactic as well as larger regions with the observed, extinction-corrected, rest-frame V-band galaxy luminosity function. The simulations have been performed with three different stellar initial mass functions (IMFs), a Kroupa (K98), a Salpeter (S) and an Arimoto–Yoshii (AY), spanning a range of a factor of 5 in chemical yield and specific supernova type II energy feedback. Gas-phase oxygen is binned according to T as log(T) ≲ 4.0 (‘cold’), log(T) ∼ 4.5 (‘warm’) and log(T) ∼ 5.0, 5.5, 6.0, 6.5, 7.0 (‘hot’ phases). Oxygen is found to be distributed over all T phases, in particular for the top-heavy AY IMF. But, at variance with previous works, it is found that for the K98 and S IMFs the cold phase is the most important. For these IMFs it contains 47 and 37 per cent, respectively, of all gas-phase oxygen, mainly at fairly high density, nH≳ 0.1 cm−3. The implications of this in relation to observational damped Lyα absorber studies are discussed. In relation to ‘missing metals’ it is found that a significant fraction of the oxygen is located in a warm/hot phase that may be very difficult to detect. Moreover, it is found that less than about 20–25 per cent of the cosmic oxygen is associated with galaxies brighter than MV∼−22, i.e. the faintest galaxy luminosities probed by current metallicity determinations for Lyman-break galaxies (LBGs). Hence, 75–80 per cent of the oxygen is also in this sense ‘missing’. From the LBG-based, λ∼ 1500Å ultraviolet luminosity density history at z≥ 3, we obtain an essentially IMF-independent constraint on the mean oxygen density at z= 3. We compare this to what is obtained from our models, for the three different IMFs. We find that the K98 IMF is strongly excluded, as the chemical yield is simply too small, the Salpeter is marginally excluded, and the AY matches the constraint well. The K98 IMF can only match the data if the λ∼ 1500Å extinction corrections have been overestimated by factor of ∼4, which seems highly unlikely. The yields for K98 are also far too small to match the observational data for C iv. The optimal IMF should have a yield intermediate between the S and AY.

The Hubble Ultra Deep Field

This paper presents the Hubble Ultra Deep Field (HUDF), a 1 million s exposure of an 11 arcmin2 region in the southern sky with the Advanced Camera for Surveys on the Hubble Space Telescope using Director's Discretionary Time. The exposure time was divided among four filters, F435W (B435), F606W (V606), F775W (i 775), and F850LP (z850), to give approximately uniform limiting magnitudes mAB ~ 29 for point sources. The image contains at least 10,000 objects, presented here as a catalog, the vast majority of which are galaxies. Visual inspection of the images shows few if any galaxies at redshifts greater than ~4 that resemble present-day spiral or elliptical galaxies. The image reinforces the conclusion from the original Hubble Deep Field that galaxies evolved strongly during the first few billion years in the infancy of the universe. Using the Lyman break dropout method to derive samples of galaxies at redshifts between 4 and 7, it is possible to study the apparent evolution of the galaxy luminosity function and number density. Examination of the catalog for dropout sources yields 504 B435 dropouts, 204 V 606 dropouts, and 54 i775 dropouts. The i775 dropouts are most likely galaxies at redshifts between 6 and 7. Using these samples, which are at different redshifts but derived from the same data, we find no evidence for a change in the characteristic luminosity of galaxies but some evidence for a decrease in their number densities between redshifts of 4 and 7. Assessing the factors needed to derive the luminosity function from the data suggests that there is considerable uncertainty in parameters from samples discovered with different instruments and derived using independent assumptions about the source populations. This assessment calls into question some of the strong conclusions of recently published work on distant galaxies. The ultraviolet luminosity density of these samples is dominated by galaxies fainter than the characteristic luminosity, and the HUDF reveals considerably more luminosity than shallower surveys. The apparent ultraviolet luminosity density of galaxies appears to decrease from redshifts of a few to redshifts greater than 6, although this decrease may be the result of faint-end incompleteness in the most distant samples. The highest redshift samples show that star formation was already vigorous at the earliest epochs at which galaxies have been observed, less than 1 billion years after the big bang.

GEMS: Which Galaxies Dominate thez ∼ 0.7 Ultraviolet Luminosity Density?

