HI Gas and Star Formation in Major Galaxy Pairs from the FAST All-Sky HI Survey (FASHI)

Abstract

Atomic hydrogen (HI) plays a fundamental role in fueling star formation in galaxies. However, the behavior of HI gas in interacting systems, particularly galaxy pairs, remains elusive. In this work, we investigate the HI content of major mergers by cross-matching the extragalactic HI catalog from the FAST All-Sky HI Survey (FASHI) with a previously established sample of isolated galaxy pairs. With the superior sensitivity of FAST, we have constructed the largest sample of major mergers with HI detections, consisting of $440$ galaxy pairs: $364$ spiral-spiral (S+S) and $76$ spiral-elliptical (S+E) systems. We examine the HI gas fraction ($f_{\mathrm{HI}}$), star formation rate (SFR) and HI star formation efficiency ($\mathrm{SFE_{HI}}=\mathrm{SFR}/M_{\rm HI}$) for individual galaxies in pairs. The control sample is matched in both stellar mass and redshift. We find that paired galaxies, particularly those in pairs with small projected separations ($d_{\mathrm{p}}<50\ h^{-1}\mathrm{kpc}$), exhibit systematically lower (by $8.8\%$) HI gas fractions compared to the control galaxies. The SFR is enhanced for galaxies in S+S pairs. $\mathrm{SFE_{HI}}$ is $\sim15\%$ higher for galaxies in S+S pairs than in the control galaxies, while spiral galaxies in S+E pairs show no significant difference in $\mathrm{SFE_{HI}}$ compared to the control sample. These findings suggest that the merging process triggers efficient HI gas depletion and enhances star formation, especially in close S+S pairs. Notably, our sample includes $26$ red spirals in paired systems. These galaxies exhibit HI deficiency and suppressed star formation activity compared to the isolated galaxies, indicating that interactions may affect quiescent spirals differently, potentially due to mechanisms similar to ellipticals.

Figures (9)

• Figure 1: Distribution of the paired galaxies (red) and the control sample (black with slash), matched in mass (a) and redshift (b).
• Figure 2: (a) H i mass versus stellar mass. (b) H i fraction versus stellar mass. (c) SFR versus stellar mass. Each point represents the S galaxy in one galaxy pair. The galaxies in S+S pairs are shown in blue dots, and those in S+E galaxies are shown in red dots, respectively. The triangles with black edges represent the red spirals. The control galaxies are shown using gray dots in the background.
• Figure 3: Panel (a) shows histograms of $\Delta f_{\mathrm{HI}}$ in paired galaxies. Black dotted columns represent $\Delta f_{\mathrm{HI}}$ for galaxies in all pairs, regardless of pair type. The galaxies in S+S and S+E galaxy pairs are indicated by blue and red shaded columns, respectively. The black, blue, and red dashed lines are the mean value of $\Delta f_{\mathrm{HI}}$ for the galaxies in all, S+S and S+E pairs, respectively. Panel (b) shows the evolution of $\Delta f_{\mathrm{HI}}$ at different projection separations ($d_{\mathrm{p}}$). Panel (c) represents $\Delta f_{\mathrm{HI}}$ against stellar mass. The galaxies in all pairs, regardless of pair type, are shown as black points with error bars in panels (b) and (c). The galaxies in S+S and S+E pairs of this work are indicated by blue and red points with error bar in (b) and (c), respectively. The black dashed line in each panel indicates the zero value. Red spirals in our sample are indicated by black-edged triangles in panel (c). The error bars represent the standard error of the mean. Results from Yu22 are shown as yellow diamonds with black edges in panel (b) and (c). The pink dashed line with $1\sigma$ error region in panel (b) and (c) shows the results from Bok22.
• Figure 4: Panel (a) shows histograms of $\Delta$SFR in paired galaxies. Panel (b) shows the evolution of $\Delta$SFR in paired galaxies with different projection separation ($d_{\mathrm{p}}$). Panel (c) represents $\Delta$SFR against stellar mass. Results from Scudder15 and Huang24 are represented by purple diamonds and gray squares in panel (b) and (c), respectively. In panel (b) and (c), the gray dashed line with $1\sigma$ error region corresponds to $\Delta\mathrm{SFR}$ for major mergers from Huang24. Green diamonds represent the results from Ferreira25. Other color scheme and labels correspond to those used in Figure \ref{['fig:delta_fh1']}.
• Figure 5: Plot of log(SFR) versus H i fraction (a) and log($\mathrm{SFE_{HI}}$) versus H i fraction (b). The color scheme and labels correspond to those used in Figure \ref{['fig:HI_data']}.