Darg et al. Article -- Utilizing Galaxy Zoo to examine properties of merging galaxies

Title: Galaxy Zoo: the properties of merging galaxies in the nearby Universe – local environments, colours, masses, star formation rates, and AGN activity

Authors: D. W. Darg, S. Kaviraj, C. J. Lintott, K. Schawinski, M. Sarzi, S. Bamford, J. Silk, D. Andreescu, P. Murray, R. C. Nichol, M. J. Raddick, A. Slosar, A. S. Szalay, D. Thomas, and J. Vandenberg

First Author’s Institution:  University of Oxford, Department of Physics

To read the full article, please click here.

Article Summary: 

Examining large-scale morphological properties of galaxy mergers proved to be a trying feat until the Galaxy Zoo project was set in motion.  Because of the great variety of configurations of mergers, visually examining images of galaxies is a much better method for identifying and classifying these specimens than using structural parameters.  Using classifications on the Galaxy Zoo interface, one can determine how ‘merger-like’ a Sloan Digital Sky Survey (SDSS) image appears to be based on the percentage of volunteers that flagged the particular image as a merger.  By utilizing the morphological data from Galaxy Zoo, Darg et al. were able to delve into important properties of merging galaxies, such as the structure of progenitor galaxies, internal properties of interacting galaxies, time-scales of merger events, local environments of mergers, star formation histories, and AGN activity. 

In this study, 3003 merging pairs were classified as well as a redshift-matched control sample.  The galaxies lie in the relatively local universe, with redshift range 0.005 < z < 0.1.  Binned redshifts for the merger and control samples are shown in figure 1.  Galaxies were classified both by their morphologies (Elliptical, Spiral, Unclear but probably Elliptical, and Unclear but probably a Spiral) and their merger stages.  Mergers could either be classified as ‘separated’, ‘interacting’, or ‘approaching post-merger’.  Of the 3003 samples, ~84% were classified as interacting, ~6% as separated, and ~10% as approaching post-merger. 

Figure 1. Binned redshift distributions for the merger and control samples.  

Figure 2. Number density of galaxies within 2.0 Mpc
of the merger and control samples.  Rho symbolizes
the Adaptive-Gaussian-environment parameter.  White
background shows galaxies in the field, dark gray shows
galaxies in clusters, and light gray shows the
intermediate regime.  

Of the merging systems visually examined in this study, there were about 3 times as many spirals than ellipticals.  This is interesting given that the ratio of spirals to ellipticals in the global galaxy population is ~1.5.  One issue that Darg et al. inquired about was the reason for this discrepancy.  Does it have to do with the environment in which these mergers take place or differences in the internal properties of these galaxies?  To parameterize the environment of these mergers, a sophisticated measure of the number of galaxies per unit volume called the ‘adaptive-Gaussion-environment parameter’ was used.  This allowed the determination of whether the merging galaxies and the control were located in the low-density field, high-density clusters, or in an intermediate regime.  Figure 2 shows that both mergers and controls peaked in a region dubbed ‘intermediate environments’.  Since mergers were also found to occupy similar if not denser environments than the control (where elliptical galaxies are more prevalent), the role of environment in causing the high spiral-to-elliptical ratio in mergers can be ruled out.  Instead, the prevalence of spirals in mergers likely arises from the longer time-scales of detectability for mergers involving spirals than for mergers involving ellipticals. 

Figure 3. Volume-limited and non-volume limited
distribution of galaxies in color space.  The graphs
on the left show the u-r color versus absolute
magnitude.  The center graphs show the frequency
of ellipticals and spirals compared to the control
sample (EU and SU stand for unsure but probably
elliptical and unsure but probably spiral,
respectively).  The right graphs show the frequency
of all merging samples compared to control.  

At least one of each of the galaxies merging had spectral data, allowing this study to do a color analysis of the samples.  In accord with earlier observations, the merging galaxies had a larger spread of colors than the control sample, supporting the notion that ‘irregular’ morphologies have a greater spread in color than ‘regular’ ones.  A volume limited (where only galaxies with M_r < -20.55 were used) and a non-volume limited color-magnitude diagram for the merger and control samples can be seen in figure 3.  A clear bimodality between the elliptical and spiral regimes can be seen in the binned color plots.  Figure 4 shows the mass-distributions of galaxies in both merger and control samples.  Across almost all environments, the spiral-galaxy stellar mass distributions appear to be the same in the mergers as in the control sample.  Ellipticals mergers on the other hand appear to be slightly more massive than their control counterparts.  When morphologies are not looked at, a very similar mass distribution for merger and control samples is attained.  The fact that mergers favor spirals (which are generally less massive) yet have an overall distribution just as massive as the control sample may indicate that galaxies involved in mergers really are more massive.  

