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Register a new userQuest for truly isolated galaxies
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Noah Brosch
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I describe attempts to identify and understand the most isolated galaxies starting from my 1983 Leiden PhD thesis, continuing through a string of graduate theses on various aspects of this topic, and concluding with an up-to-date account of the difficulty to find really isolated objects. The implication of some of the findings revealed on the way and presented here is that the nearby Universe may contain many small dark-matter haloes, and that some such haloes may possibly be accreting intergalactic gas to form dwarf galaxies.
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We analyze a suite of 33 cosmological simulations of the evolution of Milky Way-mass galaxies in low-density environments. Our sample spans a broad range of Hubble types at z=0, from nearly bulgeless disks to bulge-dominated galaxies. Despite the fact that a large fraction of the bulge is typically in place by z=1, we find no significant correlation between the morphology at z=1 and at z=0. The z=1 progenitors of disk galaxies span a range of morphologies, including smooth disks, unstable disks, interacting galaxies and bulge-dominated systems. By z=0.5, spiral arms and bars are largely in place and the progenitor morphology is correlated with the final morphology. We next focus on late-type galaxies with a bulge-to-total ratio B/T<0.3 at z=0. These show a correlation between B/T at z=0 and the mass ratio of the largest merger at z<2, as well as with the gas accretion rate at z>1. We find that the galaxies with the lowest B/T tend to have a quiet baryon input history, with no major mergers at z<2, and with a low and constant gas accretion rate that keeps a stable angular-momentum direction. More violent merger or gas accretion histories lead to galaxies with more prominent bulges. Most disk galaxies have a bulge Sersic index n<2. The galaxies with the highest bulge Sersic index tend to have histories of intense gas accretion and disk instability rather than active mergers.
Isolated galaxies have not been a hot topic over the past four decades. This is partly due to uncertainties about their existence. Are there galaxies isolated enough to be interesting? Do they exist in sufficient numbers to be statistically useful? Most attempts to compile isolated galaxy lists were marginally successful--too small number and not very isolated galaxies. If really isolated galaxies do exist then their value becomes obvious in a Universe where effects of interactions and environment (i.e. nurture) are important. They provide a means for better quantifying effects of nurture. The Catalog of Isolated Galaxies (CIG) compiled by Valentina Karachentseva appeared near the beginning of the review period. It becomes the focus of this review because of its obvious strengths and because the AMIGA project has increased its utility through a refinement (a vetted CIG). It contains almost 1000 galaxies with nearest neighbor crossing times of 1-3Gyr. It is large enough to serve as a zero-point or control sample. The galaxies in the CIG (and the distribution of galaxy types) may be significantly different than those in even slightly richer environments. The AMIGA-CIG, and future iterations, may be able to tell us something about galaxy formation. It may also allow us to better define intrinsic (natural) correlations like e.g. Fisher-Tully and FIR-OPTICAL. Correlations can be better defined when the dispersion added by external stimuli (nurture) is minimized or removed.
The basic properties of galaxies can be affected by both nature (internal processes) or nurture (interactions and effects of environment). Deconvolving the two effects is an important current effort in astrophysics. Observed properties of a sample of isolated galaxies should be largely the result of internal (natural) evolution. It follows that nurture-induced galaxy evolution can only be understood through comparative study of galaxies in different environments. We take a first look at SDSS (g-r) colors of galaxies in the AMIGA sample involving many of the most isolated galaxies in the local Universe. This leads us to simultaneously consider the pitfalls of using automated SDSS colors. We focus on median values for the principal morphological subtypes found in the AMIGA sample (E/S0 and Sb-Sc) and compare them with equivalent measures obtained for galaxies in denser environments. We find a weak tendency for AMIGA spiral galaxies to be redder than objects in close pairs. We find no clear difference when we compare with galaxies in other (e.g. group) environments. However, the (g-r) color of isolated galaxies shows a Gaussian distribution as might be expected assuming nurture-free evolution. We find a smaller median absolute deviation in colors for isolated galaxies compared to both wide and close pairs. The majority of the deviation on median colors for spiral subtypes is caused by a color-luminosity correlation. Surprisingly isolated and non-isolated early-type galaxies show similar (g-r). We see little evidence for a green valley in our sample with most spirals redder than (g-r)=0.7 having spurious colors. The redder colors of AMIGA spirals and lower color dispersions for AMIGA subtypes -compared with close pairs- is likely due to a more passive star formation in very isolated galaxies.
Using a suite of simulations (Governato et al. 2010) which successfully produce bulgeless (dwarf) disk galaxies, we provide an analysis of their associated cold interstellar media (ISM) and stellar chemical abundance patterns. A preliminary comparison with observations is undertaken, in order to assess whether the properties of the cold gas and chemistry of the stellar components are recovered successfully. To this end, we have extracted the radial and vertical gas density profiles, neutral hydrogen velocity dispersion, and the power spectrum of structure within the ISM. We complement this analysis of the cold gas with a brief examination of the simulations metallicity distribution functions and the distribution of alpha-elements-to-iron.
We study the comoving relative velocities, v12, of model isolated galaxy pairs at z=0.5. For this purpose, we use the predictions from the GALFORM semi-analytical model of galaxy formation and evolution based on a Lambda cold dark matter cosmology consistent with the results from WMAP7. In real space, we find that isolated pairs of galaxies are predicted to form an angle t with the line-of-sight that is uniformily distributed as expected if the Universe is homogeneous and isotropic. We also find that isolated pairs of galaxies separated by a comoving distance between 1 and 3 Mpc/h are predicted to have <v12>=0. For galaxies in this regime, the distribution of the angle t is predicted to change minimally from real to redshift space, with a change smaller than 5% in <sin^2 t>. However, the distances defining the comoving regime strongly depends on the applied isolation criteria.
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