According to the current paradigm, galaxies initially form as disc galaxies at the centres of their own dark matter haloes. During their subsequent evolution, they may undergo a transformation to a ...red, early-type galaxy, thus giving rise to the build-up of the red sequence. Two important, outstanding questions are (i) which transformation mechanisms are most important and (ii) in what environment do they occur. In this paper, we study the impact of transformation mechanisms that operate only on satellite galaxies, such as strangulation, ram-pressure stripping and galaxy harassment. Using a large galaxy group catalogue constructed from the Sloan Digital Sky Survey, we compare the colours and concentrations of satellites galaxies to those of central galaxies of the same stellar mass, adopting the hypothesis that the latter are the progenitors of the former. On average, satellite galaxies are redder and more concentrated than central galaxies of the same stellar mass, indicating that satellite-specific transformation processes do indeed operate. Central-satellite pairs that are matched in both stellar mass and colour, however, show no average concentration difference, indicating that the transformation mechanisms operating on satellites affect colour more than morphology. We also find that the colour and concentration differences of matched central-satellite pairs are completely independent of the mass of the host halo (not to be confused with the subhalo) of the satellite galaxy, indicating that satellite-specific transformation mechanisms are equally efficient in host haloes of all masses. This strongly rules against mechanisms that are thought to operate only in very massive haloes, such as ram-pressure stripping or harassment. Instead, we argue that strangulation is the main transformation mechanism for satellite galaxies. Finally, we determine the relative importance of satellite quenching for the build-up of the red sequence. We find that roughly 70 per cent of red-sequence satellite galaxies with M*∼ 109h−2M⊙ had their star formation quenched as satellites. This drops rapidly with increasing stellar mass, reaching virtually zero at M*∼ 1011h−2M⊙. Therefore, a very significant fraction of red satellite galaxies were already quenched before they became a satellite.
We use the Wide-field Infrared Survey Explorer (WISE) and the Sloan Digital Sky Survey (SDSS) to confirm a connection between dust-obscured active galactic nuclei (AGNs) and galaxy merging. Using a ...new, volume-limited (z less than or equal to 0.08) catalogue of visually selected major mergers and galaxy-galaxy interactions from the SDSS, with stellar masses above 2 x 10 super( 10) M..., we find that major mergers (interactions) are 5-17 (3-5) times more likely to have red 3.4 - 4.6 colours associated with dust-obscured or 'dusty' AGNs, compared to non-merging galaxies with similar masses. Using published fibre spectral diagnostics, we map the 3.4 - 4.6 versus 4.6 - 12 colours of different emission-line galaxies and find that one-quarter of Seyferts have colours indicative of a dusty AGN. We find that AGNs are five times more likely to be obscured when hosted by a merging galaxy, half of AGNs hosted by a merger are dusty, and we find no enhanced frequency of optical AGNs in merging over non-merging galaxies. We conclude that undetected AGNs missed at shorter wavelengths are at the heart of the ongoing AGN-merger connection debate. The vast majority of mergers hosting dusty AGNs are star forming and located at the centres of M sub( halo) < 10 super( 13) M... groups. Assuming plausibly short-duration dusty-AGN phases, we speculate that a large fraction of gas-rich mergers experience a brief obscured AGN phase, in agreement with the strong connection between central star formation and black hole growth seen in merger simulations. (ProQuest: ... denotes formulae/symbols omitted.)
