Elizabeth R. Stanway

The star formation rate of the Universe at z≈ 6 from the Hubble Ultra-Deep Field

We determine the abundance of i′-band dropouts in the recently released HST/ACS Hubble Ultra-Deep Field (UDF). Because the majority of these sources are likely to be z≈ 6 galaxies whose flux decrement between the F775W i′-band and F850LP z′-band arises from Lyman-α absorption, the number of detected candidates provides a valuable upper limit to the unextincted star formation rate at this redshift. We demonstrate that the increased depth of UDF enables us to reach an 8 σ limiting magnitude of z′AB= 28.5 (equivalent to 1.5 h−270 M⊙ yr−1 at z= 6.1, or 0.1 L⋆UV for the z≈ 3U-drop population), permitting us to address earlier ambiguities arising from the unobserved form of the luminosity function. We identify 54 galaxies (and only one star) at z′AB 1.3 over the deepest 11-arcmin2 portion of the UDF. The characteristic luminosity (L⋆) is consistent with values observed at z≈ 3. The faint end slope (α) is less well constrained, but is consistent with only modest evolution. The main change appears to be in the number density (Φ*). Specifically, and regardless of possible contamination from cool stars and lower-redshift sources, the UDF data support our previous result that the star formation rate at z≈ 6 was approximately six times less than at z≈ 3. This declining comoving star formation rate [0.005 h70 M⊙ yr−1 Mpc−3 at z≈ 6 at LUV > 0.1 L⋆ for a Salpeter initial mass function (IMF)] poses an interesting challenge for models which suggest that LUV > 0.1 L⋆ star-forming galaxies at z≃ 6 reionized the Universe. The short-fall in ionizing photons might be alleviated by galaxies fainter than our limit, or a radically different IMF. Alternatively, the bulk of reionization might have occurred at z≫ 6.

Spectral population synthesis including massive binaries

We have constructed a new code to produce synthetic spectra of stellar populations that includes massive binaries. We have tested this code against the broad-band colours of unresolved young massive stellar clusters in nearby galaxies, the equivalent widths of the Red and Blue Wolf–Rayet bumps in star-forming Sloan Digital Sky Survey galaxies and the ultraviolet and optical spectra of the star-forming regions Tol-A and B in NGC 5398. In each case, we find a good agreement between our models and observations. We find that in general binary populations are bluer and have fewer red supergiants, and thus significantly less flux in the I band and at longer wavelengths, than single star populations. Also we find that Wolf–Rayet stars occur over a wider range of ages up to 107 yr in a stellar population including binaries, increasing the ultraviolet flux and Wolf–Rayet spectral features at later times. In addition, we find that nebula emission contributes significantly to these observed properties and must be considered when comparing stellar models with observations of unresolved stellar populations. We conclude that incorporation of massive stellar binaries can improve the agreement between observations and synthetic spectral synthesis codes, particularly for systems with young stellar populations.

Lyman break galaxies and the star formation rate of the Universe at z≈ 6

We determine the space density of UV-luminous starburst galaxies at z≈ 6 using deep HST ACS SDSS-i′ (F775W) and SDSS-z′ (F850LP) and VLT ISAAC J and Ks band imaging of the Chandra Deep Field South. We find eight galaxies and one star with (i′−z′) > 1.5 to a depth of z′AB= 25.6 (an 8σ detection in each of the 3 available ACS epochs). This corresponds to an unobscured star formation rate of ≈15 h−270 M⊙ yr−1 at z= 5.9, equivalent to L* for the Lyman-break population at z= 3–4 (ΩΛ= 0.7, ΩM= 0.3). We are sensitive to star-forming galaxies at 5.6 ≲z≲ 7.0 with an effective comoving volume of ≈1.8 × 105h−370 Mpc3 after accounting for incompleteness at the higher redshifts due to luminosity bias. This volume should encompass the primeval subgalactic-scale fragments of the progenitors of about a thousand L* galaxies at the current epoch. We determine a volume-averaged global star formation rate of (6.7 ± 2.7) × 10−4h70 M⊙ yr−1 Mpc−3 at z∼ 6 from rest-frame UV selected starbursts at the bright end of the luminosity function: this is a lower limit because of dust obscuration and galaxies below our sensitivity limit. This measurement shows that at z∼ 6 the star formation density at the bright end is a factor of ∼6 times less than that determined by Steidel et al. for a comparable sample of UV-selected galaxies at z= 3–4, and so extends our knowledge of the star formation history of the Universe to earlier times than previous work and into the epoch where reionization may have occurred.

