Martin Kümmel

EVIDENCE FOR A MASSIVE POSTSTARBURST GALAXY AT z 6.5

We describe results from a search for high-redshift J-band dropout galaxies in the portion of the Great Observatories Origins Deep Survey (GOODS) southern field that is covered by extremely deep imaging from the Hubble Ultradeep Field (HUDF). Using observations at optical, near-infrared, and mid-infrared wavelengths from the Hubble and Spitzer Space Telescopes and the European Southern Observatory Very Large Telescope, we find one particularly remarkable candidate, which we designate HUDF-JD2. Its spectral energy distribution has distinctive features that are consistent with those of a galaxy at z ~ 6.5, observed several hundred million years after a powerful burst of star formation that produced a stellar mass of 6 × 1011 M☉ (for a Salpeter IMF). We interpret a prominent photometric break between the near-infrared and Spitzer bandpasses as the 3646 A Balmer discontinuity. The best-fitting models have low reddening and ages of several hundred Myr, placing the formation of the bulk of the stars at z > 9. Alternative models of dusty galaxies at z ≈ 2.5 are possible, but provide significantly poorer fits to the photometric data. The object is detected with Spitzer at 24 μm. We consider interpretations of the 24 μm emission as originating either from an obscured active nucleus or from star formation and find that the 24 μm detection does not help to uniquely discriminate between the z = 6.5 and 2.5 alternatives. We present optical and near-infrared spectroscopy that has, thus far, failed to detect any spectral features. This nondetection helps limit the solution in which the galaxy is a starburst or active galaxy at z ≈ 2.5, but does not rule it out. If the high-redshift interpretation is correct, HUDF-JD2 is an example of a galaxy that formed by a process strongly resembling traditional models of monolithic collapse, in which a very large mass of stars formed within a remarkably short period of time, at very high redshift.

Empirical Hα emitter count predictions for dark energy surveys

Future galaxy redshift surveys aim to measure cosmological quantities from the galaxy power spectrum. A prime example is the detection of baryonic acoustic oscillations, providing a standard ruler to measure the dark energy equation of state, w(z), to high precision. The strongest practical limitation for these experiments is how quickly accurate redshifts can be measured for sufficient galaxies to map the large-scale structure. A promising strategy is to target emission-line (i.e. star-forming) galaxies at high redshift (z∼ 0.5–2); not only is the space density of this population increasing out to z∼ 2, but also emission lines provide an efficient method of redshift determination. Motivated by the prospect of future dark energy surveys targeting Hα emitters at near-infrared wavelengths (i.e. z > 0.5), we use the latest empirical data to model the evolution of the Hα luminosity function out to z∼ 2 and thus provide predictions for the abundance of Hα emitters for practical limiting fluxes. We caution that the estimates presented in this work must be tempered by an efficiency factor, e, giving the redshift success rate from these potential targets. For a range of practical efficiencies and limiting fluxes, we provide an estimate of nP0.2 where n is the 3D galaxy number density and P0.2 is the galaxy power spectrum evaluated at k= 0.2 h Mpc−1. Ideal surveys must provide nP0.2>1 in order to balance shot-noise and cosmic variance errors. We show that a realistic emission-line survey (e= 0.5) could achieve nP0.2=1 out to z∼ 1.5 with a limiting flux of 10−16 erg s−1 cm−2. If the limiting flux is a factor of 5 brighter, then this goal can only be achieved out to z∼ 0.5, highlighting the importance of survey depth and efficiency in cosmological redshift surveys.

GRAPES, Grism Spectroscopy of the Hubble Ultra Deep Field: Description and Data Reduction

We present deep unbiased spectroscopy of the Hubble Ultra Deep Field (UDF) carried out using the slitless grism spectroscopy mode of the Advanced Camera for Surveys on the Hubble Space Telescope (HST). The Grism ACS Program for Extragalactic Science (GRAPES) achieves continuum detection as faint as zAB = 27.2 using 40 orbits (9.2 × 104 s) on HST. The data were taken at four orientation angles to correct for the overlap of spectra. GRAPES data provide a unique, uninterrupted, low-resolution (R = 100) spectral coverage for 5500 A < λ < 10500 A and allow us to detect high-redshift galaxies at 4 < z < 7 whether they have Lyα lines or just show the Lyman break, as well as find low-luminosity active galactic nuclei in an unbiased fashion. This paper describes in detail the observations and the data reduction and examines the quality of the extracted spectra. Subsequent papers will deal with the analysis of the data. The extracted and calibrated GRAPES spectra will be available from MAST at STScI.

