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Register a new userPredicting Future Space Near-IR Grism Surveys using the WFC3 Infrared Spectroscopic Parallels Survey
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We present near-infrared emission line counts and luminosity functions from the HST WFC3 Infrared Spectroscopic Parallels (WISP) program for 29 fields (0.037 deg^2) observed using both the G102 and G141 grisms. Altogether we identify 1048 emission line galaxies with observed equivalent widths greater than 40 Angstroms, 467 of which have multiple detected emission lines. The WISP survey is sensitive to fainter flux levels (3-5x10^-17 ergs/s/cm^2) than the future space near-infrared grism missions aimed at baryonic acoustic oscillation cosmology (1-4x10^-16 ergs/s/cm^2), allowing us to probe the fainter emission line galaxies that the shallower future surveys may miss. Cumulative number counts of 0.7<z<1.5 galaxies reach 10,000 deg^-2 above an H-alpha flux of 2x10^-16 ergs/s/cm^2. H-alpha-emitting galaxies with comparable [OIII] flux are roughly 5 times less common than galaxies with just H-alpha emission at those flux levels. Galaxies with low H-alpha/[OIII] ratios are very rare at the brighter fluxes that future near-infrared grism surveys will probe; our survey finds no galaxies with H-alpha/[OIII] < 0.95 that have H-alpha flux greater than 3x10^-16 ergs/s/cm^2. Our H-alpha luminosity function contains a comparable number density of faint line emitters to that found by the NICMOS near-infrared grism surveys, but significantly fewer (factors of 3-4 less) high luminosity emitters. We also find that our high redshift (z=0.9-1.5) counts are in agreement with the high redshift (z=1.47) narrow band H-alpha survey of HiZELS (Sobral et al. 2013), while our lower redshift luminosity function (z=0.3-0.9) falls slightly below their z=0.84 result. The evolution in both the H-alpha luminosity function from z=0.3--1.5 and the [OIII] luminosity function from z=0.7-2.3 is almost entirely in the L* parameter, which steadily increases with redshift over those ranges.
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HST is commonly thought of as an optical-IR imaging or UV-spectroscopy observatory. However, the advent of WFC3-IR made it possible to do slitless infrared spectroscopic surveys over an area significant for galaxy evolution studies (~0.15 deg^2). Slitless infrared spectroscopy is uniquely possible from space due to the reduced background. Redshift surveys with WFC3-IR offer probes of the astrophysics of the galaxy population at z=1-3 from line features, and the true redshift and spatial distribution of galaxies, that cannot be done with photometric surveys alone. While HST slitless spectroscopy is low spectral resolution, its high multiplex advantage makes it competitive with future ground based IR spectrographs, its flux calibration is stable, and its high spatial resolution allows measuring the spatial extent of emission lines, which only HST can do currently for large numbers of objects. A deeper slitless IR spectroscopic survey over hundreds of arcmin^2 (eg one or more GOODS fields) is one of the remaining niches for large galaxy evolution studies with HST, and would produce a sample of thousands of spectroscopically confirmed galaxies at 1<z<3 to H=25 and beyond, of great interest to a large community of investigators. Finally, although JWST multislit spectroscopy will outstrip HST in resolution and sensitivity, I believe it is critical to have a spectroscopic sample in hand before JWST flies. This applies scientifically, to be prepared for the questions we want to answer with JWST, and observationally, because JWSTs lifetime is limited and a classic problem in targeted spectroscopy has been the turn-around time for designing surveys and for deciding which classes of objects to target. This white paper is released publicly to stimulate open discussion of future large HST programs.
We present the discovery of three late type (>T4) brown dwarfs, including a probable Y dwarf, in the WFC3 Infrared Spectroscopic Parallels (WISP) Survey. We use the G141 grism spectra to determine the spectral types of the dwarfs and derive distance estimates based on a comparison with nearby T dwarfs with known parallaxes. These are the most distant spectroscopically confirmed T/Y dwarfs, with the farthest at an estimated distance of ~400 pc. We compare the number of cold dwarfs found in the WISP survey with simulations of the brown dwarf mass function. The number found is generally consistent with an initial stellar mass function dN/dM propto M^{-alpha} with alpha = 0.0--0.5, although the identification of a Y dwarf is somewhat surprising and may be indicative of either a flatter absolute magnitude/spectral type relation than previously reported or an upturn in the number of very late type brown dwarfs in the observed volume.