Stellar population effects on the inferred photon density at reionization

The relationship between stellar populations and the ionizing flux with which they irradiate their surroundings has profound implications for the evolution of the intergalactic medium (IGM). We quantify the ionizing flux arising from synthetic stellar populations which incorporate the evolution of interacting binary stars. We determine that these show ionizing flux boosted by 60 per cent at 0.05 ≤ Z ≤ 0.3 Z⊙ and a more modest 10–20 per cent at near-solar metallicities relative to star-forming populations in which stars evolve in isolation. The relation of ionizing flux to observables such as 1500 A continuum and ultraviolet spectral slope is sensitive to attributes of the stellar population including age, star formation history and initial mass function (IMF). For a galaxy forming 1 M⊙ yr−1, observed at >100 Myr after the onset of star formation, we predict a production rate of photons capable of ionizing hydrogen, Nion = 1.4 × 1053 s−1 at Z = Z⊙ and 3.5 × 1053 s−1 at 0.1 Z⊙, assuming a Salpeter-like IMF. We evaluate the impact of these issues on the ionization of the IGM, finding that the known galaxy populations can maintain the ionization state of the Universe back to z ∼ 9, assuming that their luminosity functions continue to MUV = −10, and that constraints on the IGM at z ∼ 2–5 can be satisfied with modest Lyman-continuum photon escape fractions of 4–24 per cent depending on assumed metallicity.

Spitzer imaging of i'-drop galaxies: Old stars at z ~ 6

We present new evidence for mature stellar populations with ages >100 Myr in massive galaxies (Mstellar > 1010 M⊙) seen at a time when the Universe was less than 1 Gyr old. We analyse the prominent detections of two z≈ 6 star-forming galaxies (SBM03#1 and #3) made at wavelengths corresponding to the rest-frame optical using the Infrared Array Camera camera onboard the Spitzer Space Telescope. We had previously identified these galaxies in Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) / Great Observatories Origins Deep Survey (GOODS) images of Chandra Deep Field South through the ‘i′-drop’ Lyman-break technique, and subsequently confirmed the identification spectroscopically with the Keck telescope. The new Spitzer photometry reveals significant Balmer/4000-A discontinuities, indicative of dominant stellar populations with ages >100 Myr. Fitting a range of population synthesis models (for normal initial mass functions) to the HST/Spitzer photometry yields ages of 250–650 Myr and implied formation redshifts zf≈ 7.5–13.5 in presently-accepted world models. Remarkably, our sources have best-fitting stellar masses of 1.3–3.8 × 1010 M⊙ (95 per cent confidence) assuming a Salpeter IMF. This indicates that at least some galaxies with stellar masses >20 per cent of those of a present-day L* galaxy had already assembled within the first Gyr after the Big Bang. We also deduce that the past average star formation rate must be comparable to the current observed rate (SFRUV~5–30 M⊙ yr−1), suggesting that there may have been more vigorous episodes of star formation in such systems at higher redshifts. Although a small sample, limited primarily by Spitzers detection efficiency, our result lends support to the hypothesis advocated in our earlier analyses of the Ultra Deep Field and GOODS HST/ACS data. The presence of established systems at z≈ 6 suggests that long-lived sources at earlier epochs (z > 7) played a key role in reionizing the Universe.

Near-infrared properties of i-drop galaxies in the Hubble Ultra Deep Field

We analyse near-infrared Hubble Space Telescope (HST)/Near-Infrared Camera and Multi-Object Spectrometer F110W (J) and F160W (H) band photometry of a sample of 27 i′-drop candidate z≃ 6 galaxies in the central region of the HST/Advanced Camera for Surveys Ultra Deep Field. The infrared colours of the 20 objects not affected by near neighbours are consistent with a high-redshift interpretation. This suggests that the low-redshift contamination of this i′-drop sample is smaller than that observed at brighter magnitudes, where values of 10–40 per cent have been reported. The J–H colours are consistent with a slope flat in fν(fλ∝λ−2), as would be expected for an unreddened starburst. However, there is evidence for a marginally bluer spectral slope (fλ∝λ−2.2), which is perhaps indicative of an extremely young starburst (∼10 Myr old) or a top heavy initial mass function and little dust. The low levels of contamination, median photometric redshift of z∼ 6.0 and blue spectral slope, inferred using the near-infrared data, support the validity of the assumptions in our earlier work in estimating the star formation rates, and that the majority of the i-drop candidates galaxies lie at z∼ 6.