THE ROAD TO THE RED SEQUENCE: A DETAILED VIEW OF THE FORMATION OF A MASSIVE GALAXY AT z ∼ 2

Over half of the census of massive galaxies at z 2 are dominated by quiescent stellar populations. The formation mechanism for these galaxies is still under debate, with models relying either on massive and early mergers or cold accretion. It is therefore imperative to understand in detail the properties of these galaxies. We present here a detailed analysis of the star formation history (SFH) of FW4871, a massive galaxy at z = 1.893 ± 0.002. We compare rest-frame optical and NUV slitless grism spectra from the Hubble Space Telescope with a large set of composite stellar populations to constrain the underlying SFH. Even though the morphology features prominent tidal tails, indicative of a recent merger, there is no sign of ongoing star formation within an aperture encircling one effective radius, which corresponds to a physical extent of 2.6kpc. A model assuming truncation of an otherwise constant SFH gives a formation epoch z F 10 with a truncation after 2.7Gyr, giving a mass-weighted age of 1.5Gyr and a stellar mass of (0.8-3) × 10 11 M · (the intervals representing the output from different population synthesis models), implying star formation rates of 30-110M · yr -1. A more complex model including a recent burst of star formation places the age of the youngest component at 145 +450- 70Myr, with a mass contribution lower than 20%, and a maximum amount of dust reddening of E(B - V) < 0.4mag (95% confidence levels). This low level of dust reddening is consistent with the low emission observed at 24 μm, corresponding to rest-frame 8 μm, where polycyclic aromatic hydrocarbon emission should contribute significantly if a strong formation episode were present. The color profile of FW4871 does not suggest a significant radial trend in the properties of the stellar populations out to 3 R e. We suggest that the recent merger that formed FW4871 is responsible for the quenching of its star formation.

Redshifts of Emission-Line Objects in the Hubble Ultra Deep Field

We present redshifts for 115 emission-line objects in the Hubble Ultra Deep Field identified through the Grism ACS Program for Extragalactic Science (GRAPES) project using the slitless grism spectroscopy mode of the Advanced Camera for Surveys on the Hubble Space Telescope (HST). The sample was selected by an emission-line search on all extracted one-dimensional GRAPES spectra. We identify the emission lines using line wavelength ratios where multiple lines are detected in the grism wavelength range (5800 A ≲ λ ≲ 9600 A), and using photometric redshift information where multiple lines are unavailable. We then derive redshifts using the identified lines. Our redshifts are accurate to δz ≈ 0.009, based on both statistical uncertainty estimates and comparison with published ground-based spectra. Over 40% of our sample is fainter than typical magnitude limits for ground-based spectroscopy (with i_(AB) > 25 mag). Such emission lines would likely remain undiscovered without our deep survey. The emission-line objects fall into three categories: (1) most are low- to moderate-redshift galaxies (0 ≤ z ≤ 2), including many actively star-forming galaxies with strong H II regions; (2) nine are high-redshift (4 ≤ z ≤ 7) Lyα emitters; and (3) at least three are candidate active galactic nuclei.

A Lyman Break Galaxy in the Epoch of Reionization from Hubble Space Telescope Grism Spectroscopy

We present observations of a luminous galaxy at z = 6.573—the end of the reionization epoch—which has been spectroscopically confirmed twice. The first spectroscopic confirmation comes from slitless Hubble Space Telescope Advanced Camera for Surveys grism spectra from the PEARS survey (Probing Evolution And Reionization Spectroscopically), which show a dramatic continuum break in the spectrum at rest frame 1216 A. The second confirmation is done with Keck + DEIMOS. The continuum is not clearly detected with ground-based spectra, but high wavelength resolution enables the Lyα emission line profile to be determined. We compare the line profile to composite line profiles at z = 4.5. The Lyα line profile shows no signature of a damping wing attenuation, confirming that the intergalactic gas is ionized at z = 6.57. Spectra of Lyman breaks at yet higher redshifts will be possible using comparably deep observations with IR-sensitive grisms, even at redshifts where Lyα is too attenuated by the neutral intergalactic medium to be detectable using traditional spectroscopy from the ground.

