* Astronomy

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RE: Massive galaxies

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Title: IAUS295 -- The Intriguing Life of Massive Galaxies: Introducing the Final Discussion
Authors: Alvio Renzini

This is a brief introduction to the closing discussion of the IAU Symposium 295, "The Intriguing Life of Massive Galaxies", that was held in Beijing from August 27 through 31, 2012. The discussion was focused on only four hot items, namely 1) the redshift evolution of the size of passively evolving galaxies, 2) the evolution with redshift of the specific star formation rate, 3) quenching of star formation in galaxies and dry merging, and 4) the IMF.

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Title: Evolution of the Most Massive Galaxies to z ~ 0.6: II. The link between radio AGN activity and star formation
Authors: Yan-Mei Chen, Guinevere Kauffmann, Timothy M. Heckman, Christy A. Tremonti, Simon White, Hong Guo, David Wake, Donald P. Schneider, Kevin Schawinski

We analyse the optical spectra of massive (log M*/solar masses > 11.4) radio-loud galaxies at z~0.2 and z~0.6. By comparing stellar population parameters of these radio-loud samples with radio-quiet control samples, we investigate how the presence of a radio-emitting jet relates to the recent star formation history of the host galaxy. We also investigate how the emission-line properties of the radio galaxies evolve with redshift by stacking their spectra. Our main results are the following. (1) Both at low and at high redshift, half as many radio-loud as radio-quiet galaxies have experienced significant star formation in the past Gyr. (2) The Balmer absorption line properties of massive galaxies that have experienced recent star formation show that star formation occurred as a burst in many of these systems. (3) Both the radio and the emission-line luminosity of radio AGN evolve significantly with redshift. However, radio galaxies with similar stellar population parameters, have similar emission-line properties both at high- and at low-redshift. These results suggest that massive galaxies experience cyclical episodes of gas accretion, star formation and black hole growth, followed by the production of a radio jet that shuts down further activity. The behaviour of galaxies with log M*/solar masses > 11.4 is the same at z = 0.6 as it is at z = 0.2, except that higher redshift galaxies experience more star formation and black hole growth and produce more luminous radio jets during each accretion cycle.

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Title: HST/WFC3 Confirmation of the Inside-Out Growth of Massive Galaxies at 0<z<2 and Identification of their Star Forming Progenitors at z\sim3
Authors: Shannon G. Patel, Pieter G. van Dokkum, Marijn Franx, Ryan F. Quadri, Adam Muzzin, Danilo Marchesini, Rik J. Williams, Bradford P. Holden, Mauro Stefanon

We study the structural evolution of massive galaxies by linking progenitors and descendants at a constant cumulative number density of n_c=1.4x10^{-4} Mpc^{-3} to z\sim3. Structural parameters were measured by fitting Sersic profiles to high resolution CANDELS HST WFC3 J_{125} and H_{160} imaging in the UKIDSS-UDS at 1<z<3 and ACS I_{814} imaging in COSMOS at 0.25<z<1. At a given redshift, we selected the HST band that most closely samples a common rest-frame wavelength so as to minimise systematics from color gradients in galaxies. At fixed n_c, galaxies grow in stellar mass by a factor of \sim3 from z\sim3 to z\sim0. The size evolution is complex: galaxies appear roughly constant in size from z\sim3 to z\sim2 and then grow rapidly to lower redshifts. The evolution in the surface mass density profiles indicates that most of the mass at r<2 kpc was in place by z\sim2, and that most of the new mass growth occurred at larger radii. This inside-out mass growth is therefore responsible for the larger sizes and higher Sersic indices of the descendants towards low redshift. At z<2, the effective radius evolves with the stellar mass as r_e\simM^{2.0}, consistent with scenarios that find dissipationless minor mergers to be a key driver of size evolution. The progenitors at z\sim3 were likely star forming thin disks with r_e\sim2 kpc, based on their low Sersic index of n\sim1, low median axis ratio of b/a\sim0.52, and typical location in the star forming region of the U-V vs. V-J diagram. By z\sim1.5, many of these star forming disks disappeared, giving rise to compact quiescent galaxies. Towards lower redshifts, these galaxies continued to assemble mass at larger radii and became the local ellipticals that dominate the high mass end of the mass function at the present epoch.