STAR FORMATION IN THE EARLY UNIVERSE: BEYOND THE TIP OF THE ICEBERG

We present late-time Hubble Space Telescope (HST) imaging of the fields of six Swift gamma-ray bursts (GRBs) lying at 5.0 lsim z lsim 9.5. Our data include very deep observations of the field of the most distant spectroscopically confirmed burst, GRB 090423, at z = 8.2. Using the precise positions afforded by their afterglows, we can place stringent limits on the luminosities of their host galaxies. In one case, that of GRB 060522 at z = 5.11, there is a marginal excess of flux close to the GRB position which may be a detection of a host at a magnitude J AB ≈ 28.5. None of the others are significantly detected, meaning that all the hosts lie below L* at their respective redshifts, with star formation rates (SFRs) lsim 4 M ☉ yr-1 in all cases. Indeed, stacking the five fields with WFC3-IR data, we conclude a mean SFR <0.17 M ☉ yr-1 per galaxy. These results support the proposition that the bulk of star formation, and hence integrated UV luminosity, at high redshifts arises in galaxies below the detection limits of deep-field observations. Making the reasonable assumption that GRB rate is proportional to UV luminosity at early times allows us to compare our limits with expectations based on galaxy luminosity functions (LFs) derived from the Hubble Ultra-Deep Field and other deep fields. We infer that an LF, which is evolving rapidly toward steeper faint-end slope (α) and decreasing characteristic luminosity (L*), as suggested by some other studies, is consistent with our observations, whereas a non-evolving LF shape is ruled out at gsim 90% confidence. Although it is not yet possible to make stronger statements, in the future, with larger samples and a fuller understanding of the conditions required for GRB production, studies like this hold great potential for probing the nature of star formation, the shape of the galaxy LF, and the supply of ionizing photons in the early universe.

The ultraviolet properties of star-forming galaxies - I. HST WFC3 observations of very high redshift galaxies

The acquisition of deep near-IR imaging with Wide Field Camera 3 on the Hubble Space Telescope has provided the opportunity to study the very high redshift Universe. For galaxies up to z≈ 7.7 sufficient wavelength coverage exists to probe the rest-frame ultraviolet (UV) continuum without contamination from either Lyman α emission or the Lyman α break. In this work we use near-infrared (near-IR) imaging to measure the rest-frame UV continuum colours of galaxies at 4.7 < z < 7.7. We have carefully defined a colour–colour selection to minimize any inherent bias in the measured UV continuum slope for the drop-out samples. For the highest redshift sample (6.7 < z < 7.7), selected as zf850lp-band drop-outs, we find mean UV continuum colours approximately equal to zero (AB), consistent with a dust-free, solar metallicity, star-forming population (or a moderately dusty population of low metallicity). At lower redshift we find that the mean UV continuum colours of galaxies (over the same luminosity range) are redder, and that galaxies with higher luminosities are also slightly redder on average. One interpretation of this is that lower redshift and more luminous galaxies are dustier; however, this interpretation is complicated by the effects of the star formation history and metallicity and potentially the initial mass function on the UV continuum colours.

Spectroscopy of z ∼ 7 candidate galaxies: using Lyman α to constrain the neutral fraction of hydrogen in the high-redshift universe

Following our previous spectroscopic observations of z > 7 galaxies with Gemini/Gemini Near Infra-Red Spectrograph (GNIRS) and Very Large Telescope (VLT)/XSHOOTER, which targeted a total of eight objects, we present here our results from a deeper and larger VLT/FOcal Reducer and Spectrograph (FORS2) spectroscopic sample of Wide Field Camera 3 selected z > 7 candidate galaxies. With our FORS2 setup we cover the 737–1070 nm wavelength range, enabling a search for Lyman α in the redshift range spanning 5.06–7.80. We target 22 z-band dropouts and find no evidence of Lyman α emission, with the exception of a tentative detection (<5σ, which is our adopted criterion for a secure detection) for one object. The upper limits on Lyman α flux and the broad-band magnitudes are used to constrain the rest-frame equivalent widths for this line emission. We analyse our FORS2 observations in combination with our previous GNIRS and XSHOOTER observations, and suggest that a simple model where the fraction of high rest-frame equivalent width emitters follows the trend seen at z = 3-6.5 is inconsistent with our non-detections at z ∼ 7.8 at the 96 per cent confidence level. This may indicate that a significant neutral H I fraction in the intergalactic medium suppresses Lyman α, with an estimated neutral fraction χHI∼0.5, in agreement with other estimates.

Three Lyα emitters at z ≈ 6: Early GMOS/gemini data from the GLARE project

We report spectroscopic detection of three z ∼ 6 Lyα-emitting galaxies, in the vicinity of the Hubble Ultra Deep Field, from the early data of the Gemini Lyman Alpha at Reionisation Era (GLARE) project. Two objects, GLARE 3001 (z = 5.79) and GLARE 3011 (z = 5.94), are new detections and are fainter in z′ (z = 26.37 and 27.15) than any Lyman break galaxy previously detected in Lyα. A third object, GLARE 1042 (z = 5.83), has previously been detected in line emission from the ground; we report here a new spectroscopic continuum detection. Gemini/GMOS-South spectra of these objects, obtained using nod and shuffle, are presented together with a discussion of their photometric properties. All three objects were selected for spectroscopy via the i-drop Lyman break technique, the two new detections from the GOODS version 1.0 imaging data. The red i′-z′ colors and high equivalent widths of these objects suggest a high-confidence z > 5 Lyα identification of the emission lines. This brings the total number of known z > 5 galaxies within 9a of the Hubble Ultra Deep Field to four, of which three are at the same redshift (z = 5.8 within 2000 km s-1), suggesting the existence of a large-scale structure at this redshift.