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Register a new userGalaxy Merger Rates up to z $sim$ 3 using a Bayesian Deep Learning Model $-$ A Major-Merger classifier using IllustrisTNG Simulation data
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Added by
Leonardo de Albernaz Ferreira
Publication date
2020
fields
Physics
and research's language is
English
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Merging is potentially the dominate process in galaxy formation, yet there is still debate about its history over cosmic time. To address this we classify major mergers and measure galaxy merger rates up to z $sim$ 3 in all five CANDELS fields (UDS, EGS, GOODS-S, GOODS-N, COSMOS) using deep learning convolutional neural networks (CNNs) trained with simulated galaxies from the IllustrisTNG cosmological simulation. The deep learning architecture used is objectively selected by a Bayesian Optmization process over the range of possible hyperparameters. We show that our model can achieve 90% accuracy when classifying mergers from the simulation, and has the additional feature of separating mergers before the infall of stellar masses from post mergers. We compare our machine learning classifications on CANDELS galaxies and compare with visual merger classifications from Kartaltepe et al. (2015), and show that they are broadly consistent. We finish by demonstrating that our model is capable of measuring galaxy merger rates, $mathcal{R}$, that are consistent with results found for CANDELS galaxies using close pairs statistics, with $mathcal{R}(z) = 0.02 pm 0.004 times (1 +z) ^ {2.76 pm 0.21}$. This is the first general agreement between major mergers measured using pairs and structure at z < 3.
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A fundamental feature of galaxies is their structure, yet we are just now understanding the evolution of structural properties in quantitative ways. As such, we explore the quantitative non-parametric structural evolution of 16,778 galaxies up to $zsim3$ in all five CANDELS fields, the largest collection of high resolution images of distant galaxies to date. Our goal is to investigate how the structure, as opposed to size, surface brightness, or mass, changes with time. In particular, we investigate how the concentration and asymmetry of light evolve in the rest-frame optical. To interpret our galaxy structure measurements, we also run and analyse 300 simulation realisations from IllustrisTNG to determine the timescale of mergers for the CAS system. We measure that from $z=0-3$, the median asymmetry merger timescale is $0.56^{+0.23}_{-0.18}$Gyr, and find it does not vary with redshift. Using this data, we find that galaxies become progressively asymmetric at a given mass at higher redshifts and we derive merger rates which scale as $sim(1+z)^{1.87pm0.04}$Gyr$^{-1}$, which agrees well with recent machine learning and galaxy pair approaches, removing previous inconsistencies. We also show that far-infrared selected galaxies that are invisible to textit{HST} have a negligible effect on our measurements. We also find that galaxies are more concentrated at higher redshifts. We interpret this as a sign of how their formation occurs from a smaller initial galaxy that later grows into a larger one through mergers, consistent with the size growth of galaxies from `inside-out, suggesting that the centres are the oldest parts of most galaxies.
Calculating the galaxy merger rate requires both a census of galaxies identified as merger candidates, and a cosmologically-averaged `observability timescale T_obs(z) for identifying galaxy mergers. While many have counted galaxy mergers using a variety of techniques, T_obs(z) for these techniques have been poorly constrained. We address this problem by calibrating three merger rate estimators with a suite of hydrodynamic merger simulations and three galaxy formation models. We estimate T_obs(z) for (1) close galaxy pairs with a range of projected separations, (2) the morphology indicator G-M20, and (3) the morphology indicator asymmetry A. Then we apply these timescales to the observed merger fractions at z < 1.5 from the recent literature. When our physically-motivated timescales are adopted, the observed galaxy merger rates become largely consistent. The remaining differences between the galaxy merger rates are explained by the differences in the range of mass-ratio measured by different techniques and differing parent galaxy selection. The major merger rate per unit co-moving volume for samples selected with constant number density evolves much more strongly with redshift (~ (1+z)^(+3.0 pm 1.1)) than samples selected with constant stellar mass or passively evolving luminosity (~ (1+z)^(+0.1 pm 0.4)). We calculate the minor merger rate (1:4 < M_{sat}/M_{primary} <~ 1:10) by subtracting the major merger rate from close pairs from the `total merger rate determined by G-M20. The implied minor merger rate is ~3 times the major merger rate at z ~ 0.7, and shows little evolution with redshift.
