Bar ages derived for the first time in nearby galaxies: Insights on secular evolution from the TIMER sample

TL;DR

This work measures bar ages for a substantial nearby sample (20 galaxies, expanding prior work to 20 objects) by dating the co-eval buildup of bar-driven nuclear discs using MUSE/VLT IFU data and spectral- or SFH-based disentangling of nuclear and main-disc histories. Bar ages span $1$–$13$ Gyr, implying discs became dynamically mature early in cosmic time ($z\sim6$) and that bar-driven secular evolution is ongoing. Older bars correlate with larger, longer, and more massive nuclear discs and stronger quenching signatures, while younger bars show more active central star formation; bar age shows little dependence on galaxy stellar mass, challenging a pure downsizing picture. The results provide a direct, observational handle on the timing of secular evolution and predict bar-frac­tion evolution compatible with high-redshift measurements, establishing a benchmark for how bars shape disc galaxies over cosmic time.

Abstract

Once galaxies settle their discs and become self-gravitating, stellar bars can form, driving the subsequent evolution of their host galaxy. Determining the ages of bars can therefore shed light on the epoch of the onset of secular evolution. In this work, we apply the first broadly applicable methodology to derive bar ages to a sample of 20 nearby galaxies. The method is based on the co-eval build-up of nuclear structures and bars and involves using IFS data from the MUSE instrument on VLT to disentangle the SFH of the nuclear disc from the background population. This allows us to derive the formation epoch of the nuclear disc and, thus, of the bar. We estimate the bar formation epoch of nearby galaxies - mostly from the TIMER survey-, creating the largest sample of galaxies with known bar ages to date. We find bar formation epochs between 1 and 13 Gyr ago, illustrating how disc-settling and bar formation are processes that first took place in the early Universe and are still taking place in some galaxies. We infer the bar fraction over cosmological time with our sample, finding remarkable agreement with that obtained from direct studies of galaxies at high redshifts. Additionally, for the first time, we can investigate secular evolution processes taking into account the ages of bars. Our results agree with the scenario in which bars aid the quenching of the host galaxy, with galaxies hosting older bars tending to be more "quenched". We also find that older bars tend to be longer, stronger, and host larger nuclear discs. Furthermore, we find evidence of the nuclear disc stellar mass build-up over time. On the other hand, we find no evidence of downsizing playing a role in bar formation, since we find that bar age is independent of galaxy stellar mass. With the means to estimate bar ages, we can begin to understand better when and how bars shape the observed properties of disc galaxies.

Figures (16)

• Figure 1: Nuclear disc sizes are visually defined in this work as a function of bar length (see Table \ref{['table_sample']}). We bootstrapped our sample $1000$ times to robustly estimate the Pearson correlation coefficient and the $p-value$, with associated uncertainties. By different works (e.g., gadotti2020kinematic), we find a strong correlation ($r=0.750\pm0.003$) between the structures, as expected in a bar-built scenario for nuclear discs.
• Figure 2: Illustration of the bar age measurement method. In this Figure, we illustrate the method described in Section \ref{['sec_Methodology']}, analogous to Figure 2 in de2023new. We show the nuclear disc and representative ring regions, from which we derive the SFH of the original data in the nuclear disc region (red) and the representative region (green). We proceed to subtract the main disc SFH from the original one, and the difference is considered the nuclear disc SFH (blue). We then use the ratio ND/MD to time the moment of bar formation (right plot, orange).
• Figure 3: Star formation rate surface density map of NGC 1097 ($\Sigma_\mathrm{SFR}$ in units of M$_\odot~yr^{-1}~pc^{-2}$). We present the limits of the nuclear disc defined in this work (Table \ref{['table_sampleResults']} -- solid-black contour) and the ring region from which we extract the representative spectrum in dashed-black contour -- see illustration in Fig. \ref{['fig_loom']}. Additionally, we present the $\Sigma_\mathrm{SFR} = 2\times10^{6}$ mask (solid-white countour). As one can see, the mask is limited to the star-forming ring. Since we are interested in the oldest stars that belong to the nuclear disc, masking the active star-forming regions will not affect our methodology.
• Figure 4: Distribution of bar ages derived in this work. We display our main results (total in black line), colour-coded by low- and high-star-formation sub-samples (yellow and purple, respectively). On the top, we present the median bar age of each sub-sample, together with the standard deviation of each distribution (high-SF: $4.0\pm2.0~\mathrm{Gyr}$; low-SF: $9.3\pm3.6~\mathrm{Gyr}$). It is clear that low-SF nuclear discs are hosted by older bars (typically older than $9~\mathrm{Gyr}$), whereas high-SF nuclear discs are hosted by younger bars. Additionally, we derive a large range of bar ages, varying from $1-13~\mathrm{Gyr}$, illustrating that bars started to form in a young Universe ($\sim1~\mathrm{Gyr}$), and this is an ongoing process in the Local Universe.
• Figure 5: Stellar masses as a function of bar ages. We consider the stellar masses from the S$^4$G survey, obtained with the $3.6\mu\mathrm{m}$ band (munoz2015spitzer) and the bar ages derived in this work. We bootstrapped our sample $1000$ times to robustly estimate the Pearson correlation coefficient and the $p-value$, with associated uncertainties. Lastly, we display the mean errors in the bar age and stellar mass estimates (black-dot). As discussed in munoz2015spitzer, the main source of error in the galaxy stellar mass comes from errors in distance estimates, which translate to $\pm0.32$ dex error in mass. Contrary to downsizing predictions, we find no correlation between the two properties, with a weak Pearson coefficient of $r=-0.180\pm0.007$ -- illustrated by the dotted line.