We combine high-resolution images from GEMS with redshifts and spectral energy distributions from COMBO-17 to explore the morphological types of galaxies that dominate the z ~ 0.7 UV luminosity density. We analyzed rest-frame 280 nm and V-band luminosities of 1483 galaxies with 0.65 < z < 0.75, combining these with visual morphological classifications from F850LP images (approximately rest-frame V band) taken with HST ACS on the Extended Chandra Deep Field-South. We derive UV luminosity functions and j280 luminosity densities for spheroid-dominated galaxies, spiral galaxies, Magellanic irregular galaxies, and clearly interacting galaxies with morphologies suggestive of ongoing major mergers. We check the reliability of GEMS morphologies against the deeper GOODS images and quantify an incompleteness of the GEMS merger identification at the faint end. We derive the fractions of the global UV luminosity density j280 originating from the galaxy types and find that spiral and Magellanic irregular galaxies dominate with about 40% each. Interacting and merging galaxies account for roughly 20% of j280, while the contribution of early types is negligible. These results imply that the strong decline in the UV luminosity density of the universe observed from z ~ 1 until today is dominated by the decreasing UV luminosities of normal spiral galaxies, accompanied by the migration of UV-luminous star formation in irregular galaxies to systems of progressively lower mass and luminosity. These conclusions suggest that major-merger-driven star formation cannot dominate the declining cosmic star formation rate, unless major mergers are substantially more obscured than intensely star-forming spiral galaxies and the decline in observed cosmic star formation rate is substantially stronger than the already precipitous decline in uncorrected UV luminosity density.

Star Formation History since [ITAL][CLC]z[/CLC][/ITAL] = 1.5 as Inferred from Rest-Frame Ultraviolet Luminosity Density Evolution

We investigate the evolution of the universal rest-frame ultraviolet luminosity density from z = 1.5 to the present. We analyze an extensive sample of multicolor data (U, BAB, VAB = 24.5) plus spectroscopic redshifts from the Hawaii Survey Fields and the Hubble Deep Field. Our multicolor data allow us to select our sample in the rest-frame ultraviolet (2500 A) over the entire redshift range to z = 1.5. We conclude that the evolution in the luminosity density is a function of the form (1 + z)1.7 ± 1.0 for a flat lambda (Ωm0 = 0.3, Ωλ0 = 0.7) cosmology and (1 + z)2.4 ± 1.0 for an Einstein–de Sitter cosmology.

The Star Formation Rate Intensity Distribution Function: Implications for the Cosmic Star Formation Rate History of the Universe

We address the effects of cosmological surface brightness dimming on observations of faint galaxies by examining the distribution of unobscured star formation rate intensities versus redshift. We use the star formation rate intensity distribution function to assess the ultraviolet luminosity density versus redshift, based on our photometry and photometric redshift measurements of faint galaxies in the Hubble Deep Field (HDF) and the Hubble Deep Field-South (HDF-S) Wide Field Planetary Camera 2 and Near-Infrared Camera and Multi-Object Spectrometer fields. We find that (1) previous measurements have missed a dominant fraction of the ultraviolet luminosity density of the universe at high redshifts by neglecting cosmological surface brightness dimming effects, which are important at redshifts larger than z ≈ 2; (2) the incidence of the highest intensity star-forming regions increases monotonically with redshift; and (3) the ultraviolet luminosity density plausibly increases monotonically with redshift through the highest redshifts observed. By measuring the spectrum of the luminosity density versus redshift, we also find that (4) previous measurements of the ultraviolet luminosity density at redshifts z < 2 must be reduced by a factor of ≈2 to allow for the spectrum of the luminosity density between rest-frame wavelengths 1500 and 2800 A. And, by comparing with observations of high-redshift damped Lyα absorption systems detected toward background quasi-stellar objects, we further find that (5) the distribution of star formation rate intensities matches the distribution of neutral hydrogen column densities at redshifts z ≈ 2-5, which establishes a quantitative connection between high-redshift galaxies and high column density gas and suggests that high-redshift damped Lyα absorption systems trace lower star formation rate intensity regions of the same galaxies detected in starlight in the HDF and HDF-S. Because our measurements neglect the effects of obscuration by dust, they represent lower limits to the total star formation rate density.

Exploring Galaxy Evolution from Infrared Number Counts and Cosmic Infrared Background

Abstract Recently reported infrared (IR) galaxy number counts and cosmic infrared background (CIRB) all suggest that galaxies have experienced strong evolution sometime in their lifetime. We statistically estimate the galaxy evolution history from these data. We find that an order-of-magnitude increase of the far-infrared (FIR) luminosity at redshift $ z = 0.5$–$ 1.0$ is necessary to reproduce the very high CIRB intensity at 140 $ \mu m$ reported by Hauser et al. (1998, AAA 070.161.559) and decreases to, even at most, a factor of 10 toward $ z \sim 5$, though many variants are allowed within these constraints. This evolution history also satisfies the constraints from the galaxy number counts obtained by IRAS, ISO and, roughly, SCUBA. The rapid evolution of the comoving IR luminosity density required from the CIRB well reproduces the very steep slope of galaxy number counts obtained by ISO. We also estimate the cosmic star formation history (SFH) from the obtained FIR luminosity density, considering the effect of the metal enrichment in galaxies. The derived SFH increases steeply with redshift in $ 0 \lt z \lt 0.75$, and becomes flat or even declines at $ z \gt 0.75$. This is consistent with the SFH estimated from the reported ultraviolet luminosity density. In addition, we present the performance of the Japanese ASTRO-F FIR galaxy survey. We show the expected number counts in the survey. We also evaluate how large a sky area is necessary to derive secure information of galaxy evolution up to $ z \sim 1$ from the survey, and find that at least $ 50$–$ 300\;\mathrm{deg}^2$ is required.

