Gas-rich dwarf galaxy multiples in the Apertif HI survey

TL;DR

This study uses the untargeted Apertif HI survey to quantify how frequently gas-rich dwarf galaxies occur in close multiples, finding a companion fraction of about $\sim 12$–$13\%$ depending on the mass-scaled vs constant thresholds. By comparing HI-based pair statistics with optical surveys, the work shows that HI selection uncovers a significantly larger population of dwarf multiples, largely because it avoids fiber-collision and surface-brightness biases. The authors also demonstrate a significant SFR enhancement in close, high mass-ratio dwarf pairs (up to $\sim0.33$ dex on average, with some systems up to $\sim1$ dex) and find no clear suppression in SFR for dwarfs in pairs. Methodologically, they implement a two-tier companion definition using constant and mass-scaled thresholds, propagate uncertainties across 500 realizations, and account for merged/double/split HI detections, providing robust constraints for dwarf-dwarf interaction rates and their role in dwarf galaxy evolution.

Abstract

Dwarf-dwarf galaxy encounters are a key aspect of galaxy evolution as they can ignite or temporarily suppress star formation in dwarfs and can lead to dwarf mergers. However, the frequency and impact of dwarf encounters remain poorly constrained due to limitations of spectroscopic studies, e.g. surface-brightness incompleteness of optical studies and poor spatial resolution of single-dish neutral hydrogen (HI) surveys. We aim to quantify the frequency of isolated gas-rich dwarf galaxy multiples using the untargeted, interferometric Apertif HI survey and study the impact of the interaction on star formation rates of galaxies as a function of the on-sky separation. Our parent dwarf sample consists of 2481 gas-rich galaxies with stellar masses 10^6 < M_* / M_Sun < 5*10^9, for which we identify close companions based on projected separation (r_p) and systemic velocity difference (Del_V_sys). We explore both constant thresholds for r_p and Del_V_sys corresponding to 150 kpc and 150 km/s on all galaxies in our sample, and mass-dependent thresholds based on a stellar-to-halo mass relation. We find the average number of companions per dwarf in our sample to be 13% (20%) when considering mass-dependent (constant) thresholds. In the stellar mass regime of 2*10^8 < M_* / M_Sun < 5*10^9, we find a three times higher frequency (11.6%) of dwarf companions than previously determined from optical spectroscopic studies, highlighting the power of HI for finding dwarf multiples. Furthermore, we find evidence for an increase in star formation rates (SFRs) of close dwarf galaxy pairs of galaxies with similar stellar masses.

Figures (20)

• Figure 1: Selection of dwarf galaxy samples from Apertif. The flowchart denotes most relevant steps in our selection procedure starting from the Apertif source list to dwarf galaxy samples used in this work. Boxes with bold text denote final samples that we use in our analysis.
• Figure 2: Properties of the parent dwarf galaxy sample. Left: Histograms of H i (blue) and stellar (orange) masses of the sample. The vertical line denotes the dwarf stellar mass limit of $5\times 10^9$$M_\odot$. Right: Systemic velocity versus the stellar mass of dwarfs in the sample, color coded by the gas fraction. We show typical stellar mass error in the upper left corner. The vertical line is the same as in the left plot. On the right, we plot the histogram along the $V_{\rm sys}$ axis of our sample.
• Figure 3: Sketch of the classification of multiples. For simplicity, we show a case where all galaxies have the same stellar mass. The three galaxies are denoted with numbers one to three, and the selection criteria (both spatial and spectral) for companions are visualized as dashed (for galaxies 1 and 3) and dotted circles (for galaxy 2), centered at each galaxy. In this case, the number of companions is one for both galaxy 1 and 3, and two for galaxy 2. We classify this system as a triplet.
• Figure 4: Comparison of galaxy properties between the isolated and non-isolated subsamples of our parent dwarf sample. Left:$M_{\star}$ - $M_{\text{H\,{\sc i}}}$ relation. Isolated galaxies are plotted as purple squares, and non-isolated as orange circles. The error bars denote the median values and percentile errors, with isolated sample denoted as full purple lines, and non-isolated as orange dashed lines. The black vertical line denotes the stellar mass limit of $5\times 10^9$$M_\odot$. Right: The $M_{\star}$ - SFR relation. The markings are the same as in the left panel.
• Figure 5: Mean number of companions (of any stellar mass) per dwarf galaxy in the logarithmic stellar mass bin, normalized by the number of dwarfs in the same mass bin. Values represent the mean between different realizations of the sample in each mass bin. The black line represents results obtained using constant thresholds in projected distance and systemic velocity difference (150 kpc and 150 km s$^{-1}$, respectively) for selection of companions, while the blue line represent results obtained using the mass-scaled thresholds (see Sect. \ref{['p1:sec:selection_companions']}). Both lines represent results for the total drawn dwarf galaxy sample. Shaded regions and error bars represent 1$\sigma$ errors.