Amount Of Merging Research Articles

RESOLVING THE DISCREPANCY OF GALAXY MERGER FRACTION MEASUREMENTS AT z ∼ 0–3

ABSTRACT We measure the merger fraction of massive galaxies using the UltraVISTA/COSMOS catalog, complemented with the deeper, higher resolution 3DHST+CANDELS catalog, presenting the largest mass-complete photometric merger sample up to . We find that the variation in the mass ratio probe can explain the discrepant redshift evolution of the merger fraction in the literature: selecting mergers using the H 160-band flux ratio leads to an increasing merger fraction with redshift, while selecting mergers using the stellar mass ratio reveals a merger fraction with little redshift dependence at . Defining major and minor mergers as having stellar mass ratios of 1:1–4:1 and 4:1–10:1, respectively, the results imply ∼1 major merger and ∼0.7 minor merger on average for a massive (log ) galaxy during . There may be an additional major (minor) merger if we use the H-band flux ratio selection. The observed amount of major merging alone is sufficient to explain the observed number density evolution for the very massive (log ) galaxies. The observed number of major and minor mergers can increase the size of a massive quiescent galaxy by a factor of two at most. This amount of merging is enough to bring the compact quiescent galaxies formed at to lie at below the mean of the stellar mass–size relation as measured in some works (e.g., Newman et al.), but additional mechanisms are needed to fully explain the evolution, and to be consistent with works suggesting stronger evolution.

THE COSMOS-WIRCam NEAR-INFRARED IMAGING SURVEY. I.BzK-SELECTED PASSIVE AND STAR-FORMING GALAXY CANDIDATES ATz≳ 1.4

(abridged) We present a new near-infrared survey covering the 2 deg sq COSMOS field. Combining our survey with Subaru B and z images we construct a deep, wide-field optical-infrared catalogue. At Ks<23 (AB magnitudes) our survey completeness is greater than 90% and 70% for stars and galaxies respectively and contains 143,466 galaxies and 13,254 stars. At z~2 our catalogues contain 3931 quiescent and 25,757 star-forming BzK-selected galaxies representing the largest and most secure sample of these objects to date. Our counts of quiescent galaxies turns over at Ks~22 an effect which we demonstrate cannot be due to sample incompleteness. In our survey both the number of faint and bright quiescent objects exceeds the predictions of a semi-analytic galaxy formation model, indicating potentially the need for further refinements in the amount of merging and AGN feedback at z~2 in these models. We measure the angular correlation function for each sample and find that at small scales the correlation function for passive BzK galaxies exceeds the clustering of dark matter. We use 30-band photometric redshifts to derive the spatial correlation length and the redshift distributions for each object class. At Ks<22 we find r_0^{\gamma/1.8}=7.0 +/-0.5h^{-1} Mpc for the passive BzK candidates and 4.7+/-0.8h^{-1} Mpc for the star-forming BzK galaxies. Our pBzK galaxies have an average photometric redshift of z_p~1.4, in approximate agreement with the limited spectroscopic information currently available. The stacked Ks image will be made publicly available from IRSA.

Discriminating between the physical processes that drive spheroid size evolution

Massive galaxies at high-z have smaller effective radii than those today, but similar central densities. Their size growth therefore relates primarily to the evolving abundance of low-density material. Various models have been proposed to explain this evolution, which have different implications for galaxy, star, and BH formation. We compile observations of spheroid properties as a function of redshift and use them to test proposed models. Evolution in progenitor gas-richness with redshift gives rise to initial formation of smaller spheroids at high-z. These systems can then evolve in apparent or physical size via several channels: (1) equal-density 'dry' mergers, (2) later major or minor 'dry' mergers with less-dense galaxies, (3) adiabatic expansion, (4) evolution in stellar populations & mass-to-light-ratio gradients, (5) age-dependent bias in stellar mass estimators, (6) observational fitting/selection effects. If any one of these is tuned to explain observed size evolution, they make distinct predictions for evolution in other galaxy properties. Only model (2) is consistent with observations as a dominant effect. It is the only model which allows for an increase in M_BH/M_bulge with redshift. Still, the amount of merging needed is larger than that observed or predicted. We therefore compare cosmologically motivated simulations, in which all these effects occur, & show they are consistent with all the observational constraints. Effect (2), which builds up an extended low-density envelope, does dominate the evolution, but effects 1,3,4, & 6 each contribute ~20% to the size evolution (a net factor ~2). This naturally also predicts evolution in M_BH-sigma similar to that observed.