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An approximately extended component surrounding a galactic disk, distinguished according to whether one means the stellar halo, gaseous halo or dark matter halo. These components have different constituents and scales.

The Milky Way is the barred spiral galaxy containing the Solar system. Its galactic disk, central stellar bar and surrounding stellar halo are viewed from inside the disk, so distance and obscuration complicate attempts to reconstruct the whole system. Radio astronomy supplies a geometric route through the obscuring interstellar dust: masers near young massive stars act as compact markers whose parallaxes and proper motions can be measured by very-long-baseline interferometry. A 2019 analysis of about 200 such markers mapped spiral structure and constrained the rotation curve using their three-dimensional motions. These measurements connect the familiar bright band along the galactic plane with a spatial and dynamical model.^[ref]

The qualitative prediction of galactic chemical evolution is useful: later stellar populations tend to inherit more enriched gas, and old stellar populations can preserve low stellar metallicity. The closed-box metallicity distribution, however, predicts many more metal-poor long-lived dwarfs in the solar neighbourhood than are observed. This is the G-dwarf problem, also apparent with other long-lived tracers. A single well-mixed box cannot describe the different age and abundance distributions of the galactic disk, galactic bulge, and stellar halo of the Milky Way. Nor need its exponentially declining star formation rate match the history of ongoing disk star formation.

Prolonged relatively metal-poor galactic gas inflow supplies fuel while diluting the enrichment, allowing many stars to form after the earliest enrichment stage; pre-enrichment of the initial reservoir also removes a pristine tail. Galactic outflows lower the effective yield, but a constant well-mixed outflow alone still gives an exponential metallicity distribution function, and is not automatically a cure for the G-dwarf problem. Spatially varying star formation, radial transport and migration, and selection of local survivors complicate a comparison with one global box. A varying initial mass function or stellar yield changes the enrichment law. Finally, finite stellar lifetimes and Type Ia supernovae make iron enrichment delayed, so an observed iron-abundance distribution is not interchangeable with the instantaneous total-metal distribution. Gas exchange, spatially distinct populations, and delayed enrichment are essential qualifications to the closed-box prediction.

The relevant comparison is continuing hierarchical galaxy formation in the Lambda-CDM model against an idealized monolithic galaxy formation model in which both formation and assembly were essentially complete early. Each observation below concerns a population or a fossil record, rather than a currently merging pair. None individually proves that every galaxy formed through the same route.

Fossil substructure in stellar halos. Old stars in the Milky Way's stellar halo include coherent groups in orbital-integral space, and external halos contain extended debris and shells. Phase mixing can erase spatial concentration while retaining correlations of orbital energy or angular momentum. Such stellar streams, when linked to a disrupted dwarf progenitor, record accretion of an already formed stellar population. A smooth single collapse followed by quiet passive evolution has no equally direct mechanism for creating several dynamically and chemically distinct accreted populations. This supports a composite assembly history, without requiring that every halo star was accreted or counting a current merging pair as evidence.

Growth of the quiescent population. The galaxy red sequence contains old stellar populations, yet its inferred stellar mass density grows between approximately redshift one and the present. Pure fading of an already fixed early population changes luminosities and colours, but cannot increase the mass in that population. Quenching adds newly passive systems, while subsequent assembly combines older stellar populations into larger galaxies. The observed evolution therefore requires demographic evolution after the putative completed collapse epoch. It is consistent with hierarchical galaxy formation plus quenching, although red-sequence growth alone does not distinguish assembly from all possible histories of in-situ star formation; stellar-mass estimates and survey selection must be treated consistently.

Evolution of galaxy morphology. Distant galaxy samples contain a larger prevalence of irregular and peculiar structures than a simple non-evolving extrapolation of the local population. Settled discs and smooth spheroids become common outcomes as time proceeds. Continued gas supply, dynamical interactions, and internal rearrangement in hierarchical galaxy formation naturally allow this galaxy morphological evolution. A model in which structure became fixed during one early collapse and then only stellar light faded has difficulty producing the population change. Rest-frame wavelength, resolution, and surface-brightness selection can affect the observed classifications, and clumpiness is not unique evidence for a galaxy merger: unstable gas-rich discs can also be irregular.

