--- title: Compact and Massive Galaxies (CMGs) url: https://www.emergentmind.com/topics/compact-and-massive-galaxies-cmgs type: topic --- # Compact and Massive Galaxies (CMGs) Searching arXiv for recent and foundational papers on compact and massive galaxies to ground the article. Searching arXiv for local compact massive galaxies, relics, and MaNGA/SDSS studies. Compact and Massive Galaxies (CMGs) are galaxies with unusually small effective radii for their stellar masses, commonly in the regime of \(M_\star \sim 10^{11}\,M_\odot\) and \(R_{\rm e}\sim 1\)–\(2\) kpc. In current usage, the category spans several closely related populations: the compact quiescent galaxies at high redshift often called “red nuggets,” local massive compact quiescent systems, compact post-starburst galaxies, and relic galaxies that appear to have undergone little subsequent structural growth. Across these contexts, CMGs are central to work on compaction, quenching, merger-driven size evolution, progenitor bias, and the coupling between stellar and halo assembly [1411.0667] [1605.09734]. ## 1. Definitions and taxonomic scope There is no single universal definition of a CMG. At low redshift, one widely used purely structural selection is \(0 9.2 \times 10^{10}\,M_\odot\), and \(R_e < 1.5\,\mathrm{kpc}\), with no additional cut on morphology, color, or star-formation rate [1203.2623]. At intermediate redshift, compact quiescent galaxies are often defined through the compactness parameter \[ \Sigma_{1.5} \equiv \log\left(\frac{M}{R_{\rm e}^{1.5}}\right)\;[M_\odot\,{\rm kpc}^{-1.5}], \] with the threshold \[ \Sigma_{1.5} > 10.3, \] combined with spectroscopic quiescence, for example weak [O II], H\(\beta\), and H\(\alpha\) emission [1605.09734]. In Illustris at \(z=2\), a stricter CMG selection used for formation-history analysis is \[ M_\star > 10^{11}\,M_\odot,\qquad R_{1/2}<2\,\mathrm{kpc}, \] with quiescence treated as a secondary classification rather than a formal requirement [1411.0667]. Other low-redshift studies define MCGs through joint outlier status in structural and dynamical scaling relations. One SDSS-based framework selects quiescent galaxies that lie \(2\sigma_{\rm fit}\) below \[ \log R_e = 0.8784\,\log \sigma_e - 1.2265 \] and \(2\sigma_{\rm fit}\) above \[ \log \sigma_e = 0.2538\,\log M_\star - 0.5622, \] thereby isolating galaxies that are simultaneously too small for their \(\sigma_e\) and too high-\(\sigma_e\) for their \(M_\star\) [2605.09733]. This suggests that “compactness” is operational rather than singular: some studies emphasize size, some stellar surface density, and some a joint structural-dynamical extremeness. The taxonomic breadth matters because different selections recover physically distinct subpopulations. Some compact systems are ancient relics, some are intermediate-redshift post-starburst progenitors of compact quiescent galaxies, and some are later-forming or rejuvenated compact systems. The term CMG therefore denotes a family of dense galaxies rather than a single evolutionary class. ## 2. Structural and kinematic phenomenology Nearby compact massive galaxies selected by size and mass alone have \(R_e \sim 0.8\)–\(1.6\,\mathrm{kpc}\), velocity dispersions \(\sigma \sim 135\)–\(233\,\mathrm{km\,s^{-1}}\), Sérsic indices \(n \sim 2.4\)–\(5.8\), and mostly elongated morphologies with \(b/a \lesssim 0.6\); five of seven systems in the detailed sample show significant rotation, and most occupy the fast-rotator regime [1203.2623]. In high-redshift observations, the canonical compact quiescent systems at \(z\sim 2\) have typical \(M_* \sim 1.7 \times 10^{11}\,M_\odot\), median \(r_e \sim 0.9\,\mathrm{kpc}\), \(\langle n\rangle \sim 2.3\), and a broad ellipticity distribution including objects with \(\epsilon > 0.6\), consistent with substantial rotational flattening [1008.4127]. Integral-field work strengthens the kinematic picture. In MaNGA, low-redshift massive compact galaxies are predominantly