Apertif HI Survey Overview

• Apertif HI Survey is an untargeted, interferometric study using WSRT with phased array feed technology to map HI content in the northern sky.
• The survey employs advanced methods, including SoFiA-2 source-finding and adaptive companion criteria, to robustly identify and quantify dwarf galaxy interactions.
• It reveals that gas-rich dwarf pairs show enhanced star formation, with near-equal mass pairs boosting SFR by +0.33 dex, highlighting dynamic evolutionary processes.

The Apertif HI Survey is an untargeted, interferometric survey of neutral atomic hydrogen (HI) in the northern sky, conducted with the Westerbork Synthesis Radio Telescope (WSRT) upgraded with phased array feed (PAF) technology. By exploiting a wide instantaneous field of view, high angular resolution, and advanced digital backend, the Apertif HI Survey delivers a transformative capability for mapping HI content, kinematics, and the incidence of galaxy interactions across a broad dynamic range of galaxy mass, with an emphasis on low-mass and gas-rich systems. The survey provides new empirical constraints on galaxy evolution, star formation, and the dynamics of dwarf galaxy multiples, which have been previously inaccessible owing to the limitations of optical and single-dish HI studies (Šiljeg et al., 11 Sep 2025).

1. Survey Design, Sample Selection, and Technical Methodology

The Apertif HI Survey utilizes the PAF-enabled WSRT, which forms 40 partially overlapping beams per pointing, yielding an instantaneous survey area of up to 8 deg² (Cappellen et al., 2021). Each field is observed for 11.5 hr, producing deep spectral line cubes in the 1292.5–1429.3 MHz band (HI velocities from 424 to 10,170 km s⁻¹). The source-finding is performed by SoFiA-2, which uses multiple spatial and spectral smoothing kernels to define signal masks, followed by rigorous post-processing to address artefacts such as merged or split source detections.

The parent dwarf galaxy sample consists of 2,481 gas-rich galaxies, selected by their stellar masses: $10^6 < M_*/M_\odot < 5 \times 10^9.$ Stellar masses are obtained from cross-matches with optical catalogs (e.g., Pan-STARRS1 and SDSS ASC) or computed using pipeline-derived $g$ and $i$ magnitudes. HI masses, $M_{\mathrm{HI}}$, are integrated from the spectral cubes as per

$$M_{\mathrm{HI}} = 2.343 \times 10^5 \left(\frac{D}{\mathrm{Mpc}}\right)^2 F_{\mathrm{HI}},$$

where $F_{\mathrm{HI}}$ is the total flux and $D$ is the flow-corrected distance.

The sample is further refined to remove ambiguous detections and to resolve close pairs, resulting in a final robust catalog for multiplicity studies.

2. Quantifying Dwarf-Dwarf Multiplicity: Criteria and Implementation

The identification of galaxy companions (“multiples”) is based on both constant and mass-dependent spatial and kinematic thresholds applied pairwise:

• Constant criteria: Any galaxy within
  • $r_p < 150$ kpc (projected separation)
  • $|\Delta V_\mathrm{sys}| < 150$ km s⁻¹ (systemic velocity difference)
  • is considered a companion.
• Mass-dependent criteria: The thresholds are scaled by the dark-matter halo properties of the primary, using the stellar-to-halo mass relation

$\log(M_{\mathrm{vir}}) = 0.57 \log(M_*) + 6.08,$

which sets the physical virial radius and corresponding escape velocity as adaptive selection cuts.

Isolation is strictly defined: a dwarf is deemed “isolated” if it is not within a tidal index $\Theta > -11.5$ of, or $|\Delta V_\mathrm{sys}| < 1000$ km s⁻¹ from, a massive galaxy ($M_* > 5 \times 10^9\,M_\odot$), where

$$\Theta = \log \left[\frac{M_*\,(\text{in } 10^{11}\,M_\odot)}{r_p^3~(\text{Mpc}^3)}\right] - 10.97.$$

This ensures that the analysis focuses on truly dwarf-dominated environments where interactions are not driven by massive neighbors.

3. Results: Multiplicity Frequency and Comparison to Optical Studies

The survey reveals a significantly higher multiplicity among HI-selected dwarf galaxies than previously inferred from optical samples:

Selection Criterion	Multiplicity Fraction (%)	Reference Sample
Constant (rₚ, ΔV_sys)	20	All Apertif dwarfs
Mass-scaled (via SHMR)	13	All Apertif dwarfs
Mass range $2\times10^8<M_*<5\times10^9$	11.6	B18 optical spectroscopy
B18 (optical)	~4	Optical spectroscopy studies

The factor-of-three excess (11.6% versus ~4%) in the same $M_*$ range underscores the greater efficacy of HI surveys in detecting gas-rich low-surface-brightness (LSB) dwarfs that optical studies systematically miss due to surface-brightness limits and spectroscopic fiber collisions. The result is robust to the use of adaptive, physically motivated companion criteria and removal of galaxies influenced by massive halos (Šiljeg et al., 11 Sep 2025).

