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hyacinthwj edited this page Feb 20, 2026 · 3 revisions

Promise of FLOWER

The FLOWER, FAST Long-exposure Observation of Wide-field HI Emission around/behind Resolved galaxies, is a second-round Priority and Key Projects at FAST. It is assigned 500 hours (2025-2027) to

  1. map the HI of a few tens central galaxies, together with their surroundings within the virial radius, of Milky-Way like halos in the local universe ($d$ <20 Mpc). The $3\sigma$ NHI depth (20 km/s line) will be $10^{17.8} \mathrm{cm}^{-2}$.

  2. blindly survey the HI of ~250 ${\rm deg}^2$ in the universe volume of redshift >0.01 behind those local halos. The $3\sigma$ MHI depth (100 km/s line) will be $10^{5.7}\times(d/5 \mathrm{Mpc})^2 M_{\odot}$.

Link and Difference to Its Pilot Survey FEASTS

Its pilot survey FEASTS studied properties of diffuse HI in/around nearby HI-rich galaxies. It combined FAST images with interferometric images to obtain full HI images, and revisit scaling relations of HI surface densities. FLOWER will extend and expand the FEASTS science, since its design differs from that of FEASTS in the following aspect:

  1. The FLOWER sample is selected purely based on halo mass (group mass), central galaxy stellar mass, systematic velocity, and distance of foreground targets; the FEASTS sample was selected by the HI fluxes.

  2. The FLOWER field center and size will be determined by the halo (group) properties; those of FEASTS were by the HI disk properties.

  3. The FLOWER expands the exploration to the background sky in the redshift range z=0.01-0.06. There, the survey, which is essentially a blind survey of integral HI spectra of galaxies, will reach >50% detection completeness for over 10,000 galaxies in thousands of completely covered galaxy groups. For the same stellar mass range, it will expand the previous xGASS by at least 4 times, and extend the xGASS stellar mass range to the low and high ends.

The FEASTS data will add to FLOWER in the following aspects, for the two surveys have the same survey mode and depth and will make a uniform dataset.

  1. it will deepen the $N_{\rm HI}$ depth of HI images by 1.414 times for ~20 foreground galaxies, making the disk-halo interface more visible.
  2. it will increase the sky area of the background integral HI survey by 1.35 times, further reducing cosmic variance and increasing statiscs.

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