Mark Dickinson

TYPE Ia SUPERNOVA DISCOVERIES AT Z > 1 FROM THE HUBBLE SPACE TELESCOPE: EVIDENCE FOR PAST DECELERATION AND CONSTRAINTS ON DARK ENERGY EVOLUTION 1

We have discovered 16 Type Ia supernovae (SNe Ia) with the Hubble Space Telescope (HST) and have used them to provide the first conclusive evidence for cosmic deceleration that preceded the current epoch of cosmic acceleration. These objects, discovered during the course of the GOODS ACS Treasury program, include 6 of the 7 highest redshift SNe Ia known, all at z > 1.25, and populate the Hubble diagram in unexplored territory. The luminosity distances to these objects and to 170 previously reported SNe Ia have been determined using empirical relations between light-curve shape and luminosity. A purely kinematic interpretation of the SN Ia sample provides evidence at the greater than 99% confidence level for a transition from deceleration to acceleration or, similarly, strong evidence for a cosmic jerk. Using a simple model of the expansion history, the transition between the two epochs is constrained to be at z = 0.46 ± 0.13. The data are consistent with the cosmic concordance model of ΩM ≈ 0.3, ΩΛ ≈ 0.7 (χ = 1.06) and are inconsistent with a simple model of evolution or dust as an alternative to dark energy. For a flat universe with a cosmological constant, we measure ΩM = 0.29 ± (equivalently, ΩΛ = 0.71). When combined with external flat-universe constraints, including the cosmic microwave background and large-scale structure, we find w = -1.02 ± (and w < -0.76 at the 95% confidence level) for an assumed static equation of state of dark energy, P = wρc2. Joint constraints on both the recent equation of state of dark energy, w0, and its time evolution, dw/dz, are a factor of ~8 more precise than the first estimates and twice as precise as those without the SNe Ia discovered with HST. Our constraints are consistent with the static nature of and value of w expected for a cosmological constant (i.e., w0 = -1.0, dw/dz = 0) and are inconsistent with very rapid evolution of dark energy. We address consequences of evolving dark energy for the fate of the universe.

High-redshift galaxies in the Hubble Deep Field: colour selection and star formation history to z ∼ 4

The Lyman decrement associated with the cumulative effect of H I in QSO absorption systems along the line of sight provides a distinctive feature for identifying galaxies at z ≳ 2.5. Colour criteria, which are sensitive to the presence of a Lyman continuum break superposed on an otherwise flat UV spectrum, have been shown, through Keck spectroscopy, to successfully identify a substantial population of star-forming galaxies at 3 ≲ z ≲ 3.5. Such objects have proven to be surprisingly elusive in field galaxy redshift surveys; quantification of their surface densities and morphologies is crucial for determining how and when galaxies formed. The Hubble Deep Field (HDF) observations offer the opportunity to exploit the ubiquitous effect of intergalactic absorption and obtain useful statistical constraints on the redshift distribution of galaxies to considerably fainter limits than the current spectroscopic limits. We model the H I cosmic opacity as a function of redshift, including scattering in resonant lines of the Lyman series and Lyman continuum absorption, and use stellar population synthesis models with a wide variety of ages, metallicities, dust contents and redshifts to derive colour selection criteria that provide a robust separation between high-redshift and low-redshift galaxies. From the HDF images we construct a sample of star-forming galaxies at 2 ≲z ≲ 4.5. While none of the ∼ 60 objects in the HDF having known Keck/Low-Resolution Imaging Spectrograph (LRIS) spectroscopic redshifts in the range 0 ≲ z ≲1.4 is found to contaminate our high-redshift sample, our colour criteria are able to efficiently select the 2.6 ≲ z ≲ 3.2 galaxies identified by Steidel et al. The ultraviolet (and blue) dropout technique opens up the possibility of investigating cosmic star and element formation in the early Universe. We set a lower limit to the ejection rate of heavy elements per unit comoving volume from Type II supernovae at 〈z〉 = 2.75 of ≈ 3.6 × 10^(−4) M_⊙ yr^(−1) Mpc^(−3) (for q_0 = 0.5 and H_0 = 50 km s^(−1) Mpc^(−1)), which is 3 times higher than the local value but still 4 times lower than the rate observed at z ≈ 1. At 〈z〉 = 4, our lower limit to the cosmic metal ejection rate is ≈ 3 times lower than the 〈z〉 = 2.75 value. We discuss the implications of these results on models of galaxy formation, and on the chemical enrichment and ionization history of the intergalactic medium.