Multiplicity extends up to triplets and quadruplets, but the vast majority of systems are pairs, reflecting the hierarchical nature of dwarf encounters in the field.

4. Star Formation Rate Offsets in Dwarf Multiples

By combining the Apertif HI catalog with FUV-derived SFRs (from GALEX or ancillary catalogs), the impact of close interactions on star formation is analyzed using a bootstrapped, HI/stellar mass-matched control sample. The key empirical results are:

• Close primary–primary (near-equal mass) pairs at separations $<0.2\,R_\mathrm{vir}$ show a median SFR offset of $+0.33$ dex relative to the control sample.
• In highly unequal mass pairs (mass ratio $<1/3$):
  • The high-mass (“primary”) shows a median SFR enhancement of $\approx0.09$ dex.
  • The low-mass (“secondary”) shows tentative suppression of SFR at very close separations.

These diagnostics are expressed as

$$\Delta\log \mathrm{SFR} = \log \left( \frac{\mathrm{SFR}_\mathrm{pair}}{\mathrm{median}(\mathrm{SFR}_\mathrm{control})} \right),$$

and are further analyzed as a function of projected separation normalized by $R_\mathrm{vir}$.

The analysis supports the interpretation—well documented in higher-mass systems—that gravitational interactions between gas-rich dwarfs trigger SFR elevation when the mass ratio is close to unity and the pairs are at small separations, but can lead to suppression in the subordinate system at extreme mass ratios.

5. Astrophysical Implications and Galaxy Evolution Context

The higher incidence of multiplicity among HI-selected dwarfs implies that interactions and mergers are a more important driver of dwarf galaxy evolution than previously captured by spectroscopic or single-dish HI surveys. Notably:

• Frequency of interactions: The $~13\%$–$20\%$ companion fraction suggests that dwarf-dwarf encounters are common in the gas-rich field population, providing a reservoir of dynamically evolving systems relevant to both hierarchical galaxy assembly and the low-mass end of the merger tree.
• Star formation regulation: The demonstrated SFR boost in close pairs, and evidence for suppression in subordinate members, highlights the complex, environment-dependent interplay between gas dynamics and star formation feedback during encounters.
• Limitations of optical studies: The systematic undercounting of LSB dwarf multiples in optical surveys is quantitatively demonstrated. HI selection yields a less biased census of faint, gas-rich dwarfs and their interactions—a crucial point for models of star formation efficiency and feedback in low-mass halos in $\Lambda$CDM cosmology.
• Benchmark for simulations: The Apertif results provide empirical multiplicity and SFR statistics that can be directly compared to predictions from hydrodynamical simulations and semi-analytic models, motivating further refinement of subgrid physics and feedback prescriptions.

6. Future Prospects and Remaining Challenges

The Apertif HI Survey establishes a benchmark for the census and characterization of dwarf-dwarf systems. A plausible implication is that future, deeper HI surveys with increased angular resolution and larger sky coverage (e.g., with the SKA pathfinder programs) will further extend the dynamic range and completeness of such studies.

Challenges remain:

• Completeness at the lowest masses and highest distances: Even with Apertif’s resolution and depth, the HI-detection limit and blending in crowded regions may bias against low-mass companions.
• Three-dimensional isolation: The lack of precise distances for all dwarfs introduces projection effects; however, the inclusion of stringent velocity cuts and tidal indices reduces this to a minimum.
• Complementarity with multiwavelength data: Combining HI-based catalogs with deep optical and ultraviolet surveys (e.g. as for photometric and UDG studies in (Šiljeg et al., 27 Sep 2024)) will yield a more complete picture of interaction-driven dynamical and star formation histories.

7. Summary Table: Dwarf Multiplicity and SFR in the Apertif HI Survey

Parameter	Value or Trend	Context
Sample size (dwarfs)	2,481	HI-selected ($10^6<M_*<5\times10^9$ $M_\odot$)
Constant-threshold companion rate	20%	$r_p<150$ kpc, $|\Delta V|<150$ km s⁻¹
Mass-scaled threshold rate	13%	Scaled by stellar-to-halo mass
Comparison to optical	$3\times$ higher multiplicity than B18 optical sample	Robust to selection method
SFR enhancement (equal-mass pairs, $<0.2 R_\mathrm{vir}$)	+0.33 dex (median)	Relative to HI/stellar-matched controls
SFR suppression (low-mass companion)	Possible, at very close separation	Unequal-mass pairs

These comprehensive results confirm that the interferometric, wide-field, and sensitivity advantages of the Apertif HI Survey deliver new empirical insights into dwarf galaxy multiplicity and the regulation of star formation through low-mass interactions (Šiljeg et al., 11 Sep 2025).