Aims: We study the major merger fraction in a SPITZER/IRAC-selected catalogue in the GOODS-S field up to z ~ 1 for luminosity- and mass-limited samples. Methods: We select disc-disc merger remnants on the basis of morphological asymmetries, and address three main sources of systematic errors: (i) we explicitly apply morphological K-corrections, (ii) we measure asymmetries in galaxies artificially redshifted to z_d = 1.0 to deal with loss of morphological information with redshift, and (iii) we take into account the observational errors in z and A, which tend to overestimate the merger fraction, though use of maximum likelihood techniques. Results: We obtain morphological merger fractions (f_m) below 0.06 up to z ~ 1. Parameterizing the merger fraction evolution with redshift as f_m(z) = f_m(0) (1+z)^m, we find that m = 1.8 +/- 0.5 for M_B <= -20 galaxies, while m = 5.4 +/- 0.4 for M_star >= 10^10 M_Sun galaxies. When we translate our merger fractions to merger rates (R_m), their evolution, parameterized as R_m(z) = R_m(0) (1+z)^n, is quite similar in both cases: n = 3.3 +/- 0.8 and n = 3.5 +/- 0.4, respectively. Conclusions: Our results imply that only ~8% of todays M_star >= 10^10 M_Sun galaxies have undergone a disc-disc major merger since z ~ 1. In addition, ~21% of this mass galaxies at z ~ 1 have undergone one of these mergers since z ~ 1.5. This suggests that disc-disc major mergers are not the dominant process in the evolution of M_star >= 10^10 M_Sun galaxies since z ~ 1, but may be an important process at z > 1.
We derive the close, kinematic pair fraction and merger rate up to z ~ 1.2 from the initial data of the DEEP2 Redshift Survey. Assuming a mild luminosity evolution, the number of companions per luminous galaxy is found to evolve as (1+z)^{m}, with m = 0.51+-0.28; assuming no evolution, m = 1.60+-0.29. Our results imply that only 9% of present-day $L^{*}$ galaxies have undergone major mergers since z ~ 1.2 and that the average major merger rate is about 4* 10^{-4} h^{3} Mpc^{-3} Gyr^{-1} for z ~ 0.5 - 1.2. Most previous studies have yielded higher values.
We present a study of the largest available sample of near-infrared selected (i.e., stellar mass selected) dynamically close pairs of galaxies at low redshifts ($z<0.3$). We combine this sample with new estimates of the major-merger pair fraction for stellar mass selected galaxies at $z<0.8$, from the Red Sequence Cluster Survey (RCS1). We construct our low-redshift $K-$band selected sample using photometry from the UKIRT Infrared Deep Sky Survey (UKIDSS) and the Two Micron All Sky Survey (2MASS) in the $K-$band ($sim 2.2~mu$m). Combined with all available spectroscopy, our $K-$band selected sample contains $sim 250,000$ galaxies and is $> 90%$ spectroscopically complete. The depth and large volume of this sample allow us to investigate the low-redshift pair fraction and merger rate of galaxies over a wide range in $K-$band luminosity. We find the major-merger pair fraction to be flat at $sim 2%$ as a function of $K-$band luminosity for galaxies in the range $10^8 - 10^{12} L_{odot}$, in contrast to recent results from studies in the local group that find a substantially higher low-mass pair fraction. This low-redshift major-merger pair fraction is $sim 40-50%$ higher than previous estimates drawn from $K-$band samples, which were based on 2MASS photometry alone. Combining with the RCS1 sample we find a much flatter evolution ($m = 0.7 pm 0.1$), in the relation $f_{rm{pair}} propto (1+z)^m$, than indicated in many previous studies. These results indicate that a typical $Lsim L^*$ galaxy has undergone $sim 0.2-0.8$ major mergers since $z=1$ (depending on the assumptions of merger timescale and percentage of pairs that actually merge).
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