Red Mergers Research Articles

Understanding jets in post-AGB close binaries

AbstractWe have discovered jets in post-AGB binaries. The orbital motion allows us to carry out tomography of the jet as light from the primary star shines through the jet cone. Jets play a major role in many astrophysical environments, from young stellar objects to galaxies. They are also used to study the energetics of accretion phenomena in systems such as red transients and stellar mergers. We use high-resolution, optical, time-series spectra to constrain theories of jet launching, and the impact of jets on the evolution of these post-AGB binaries.

THE AGE SPREAD OF QUIESCENT GALAXIES WITH THE NEWFIRM MEDIUM-BAND SURVEY: IDENTIFICATION OF THE OLDEST GALAXIES OUT TOz∼ 2

With a complete, mass-selected sample of quiescent galaxies from the NEWFIRM Medium-Band Survey, we study the stellar populations of the oldest and most massive galaxies (>1011 M☉) to high redshift. The sample includes 570 quiescent galaxies selected based on their extinction-corrected U − V colors out to z = 2.2, with accurate photometric redshifts, σz/(1 + z) ∼ 2%, and rest-frame colors, σU−V ∼ 0.06 mag. We measure an increase in the intrinsic scatter of the rest-frame U − V colors of quiescent galaxies with redshift. This scatter in color arises from the spread in ages of the quiescent galaxies, where we see both relatively quiescent red, old galaxies and quiescent blue, younger galaxies toward higher redshift. The trends between color and age are consistent with the observed composite rest-frame spectral energy distributions (SEDs) of these galaxies. The composite SEDs of the reddest and bluest quiescent galaxies are fundamentally different, with remarkably well-defined 4000 Å and Balmer breaks, respectively. Some of the quiescent galaxies may be up to four times older than the average age and up to the age of the universe, if the assumption of solar metallicity is correct. By matching the scatter predicted by models that include growth of the red sequence by the transformation of blue galaxies to the observed intrinsic scatter, the data indicate that most early-type galaxies formed their stars at high redshift with a burst of star formation prior to migrating to the red sequence. The observed U − V color evolution with redshift is weaker than passive evolution predicts; possible mechanisms to slow the color evolution include increasing amounts of dust in quiescent galaxies toward higher redshift, red mergers at z ≲ 1, and a frosting of relatively young stars from star formation at later times.

The Detection of a Red Sequence of Massive Field Galaxies atz∼ 2.3 and Its Evolution toz∼ 0

The existence of massive galaxies with strongly suppressed star formation at z similar to 2.3, identified in a previous paper, suggests that a red sequence may already be in place beyond z = 2. In order to test this hypothesis, we study the rest-frame U - B color distribution of massive galaxies at 2 3 sigma) red sequence, which hosts similar to 60% of the stellar mass at the high-mass end. The red-sequence galaxies have little or no ongoing star formation, as inferred from both emission-line diagnostics and stellar continuum shapes. Their strong Balmer breaks and their location in the rest-frame U - B, B - V plane indicate that they are in a poststarburst phase, with typical ages of similar to 0.5-1.0 Gyr. In order to study the evolution of the red sequence, we compare our sample with spectroscopic massive galaxy samples at 0.02 10(11)M(circle dot)) of the red sequence grow by factors of similar to 8 and similar to 6, respectively. We explore simple models to explain the observed evolution. Passive evolution models predict too-strong Delta(U - B) and produce z similar to 0 galaxies that are too red. More complicated models that include aging, galaxy transformations, and red mergers can explain both the number density and color evolution of the massive end of the red sequence between z similar to 2.3 and the present.

Hubble Space Telescope Images of Red Mergers: How Dry Are They?

Mergers between red galaxies are observed to be common in the nearby universe, and are thought to be the dominant mechanism by which massive galaxies grow their mass at late times. These "dry" mergers can be readily identified in very deep ground-based images, thanks to their extended low surface brightness tidal features. However, ground-based images lack the required resolution to determine the morphologies of the merging galaxies, and to measure the amount of dust and associated gas. We present HST ACS and WFPC2 observations of a sample of 31 bulge-dominated red-sequence galaxies at z ∼ 0.1, comprised of ongoing mergers, merger remnants, and undisturbed galaxies. Nearly all galaxies have early-type morphologies and most are well-fit by r1/4 law surface brightness profiles. We find that only 10% of the galaxies show evidence for the presence of dust. The amount of cold gas (or its upper limit) is calculated from the mean color excess, assuming a simple relation between gas mass and dust mass. The gas mass is low for all galaxies, and we find that Mgas/Mstellar ≲ 3 × 104. We infer that red mergers in the nearby universe mostly involve early-type galaxies containing little cold gas and dust. This may imply that the progenitors were mostly devoid of gas and/or that feedback mechanisms are very effective in preventing the gas to cool. The lack of gas in these objects may also imply a relatively large fraction of binary black holes in the centers of massive ellipticals.

