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Register a new userHST Grism-derived Forecasts for Future Galaxy Redshift Surveys
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Micaela B. Bagley
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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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Knowledge of the number density of H$alpha$ emitting galaxies is vital for assessing the scientific impact of the Euclid and WFIRST missions. In this work we present predictions from a galaxy formation model, Galacticus, for the cumulative number counts of H$alpha$-emitting galaxies. We couple Galacticus to three different dust attenuation methods and examine the counts using each method. A $chi^2$ minimisation approach is used to compare the model predictions to observed galaxy counts and calibrate the dust parameters. We find that weak dust attenuation is required for the Galacticus counts to be broadly consistent with the observations, though the optimum dust parameters return large values for $chi^2$, suggesting that further calibration of Galacticus is necessary. The model predictions are also consistent with observed estimates for the optical depth and the H$alpha$ luminosity function. Finally we present forecasts for the redshift distributions and number counts for two Euclid-like and one WFIRST-like survey. For a Euclid-like survey with redshift range $0.9leqslant zleqslant 1.8$ and H$alpha+{rm [NII]}$ blended flux limit of $2times 10^{-16}{rm erg},{rm s}^{-1},{rm cm}^{-2}$ we predict a number density between 3900--4800 galaxies per square degree. For a WFIRST-like survey with redshift range $1leqslant zleqslant 2$ and blended flux limit of $1times 10^{-16}{rm erg},{rm s}^{-1},{rm cm}^{-2}$ we predict a number density between 10400--15200 galaxies per square degree.
We present deep spectroscopic observations of a Lyman-break galaxy candidate (hereafter MACS1149-JD) at $zsim9.5$ with the $textit{Hubble}$ Space Telescope ($textit{HST}$) WFC3/IR grisms. The grism observations were taken at 4 distinct position angles, totaling 34 orbits with the G141 grism, although only 19 of the orbits are relatively uncontaminated along the trace of MACS1149-JD. We fit a 3-parameter ($z$, F160W mag, and Ly$alpha$ equivalent width) Lyman-break galaxy template to the three least contaminated grism position angles using an MCMC approach. The grism data alone are best fit with a redshift of $z_{mathrm{grism}}=9.53^{+0.39}_{-0.60}$ ($68%$ confidence), in good agreement with our photometric estimate of $z_{mathrm{phot}}=9.51^{+0.06}_{-0.12}$ ($68%$ confidence). Our analysis rules out Lyman-alpha emission from MACS1149-JD above a $3sigma$ equivalent width of 21 AA{}, consistent with a highly neutral IGM. We explore a scenario where the red $textit{Spitzer}$/IRAC $[3.6] - [4.5]$ color of the galaxy previously pointed out in the literature is due to strong rest-frame optical emission lines from a very young stellar population rather than a 4000 AA{} break. We find that while this can provide an explanation for the observed IRAC color, it requires a lower redshift ($zlesssim9.1$), which is less preferred by the $textit{HST}$ imaging data. The grism data are consistent with both scenarios, indicating that the red IRAC color can still be explained by a 4000 AA{} break, characteristic of a relatively evolved stellar population. In this interpretation, the photometry indicate that a $340^{+29}_{-35}$ Myr stellar population is already present in this galaxy only $sim500~mathrm{Myr}$ after the Big Bang.