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'Starbursts' and black holes lead to biggest galaxies

Frenetic star-forming activity in the early Universe is linked to the most massive galaxies in today's cosmos, new research suggests.
This "starbursting" activity when the Universe was just a few billion years old appears to have been clamped off by the growth of supermassive black holes.
An international team gathered hints of the mysterious "dark matter" in early galaxies to confirm the link.
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The Wild Early Lives of Today's Most Massive Galaxies

Using the APEX telescope, a team of astronomers has found the strongest link so far between the most powerful bursts of star formation in the early Universe, and the most massive galaxies found today. The galaxies, flowering with dramatic starbursts in the early Universe, saw the birth of new stars abruptly cut short, leaving them as massive - but passive - galaxies of aging stars in the present day. The astronomers also have a likely culprit for the sudden end to the starbursts: the emergence of supermassive black holes.
Astronomers have combined observations from the LABOCA camera on the ESO-operated 12-metre Atacama Pathfinder Experiment (APEX) telescope with measurements made with ESO's Very Large Telescope, NASA's Spitzer Space Telescope, and others, to look at the way that bright, distant galaxies are gathered together in groups or clusters.
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Title: Satellites around massive galaxies since z~2
Authors: E. Mármol-Queraltó, I. Trujillo, P.G. Pérez-González, J. Varela, G. Barro

Accretion of minor satellites has been postulated as the most likely mechanism to explain the significant size evolution of the massive galaxies over cosmic time. Using a sample of 629 massive (Mstar~10^11 Msun) galaxies from the near-infrared Palomar/DEEP-2 survey, we explore which fraction of these objects has satellites with 0.01 Msat < Mcentral < 1 (1:100) up to z=1 and which fraction has satellites with 0.1 Msat < Mcentral < 1 (1:10) up to z=2 within a projected radial distance of 100 kpc. We find that the fraction of massive galaxies with satellites, after the background correction, remains basically constant and close to ~30% for satellites with a mass ratio down to 1:100 up to z=1, and ~15% for satellites with a 1:10 mass ratio up to z=2. The family of spheroid-like massive galaxies presents a 2-3 times larger fraction of objects with satellites than the group of disk-like massive galaxies. A crude estimation of the number of 1:3 mergers a massive spheroid-like galaxy experiences since z~2 is around 2. For a disk-like galaxy this number decreases to ~1.

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Large galaxies stopped growing 7 billion years ago

Galaxies are thought to develop by the gravitational attraction between and merger of smaller 'sub-galaxies', a process that standard cosmological ideas suggest should be ongoing. But new data from a team of scientists from Liverpool John Moores University directly challenges this idea, suggesting that the growth of some of the most massive objects stopped 7 billion years ago when the Universe was half its present age. On Monday 18 April team member Claire Burke will present their work at the Royal Astronomical Society's National Astronomy Meeting (NAM 2011) in Llandudno, Wales.
How galaxies form and then evolve is still a major unanswered question in astronomy. The sub-galaxy units thought to have merged to make galaxies, are themselves associated with fluctuations in the density of material in the cosmos left over from the Big Bang and seen today as temperature 'ripples' in the cosmic background radiation.
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Title: Little change in the sizes of the most massive galaxies since z = 1
Authors: J.P. Stott, C. A. Collins, C. Burke, V. Hamilton-Morris, G. P. Smith

Recent reports suggest that elliptical galaxies have increased their size dramatically over the last ~8 Gyr. This result points to a major re-think of the processes dominating the latetime evolution of galaxies. In this paper we present the first estimates for the scale sizes of brightest cluster galaxies (BCGs) in the redshift range 0.8 < z < 1.3 from an analysis of deep Hubble Space Telescope imaging, comparing to a well matched local sample taken from the Local Cluster Substructure Survey at z ~ 0.2. For a small sample of 5 high redshift BCGs we measure half-light radii ranging from 14 - 53 kpc using de Vaucuoleurs profile fits, with an average determined from stacking of 32.1 ±2.5 kpc compared to a value 43.2 ±1.0 kpc for the low redshift comparison sample. This implies that the scale sizes of BCGs at z = 1 are ~ 30% smaller than at z = 0.25. Analyses comparing either Sersic or Petrosian radii also indicate little or no evolution between the two samples. The detection of only modest evolution at most out to z = 1 argues against BCGs having undergone the large increase in size reported for massive galaxies since z = 2 and in fact the scale-size evolution of BCGs appears closer to that reported for radio galaxies over a similar epoch. We conclude that this lack of size evolution, particularly when coupled with recent results on the lack of BCG stellar mass evolution, demonstrates that major merging is not an important process in the late time evolution of these systems. The homogeneity and maturity of BCGs at z = 1 continues to challenge galaxy evolution models.

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Massive Galaxies Formed When Universe Was Young
New Findings Disagree with Current Models

Some of the universe's most massive galaxies may have formed billions of years earlier than current scientific models predict, according to surprising new research led by Tufts University. The findings appear in the Astrophysical Journal online Nov. 24 and in print Dec. 10, 2010.
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