The MUSE Hubble Ultra Deep Field Survey: I. Survey description, data reduction and source detection

Abstract: We present the MUSE Hubble Ultra Deep Survey, a mosaic of nine MUSE fields covering 90\% of the entire HUDF region with a 10-hour deep exposure time, plus a deeper 31-hour exposure in a single 1.15 arcmin2 field. The improved observing strategy and advanced data reduction results in datacubes with sub-arcsecond spatial resolution (0.65 arcsec at 7000 A) and accurate astrometry (0.07 arcsec rms). We compare the broadband photometric properties of the datacubes to HST photometry, finding a good agreement in zeropoint up to mAB=28 but with an increasing scatter for faint objects. We have investigated the noise properties and developed an empirical way to account for the impact of the correlation introduced by the 3D drizzle interpolation. The achieved 3 sigma emission line detection limit for a point source is 1.5 and 3.1 10-19 erg.s-1.cm-2 for the single ultra-deep datacube and the mosaic, respectively. We extracted 6288 sources using an optimal extraction scheme that takes the published HST source locations as prior. In parallel, we performed a blind search of emission line galaxies using an original method based on advanced test statistics and filter matching. The blind search results in 1251 emission line galaxy candidates in the mosaic and 306 in the ultradeep datacube, including 72 sources without HST counterparts (mAB>31). In addition 88 sources missed in the HST catalog but with clear HST counterparts were identified. This data set is the deepest spectroscopic survey ever performed. In just over 100 hours of integration time, it provides nearly an order of magnitude more spectroscopic redshifts compared to the data that has been accumulated on the UDF over the past decade. The depth and high quality of these datacubes enables new and detailed studies of the physical properties of the galaxy population and their environments over a large redshift range.

Analysis of the MUSE Hubble Ultra Deep Field Survey

The research paper titled "The MUSE Hubble Ultra Deep Field Survey: I. Survey description, data reduction and source detection" presents an extensive study conducted using the Multi Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope (VLT). Covering approximately 90% of the Hubble Ultra Deep Field (HUDF), this survey employs MUSE to achieve a depth and quality of data unprecedented in spectroscopic studies of this region.

The observations encompass a mosaic of nine MUSE fields with a median depth of 10 hours each, paired with an ultra-deep 31-hour single field exposure. This setup extends the coverage of the HUDF, making extensive use of MUSE's capability to perform deep integral field spectroscopy. The data reduction incorporated advancements such as a refined observational strategy and sophisticated self-calibration techniques. These improvements have resulted in significant enhancements in spatial resolution and signal-to-noise ratio.

Astrometric accuracy is robust, achieving a precision better than 0\farcs07 rms for galaxies brighter than AB 27. In terms of photometric accuracy, there is consistency with Hubble Space Telescope (HST) magnitudes up to AB 28, with increasing scatter for fainter objects. These improvements are crucial in adequately resolving sources and ensuring reliability in subsequent analyses of spectral features and inferred redshifts.

One notable advancement is the modeling of the Field Spatial Point Spread Function (FSF) using a Moffat profile, which is integral for assessing the spatial resolution across the field. Deriving accurate FSF estimates is paramount for reliable detection and characterization of faint astronomical sources.

Concerning noise properties, particularly important for emission line studies, the propagation and estimation methodologies used confirm that the variance estimated is consistent with the data, enabling a more precise flux measurement. The achieved 3σ line detection limits for point sources are (1.5 \times 10^{-19} \, \text{erg cm}^{-2} \text{s}^{-1}) for the ultra-deep field and (3.1 \times 10^{-19} \, \text{erg cm}^{-2} \text{s}^{-1}) for the mosaic. This sensitivity marks an improvement over previous datasets and provides a robust basis for identifying faint high-redshift galaxies.

The survey employed two complementary methods of source detection: HST-prior extraction using the Rafelski et al. (2015) catalog and a blind search approach leveraging the ORIGIN algorithm. The former yields 6288 extractions partly mitigated from blending issues via narrow-band imaging techniques. The latter detects sources independently by analyzing the MUSE datacube's entire spatial-spectral scope. By incorporating spatio-spectral filtering techniques, ORIGIN demonstrates considerable detection efficiency and purity, identifying 306 emission line sources in the ultra-deep field, of which some are notably without HST counterparts at magnitudes beyond (m_{AB}>31).

This work sets the stage for rich scientific inquiry into the formation and evolution of galaxies. The spectroscopic redshift catalog resulting from this survey substantially increases the redshift data available from the HUDF, enhancing the capability for studies into galaxy kinematics, star formation histories, and environmental influences over cosmic time. Furthermore, the methodologies developed hold promise for leveraging upcoming deep-field surveys and maximizing data utility from similar instruments, forecasting a refreshing scope for future advancements in extragalactic astronomy.