Using the ligthcone from the cosmological hydrodynamical simulation Horizon-AGN, we produced a photometric catalogue over $0<z<4$ with apparent magnitudes in COSMOS, DES, LSST-like, and Euclid-like filters at depths comparable to these surveys. The virtual photometry accounts for the complex star formation history and metal enrichment of Horizon-AGN galaxies, and consistently includes magnitude errors, dust attenuation and absorption by inter-galactic medium. The COSMOS-like photometry is fitted in the same configuration as the COSMOS2015 catalogue. We then quantify random and systematic errors of photometric redshifts, stellar masses, and star-formation rates (SFR). Photometric redshifts and redshift errors capture the same dependencies on magnitude and redshift as found in COSMOS2015, excluding the impact of source extraction. COSMOS-like stellar masses are well recovered with a dispersion typically lower than 0.1 dex. The simple star formation histories and metallicities of the templates induce a systematic underestimation of stellar masses at $z<1.5$ by at most 0.12 dex. SFR estimates exhibit a dust-induced bimodality combined with a larger scatter (typically between 0.2 and 0.6 dex). We also use our mock catalogue to predict photometric redshifts and stellar masses in future imaging surveys. We stress that adding Euclid near-infrared photometry to the LSST-like baseline improves redshift accuracy especially at the faint end and decreases the outlier fraction by a factor $sim$2. It also considerably improves stellar masses, reducing the scatter up to a factor 3. It would therefore be mutually beneficial for LSST and Euclid to work in synergy.
We present a study of photometric redshift accuracy in the 3D-HST photometric catalogs, using 3D-HST grism redshifts to quantify and dissect trends in redshift accuracy for galaxies brighter than $H_{F140W}<24$ with an unprecedented and representative high-redshift galaxy sample. We find an average scatter of $0.0197pm0.0003(1+z)$ in the Skelton et al. (2014) photometric redshifts. Photometric redshift accuracy decreases with magnitude and redshift, but does not vary monotonically with color or stellar mass. The 1-$sigma$ scatter lies between $0.01-0.03$(1+z) for galaxies of all masses and colors below $z<2.5$ (for $H_{F140W}{<}24$), with the exception of a population of very red ($U-V > 2$), dusty star-forming galaxies for which the scatter increases to $sim0.1(1+z)$. Although the overall photometric redshift accuracy for quiescent galaxies is better than for star-forming galaxies, scatter depends more strongly on magnitude and redshift than on galaxy type. We verify these trends using the redshift distributions of close pairs and extend the analysis to fainter objects, where photometric redshift errors further increase to $sim0.046(1+z)$ at $H_{F160W}=26$. We demonstrate that photometric redshift accuracy is strongly filter-dependent and quantify the contribution of multiple filter combinations. We evaluate the widths of redshift probability distribution functions and find that error estimates are underestimated by a factor of $sim1.1-1.6$, but that uniformly broadening the distribution does not adequately account for fitting outliers. Finally, we suggest possible applications of these data in planning for current and future surveys and simulate photometric redshift performance in the LSST, DES, and combined DES and VHS surveys.
We exploit the synergy between low-resolution spectroscopy and photometric redshifts to study environmental effects on galaxy evolution in slitless spectroscopic surveys from space. As a test case, we consider the future Euclid Deep survey (~40deg$^2$), which combines a slitless spectroscopic survey limited at H$alpha$ flux $geq5times 10^{-17}$ erg cm$^{-2}$ s$^{-1}$ and a photometric survey limited in H-band ($Hleq26$). We use Euclid-like galaxy mock catalogues, in which we anchor the photometric redshifts to the 3D galaxy distribution of the available spectroscopic redshifts. We then estimate the local density contrast by counting objects in cylindrical cells with radius from 1 to 10 h$^{-1}$Mpc over the redshift range 0.9<z<1.8. We compare this density field with the one computed in a mock catalogue with the same depth as the Euclid Deep survey (H=26) but without redshift measurement errors. We find that our method successfully separates high from low density environments (the last from the first quintile of the density distribution), with higher efficiency at low redshift and large cell: the fraction of low density regions mistaken by high density peaks is <1% for all scales and redshifts explored, but for scales of 1 h$^{-1}$Mpc for which is a few percent. These results show that we can efficiently study environment in photometric samples if spectroscopic information is available for a smaller sample of objects that sparsely samples the same volume. We demonstrate that these studies are possible in the Euclid Deep survey, i.e. in a redshift range in which environmental effects are different from those observed in the local universe, hence providing new constraints for galaxy evolution models.
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