Compact and Massive Galaxies (CMGs)

• CMGs are dense galaxies characterized by high stellar masses (around 10^11 M☉) and compact sizes (typically 1–2 kpc), spanning a variety of subpopulations such as red nuggets, relics, and post‐starburst systems.
• They exhibit distinct structural and kinematic properties, including high velocity dispersions, diverse Sérsic profiles, and a significant amount of ordered rotation, which collectively inform their evolutionary histories.
• Formation channels of CMGs include early assembly in a denser universe and merger-driven compaction, with subsequent evolution influenced by environmental factors and accretion through minor mergers.

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 (Wellons et al., 2014, Zahid et al., 2016).

1. Definitions and taxonomic scope

There is no single universal definition of a CMG. At low redshift, one widely used purely structural selection is $0$M_\star > 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 (Ferré-Mateu et al., 2012). 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 (Zahid et al., 2016). In Illustris at $R_{\rm e}\sim 1$0, a stricter CMG selection used for formation-history analysis is

$R_{\rm e}\sim 1$1

with quiescence treated as a secondary classification rather than a formal requirement (Wellons et al., 2014).

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 $R_{\rm e}\sim 1$2 below

$R_{\rm e}\sim 1$3

and $R_{\rm e}\sim 1$4 above

$R_{\rm e}\sim 1$5

thereby isolating galaxies that are simultaneously too small for their $R_{\rm e}\sim 1$6 and too high-$R_{\rm e}\sim 1$7 for their $R_{\rm e}\sim 1$8 (Clerici et al., 10 May 2026). 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_{\rm e}\sim 1$9–$2$0, velocity dispersions $2$1–$2$2, Sérsic indices $2$3–$2$4, and mostly elongated morphologies with $2$5; five of seven systems in the detailed sample show significant rotation, and most occupy the fast-rotator regime (Ferré-Mateu et al., 2012). In high-redshift observations, the canonical compact quiescent systems at $2$6 have typical $2$7, median $2$8, $2$9, and a broad ellipticity distribution including objects with $0Wuyts et al., 2010).

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 $0Schnorr-Müller et al., 2021). 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 $0$M_\star > 9.2 \times 10^{10}\,M_\odot$0, $M_\star > 9.2 \times 10^{10}\,M_\odot$1 are morphologically S0 and $M_\star > 9.2 \times 10^{10}\,M_\odot$2 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 $M_\star > 9.2 \times 10^{10}\,M_\odot$3 versus $M_\star > 9.2 \times 10^{10}\,M_\odot$4 (Clerici et al., 10 May 2026). 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-$M_\star > 9.2 \times 10^{10}\,M_\odot$5 compact quiescent galaxies predict strong age, metallicity, and extinction gradients such that the median ratio $M_\star > 9.2 \times 10^{10}\,M_\odot$6 is $M_\star > 9.2 \times 10^{10}\,M_\odot$7 during the quiescent remnant phase, with red cores and wavelength-dependent sizes (Wuyts et al., 2010). 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 $M_\star > 9.2 \times 10^{10}\,M_\odot$8 already show that the channel must be active above $M_\star > 9.2 \times 10^{10}\,M_\odot$9. In NMBS plus CANDELS, four compact massive quiescent galaxies were identified at $R_e < 1.5\,\mathrm{kpc}$0, together with five compact star-forming galaxies at $R_e < 1.5\,\mathrm{kpc}$1 with $R_e < 1.5\,\mathrm{kpc}$2 and $R_e < 1.5\,\mathrm{kpc}$3, likely progenitors of the $R_e < 1.5\,\mathrm{kpc}$4 compact quiescent population; the observed number densities require that additional compact quiescent systems be created in the $R_e < 1.5\,\mathrm{kpc}$5 Gyr interval between $R_e < 1.5\,\mathrm{kpc}$6 and $R_e < 1.5\,\mathrm{kpc}$7, and constant star-formation histories fail to reproduce the observed $R_e < 1.5\,\mathrm{kpc}$8 quiescent population (Stefanon et al., 2013).

