The effect of mass and morphology on the mass assembly of galaxies

TL;DR

Using full spectral fitting of MaNGA IFU data with $pyPipe3D$, the paper reconstructs star formation histories and mass assembly histories for thousands of local galaxies to study how the sSFR–$M_\star$ relation evolves with cosmic time and how present morphology correlates with past SFH. They find that the sSFR–$M_\star$ relation was shallower at look-back times, and the current negative correlation arises because more massive galaxies quenched earlier, while less massive ones continued forming stars longer. Morphology-based segregation in the sSFR–$M_\star$ relation is imprinted from early times, indicating initial conditions in formation drive present morphology; results are compared with eight cosmic surveys and theoretical models, showing partial agreement with redshift offsets. Overall, the study provides a time-resolved view of downsizing, quenching timescales, and the origin of present-day morphologies, highlighting the utility and limitations of full spectral fitting for cosmic star formation histories.

Abstract

The pace at which galaxies grew into their current stellar masses and how this growth is regulated is still not fully understood, nor is the role that morphology plays in this process. We applied full spectral fitting techniques with pyPipe3D to the MaNGA sample to obtain its star formation and stellar mass histories and used these to investigate the mass assembly of galaxies by measuring how their specific star formation correlates to their stellar mass at different look-back times. We find that the correlation between these two parameters was shallower in the past. Galaxies used to have similar mass doubling times and the current negative correlation between the specific star formation and M* is primarily due to more massive galaxies 'dropping' off the main sequence earlier than less massive ones. Additionally, selecting the galaxies into bins based on their present-day morphology shows a segregation in specific star formation rate (sSFR) that is maintained even at high look-back times, showing that the factors that determine which morphology a galaxy ends up in are in place at very early times. Similarly, selecting them based on their current star formation status shows that, on average, currently retired galaxies used to have slightly a higher sSFR before the drop-off, whereas galaxies that have continued to form stars until today had a lower sSFR initially. We compare our results to a set of cosmic surveys, finding partial agreement in our results with several of them, though with significant offsets in redshift. Finally, we discuss how our results fit with certain theoretical models on galaxy evolution as well as cosmological simulations.

Figures (8)

• Figure 1: Currently observed sSFR of MaNGA galaxies compared to their M$_\star$ (bottom left). The black contours enclose 95%, 65% and 35% of the sample respectively. The green line is the median sSFR within 0.25 dex wide M$_\star$ bins. In the top and right panels, the distributions in M$_\star$ and sSFR, respectively, are shown as histograms, with the three morphological bins shown in lines of the corresponding colour.
• Figure 2: Evolution of the sSFR over cosmic time for the galaxies in the MaNGA sample. In each panel the M$_\star$ and sSFR of the galaxies measured at different LBT are shown. The colour of the data points corresponds to the currently observed morphology of the galaxies. The black contours enclose 95%, 65% and 35% of the sample, respectively. The cyan and violet dots and lines show the median sSFR within 0.5 dex wide M$_\star$ bins, selecting galaxies above and below 6 Å in EW$_\mathrm{H\alpha}$, respectively. The error bars for the dots show the 25th and 75th percentile of the distribution within each M$_\star$ bin.
• Figure 3: Evolution of the sSFR over cosmic time for the galaxies in the MaNGA sample. In each panel the M$_\star$ and sSFR of the galaxies measured at different LBT are shown. There are three sets of contours whose colours correspond to the current morphology of the galaxies and whose levels enclose 95% and 50% of each sub-sample.
• Figure 4: Two-sample Kolmogorov-Smirnov test for each pair of galaxy populations considered in the article. In each panel we show the p-value for each pair of populations performed individually at each of the 0.5 dex wide M$_\star$ bins. The dotted line shows the limit of p-value$\,= 0.05$, below which the samples are considered to be statistically significantly different. The colours identify the pairs of populations used.
• Figure 5: sSFR-M$_\star$ relation for eight cosmic surveys. The data have been adapted into the M$_\star$-sSFR plane in some cases. The colour of each line corresponds to the average redshift of its bin and the redshift colour scale is the same for all panels. The dotted lines correspond to M$_\star$ intervals where sample completeness is not guaranteed as reported in each work. The lower right corner of each field indicates the median scatter of the data.