Lyman Break Galaxies at z>4 and the Evolution of the UV Luminosity Density at High Redshift

We present initial results of a survey for star-forming galaxies in the redshift range 3.8 z 4.5. This sample consists of a photometric catalog of 244 galaxies culled from a total solid angle of 0.23 deg2 to an apparent magnitude of IAB = 25.0. Spectroscopic redshifts in the range 3.61 ? z ? 4.81 have been obtained for 48 of these galaxies; their median redshift is z = 4.13. Selecting these galaxies in a manner entirely analogous to our large survey for Lyman-break galaxies at smaller redshift (2.7 z 3.4) allows a relatively clean differential comparison between the populations and integrated luminosity density at these two cosmic epochs. Over the same range of UV luminosity, the spectroscopic properties of the galaxy samples at z ~ 4 and z ~ 3 are indistinguishable, as are the luminosity function shapes and the total integrated UV luminosity densities [?UV(z = 3)/?UV(z = 4) = 1.1 ? 0.3]. We see no evidence at these bright magnitudes for the steep decline in the star formation density inferred from fainter photometric Lyman-break galaxies in the Hubble deep field (HDF). The HDF provides the only existing data on Lyman-break galaxy number densities at fainter magnitudes. We have reanalyzed the z ~ 3 and z ~ 4 Lyman-break galaxies in the HDF using our improved knowledge of the spectral energy distributions of these galaxies, and we find, like previous authors, that faint Lyman-break galaxies appear to be rarer at z ~ 4 than z ~ 3. This might signal a large change in the faint-end slope of the Lyman-break galaxy luminosity function between redshifts z ~ 3 and z ~ 4, or, more likely, be due to significant variance in the number counts within the small volumes probed by the HDF at high redshifts (~160 times smaller than the ground-based surveys discussed here). If the true luminosity density at z ~ 4 is somewhat higher than implied by the HDF, as our ground-based sample suggests, then the emissivity of star formation as a function of redshift would appear essentially constant for all z > 1 once internally consistent corrections for dust are made. This suggests that there may be no obvious peak in star formation activity and that the onset of substantial star formation in galaxies might occur at z 4.5.

New Hubble Space Telescope Discoveries of Type Ia Supernovae at z ≥ 1: Narrowing Constraints on the Early Behavior of Dark Energy*

We have discovered 21 new Type Ia supernovae (SNe Ia) with the Hubble Space Telescope (HST) and have used them to trace the history of cosmic expansion over the last 10 billion yr. These objects, which include 13 spectroscopicallyconfirmedSNeIaat z � 1,werediscoveredduring14epochsofreimagingoftheGOODSfieldsNorthand South over 2 yr with the Advanced Camera for Surveys on HST. Together with a recalibration of our previous HSTdiscovered SNe Ia, the full sample of 23 SNe Ia at z � 1 provides the highest redshift sample known. Combining these data with previous SN Ia data sets, we measured Hz ðÞ at discrete, uncorrelated epochs, reducing the uncertainty of Hz >1 ðÞ from 50% to under 20%, strengthening the evidence for a cosmic jerk—the transition from deceleration in the past to acceleration in thepresent. The uniqueleverage of theHSThigh-redshift SNe Ia provides thefirstmeaningful constraint on the dark energy equation-of-state parameter at z � 1. The result remains consistent with a cosmological constant [ wz ðÞ ¼� 1] and rules out rapidly evolving dark energy (dw/dz 31). The defining property of dark energy, its negative pressure, appears to be present at z > 1, in the epoch preceding acceleration, with � 98% confidenceinourprimaryfit.Moreover,thez > 1sample-averagedspectralenergydistributionisconsistentwiththat of thetypicalSNIaoverthelast10Gyr,indicatingthatanyspectralevolutionofthepropertiesof SNeIawithredshift is still below our detection threshold.

