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Register a new userThe Galaxy Major Merger Fraction to z ~ 1
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Carlos L\\'opez-Sanjuan
Publication date
2009
fields
Physics
and research's language is
English
Authors
C. Lopez-Sanjuan
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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.
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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).
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.
The infrared (IR) emission of M_* galaxies (10^{10.4} < M_{star} < 10^{11.0} M_sun) in galaxy pairs, derived using data obtained in Herschel (PEP/HerMES) and Spitzer (S-COSMOS) surveys, is compared to that of single disk galaxies in well matched control samples to study the cosmic evolution of the star-formation enhancement induced by galaxy-galaxy interaction. Both the mean IR SED and mean IR luminosity of star-forming galaxies (SFGs) in SFG+SFG (S+S) pairs in the redshift bin of 0.6 < z < 1 are consistent with no star-formation enhancement. SFGs in S+S pairs in a lower redshift bin of 0.2 < z < 0.6 show marginal evidence for a weak star-formation enhancement. Together with the significant and strong sSFR enhancement shown by SFGs in a local sample of S+S pairs (obtained using previously published Spitzer observations), our results reveal a trend for the star-formation enhancement in S+S pairs to decrease with increasing redshift. Between z=0 and z=1, this decline of interaction-induced star-formation enhancement occurs in parallel with the dramatic increase (by a factor of ~10) of the sSFR of single SFGs, both can be explained by the higher gas fraction in higher z disks. SFGs in mixed pairs (S+E pairs) do not show any significant star-formation enhancement at any redshift. The difference between SFGs in S+S pairs and in S+E pairs suggests a modulation of the sSFR by the inter-galactic medium IGM in the dark matter halos (DMH) hosting these pairs.
Using high resolution SPH simulations in a fully cosmological Lambda CDM context we study the formation of a bright disk dominated galaxy that originates from a wet major merger at z=0.8. The progenitors of the disk galaxy are themselves disk galaxies that formed from early major mergers between galaxies with blue colors. A substantial thin stellar disk grows rapidly following the last major merger and the present day properties of the final remnant are typical of early type spiral galaxies, with an i band B/D ~0.65, a disk scale length of 7.2 kpc, g-r = 0.5 mag, an HI line width (W_{20}/2) of 238 km/sec and total magnitude i = -22.4. The key ingredients for the formation of a dominant stellar disk component after a major merger are: i) substantial and rapid accretion of gas through cold flows followed at late times by cooling of gas from the hot phase, ii) supernova feedback that is able to partially suppress star formation during mergers and iii) relative fading of the spheroidal component. The gas fraction of the progenitors disks does not exceed 25% at z<3, emphasizing that the continuous supply of gas from the local environment plays a major role in the regrowth of disks and in keeping the galaxies blue. The results of this simulation alleviate the problem posed for the existence of disk galaxies by the high likelihood of interactions and mergers for galaxy sized halos at relatively low z.
Rapid mass assembly, likely from mergers or smooth accretion, has been predicted to play a vital role in star-formation in high-redshift Lyman-alpha (Lya) emitters. Here we predict the major merger, minor merger, and smooth accreting Lya emitter fraction from z~3 to z~7 using a large dark matter simulation, and a simple physical model that is successful in reproducing many observations over this large redshift range. The central tenet of this model, different from many of the earlier models, is that the star-formation in Lya emitters is proportional to the mass accretion rate rather than the total halo mass. We find that at z~3, nearly 35% of the Lya emitters accrete their mass through major (3:1) mergers, and this fraction increases to about 50% at z~7. This imply that the star-formation in a large fraction of high-redshift Lya emitters is driven by mergers. While there is discrepancy between the model predictions and observed merger fractions, some of this difference (~15%) can be attributed to the mass-ratio used to define a merger in the simulation. We predict that future, deeper observations which use a 3:1 definition of major mergers will find >30% major merger fraction of Lya emitters at redshifts >3.
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