Cosmological evolution and hierarchical galaxy formation

We calculate the rate at which dark matter haloes merge to form higher mass systems. Two complementary derivations using Press—Schechter theory are given, both of which result in the same equation for the formation rate. First, a derivation using the properties of the Brownian random walks within the framework of Press—Schechter theory is presented. We then use Bayes' theorem to obtain the same result from the standard Press—Schechter mass function. The rate obtained is shown to be in good agreement with results from Monte Carlo and N-body simulations. We illustrate the usefulness of this formula by calculating the expected cosmological evolution in the rate of star formation that is due to short-lived, merger-induced starbursts. The calculated evolution is well-matched to the observed evolution in ultraviolet luminosity density, in contrast to the lower rates of evolution that are derived from semi-analytic models that do not include a dominant contribution from starbursts. Hence we suggest that the bulk of the observed ultraviolet starlight at z>1 arises from merger-induced starbursts. Finally, we show that a simple merging-halo model can also account for the bulk of the observed evolution in the comoving quasar space density.

An ultraviolet-selected galaxy redshift survey -- II. The physical nature of star formation in an enlarged sample

We present further spectroscopic observations for a sample of galaxies selected in the vacuum ultraviolet (UV) at 2000 \AA from the FOCA balloon-borne imaging camera of Milliard et al. (1992). This work represents an extension of the initial study of Treyer et al. (1998). Our enlarged catalogue contains 433 sources; 273 of these are galaxies, nearly all with redshifts z=0-0.4. Nebular emission line measurements are available for 216 galaxies, allowing us to address issues of reddening and metallicity. The UV and Halpha luminosity functions strengthen our earlier assertions that the local volume-averaged star formation rate is higher than indicated from earlier surveys. Moreover, internally within our sample, we do not find a steep rise in the UV luminosity density with redshift over 0<z<0.4. Our data is more consistent with a modest evolutionary trend as suggested by recent redshift survey results. We find no evidence for a significant number of AGN in our sample. We find the UV flux indicates a consistently higher mean star formation rate than that implied by the Halpha luminosity for typical constant or declining star formation histories. Following Glazebrook et al. (1999), we interpret this discrepancy in terms of a starburst model for our UV-luminous sources. Whilst we can explain most of our observations in this way, there remains a small population with extreme UV-optical colours which cannot be understood.

Dust Absorption and the Ultraviolet Luminosity Density atz ≈ 3 as Calibrated by Local Starburst Galaxies

We refine a technique to measure the absorption-corrected ultraviolet (UV) luminosity of starburst galaxies using rest-frame UV quantities alone and apply it to Lyman-limit U dropouts at z ? 3 found in the Hubble Deep Field (HDF). The method is based on an observed correlation between the ratio of far-infrared (FIR) to UV fluxes with spectral slope ? (a UV color). A simple fit to this relation allows the UV flux absorbed by dust and reprocessed to the FIR to be calculated, and hence the dust-free UV luminosity to be determined. International Ultraviolet Explorer spectra and Infrared Astronomical Satellite fluxes of local starbursts are used to calibrate the FFIR/F1600 versus ? relation in terms of A1600 (the dust absorption at 1600 ?) and the transformation from broadband photometric color to ?. Both calibrations are almost completely independent of theoretical stellar-population models. We show that the recent marginal and nondetections of HDF U dropouts at radio and submillimeter wavelengths are consistent with their assumed starburst nature and our calculated A1600. This is also true of recent observations of the ratio of optical emission-line flux to UV flux density in the brightest U dropouts. This latter ratio turns out not to be a good indicator of dust extinction. In U dropouts, absolute magnitude M1600,0 correlates with ?: brighter galaxies are redder, as is observed to be the case for local starburst galaxies. This suggests that a mass-metallicity relationship is already in place at z ? 3. The absorption-corrected UV luminosity function of U dropouts extends up to M1600,0 ? -24 AB mag, corresponding to a star formation rate ~200 ? yr-1 (H0 = 50 km s-1 Mpc-3 and q0 = 0.5 are assumed throughout). The absorption-corrected UV luminosity density at z ? 3 is ?1600,0 ? 1.4 ? 1027 ergs-1 Hz-1 Mpc-1. It is still a lower limit since completeness corrections have not been done and because only galaxies with A1600 3.6 mag are blue enough in the UV to be selected as U dropouts. The luminosity-weighted mean dust-absorption factor of our sample is 5.4 ? 0.9 at 1600 ?.