The Great Observatories Origins Deep Survey: Initial Results from Optical and Near-Infrared Imaging

This special issue of the Astrophysical Journal Letters is dedicated to presenting initial results from the Great Observatories Origins Deep Survey (GOODS) that are primarily, but not exclusively, based on multiband imaging data obtained with the Hubble Space Telescope and the Advanced Camera for Surveys (ACS). The survey covers roughly 320 arcmin2 in the ACS F435W, F606W, F814W, and F850LP bands, divided into two well-studied fields. Existing deep observations from the Chandra X-Ray Observatory and ground-based facilities are supplemented with new, deep imaging in the optical and near-infrared from the European Southern Observatory and from the Kitt Peak National Observatory. Deep observations with the Space Infrared Telescope Facility are scheduled. Reduced data from all facilities are being released worldwide within 3-6 months of acquisition. Together, this data set provides two deep reference fields for studies of distant normal and active galaxies, supernovae, and faint stars in our own Galaxy. This Letter serves to outline the survey strategy and describe the specific data that have been used in the accompanying letters, summarizing the reduction procedures and sensitivity limits.

Lyman break galaxies at redshift Z ~ 3: Survey description and full data set

We present the basic data for a large ground-based spectroscopic survey for z ~ 3 Lyman break galaxies (LBGs), photometrically selected using rest-UV colors from very deep images in 17 high Galactic latitude fields. The total survey covers an area of 0.38 deg2 and includes 2347 photometrically selected candidate LBGs to an apparent AB magnitude limit of 25.5. Approximately half of these objects have been observed spectroscopically using the Keck telescopes, yielding 940 redshifts with z = 2.96 ± 0.29. We discuss the images, photometry, target selection, and spectroscopic program in some detail and present catalogs of the photometric and spectroscopic data, made available in electronic form. We discuss the general utility of conducting nearly volume-limited redshift surveys in prescribed redshift intervals using judicious application of photometric preselection.

The Evolution of Early-Type Galaxies in Distant Clusters*

We present results from an optical-infrared photometric study of early-type (E+S0) galaxies in 19 galaxy clusters out to z = 0.9. The galaxy sample is selected on the basis of morphologies determined from Hubble Space Telescope (HST) WFPC2 images and is photometrically defined in the K band in order to minimize redshift-dependent selection biases. Using new ground-based photometry in five optical and infrared bands for each cluster, we examine the evolution of the color-magnitude relation for early-type cluster galaxies, considering its slope, intercept, and color scatter around the mean relation. New multiwavelength photometry of galaxies in the Coma Cluster is used to provide a baseline sample at z ≈ 0 with which to compare the distant clusters. The optical-IR colors of the early-type cluster galaxies become bluer with increasing redshift in a manner consistent with the passive evolution of an old stellar population formed at an early cosmic epoch. The degree of color evolution is similar for clusters at similar redshift and does not depend strongly on the optical richness or X-ray luminosity of the cluster, which suggests that the history of early-type galaxies is relatively insensitive to environment, at least above a certain density threshold. The slope of the color-magnitude relationship shows no significant change out to z = 0.9, which provides evidence that it arises from a correlation between galaxy mass and metallicity, not age. Finally, the intrinsic scatter in the optical-IR colors of the galaxies is small and nearly constant with redshift, which indicates that the majority of giant, early-type galaxies in clusters share a common star formation history, with little perturbation due to uncorrelated episodes of later star formation. Taken together, our results are consistent with models in which most early-type galaxies in rich clusters are old, formed the majority of their stars at high redshift in a well-synchronized fashion, and evolved quiescently thereafter. We consider several possible effects that may be introduced by the choice of morphologically recognizable elliptical and S0 galaxies in dense environments as a subject for study. In particular, the inclusion of S0 galaxies, which might be undergoing morphological transformation in clusters as part of the Butcher-Oemler effect, may influence the results of our investigation.