The Evolution of Galaxy Mergers and Morphology atz< 1.2 in the Extended Groth Strip

We present the quantitative rest-frame B morphological evolution and galaxy merger fractions at 0.2 < z < 1.2 as observed by the All-wavelength Extended Groth Strip International Survey (AEGIS). We use the Gini coefficent and M_20 to identify major mergers and classify galaxy morphology for a volume-limited sample of 3009 galaxies brighter than 0.4 L_B^*, assuming pure luminosity evolution of 1.3 M_B per unit redshift. We find that the merger fraction remains roughly constant at 10 +/- 2% for 0.2 < z < 1.2. The fraction of E/S0/Sa increases from 21+/- 3% at z ~ 1.1 to 44 +/- 9% at z ~ 0.3, while the fraction of Sb-Ir decreases from 64 +/- 6% at z ~ 1.1 to 47 +/- 9% at z ~ 0.3. The majority of z < 1.2 Spitzer MIPS 24 micron sources with L(IR) > 10^11 L_sun are disk galaxies, and only ~ 15% are classified as major merger candidates. Edge-on and dusty disk galaxies (Sb-Ir) are almost a third of the red sequence at z ~ 1.1, while E/S0/Sa makeup over 90% of the red sequence at z ~ 0.3. Approximately 2% of our full sample are red mergers. We conclude (1) the galaxy merger rate does not evolve strongly between 0.2 < z < 1.2; (2) the decrease in the volume-averaged star-formation rate density since z ~ 1 is a result of declining star-formation in disk galaxies rather than a disappearing population of major mergers; (3) the build-up of the red sequence at z < 1 can be explained by a doubling in the number of spheroidal galaxies since z ~ 1.2.

How Dry Are Red Mergers?

The focus of current research in galaxy evolution has increasingly turned to understanding the effect that mergers have on the evolution of systems on the red sequence. For those interactions purported to occur dissipationlessly (so called mergers), it would appear that the role of gas is minimal. However, if these mergers are not completely dry, then even low levels of gas may be detectable. The purpose of our study is to test whether early type galaxies with HI in or around them, or wet ellipticals, would have been selected as dry mergers by the criteria in van Dokkum (2005, AJ, 130, 2647). To that end, we examine a sample of 20 early types from the HI Rogues Gallery with neutral hydrogen in their immediate environs. Of these, the 15 brightest and reddest galaxies match the optical dry merger criteria, but in each case, the presence of HI means that they are not truly dry.

Red mergers and the assembly of massive elliptical galaxies: the fundamental plane and its projections

Several recent observations suggest that gas-poor (dissipationless) mergers of elliptical galaxies contribute significantly to the build-up of the massive end of the red sequence at z≲ 1. We perform a series of major merger simulations to investigate the spatial and velocity structure of the remnants of such mergers. Regardless of orbital energy or angular momentum, we find that the stellar remnants lie on the fundamental plane defined by their progenitors, a result of virial equilibrium with a small tilt due to an increasing central dark matter fraction. However, the locations of merger remnants in the projections of the fundamental plane – the Faber–Jackson and Re–M* relations – depend strongly on the merger orbit, and the relations steepen significantly from the canonical scalings (L∝σ4e and Re∝M0.6*) for mergers on radial orbits. This steepening arises because stellar bulges on orbits with lower angular momentum lose less energy via dynamical friction on the dark matter haloes than do bulges on orbits with substantial angular momentum. This results in a less tightly bound remnant bulge with a smaller velocity dispersion and a larger effective radius. Our results imply that the projections of the fundamental plane – but not necessarily the plane itself – provide a powerful way of investigating the assembly history of massive elliptical galaxies, including the brightest cluster galaxies at or near the centres of galaxy clusters. We argue that most massive ellipticals are formed by anisotropic merging and that their fundamental plane projections should thus differ noticeably from those of lower mass ellipticals even though they should lie on the same fundamental plane. Current observations are consistent with this conclusion. The steepening in the L–σe relation for luminous ellipticals may also be reflected in a corresponding steepening in the MBH–σe relation for massive black holes.

The Recent and Continuing Assembly of Field Elliptical Galaxies by Red Mergers

We present a study of tidal debris associated with 126 nearby red galaxies, selected from the 1.2 deg2 Multiwavelength Survey by Yale-Chile and the 9.3 deg2 NOAO Deep Wide-Field Survey. In the full sample, 67 galaxies (53%) show morphological signatures of tidal interactions consisting of broad fans of stars, tails, and other asymmetries at very faint surface brightness levels. When restricting the sample to the 86 bulge-dominated early-type galaxies, the fraction of tidally disturbed galaxies rises to 71%, which implies that for every "normal" undisturbed elliptical there are two that show clear signs of interactions. The tidal features are red and smooth and often extend over >50 kpc. Of the tidally distorted galaxies, about two-thirds are remnants, and one-third are interacting with a companion galaxy. The companions are usually bright red galaxies as well; the median R-band luminosity ratio of the tidal pairs is 0.31, and the median color difference after correcting for the slope of the color-magnitude relation is -0.02 in B - R. If the ongoing mergers are representative for the progenitors of the remnants, ∼35% of bulge-dominated galaxies experienced a merger with mass ratio >1 : 4 in the recent past. With further assumptions it is estimated that the present-day mass accretion rate of galaxies on the red sequence ΔM/M = 0.09 ± 0.04 Gyr-1. For a constant or increasing mass accretion rate with redshift, we find that red mergers may lead to an evolution of a factor of ≳2 in the stellar mass density in luminous red galaxies over the redshift range 0 < z < 1, consistent with recent studies of the evolution of the luminosity density. We conclude that the majority of today's most luminous field elliptical galaxies were assembled at low redshift through mergers of gas-poor, bulge-dominated systems. These "dry" mergers are consistent with the high central densities of ellipticals, their old stellar populations, and the strong correlations of their properties. It will be interesting to determine whether this mode of merging only plays an important role at low redshift or is relevant for galaxies at any redshift if they exceed a critical mass scale.