fast rotators and show a strong anti-correlation between the Gauss-Hermite moment \(h_3\) and \(V/\sigma\), whereas a substantial fraction of median-sized quiescent controls are slow rotators and the fast-rotating controls generally show only a weak \(h_3\)–\(V/\sigma\) anti-correlation [2104.12737]. That combination is characteristic of systems with significant ordered rotation and a strong contribution from short-axis tube orbits. It places many local MCGs closer to compact, bulge-dominated S0-like systems than to classical giant slow-rotator ellipticals. Deep low-redshift imaging further indicates that CMG structure is often more complex than single-Sérsic fits imply. In a sample of 246 \(z\sim 0\) massive compact quiescent galaxies with \(\log M_\star \sim 10\)–11, \(\sigma_e \sim 150\)–\(350\,\mathrm{km\,s^{-1}}\), and \(R_e \sim 0.7\)–\(2.5\,\mathrm{kpc}\), \(93\%\) are morphologically S0 and \(75\%\) require a three-component decomposition consisting of bulge, disk, and envelope; bars are absent, and the main structural distinction from matched average-sized quiescent galaxies is a much more compact disk component, with \(R_{\mathrm{e,Disk}}=1.9\) versus \(3.3\,\mathrm{kpc}\) [2605.09733]. This strongly favors a picture in which many local CMGs are compact, dynamically hot, disk-bearing systems rather than miniature pure ellipticals. A separate structural issue is that light does not necessarily trace mass in a straightforward way. Hydrodynamical merger simulations of high-\(z\) compact quiescent galaxies predict strong age, metallicity, and extinction gradients such that the median ratio \(R_{e,V}/R_{e,\rm mass}\) is \(\sim 2\) during the quiescent remnant phase, with red cores and wavelength-dependent sizes [1008.4127]. This implies that CMG densities inferred from rest-frame optical light are, in many cases, lower limits to the true stellar-mass densities. ## 3. Formation channels across cosmic time Searches for the progenitors of compact quiescent galaxies at \(z\sim 2\) already show that the channel must be active above \(z=3\). In NMBS plus CANDELS, four compact massive quiescent galaxies were identified at \(z>3\), together with five compact star-forming galaxies at \(z\sim 3\) with \(R_e<1.4\,\mathrm{kpc}\) and \(M_*>10^{10.6}\,M_\odot\), likely progenitors of the \(z\sim 2\) compact quiescent population; the observed number densities require that additional compact quiescent systems be created in the \(\sim 1\) Gyr interval between \(z=3\) and \(z=2\), and constant star-formation histories fail to reproduce the observed \(z\sim 2\) quiescent population [1301.7063]. In cosmological hydrodynamical simulations, two dominant pathways emerge. In Illustris, \(z=2\) CMGs with \(M_*>10^{11}\,M_\odot\) and \(R_{1/2}<2\,\mathrm{kpc}\) arise mainly through either merger-driven starburst compaction or very early assembly in a denser universe [1411.0667]. In the starburst channel, gas-rich major mergers at \(z\sim 2\)–4 can drive star formation rates above \(\sim 500\,M_\odot\,\mathrm{yr}^{-1}\), increase the central stellar density sharply, and shrink \(R_{1/2}\) from \(\sim 6\) kpc to \(<2\) kpc. In the early-assembly channel, half the final stellar mass can already be in place by \(z=4.8\), after which the galaxy remains compact and quenches early. The two channels are not disjoint, and hybrid cases are common. Idealized merger simulations add an important constraint. Gas-rich major mergers of compact high-redshift disks can reproduce the masses, sizes, dispersions, and rotational support of compact quiescent galaxies if the gas fraction at final coalescence is high, \(f_{\rm gas}\sim 0.4\), but the resulting remnants are typically too cuspy and too wing-dominated in single-Sérsic terms, with \(n\gg 4\), compared with the observed \(n\sim 2\)–4 range [1008.4127]. This implies that forming the dense core is not the main difficulty; limiting early, low-density star formation in the progenitors is. At \(z=0\), IllustrisTNG indicates that most simulated MCGs do not require an explicit late compaction event. Instead, they generally assemble early and accrete low-angular-momentum gas, increasing mass while growing size much more slowly; only a small fraction shrink in size by \(40\%\) or more before quenching [2307.08911]. In the same models, enhanced SMBH growth accompanies this compact pathway, and MCGs reach the threshold SMBH mass of \(\log M_{\rm BH}\sim 10^{8.5}\,M_\odot\) earlier than non-compact galaxies, at which point kinetic AGN feedback becomes effective and quenching proceeds rapidly. ## 4. Descendants, relics, and environmental processing Dark-matter-only halo analyses place an environmental framework around the observed CMG phenomenon. Using NGC 1277 as an anchor, one Bolshoi-based study defines compact halo analogs by \[ \rho_{\rm s} > 0.02~{\rm M}_\odot\,{\rm pc}^{-3},\qquad M_{\rm v} > 2\times10^{12}\,M_\odot, \] finding 346 such halos in the simulation volume, a number density \(n \approx 7\times 10^{-6}\,{\rm Mpc}^{-3}\), and showing that they are the most compact \(\sim 1\%\) of halos in that mass range [1503.03078]. More than \(80\%\) are substructures at \(z=0\), about \(94\%\) are or have been influenced by a larger host halo, and the probability that a massive subhalo is this compact rises from \(\sim 5\%\) in \(10^{13}\,M_\odot\) hosts to \(\sim 20\)–40\% in \(10^{15}\,M_\odot\) clusters. Their \(z=2\) progenitors are structurally ordinary for their epoch, but subsequently experience unusually low mass accretion; about \(30\%\) even lose mass between \(z=2\) and \(z=0\). This strongly links present-day compactness to stalled halo growth and environmental processing after infall. A direct descendant study in Illustris reaches a parallel conclusion on the stellar side. Following 35 massive compact galaxies from \(z=2\) to \(z=0\), about half become the compact cores of more massive descendants, a third remain largely undisturbed, \(15\%\) are consumed in mergers, and only \(\sim 10\%\) remain compact by \(z=0\); the majority of the size growth is driven by acquired ex-situ mass, and isolated or satellite compact galaxies are the most likely to survive [1507.02291]. The implication is not that compact systems disappear, but that many cease to be identifiable as stand-alone compact galaxies because they become embedded dense cores. This interpretation is reinforced by SDSS bulge-disk decompositions at \(z\sim 0.1\). Compact cores satisfying the same compactness criteria used for red nuggets are abundant, with number densities comparable to the peak number densities of red nuggets at \(z\sim 1.5\), and their hosts are distributed across multiple morphologies: \(50.9\%\) ellipticals, \(43.4\%\) S0s, \(5.4\%\) Sab, and \(0.3\%\) Scd [1601.03920]. The standard inside-out growth picture is therefore broader than a simple red-nugget-to-elliptical mapping. A substantial fraction of high-redshift compact systems can plausibly survive as compact cores embedded in disks as well as in spheroids. ## 5. Stellar populations, star-formation histories, and population diversity Low-redshift compact systems are not uniformly old relics. In one nearby sample of seven massive compact galaxies, the mean luminosity-weighted ages are mostly \(<2\,\mathrm{Gyr}\), metallicities are solar or super-solar, age and metallicity gradients are weak, and the inferred star-formation histories include recent bursts that in some cases contributed more than \(30\%\)–\(50\%\) of the total stellar mass within the last \(\sim 2\) Gyr [1203.2623]. These galaxies are structurally similar to high-redshift CMGs but are not straightforward fossil survivors of that population. They instead appear to be late-forming or strongly rejuvenated compact systems. Intermediate-redshift compact E+A galaxies provide a direct bridge between these rejuvenated systems and older compact quiescent galaxies. A catalog of 438 compact E+A galaxies at \(0.2