Galaxy interactions and mergers are thought to play an important role in the evolution of galaxies. Studies in the nearby universe show a higher fraction of active galactic nuclei (AGNs) in ...interacting and merging galaxies than in their isolated counterparts, indicating that such interactions are important contributors to black hole growth. To investigate the evolution of this role at higher redshifts, we have compiled the largest known sample of major spectroscopic galaxy pairs (2381 with ΔV < 5000 km s−1) at 0.5 < z < 3.0 from observations in the COSMOS and CANDELS surveys. We identify X-ray and IR AGNs among this kinematic pair sample, a visually identified sample of mergers and interactions, and a mass-, redshift-, and environment-matched control sample for each in order to calculate AGN fractions and the level of AGN enhancement as a function of relative velocity, redshift, and X-ray luminosity. While we see a slight increase in AGN fraction with decreasing projected separation, overall, we find no significant enhancement relative to the control sample at any separation. In the closest projected separation bin (< 25 kpc, ΔV < 1000 km s−1), we find enhancements of a factor of and for X-ray and IR-selected AGNs, respectively. While we conclude that galaxy interactions do not significantly enhance AGN activity on average over 0.5 < z < 3.0 at these separations, given the errors and the small sample size at the closest projected separations, our results would be consistent with the presence of low-level AGN enhancement.
We examine the fraction of massive ( ) compact star-forming galaxies (cSFGs) that host an active galactic nucleus (AGN) at . These cSFGs are likely the direct progenitors of the compact quiescent ...galaxies observed at this epoch, which are the first population of passive galaxies to appear in large numbers in the early Universe. We identify cSFGs that host an AGN using a combination of Hubble WFC3 imaging and Chandra X-ray observations in four fields: the Chandra Deep Fields, the Extended Groth Strip, and the UKIDSS Ultra Deep Survey field. We find that (65/166) of cSFGs at host an X-ray detected AGN. This fraction is 3.2 times higher than the incidence of AGN in extended star-forming galaxies with similar masses at these redshifts. This difference is significant at the level. Our results are consistent with models in which cSFGs are formed through a dissipative contraction that triggers a compact starburst and concurrent growth of the central black hole. We also discuss our findings in the context of cosmological galaxy evolution simulations that require feedback energy to rapidly quench cSFGs. We show that the AGN fraction peaks precisely where energy injection is needed to reproduce the decline in the number density of cSFGs with redshift. Our results suggest that the first abundant population of massive quenched galaxies emerged directly following a phase of elevated supermassive black hole growth and further hints at a possible connection between AGN and the rapid quenching of star formation in these galaxies.
Abstract
We present the first comprehensive release of photometric redshifts (photo
-
z
's) from the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (CANDELS) team. We use statistics ...based upon the Quantile–Quantile (
Q
–
Q
) plot to identify biases and signatures of underestimated or overestimated errors in photo
-
z
probability density functions (PDFs) produced by six groups in the collaboration; correcting for these effects makes the resulting PDFs better match the statistical definition of a PDF. After correcting each group’s PDF, we explore three methods of combining the different groups’ PDFs for a given object into a consensus curve. Two of these methods are based on identifying the minimum
f
-divergence curve, i.e., the PDF that is closest in aggregate to the other PDFs in a set (analogous to the median of an array of numbers). We demonstrate that these techniques yield improved results using sets of spectroscopic redshifts independent of those used to optimize PDF modifications. The best photo
-
z
PDFs and point estimates are achieved with the minimum
f
-divergence using the best four PDFs for each object (mFDa4) and the hierarchical Bayesian (HB4) methods, respectively. The HB4 photo
-
z
point estimates produced
σ
NMAD
= 0.0227/0.0189 and ∣Δ
z
/(1 +
z
)∣ > 0.15 outlier fraction = 0.067/0.019 for spectroscopic and 3D Hubble Space Telescope redshifts, respectively. Finally, we describe the structure and provide guidance for the use of the CANDELS photo
-
z
catalogs, which are available at
https://archive.stsci.edu/prepds/candels/
.