Stellar properties of z ~ 1 Lyman-break galaxies from ACS slitless grism spectra

Lyman-break galaxies are now regularly found in the high redshift Universe by searching for the break in the galaxy spectrum caused by the Lyman-limit redshifted into the optical or even near-IR. At lower redshift, this break is covered by the GALEX UV channels and small samples of z ∼ 1 LBGs have been presented in the literature. Here we give results from fitting the spectral energy distributions of a small sub-set of low redshift LBGs and demonstrate the advantage of including photometric points derived from HST ACS slitless grism observations. The results show these galaxies to have very young, star forming populations, while still being massive and dusty. LBGs at low and high redshift show remarkable similarities in their properties, indicating that the LBG selection method picks similar galaxies throughout the Universe.

The Hubble Legacy Archive ACS grism data

A public release of slitless spectra, obtained with ACS/WFC and the G800L grism, is presented. Spectra were automatically extracted in a uniform way from 153 archival fields (or “associations”) distributed across the two Galactic caps, covering all observations to 2008. The ACS G800L grism provides a wavelength range of 0.55−1.00 μm, with a dispersion of 40 A/pixel and a resolution of ∼80 A for point-like sources. The ACS G800L images and matched direct images were reduced with an automatic pipeline that handles all steps from archive retrieval, alignment and astrometric calibration, direct image combination, catalogue generation, spectral extraction and collection of metadata. The large number of extracted spectra (73, 581) demanded automatic methods for quality control and an automated classification algorithm was trained on the visual inspection of several thousand spectra. The final sample of quality controlled spectra includes 47 919 datasets (65% of the total number of extracted spectra) for 32 149 unique objects, with a median iAB-band magnitude of 23.7, reaching 26.5 AB for the faintest objects. Each released dataset contains science-ready 1D and 2D spectra, as well as multi-band image cutouts of corresponding sources and a useful preview page summarising the direct and slitless data, astrometric and photometric parameters. This release is part of the continuing effort to enhance the content of the Hubble Legacy Archive (HLA) with highly processed data products which significantly facilitate the scientific exploitation of the Hubble data. In order to characterize the slitless spectra, emission-line flux and equivalent width sensitivity of the ACS data were compared with public ground-based spectra in the GOODS-South field. An example list of emission line galaxies with two or more identified lines is also included, covering the redshift range 0.2−4.6. Almost all redshift determinations outside of the GOODS fields are new. The scope of science projects possible with the ACS slitless release data is large, from studies of Galactic stars to searches for high redshift galaxies.

Slitless Grism Spectroscopy with the Hubble Space Telescope Advanced Camera for Surveys

The Advanced Camera for Surveys on-board HST is equipped with a set of one grism and three prisms for low-resolution, slitless spectroscopy in the range 1150 u to 10500 u The G800L grism provides optical spectroscopy between 5500 u and > 1 µm with a mean dispersion of 39 u and 24 u (in the first spectral order) when coupled with the Wide Field and the High Resolution Channels, respectively. Given the lack of any on-board calibration lamps for wavelength and narrow band flat-fielding, the G800L grism can only be calibrated using astronomical targets. In this paper, we describe the strategy used to calibrate the grism in orbit, with special attention to the treatment of the field dependence of the grism flat-field, wavelength solution and sensitivity in both Channels. Subject headings: instrumentation: spectrographs, methods: data analysis, techniques: spectroscopic

HST/WFC3 in-orbit grism performance

The Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) is fitted with three grisms for slitless spectroscopy. In the UVIS channel there is one grism, G280, for the near-UV to visible range (200 - 400nm; 1.4nm/pix). The IR channel has two grisms: G102 for the shorter (800-1150nm; 2.45nm/pix) and G141 for the longer (1100-1700nm; 4.65nm/pix) NIR wavelengths. Using Servicing Mission Observatory Verification (SMOV) and Cycle 17 calibration data we have assessed the performance of the grisms. We have measured the fielddependent trace locations and dispersion solutions and determined the throughputs. The trace and wavelength solutions for the IR grisms were found to be linear functions, varying smoothly across the field of view. The UVIS grism exhibits a highly bent trace and significantly non-linear dispersion solutions. The maximum throughputs for the G102 and G141 grisms, including the telescope optics, are 41% at 1100 nm and 48% at 1450 nm, respectively. Limiting magnitudes at S/N=5 and a 1h exposure are JAB=22.6 and HAB=22.9 for the G102 and G141 grisms, respectively. The calibration results are published in the form of sensitivity and configuration files that can be used with our dedicated extraction software aXe to reduce WFC3 slitless data.