Dynamically close galaxy pairs from the unWISE survey: Testing the merger-AGN-star formation connection

TL;DR

This study tests the merger-AGN-star formation connection by analyzing dynamically close galaxy pairs from the unWISE survey at $z<0.25$ in two stellar-mass bins, using $r_p$ and $ abla V$ to trace interaction strength. Star formation is quantified with the $NUV-r$ colour proxy, while AGN activity is identified via X-ray, radio, infrared, and optical emission-line indicators, across multiple proximity bins. The main result is that there is only weak evidence for enhanced star formation in the closest pairs ($r_p<20$ kpc, $ abla V<500$ km s$^{-1}$) and no robust increase in AGN incidence with proximity, suggesting secular processes dominate SF/AGN triggering at low redshift. These findings align with recent simulations and observational work, challenging the view that mergers are the primary drivers of local SF and AGN activity and highlighting the role of environment and post-merger stages.

Abstract

Galaxy mergers are expected to have a profound influence on the star formation histories of galaxies. It is generally expected that mergers are the main drivers of galaxy mass growth through the accretion of mass and the triggering of new star formation episodes, while the shocks and torques induced by the merger may drive gas and dust to central supermassive black holes and fuel active galactic nuclei (AGN) activity and producing both positive and negative feedback. We test whether a merger-AGN-star formation connection exists by selecting samples of galaxy pairs of stellar masses log(M/Msun) approximately 10.2 and 11.4 within the redshift of 0.25 at various projected separation and velocity differences in an increasing order, and therefore having a decreasing probability of being truly bound and interacting. We identify galaxies in close pairs and then measure their star formation rates (SFRs) (via their NUV - r colours) and the degree of AGN activity (from X-rays, radio emission at 20cm, WISE infrared colours, and emission line ratios) as a function of their projected separation and velocity difference. We find only weak evidence that galaxies in pairs have higher SFRs as galaxies become closer in projected and velocity separation, except possibly for pairs at closest separation of less than 20 kpc and velocity difference less than 500 km/s. Similarly, we see no strong evidence that AGN are more common for galaxies in closer pairs, irrespective of the method used to detect AGN. For this sample, we do not find any clear evidence that mergers and interactions may play a significant role in triggering star formation and AGN activity, opposite to expectations from theoretical models invoking feedback episodes. Secular processes may be more important, although this may depend on the selection of galaxies and indicators for star formation and AGN activity.

Figures (20)

• Figure 1: Distribution of all galaxies (left) and galaxies with redshift (right) on the sky.
• Figure 2: WISE W1 magnitude comparison of the total photometric sample (blue) and the sample with spectroscopic redshifts (red), normalised by the area under the curve.
• Figure 3: Spectroscopic redshift distribution of the 684,186 selected galaxies.
• Figure 4: $M_{W1}$ vs. spectroscopic redshift for galaxies in our sample. The red line represents the predicted evolution for a slowly evolving galaxy with $W1=14.7$ at $z=0$. The violet and orange boxes define the two volume-limited samples we search for pairs in. We refer to the violet box ($-21 < M_{W1} < -24$ and z $<$ 0.03) as Box 1 and to the orange box ($-24 < M_{W1} < -27$ and z $<$ 0.10) as Box 2.
• Figure 5: Top: Violin plots of the distribution of $(NUV-r)_0$ colours for galaxies in the lower mass sample (Box 1: $-21 < M_{W1} < -24$ and $z < 0.03$) on the left and higher mass sample (Box 2: $-24 < M_{W1} < -27$ and z $<$ 0.10) on the right as a function of pair separation (on the X-axis) and velocity difference (red for $< 500$ km s$^{-1}$ and blue for $500 < \Delta V < 1000$ km s$^{-1}$). Note that we have slightly offset the two velocity samples for clarity. Galaxies below the dashed line are classified as star-forming. Bottom: The mean fraction of star-forming galaxies as a function of pair separation and velocity difference (same colour scheme as top panels) in Box 1 (left) and Box 2 (right). For the error bars see the text.