In cosmological hydrodynamical simulations, two dominant pathways emerge. In Illustris, $R_e < 1.5\,\mathrm{kpc}$9 CMGs with $$\Sigma_{1.5} \equiv \log\left(\frac{M}{R_{\rm e}^{1.5}}\right)\;[M_\odot\,{\rm kpc}^{-1.5}],$$0 and $$\Sigma_{1.5} \equiv \log\left(\frac{M}{R_{\rm e}^{1.5}}\right)\;[M_\odot\,{\rm kpc}^{-1.5}],$$1 arise mainly through either merger-driven starburst compaction or very early assembly in a denser universe (Wellons et al., 2014). In the starburst channel, gas-rich major mergers at $$\Sigma_{1.5} \equiv \log\left(\frac{M}{R_{\rm e}^{1.5}}\right)\;[M_\odot\,{\rm kpc}^{-1.5}],$$2–4 can drive star formation rates above $$\Sigma_{1.5} \equiv \log\left(\frac{M}{R_{\rm e}^{1.5}}\right)\;[M_\odot\,{\rm kpc}^{-1.5}],$$3, increase the central stellar density sharply, and shrink $$\Sigma_{1.5} \equiv \log\left(\frac{M}{R_{\rm e}^{1.5}}\right)\;[M_\odot\,{\rm kpc}^{-1.5}],$$4 from $$\Sigma_{1.5} \equiv \log\left(\frac{M}{R_{\rm e}^{1.5}}\right)\;[M_\odot\,{\rm kpc}^{-1.5}],$$5 kpc to $$\Sigma_{1.5} \equiv \log\left(\frac{M}{R_{\rm e}^{1.5}}\right)\;[M_\odot\,{\rm kpc}^{-1.5}],$$6 kpc. In the early-assembly channel, half the final stellar mass can already be in place by $$\Sigma_{1.5} \equiv \log\left(\frac{M}{R_{\rm e}^{1.5}}\right)\;[M_\odot\,{\rm kpc}^{-1.5}],$$7, 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, $$\Sigma_{1.5} \equiv \log\left(\frac{M}{R_{\rm e}^{1.5}}\right)\;[M_\odot\,{\rm kpc}^{-1.5}],$$8, but the resulting remnants are typically too cuspy and too wing-dominated in single-Sérsic terms, with $$\Sigma_{1.5} \equiv \log\left(\frac{M}{R_{\rm e}^{1.5}}\right)\;[M_\odot\,{\rm kpc}^{-1.5}],$$9, compared with the observed $\Sigma_{1.5} > 10.3,$0–4 range (Wuyts et al., 2010). This implies that forming the dense core is not the main difficulty; limiting early, low-density star formation in the progenitors is.

At $\Sigma_{1.5} > 10.3,$1, 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 $\Sigma_{1.5} > 10.3,$2 or more before quenching (Lohmann et al., 2023). In the same models, enhanced SMBH growth accompanies this compact pathway, and MCGs reach the threshold SMBH mass of $\Sigma_{1.5} > 10.3,$3 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

$\Sigma_{1.5} > 10.3,$4

finding 346 such halos in the simulation volume, a number density $\Sigma_{1.5} > 10.3,$5, and showing that they are the most compact $\Sigma_{1.5} > 10.3,$6 of halos in that mass range (Stringer et al., 2015). More than $\Sigma_{1.5} > 10.3,$7 are substructures at $\Sigma_{1.5} > 10.3,$8, about $\Sigma_{1.5} > 10.3,$9 are or have been influenced by a larger host halo, and the probability that a massive subhalo is this compact rises from $\beta$0 in $\beta$1 hosts to $\beta$2–40\% in $\beta$3 clusters. Their $\beta$4 progenitors are structurally ordinary for their epoch, but subsequently experience unusually low mass accretion; about $\beta$5 even lose mass between $\beta$6 and $\beta$7. 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 $\beta$8 to $\beta$9, about half become the compact cores of more massive descendants, a third remain largely undisturbed, $\alpha$0 are consumed in mergers, and only $\alpha$1 remain compact by $\alpha$2; 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 (Wellons et al., 2015). 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 $\alpha$3. 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 $\alpha$4, and their hosts are distributed across multiple morphologies: $\alpha$5 ellipticals, $\alpha$6 S0s, $\alpha$7 Sab, and $\alpha$8 Scd (Rosa et al., 2016). 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 $\alpha$9, 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 $R_{\rm e}\sim 1$00–$R_{\rm e}\sim 1$01 of the total stellar mass within the last $R_{\rm e}\sim 1$02 Gyr (Ferré-Mateu et al., 2012). 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 $R_{\rm e}\sim 1$03 shows that they satisfy the same compactness criterion as compact quiescent galaxies, have SSP-equivalent ages mostly $R_{\rm e}\sim 1$04 Gyr with a distribution peaking around $R_{\rm e}\sim 1$05 Myr, and have velocity-dispersion distributions statistically consistent with older compact quiescent systems (Zahid et al., 2016). Under passive evolution and an E+A visibility time of $R_{\rm e}\sim 1$06 Gyr, the lower-limit cumulative number density of compact quiescent galaxies formed via this channel at $R_{\rm e}\sim 1$07 is comparable to the lower-limit total number density of compact quiescent galaxies in the same redshift range. This suggests ongoing replenishment of the compact population rather than pure attrition from high redshift.

