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Register a new userWhen did the Large Elliptical Galaxies Form?
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P. J. E. Peebles
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The simple reading of the evidence is that the large elliptical galaxies existed at about the present star mass and comoving number density at redshift z=2. This is subject to the usual uncertainties of measurement and interpretation in astronomy, but should be taken seriously because it is indicated by quite a few lines of evidence. And it might be a guide to a more perfect theory of galaxy formation.
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The recent LIGO detection of gravitational waves (GW150914), likely originating from the merger of two $sim 30 M_odot$ black holes suggests progenitor stars of low metallicity ($[Z/Z_odot] lesssim 0.3$), constraining when and where the progenitor of GW150914 may have formed. We combine estimates of galaxy properties (metallicity, star formation rate and merger rate) across cosmic time to predict the low redshift black hole - black hole merger rate as a function of present day host galaxy mass, $M_mathrm{gal}$, and the formation redshift of the progenitor system $z_mathrm{form}$ for different progenitor metallicities $Z_mathrm{c}$. At $Z_mathrm{c}=0.1 Z_odot$, the signal is dominated by binaries in massive galaxies with $z_mathrm{form}simeq 2$, with a small contribution from binaries formed around $z_mathrm{form}simeq 0.5$ in dwarf galaxies. For $Z_mathrm{c}=0.01Z_odot$, fast mergers are possible and very recent star formation in dwarfs likely dominates. Additional gravitational wave detections from merging massive black holes will provide constraints on the mass-metallicity relation and massive star formation at high redshifts.
We have used the Hubble Space Telescopes Advanced Camera for Surveys (Ford et al. 2003) to measure the cumulative mass density in morphologically-selected early-type galaxies over the redshift range 0.8 < z < 1.7. Our imaging data set covers four well-separated sight-lines, and is roughly intermediate (in terms of both depth and area) between the GOODS/GEMS imaging data, and the images obtained in the Hubble Deep Field campaigns. Our images contain 144 galaxies with ultra-deep spectroscopy obtained as part of the Gemini Deep Deep Survey. These images have been analyzed using a new purpose-written morphological analysis code which improves the reliability of morphological classifications by adopting a quasi-Petrosian image thresholding technique. We find that at z ~ 1 about 80% of the stars living in the most massive galaxies reside in early-type systems. This fraction is similar to that seen in the local Universe. However, we detect very rapid evolution in this fraction over the range 0.8 < z < 1.7, suggesting that over this redshift range the strong morphology-mass relationship seen in the nearby Universe is beginning to fall into place. By comparing our images to published spectroscopic classifications, we show that little ambiguity exists in connecting spectral classes to morphological classes for spectroscopically quiescent systems. However, the mass density function of early-type galaxies is evolving more rapidly than that of spectroscopically quiescent systems, which we take as further evidence that we are witnessing the formation of massive early-type galaxies over the 0.8 < z < 1.7 redshift range.
We have used the Hubble Space Telescopes Advanced Camera for Surveys to measure the mass density function of morphologically selected early-type galaxies in the Gemini Deep Deep Survey fields, over the redshift range 0.9 < z < 1.6. Our imaging data set covers four well-separated sight-lines, and is roughly intermediate (in terms of both depth and area) between the GOODS/GEMS imaging data, and the images obtained in the Hubble Deep Field campaigns. Our images contain 144 galaxies with ultra-deep spectroscopy, and they have been analyzed using a new purpose-written morphological analysis code which improves the reliability of morphological classifications by adopting a quasi-petrosian image thresholding technique. We find that at z = 1 approximately 70% of the stars in massive galaxies reside in early-type systems. This fraction is remarkably similar to that seen in the local Universe. However, we detect very rapid evolution in this fraction over the range 1.0 < z < 1.6, suggesting that in this epoch the strong color-morphology relationship seen in the nearby Universe is beginning to fall into place.
The characteristic mass that sets the peak of the stellar initial mass function (IMF) is closely linked to the thermodynamic behaviour of interstellar gas, which controls how gas fragments as it collapses under gravity. As the Universe has grown in metal abundance over cosmic time, this thermodynamic behaviour has evolved from a primordial regime dominated by the competition between compressional heating and molecular hydrogen cooling to a modern regime where the dominant process in dense gas is protostellar radiation feedback, transmitted to the gas by dust-gas collisions. In this paper we map out the primordial-to-modern transition by constructing a model for the thermodynamics of collapsing, dusty gas clouds at a wide range of metallicities. We show the transition from the primordial regime to the modern regime begins at metallicity $Zsim 10^{-4} rm{Z_odot}$, passes through an intermediate stage where metal line cooling is dominant at $Z sim 10^{-3},rm{Z_{odot}}$, and then transitions to the modern dust- and feedback-dominated regime at $Zsim 10^{-2} rm{Z_odot}$. In low pressure environments like the Milky Way, this transition is accompanied by a dramatic change in the characteristic stellar mass, from $sim 50,rm{M_odot}$ at $Z sim 10^{-6},rm{Z_{odot}}$ to $sim 0.3,rm{M_odot}$ once radiation feedback begins to dominate, which marks the appearance of the modern bottom-heavy Milky Way IMF. In the high pressure environments typical of massive elliptical galaxies, the characteristic mass for the modern, dust-dominated regime falls to $sim 0.1,rm{M_{odot}}$, thus providing an explanation for the brown dwarf rich population observed in these galaxies. We conclude that metallicity is a key driver of variations in the characteristic stellar mass, and by extension, the IMF.
Elliptical galaxies today appear aligned with the large-scale structure of the Universe, but it is still an open question when they acquire this alignment. Observational data is currently insufficient to provide constraints on the time evolution of intrinsic alignments, and hence existing models range from assuming that galaxies gain some primordial alignment at formation, to suggesting that they react instantaneously to tidal interactions with the large-scale structure. Using the cosmological hydrodynamical simulation Horizon-AGN, we measure the relative alignments between the major axes of galaxies and eigenvectors of the tidal field as a function of redshift. We focus on constraining the time evolution of the alignment of the main progenitors of massive $z=0$ elliptical galaxies, the main weak lensing contaminant at low redshift. We show that this population, which at $z=0$ has a stellar mass above $10^{10.4}$ M$_odot$, transitions from having no alignment with the tidal field at $z=3$, to a significant alignment by $z=1$. From $z=0.5$ they preserve their alignment at an approximately constant level until $z=0$. We find a mass-dependence of the alignment signal of elliptical progenitors, whereby ellipticals that are less massive today ($10^{10.4}<M/{rm M}_odot<10^{10.7}$) do not become aligned till later redshifts ($z<2$), compared to more massive counterparts. We also present an extended study of progenitor alignments in the parameter space of stellar mass and galaxy dynamics, the impact of shape definition and tidal field smoothing.
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