FATHOMER survey: III. Preliminary HI galaxy identification results

Abstract

We present the HI galaxy observation results of the FATHOMER (FAst neuTral HydrOgen intensity Mapping ExpeRiment), a pilot drift scan survey by the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The survey comprises 28 hours of observations over 7 nights in 2021, covering a $60\, °^2$ sky area in the frequency range 1.05-1.45 GHz. The HI galaxies are identified using both a matched-filtering algorithm and the SoFiA source-finding pipeline, which yield consistent detections. We derive the velocity width ($W_{50}$), flux density, and HI mass for detected galaxies. A total of 702 galaxies are identified with HI mass above $10^{6.2}\,{M_\odot}$, signal-to-noise ratio greater than 5, and redshift $z < 0.09$. Among these, 331 are previously known from the ALFALFA survey. Of the newly detected sources, 9 have spectroscopic confirmation from SDSS, 285 are matched to SDSS or DESI photometric data, and 77 lack optical counterparts--possible candidates for dark or faint galaxies. Comparison with ALFALFA shows that FAST enables detection of galaxies at higher redshifts and with lower HI fluxes, despite the radio frequency interference (RFI) and partial data masking. A preliminary HI mass function analysis reveals a higher characteristic mass and steeper low-mass slope than ALFALFA, indicating FAST's enhanced sensitivity to massive and distant HI systems. These results demonstrate FAST's strong potential for future deep HI surveys and highlight the importance of improved RFI mitigation and completeness correction.

Figures (13)

• Figure 1: The Hi galaxies detected within the FATHOMER field.
• Figure 2: Frequency dependence of the noise level (blue curve, left axis) and the corresponding fraction of RFI-masked pixels (red dashed curve, right axis). The bottom axis shows the observed frequency in MHz, while the top axis indicates the corresponding redshift of the Hi line. The secondary y-axis illustrates the fraction of RFI-masked pixels over the same frequency range. The gray curve represents the expected rms increase induced by RFI flagging, which is estimated via Eq. (\ref{['eq:rmsexp']}).
• Figure 3: Frequency spectra for galaxy AGCNr 191363 in FATHOMER (left) and ALFALFA (middle). The right panel is the sky map in FATHOMER. The signal in ALFALFA is relatively weak, with $\rm S/N=4.3$. In FATHOMER, we get a higher S/N ($\rm S/N=6.3$) and confirm that this signal should be real.
• Figure 4: Top: Comparison of the flux between FATHOMER and ALFALFA. The black dashed line represents the equality of two fluxes from two different surveys. All galaxies shown here have $\rm S/N>10$. Bottom: Histogram of $\rm \delta S_{21}$.
• Figure 5: Distribution of $\hat{S}_{21}^{3/2} {\rm d}N/{\rm d}\log \hat{S}_{21}$ versus $\log \hat{S}_{21}$ from the Hi galaxies with $\rm S/N \geq 6.5$, in the ALFALFA (blue) and FATHOMER (red) to evaluate the completeness. In the FATHOMER, we can join the Code 1 and Code 2 candidates. The vertical dashed lines represent the flux where the survey completeness is 90%. The 90% completeness is according to the fit of $\hat{S}_{21,90\%}$, and the correspond logarithmic values are 0.10 for FATHOMER and 0.034 for ALFALFA, respectively.