Figure 4. Mass distributions for galaxies in all
environments (top row), galaxies in the field (second
row), galaxies in intermediate regimes (third row) and
galaxies in dense clusters (bottom).  

Figure 5 shows the entire sample of merger-pairs in a mass-color-morphology graph.  Both color and morphology of the galaxies scale strongly with mass.  An interesting find is that there is a near absence of ellipticals with masses below 3 x 10¹⁰ solar masses, raising the question as to what happens to two low-mass spiral galaxies when they merge.  This may be due to low-mass galaxies retaining a sufficient amount of gas to reform a disc after a major merger event (gas content along with conservation of angular momentum is what leads to a flattened-out disc shape in galaxies).  More massive galaxies may be prone to more catastrophic angular momentum loss during a merger event, and the remaining gas supplies may plunge into the central core and transfer the angular momentum required for disc morphology into the stellar dispersion of the remnant bulge. 

Figure 5. A mass-color-morphology diagram.  The top three plots show the average stellar mass for (from left to right) spiral-spiral mergers, spiral-elliptical mergers, and elliptical-elliptical mergers.  The main plot shows the u-r color of each sample, the stellar mass of each galaxy (with the more massive galaxy's value on the x-axis) and the type of galaxies that are taking part in the merger event (given by a circle, asterisk, or triangle data point).  

Due to the importance of feedback mechanisms to a gas retention model, the study next examined active galactic nuclei (AGN) and star formation signatures of the mergers.  By examining the measured fluxes of emission lines in the samples, this study was able to determine the most likely sources of these emissions and separate their galaxies into 4 categories: star-forming, mixed (both star formation and AGN activity), AGN (either Seyfert nuclei or LINERs), and quiescent (or ‘weak emission-line’).  Figure 6 shows the locations of these four types of mergers in mass-color space.  In this plot, galaxies characterized by star-formation occupied the low-mass region, AGN occupied the intermediate-mass region, and quiescent types occupied the high-mass region.  This suggests that the fuel supply of high-mass galaxies has been exhausted (as to not fuel star formation or AGN activity) and low-mass galaxies may have insufficient mass to power AGN.  Alternatively, AGN signatures in low-mass galaxies may also be obscured by high gas content and high star formation rates (SFRs).  By comparing spectral signatures to a control sample, the study determined that mergers significantly enhance SFRs in spiral galaxies only, whereas ellipticals live up to being ‘red and dead’ and their SFRs not as affected by major mergers.  Using H-alpha emission strength, estimations of the SFRs (of galaxies that fell in the star-forming category) were measured to be ~5.2 solar masses per year, which was about twice the value of a control sample of non-merging star-forming galaxies.  The highest SFR of the merging galaxies was ~ 95 solar masses per year. 

Figure 6. Color-stellar mass relation for galaxies of differing spectral types.  Each plot highlights the samples that fall into each respective category.  

The use of galaxy zoo morphology classifications allowed this study to analyze the effects of mergers on different types of galaxies.  By estimating the environment around mergers, the prevalence of spiral galaxies in merger events was found to not be due to the density of the environments in which mergers occur.  Therefore, internal properties of galaxies may be the reason for the high number of spirals in mergers; spirals have large gas reservoirs that may result in longer time-scales of merger events, whereas when two elliptical galaxies merge one would expect them to produce comparatively faint tidal tails and little star formation, thus making them harder to detect.  Since detectability of mergers relates to their timescales and timescales relate to internal properties of galaxies, addressing the colors, stellar masses, and spectral emission of the samples is of importance.  This study found that colors of merging galaxies scale strongly with mass and morphology, and are spread over a larger area than control galaxies.  Ellipticals are rare below a mass of ~ 3 x 10¹⁰ solar masses, which may be due to low-mass spiral mergers surviving the event and having enough gas to reform their disc.  Moving to the feedback mechanisms of the merging samples, Darg et al. found that mergers induce intense star-formation only when they involve spiral galaxies, and AGN activity was not present in low-mass mergers.  In star-forming mergers, the SFRs were ~2 times greater than that of a control sample of star-forming galaxies.  This study also found that specific SFRs (star-formation rates per unit stellar mass), scale down with stellar mass, possibly due to gas supplies being continually drained as galaxies accumulate mass.  The results generally imply that mergers affect spirals much more than ellipticals, which in turn affects the time-scales of detectability for merger events.