We present grism spectra of emission-line galaxies (ELGs) from 0.6-1.6 microns from the Wide Field Camera 3 on the Hubble Space Telescope. These new infrared grism data augment previous optical Advanced Camera for Surveys G800L 0.6-0.95 micron grism data in GOODS-South from the PEARS program, extending the wavelength covereage well past the G800L red cutoff. The ERS grism field was observed at a depth of 2 orbits per grism, yielding spectra of hundreds of faint objects, a subset of which are presented here. ELGs are studied via the Ha, [OIII], and [OII] emission lines detected in the redshift ranges 0.2<z<1.4, 1.2<z<2.2 and 2.0<z<3.3 respectively in the G102 (0.8-1.1 microns; R~210) and G141 (1.1-1.6 microns; R~130) grisms. The higher spectral resolution afforded by the WFC3 grisms also reveals emission lines not detectable with the G800L grism (e.g., [SII] and [SIII] lines). From these relatively shallow observations, line luminosities, star-formation rates, and grism spectroscopic redshifts are determined for a total of 48 ELGs to m(AB)~25 mag. Seventeen GOODS-South galaxies that previously only had photometric redshifts now have new grism-spectroscopic redshifts, in some cases with large corrections to the photometric redshifts (Delta(z)~0.3-0.5). Additionally, one galaxy had no previously-measured redshift but now has a secure grism-spectroscopic redshift, for a total of 18 new GOODS-South spectroscopic redshifts. The faintest source in our sample has a magnitude m(AB)=26.9 mag. The ERS grism data also reflect the expected trend of lower specific star formation rates for the highest mass galaxies in the sample as a function of redshift, consistent with downsizing and discovered previously from large surveys. These results demonstrate the remarkable efficiency and capability of the WFC3 NIR grisms for measuring galaxy properties to faint magnitudes and redshifts to z>2.
The future space missions Euclid and WFIRST-AFTA will use the Halpha emission line to measure the redshifts of tens of millions of galaxies. The Halpha luminosity function at z>0.7 is one of the major sources of uncertainty in forecasting cosmological constraints from these missions. We construct unified empirical models of the Halpha luminosity function spanning the range of redshifts and line luminosities relevant to the redshift surveys proposed with Euclid and WFIRST-AFTA. By fitting to observed luminosity functions from Halpha surveys, we build three models for its evolution. Different fitting methodologies, functional forms for the luminosity function, subsets of the empirical input data, and treatment of systematic errors are considered to explore the robustness of the results. Functional forms and model parameters are provided for all three models, along with the counts and redshift distributions up to z~2.5 for a range of limiting fluxes (F_Halpha>0.5 - 3 x 10^-16 erg cm^-2 s^-1) that are relevant for future space missions. For instance, in the redshift range 0.90<z<1.8, our models predict an available galaxy density in the range 7700--13300 and 2000--4800 deg^-2 respectively at fluxes above F_Halpha>1 and 2 x 10^-16 erg cm^-2 s^-1, and 32000--48000 for F_Halpha>0.5 x 10^-16 erg cm^-2 s^-1 in the extended redshift range 0.40<z<1.8. We also consider the implications of our empirical models for the total Halpha luminosity density of the Universe, and the closely related cosmic star formation history.
The mutually complementary Euclid and Roman galaxy redshift surveys will use Halpha- and [OIII]-selected emission line galaxies as tracers of the large scale structure at $0.9 lesssim z lesssim 1.9$ (Halpha) and $1.5 lesssim z lesssim 2.7$ ([OIII]). It is essential to have a reliable and sufficiently precise knowledge of the expected numbers of Halpha-emitting galaxies in the survey volume in order to optimize these redshift surveys for the study of dark energy. Additionally, these future samples of emission-line galaxies will, like all slitless spectroscopy surveys, be affected by a complex selection function that depends on galaxy size and luminosity, line equivalent width, and redshift errors arising from the misidentification of single emission-line galaxies. Focusing on the specifics of the Euclid survey, we combine two slitless spectroscopic WFC3-IR datasets -- 3D-HST+AGHAST and the WISP survey -- to construct a Euclid-like sample that covers an area of 0.56 deg$^2$ and includes 1277 emission line galaxies. We detect 1091 ($sim$3270 deg$^{-2}$) Halpha+[NII]-emitting galaxies in the range $0.9leq z leq 1.6$ and 162 ($sim$440 deg$^{-2}$) [OIII]$lambda$5007-emitters over $1.5leq z leq 2.3$ with line fluxes $geq 2 times 10^{-16}$ erg s$^{-1}$ cm$^{-2}$. The median of the Halpha+[NII] equivalent width distribution is $sim$250r{A}, and the effective radii of the continuum and Halpha+[NII] emission are correlated with a median of $sim$0.38 and significant scatter ($sigma sim $0.2$-$0.35). Finally, we explore the prevalence of redshift misidentification in future Euclid samples, finding potential contamination rates of $sim$14-20% and $sim$6% down to $2times 10^{-16}$ and $6 times 10^{-17}$ erg s$^{-1}$ cm$^{-2}$, respectively, though with increased wavelength coverage these percentages drop to nearly zero.
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