Abstract
We explore observational and theoretical constraints on how galaxies might transition between the ‘star-forming main sequence’ (SFMS) and varying ‘degrees of quiescence’ out to z = 3. Our ...analysis is focused on galaxies with stellar mass M* > 1010 M⊙, and is enabled by GAMA and CANDELS observations, a semi-analytic model (SAM) of galaxy formation, and a cosmological hydrodynamical ‘zoom in’ simulation with momentum-driven AGN feedback. In both the observations and the SAM, transition galaxies tend to have intermediate Sérsic indices, half-light radii, and surface stellar mass densities compared to star-forming and quiescent galaxies out to z = 3. We place an observational upper limit on the average population transition time-scale as a function of redshift, finding that the average high-redshift galaxy is on a ‘fast track’ for quenching whereas the average low-redshift galaxy is on a ‘slow track’ for quenching. We qualitatively identify four physical origin scenarios for transition galaxies in the SAM: oscillations on the SFMS, slow quenching, fast quenching, and rejuvenation. Quenching time-scales in both the SAM and the hydrodynamical simulation are not fast enough to reproduce the quiescent population that we observe at z ∼ 3. In the SAM, we do not find a clear-cut morphological dependence of quenching time-scales, but we do predict that the mean stellar ages, cold gas fractions, SMBH (supermassive black hole) masses and halo masses of transition galaxies tend to be intermediate relative to those of star-forming and quiescent galaxies at z < 3.
Abstract
Galaxy morphology and its evolution over the cosmic epoch hold important clues for understanding the regulation of star formation (SF). However, studying the relationship between morphology ...and SF has been hindered by the availability of consistent data at different redshifts. Our sample, combining CANDELS (0.8 <
z
< 2.5) and the GALEX-SDSS-WISE Legacy Catalog (GSWLC;
z
∼ 0), has physical parameters derived using consistent SED fitting with flexible dust attenuation laws. We adopt visual classifications from Kartaltepe et al. and expand them to
z
∼ 0 using SDSS images matching the physical resolution of CANDELS rest-frame optical images and deep FUV GALEX images matching the physical resolution of the CANDELS rest-frame FUV images. Our main finding is that disks with SF clumps at
z
∼ 0 make a similar fraction (∼15%) of star-forming galaxies as at
z
∼ 2. The clumpy disk contribution to the SF budget peaks at
z
∼ 1, rather than
z
∼ 2, suggesting that the principal epoch of disk assembly continues to lower redshifts. Star-forming spheroids (“blue nuggets”), though less centrally concentrated than quenched spheroids, contribute significantly (∼15%) to the SF budget at
z
∼ 1–2, suggesting that compaction precedes quenching. Among green valley and quiescent galaxies, the pure spheroid fraction drops after
z
∼ 1, whereas spheroids with disks (S0-like) become dominant. Mergers at or nearing coalescence are enhanced in SFR relative to the main sequence at all redshifts by a factor of ∼2, but contribute ≲5% to the SF budget, with their contribution remaining small above the main sequence.
We examine the spheroid growth and star formation quenching experienced by galaxies since z ∼ 3 by studying the evolution with redshift of the quiescent and spheroid-dominated fractions of galaxies ...from the CANDELS (Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey) and GAMA (Galaxy and Mass Assembly) surveys. We compare the observed fractions with predictions from a semi-analytic model which includes prescriptions for bulge growth and AGN feedback due to mergers and disc instabilities. We facilitate direct morphological comparison by converting our model bulge-to-total stellar mass ratios to Sérsic indices. We then subdivide our population into the four quadrants of the specific star formation rate–Sérsic index plane and study the build-up of each of these subpopulations. We find that the fraction of star-forming discs declines steadily, while the fraction of quiescent spheroids builds up over cosmic time. The fractions of star-forming spheroids and quiescent discs are both non-negligible, and stay nearly constant over the period we have studied. Our model is qualitatively successful at reproducing the evolution of the two ‘main’ populations (star-forming discs and quiescent spheroids), and approximately reproduces the relative fractions of all four types, but predicts a stronger decline in star-forming spheroids, and increase in quiescent discs, than is seen in the observations. A model with an additional channel for bulge growth via disc instabilities agrees better overall with the observations than a model in which bulges can grow only through mergers. We also examine the relative importance of these different physical drivers of transformation (major and minor mergers and disc instabilities).
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