By contrast, a large SDSS sample of 1,858 low-redshift MCGs selected as outliers in the $R_{\rm e}\sim 1$08–$R_{\rm e}\sim 1$09 and $R_{\rm e}\sim 1$10–$R_{\rm e}\sim 1$11 planes is predominantly old, with ages $R_{\rm e}\sim 1$12 Gyr, $R_{\rm e}\sim 1$13, and solar to super-solar metallicities (Clerici et al., 2024). In that sample, metallicity increases with $R_{\rm e}\sim 1$14, while age and $R_{\rm e}\sim 1$15 vary little with $R_{\rm e}\sim 1$16; at fixed $R_{\rm e}\sim 1$17, MCGs are on average more metal-poor than typical quiescent controls, and within the SDSS fiber they have lower stellar masses at fixed velocity dispersion, raising the possibility of a bottom-heavier IMF or a larger dark matter fraction in the inner $R_{\rm e}\sim 1$18–2 kpc.

The local compact population therefore contains multiple subfamilies. In MaNGA, 37 compact galaxies that bridge the mass gap between compact ellipticals and classical CMGs split into three groups: an old, rapidly formed relic-like class with mean age $R_{\rm e}\sim 1$19 Gyr, $R_{\rm e}\sim 1$20, and $R_{\rm e}\sim 1$21; an intermediate-age class with extended star-formation histories; and a rejuvenated class with a late increase in star formation around $R_{\rm e}\sim 1$22 Gyr ago (Grèbol-Tomàs et al., 2023). This suggests that compactness is compatible with early relic survival, extended in-situ growth, and rejuvenated dissipative formation.

6. Uncertainties, competing interpretations, and emerging diagnostics

Several major caveats recur across the literature. First, selection effects are central. In cluster samples, if all non-BCG galaxies are considered regardless of density, morphology, or spectral type, the median size at $R_{\rm e}\sim 1$23 is only a factor 1.18 smaller than in local clusters; by contrast, morphologically selected early-type samples give a much larger apparent size evolution, which can be driven by morphological transformation and progenitor bias rather than by strong physical growth of the same galaxies (Valentinuzzi et al., 2010). TNG-based work reaches a related conclusion: at least a third of present-day median-sized quiescent galaxies do not have a compact progenitor, so both dry mergers and progenitor bias are required to explain the separation between compact and median-sized quiescent populations (Lohmann et al., 2023).

Second, observational size and profile measurements do not map one-to-one onto mass structure. Strong $R_{\rm e}\sim 1$24 gradients can make rest-frame optical half-light radii substantially larger than half-mass radii, and single-Sérsic fits can obscure intrinsically multi-component systems (Wuyts et al., 2010). On the theoretical side, dark-matter-only studies provide a cosmological context for compact halos, but the mapping from halo parameters $R_{\rm e}\sim 1$25 to stellar compactness is indirect, and baryons can shift $R_{\rm e}\sim 1$26 and $R_{\rm e}\sim 1$27 by factors of $R_{\rm e}\sim 1$28 even if the average expectation shift is small (Stringer et al., 2015).

Third, not all frameworks attribute CMGs to the same underlying physics. Within MOND, a single hydrodynamical collapse model of an isolated, initially non-rotating post-Big-Bang gas cloud produces a galaxy with $R_{\rm e}\sim 1$29, $R_{\rm e}\sim 1$30, $R_{\rm e}\sim 1$31, and a star-formation timescale $R_{\rm e}\sim 1$32, comparable to the properties of NGC 1277-like relics (Eappen et al., 2024). This does not resolve the broader cosmological debate, but it shows that compact, high-dispersion, fast-rotating relic analogs can be modeled outside the standard dark-matter framework.

Finally, new diagnostics are emerging beyond stellar structure alone. In TNG100-based synthetic globular-cluster systems of 17 compact massive galaxies, three strong relic analogs show high in-situ GC fractions, narrow GC metallicity distributions, and compact GC spatial distributions; the GC mass fraction traces host assembly history more robustly than GC number fraction, and the ratio of ex-situ to in-situ GC extent is tightly correlated with the host stripped fraction (Moura et al., 3 Apr 2026). This suggests that GC systems may become a practical discriminator between true relics, tidally stripped compact systems, and compact galaxies that experienced substantial late accretion.

Taken together, the evidence supports a plural rather than unitary view of CMGs. Some are direct relics of early compact formation, some are the dense cores of systems that later grew by accretion, some are intermediate-redshift post-starburst products, and some are later or rejuvenated compacts. The common denominator is exceptional central density; the distinguishing variables are assembly history, environment, and the degree to which subsequent growth, stripping, or rejuvenation altered the original compact state.