The rest-frame optical properties of Z 3 galaxies

We present the results of a near-infrared imaging survey of z ~ 3 Lyman break galaxies (LBGs). The survey covers a total of 30 arcmin2 and includes 118 photometrically selected LBGs with Ks-band measurements, 63 of which also have J-band measurements, and 81 of which have spectroscopic redshifts. Using the distribution of optical magnitudes from previous work and -Ks colors for this subsample, we compute the rest-frame optical luminosity function of LBGs. This luminosity function is described by an analytic Schechter fit with a very steep faint-end slope of ? = -1.85 ? 0.15, and it strikingly exceeds locally determined optical luminosity functions at brighter magnitudes, where it is fairly well constrained. The V-band luminosity density of only the observed bright end of the z ~ 3 LBG luminosity function already approaches that of all stars in the local universe. For the 81 galaxies with measured redshifts, we investigate the range of LBG stellar populations implied by the photometry that generally spans the range 900-5500 ? in the rest frame. The parameters under consideration are the star formation rate as a function of time, the time since the onset of star formation, and the degree of reddening and extinction by dust. While there are only weak constraints on the parameters for most of the individual galaxies, there are strong trends in the sample as a whole. With a wider wavelength baseline than most previous studies at similar redshifts, we confirm the trend that intrinsically more luminous galaxies are dustier. We also find that there is a strong correlation between extinction and the age of the star formation episode, in the sense that younger galaxies are dustier and have much higher star formation rates. The strong correlation between extinction and age, which we show is unlikely to be an artifact of the modeling procedure, has important implications for an evolutionary sequence among LBGs. A unified scenario that accounts for the observed trends in bright LBGs is one in which a relatively short period of very rapid star formation (hundreds of M? yr-1) lasts for roughly 50-100 Myr, after which both the extinction and star formation rate are considerably reduced and stars are formed at a more quiescent, but still rapid, rate for at least a few hundred megayears. In our sample, a considerable fraction (~20%) of the LBGs have best-fit star formation ages 1 Gyr, implied stellar masses of 1010 M?, and are still forming stars at ~30 M? yr-1.

The Stellar Populations and Evolution of Lyman Break Galaxies

Using deep near-infrared and optical observations of the Hubble Deep Field-North from the Hubble Space Telescope NICMOS and WFPC2 instruments and from the ground, we examine the spectral energy distributions (SEDs) of Lyman break galaxies (LBGs) at 2.0 z 3.5 in order to investigate their stellar population properties. The ultraviolet-optical rest-frame SEDs of the galaxies are much bluer than those of present-day spiral and elliptical galaxies and are generally similar to those of local starburst galaxies with modest amounts of reddening. We use stellar population synthesis models to study the properties of the stars that dominate the light from LBGs. Under the assumption that the star formation rate is continuous or decreasing with time, the best-fitting models provide a lower bound on the LBG mass estimates. LBGs with L* UV luminosities are estimated to have minimum stellar masses ~1010 ☉, or roughly 1/10 that of a present-day L* galaxy, similar to the mass of the Milky Way bulge. By considering the photometric effects of a second stellar population component of maximally old stars, we set an upper bound on the stellar masses that is ~3-8 times the minimum mass estimate. The stellar masses derived for bright LBGs are similar to published estimates of their dynamical masses based on nebular emission line widths, suggesting that such kinematic measurements may substantially underestimate the total masses of the dark matter halos. We find only loose constraints on the individual galaxy ages, extinction, metallicities, initial mass functions, and prior star formation histories. Most LBGs are well fitted by models with population ages that range from 30 Myr to ~1 Gyr, although for models with subsolar metallicities a significant minority of galaxies are well fitted by very young (10 Myr), very dusty stellar populations, A(1700 A) > 2.5 mag. We find no galaxies whose SEDs are consistent with young (108 yr), dust-free objects, which suggests that LBGs are not dominated by first-generation stars and that such objects are rare at these redshifts. We also find that the typical ages for the observed star formation events are significantly younger than the time interval covered by this redshift range (~1.5 Gyr). From this, and from the relative absence of candidates for quiescent, non-star-forming galaxies at these redshifts in the NICMOS data that might correspond to the fading remnants of galaxies formed at higher redshift, we suggest that star formation in LBGs may be recurrent, with short duty cycles and a timescale between star formation events of 1 Gyr.

The evolution of the global star formation history as measured from the Hubble Deep Field

The Hubble Deep Field (HDF) is the deepest set of multicolor optical photometric observations ever undertaken, and it offers a valuable data set with which to study galaxy evolution. Combining the optical WFPC2 data with ground-based near-infrared photometry, we derive photometrically estimated redshifts for HDF galaxies with J 2, and they bridge the redshift gap between those two samples. The overall star formation or metal enrichment rate history is consistent with the theoretical models of White and Frenk and the predictions of Pei and Fall based on the evolving H I content of Lyα QSO absorption line systems.