FastSound is a galaxy redshift survey using the near-infrared Fiber Multi-Object Spectrograph (FMOS) mounted on the Subaru Telescope, targeting H$alpha$ emitters at $z sim 1.18$--$1.54$ down to the sensitivity limit of H$alpha$ flux $sim 2 times 10^{
-16} rm erg cm^{-2} s^{-1}$. The primary goal of the survey is to detect redshift space distortions (RSD), to test General Relativity by measuring the growth rate of large scale structure and to constrain modified gravity models for the origin of the accelerated expansion of the universe. The target galaxies were selected based on photometric redshifts and H$alpha$ flux estimates calculated by fitting spectral energy distribution (SED) models to the five optical magnitudes of the Canada France Hawaii Telescope Legacy Survey (CFHTLS) Wide catalog. The survey started in March 2012, and all the observations were completed in July 2014. In total, we achieved $121$ pointings of FMOS (each pointing has a $30$ arcmin diameter circular footprint) covering $20.6$ deg$^2$ by tiling the four fields of the CFHTLS Wide in a hexagonal pattern. Emission lines were detected from $sim 4,000$ star forming galaxies by an automatic line detection algorithm applied to 2D spectral images. This is the first in a series of papers based on FastSound data, and we describe the details of the survey design, target selection, observations, data reduction, and emission line detections.
We describe the development of automated emission line detection software for the Fiber Multi-Object Spectrograph (FMOS), which is a near-infrared spectrograph fed by $400$ fibers from the $0.2$ deg$^2$ prime focus field of view of the Subaru Telesco
pe. The software, FIELD (FMOS software for Image-based Emission Line Detection), is developed and tested mainly for the FastSound survey, which is targeting H$alpha$ emitting galaxies at $z sim 1.3$ to measure the redshift space distortion as a test of general relativity beyond $z sim 1$. The basic algorithm is to calculate the line signal-to-noise ratio ($S/N$) along the wavelength direction, given by a 2-D convolution of the spectral image and a detection kernel representing a typical emission line profile. A unique feature of FMOS is its use of OH airglow suppression masks, requiring the use of flat-field images to suppress noise around the mask regions. Bad pixels on the detectors and pixels affected by cosmic-rays are efficiently removed by using the information obtained from the FMOS analysis pipeline. We limit the range of acceptable line-shape parameters for the detected candidates to further improve the reliability of line detection. The final performance of line detection is tested using a subset of the FastSound data; the false detection rate of spurious objects is examined by using inverted frames obtained by exchanging object and sky frames. The false detection rate is $< 1$% at $S/N > 5$, allowing an efficient and objective emission line search for FMOS data at the line flux level of $gtrsim 1.0 times 10^{-16}$[erg/cm$^2$/s].
The efficient selection of high-redshift emission galaxies is important for future large galaxy redshift surveys for cosmology. Here we describe the target selection methods for the FastSound project, a redshift survey for H alpha emitting galaxies a
t z=1.2-1.5 using Subaru/FMOS to measure the linear growth rate fsigma 8 via Redshift Space Distortion (RSD) and constrain the theory of gravity. To select ~400 target galaxies in the 0.2 deg^2 FMOS field-of-view from photometric data of CFHTLS-Wide (u*griz), we test several different methods based on color-color diagrams or photometric redshift estimates from spectral energy distribution (SED) fitting. We also test the improvement in selection efficiency that can be achieved by adding near-infrared data from the UKIDSS DXS (J). The success rates of H alpha detection with FMOS averaged over two observed fields using these methods are 11.3% (color-color, optical), 13.6% (color-color, optical+NIR), 17.3% (photo-z, optical), and 15.1% (photo-z, optical+NIR). Selection from photometric redshifts tends to give a better efficiency than color-based methods, although there is no significant improvement by adding J band data within the statistical scatter. We also investigate the main limiting factors for the success rate, by using the sample of the HiZELS H alpha emitters that were selected by narrow-band imaging. Although the number density of total H alpha emitters having higher H alpha fluxes than the FMOS sensitivity is comparable with the FMOS fiber density, the limited accuracy of photometric redshift and H alpha flux estimations have comparable effects on the success rate of <~20% obtained from SED fitting.