Evidence for a Gradual Decline in the Universal Rest-Frame Ultraviolet Luminosity Density for [CLC][ITAL]z[/ITAL][/CLC] &lt; 1

We have utilized various magnitude-limited samples drawn from an extremely deep and highly complete spectroscopic redshift survey of galaxies observed in seven colors in the Hawaii Survey Fields and the Hubble Deep Field to investigate the evolution of the universal rest-frame ultraviolet luminosity density from z = 1 to the present. The multicolor data (U', B, V, R, and I, J, HK') enable the sample selection to be made in the rest-frame ultraviolet for the entire redshift range. Because of the large sample size and depth (UAB = 24.75, BAB = 24.75, IAB = 23.5), we are able to accurately determine the luminosity density to z = 1. We do not confirm the very steep evolution reported by Lilly et al. but instead find a shallower slope, approximately (1 + z)1.5 for q0 = 0.5, which would imply that galaxy formation is continuing smoothly to the present time rather than peaking at z = 1. Much of the present formation is taking place in smaller galaxies. Detailed comparisons with other recent determinations of the evolution are presented.

Spectroscopic identification of a galaxy at a probable redshift of z = 6.68

The detection and identification of distant galaxies is an important goal of observational cosmology, as such galaxies are seen at a time when the Universe was very young. The development of new techniques and instrumentation permits the search for ever-fainter galaxies, and so aids attempts to determine when the first stars and galaxies formed. Here we report the identification of a galaxy at a probable redshift of 6.68, the most distant object yet detected. The galaxy's spectrum is characterized by an abrupt discontinuity at a wavelength λ≈ 9,300 A, which we interpret as arising from the absorption of light at shorter wavelengths by hydrogen gas along the line of sight (the Lyman-α decrement), and by an emission line at λ≈ 9,334 A, which we interpret as the Lyman-α line at a redshift of 6.68. The galaxy is relatively bright: the ultraviolet luminosity density contributed by this one galaxy is almost ten times the value measured at z = 3.

The Canada–United Kingdom Deep Submillimeter Survey. II. First Identifications, Redshifts, and Implications for Galaxy Evolution

Identifications are sought for 12 submillimeter sources detected in a deep submillimeter survey. Six are securely identified, two have probable identifications, and four remain unidentified with IAB > 25. Spectroscopic and estimated photometric redshifts indicate that four of the sources have z 3. The spectral energy distributions of the identifications, as defined by measurements or upper limits to the flux densities at 8000 A, at 15, 450, 850 μm, and at 6 cm, are consistent with the spectral energy distributions of high-extinction starbursts such as Arp 220. The far-IR luminosities of the sources at z > 0.5 are of order 3 × 1012 h-250 L☉, i.e., slightly larger than that of Arp 220. As with local ultraluminous infrared galaxies, the optical luminosities of the identified galaxies are comparable to present-day L*, and the optical morphologies of many of the galaxies show evidence for mergers or highly disruptive interactions. Based on this small sample, the cumulative bolometric luminosity function shows strong evolution to z~1, but weaker or possibly even negative evolution beyond. The redshift dependence of the far-IR luminosity density does not appear, at this early stage, to be inconsistent with that seen in the ultraviolet luminosity density. Although the computation of bolometric luminosities is quite uncertain, the population of very luminous galaxies that is detected in the surveys at z > 1 is already matching, in the far-IR, the bolometric output in the ultraviolet of the whole optically selected population. Assuming that the energy source in the far-IR is massive stars, this suggests that the total luminous output from star formation in the universe will be dominated by the far-IR emission once the lower luminosity sources, below the current far-IR detection threshold, are included. Furthermore, the detected systems have individual star formation rates (exceeding 300 h-250 M☉ yr-1) that are much higher than seen in the ultraviolet-selected samples and that are sufficient to form substantial stellar populations on dynamical timescales of 108 yr. The association with mergerlike morphologies and the obvious presence of dust makes it attractive to identify these systems as forming the metal-rich spheroid population, in which case we would infer that much of this activity has occurred relatively recently, at z~2.

The Ultraviolet Luminosity Density of the Universe from Photometric Redshifts of Galaxies in the Hubble Deep Field

Studies of the Hubble Deep Field (HDF) and other deep surveys have revealed an apparent peak in the ultraviolet (UV) luminosity density, and therefore the star-formation rate density, of the Universe at redshifts 1 2.