Emily MacDonald

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 Hubble deep field-north SCUBA super-map – IV. Characterizing submillimetre galaxies using deep Spitzer imaging

We present spectral energy distributions (SEDs), Spitzer colours, and infrared (IR) luminosities for 850-μm selected galaxies in the Great Observatories Origins Deep Survey Northern (GOODS-N) field. Using the deep Spitzer Legacy images and new data and reductions of the Very Large Array-Hubble Deep Field (VLA-HDF) radio data, we find statistically secure counterparts for 60 per cent (21/35) of our submillimetre (submm) sample, and identify tentative counterparts for another 12 objects. This is the largest sample of submm galaxies with statistically secure counterparts detected in the radio and with Spitzer. Half of the secure counterparts have spectroscopic redshifts, while the other half have photometric redshifts. We find that in most cases the 850-μm emission is dominated by a single 24-μm source, with a median flux density of 241 μJy, leading to a median 24-to-850-μm flux density ratio of 0.040. A composite rest-frame SED shows that the submm sources peak at longer wavelengths than those of local ultraluminous infrared galaxies (ULIRGs). Using a basic grey-body model, 850-μm selected galaxies appear to be cooler than local ULIRGs of the same luminosity. This demonstrates the strong selection effects, both locally and at high redshift, which may lead to an incomplete census of the ULIRG population. The SEDs of submm galaxies are also different from those of their high-redshift neighbours, the near-IR selected BzK galaxies, whose mid-IR-to-radio SEDs are more like those of local ULIRGs. Using 24-μm, 850-μm and 1.4-GHz observations, we fit templates that span the mid-IR through radio to derive the integrated IR luminosity (LIR) of the submm galaxies and find a median value of LIR(8–1000 μm) = 6.0 × 1012 L. By themselves, 24-μm and radio fluxes are able to predict LIR reasonably well because they are relatively insensitive to temperature. However, the submm flux by itself consistently overpredicts LIR when using spectral templates which obey the local ULIRG temperature–luminosity relation. The shorter Spitzer wavelengths sample the stellar bump at the redshifts of the submm sources, and we find that the Spitzer photometry alone provides a model-independent estimate of the redshift, σ[Δz/(1 + z)] = 0.07. The median redshift for our secure submm counterparts is 2.0. Using X-ray and mid-IR data, only 5 per cent of our secure counterparts (1/21) show strong evidence for an active galactic nucleus dominating the LIR.

The Oxford–Dartmouth Thirty Degree Survey – II. Clustering of bright Lyman break galaxies: strong luminosity‐dependent bias at z= 4

We present measurements of the clustering properties of bright (L > L * ) z ∼ 4 Lyman break galaxies (LBGs) selected from the Oxford-Dartmouth Thirty Degree Survey (ODT). We describe techniques used to select and evaluate our candidates and calculate the angular correlation function, which we find best fitted by a power law, ω(θ) = A w θ -β with A w = 15.4 (with 0 in arcsec), using a constrained slope of β = 0.8. Using a redshift distribution consistent with photometric models, we deproject this correlation function and find a comoving r 0 = 11.4 +1.7 -1.9 h -1 100 Mpc in a Ω m = 0.3 flat A cosmology for i AB ≤ 24.5. This corresponds to a linear bias value of b = 8.1 +2.0 -2.6 (assuming σ 8 = 0.9). These data show a significantly larger r 0 and b than previous studies at z ∼ 4. We interpret this as evidence that the brightest LBGs have a larger bias than fainter ones, indicating a strong luminosity dependence for the measured bias of an LBG sample. Comparing this against recent results in the literature at fainter (subL * ) limiting magnitudes, and with simple models describing the relationship between LBGs and dark matter haloes, we discuss the implications on the implied environments and nature of LBGs. It seems that the brightest LBGs (in contrast with the majority subL * population) have clustering properties, and host dark matter halo masses, which are consistent with them being progenitors of the most massive galaxies today.

Multi-object near-infrared Hα spectroscopy of z ∼ 1 star-forming galaxies in the Hubble Deep Field north

We present preliminary results from a programme to obtain multi-object near-infrared spectroscopy of galaxies at redshifts 0.7 < z < 1.5. We are using the instrument CIRPASS (the Cambridge Infra-Red PAnoramic Survey Spectrograph), in multi-object mode, to survey Ha in galaxies at z ∼ 1. We aim to address the true star formation history of the Universe at this epoch: potentially the peak period of star formation activity. Ha is the same star formation measure used at low redshift, and hence we can trace star formation without the systematic uncertainties of using different calibrators in different redshift bins, or the extreme dust extinction in the rest-ultraviolet (rest-UV). CIRPASS has been successfully demonstrated in multi-object mode on the Anglo-Australian Telescope (AAT) and the William Herschel Telescope (WHT). Here we present preliminary results from one of our fields, the Hubble Deep Field North, observed with the WHT. With 150 fibres deployed over an unvignetted field of ∼15 arcmin, we have several detections of Ha from star-forming galaxies at 0.8 < z < 1.0 and present spectra of the seven brightest of these. By pre-selecting galaxies with redshifts such that Hα will appear between the OH sky lines, we can detect star formation rates of 5 h -2 70 M ○. yr -1 (5 σ in 3 hours, Ω M = 0.3, Ω Λ = 0.7). It appears that star formation rates inferred from Ha are, on average, a factor of more than two higher than those based on the UV continuum alone.