Evolution of sizes at fixed stellar mass. Massive high-redshift galaxies, including concentrated systems, are often more compact than comparable populations at low redshift. Galaxy size evolution implies structural growth beyond passive luminosity fading. In particular, deposition of accreted stars at large radii in minor galaxy mergers can enlarge the outer stellar body while preserving an old central stellar population; gas accretion can grow discs from the inside outward. A completed early collapse with a fixed stellar structure does not naturally supply those new outskirts. Changes in sample membership and expansion associated with stellar mass loss also contribute, so this is evidence requiring continued structural evolution rather than a unique quantitative measurement of a merger rate.

Diffuse intracluster stars. Intracluster light and extended outer envelopes show that some stars no longer belong to individual cluster galaxies. Their irregular distribution and substructure retain evidence of redistribution within an assembling cluster. Tidal stripping and disruption of member systems supply stars to the common environment, as expected when smaller bound systems are incorporated into a growing hierarchy. Passive evolution of independent galaxies formed in isolated early collapses cannot itself move their stars into the space between galaxies. Early stellar formation followed by later stripping remains possible, but admitting that later dynamical assembly already departs from the strict completed-collapse picture. This evidence concerns the cumulative stellar distribution, not merely the existence of an occasional present-day merging pair.

Measure the logarithmic surface brightness slope in resolved radial annuli after accounting for instrumental blurring and foreground emission. Between a halo core and its outer boundary, dark-matter decay radiation predicts $d\ln I/d\ln R=-1$, whereas dark-matter annihilation radiation predicts $-3$; doubling radius therefore reduces the respective surface brightness by factors $2$ and $8$. These are quantitative templates to compare with independently mapped stellar and gaseous components.

An exponential galactic disk has $I_{\ast }(R)\propto e^{-R/R_d}$, so $d\ln I_{\ast }/d\ln R=-R/R_d$ and $I_{\ast }(2R)/I_{\ast }(R)=e^{-R/R_d}$. At $R=5R_d$ the latter is about $0.0067$, much faster decline than either halo template, and the projected emission should also be flattened along the galactic disk. A galactic bulge described by a Sérsic profile has $d\ln I_{\ast }/d\ln R=-(b_n/n)(R/R_e{)}^{1/n}$; for $n=4$ and $b_4\simeq 7.67$, this is about $-3.4$ at $10R_e$ and continues to steepen, whereas a pure halo power law keeps a fixed slope.

For a stellar halo with outer luminosity density proportional to $r^{-p_{\ast }}$, the projection of a spherical power-law emissivity gives $I_{\ast }\propto R^{1-p_{\ast }}$. Illustratively, $p_{\ast }=3$--$4$ gives projected slopes $-2$--$-3$: then $I_d/I_{\ast }\propto R^{p_{\ast }-2}$ rises strongly outward, but annihilation can resemble the steeper stellar template. Gas-related gamma emission must be compared with observed gas columns and the cosmic ray distribution; it is not always proportional to optical light.

A shallow, extended, approximately spherical $R^{-1}$ component is particularly distinctive; an $R^{-3}$ component needs additional evidence. Fit the stellar, gas and halo templates together over a broad radial range, including morphology and emission spectra. A measured slope alone cannot uniquely identify dark matter, since stellar or gas profiles can coincide with a halo slope over a limited interval.

Massive stars rapidly return oxygen, magnesium, neon and other alpha elements through core-collapse supernovae, whereas Type Ia supernovae supply a substantial delayed iron contribution. If the galactic bulge and stellar halo formed much of their stellar mass rapidly, their stars locked in enhanced alpha-element to iron ratios before much delayed iron enrichment, while prolonged star formation in the local galactic disk included more iron-rich gas. The abundance contrast chiefly records different enrichment timescales, with the initial mass function and gas exchange also affecting the ratios.

The Milky Way has a thin stellar galactic disk, radius roughly $15$–$20\mathrm{kpc}$ and scale height about $0.3\mathrm{kpc}$, containing gas, dust, young and old stars, spiral arms and most current star formation. Its stellar mass is a few $10^{10}{M}_{\odot }$. A thicker, older galactic disk has scale height around $1\mathrm{kpc}$ and stellar mass of order $10^{10}{M}_{\odot }$, with little cold gas.

The central galactic bulge and bar extend a few thousand parsecs and contain about $10^{10}{M}_{\odot }$, largely older stars. The central supermassive black hole has mass about $4\times 10^6{M}_{\odot }$ and is negligible in the total galactic budget. A diffuse old metal-poor stellar halo, extending tens to over $100\mathrm{kpc}$, contains globular clusters and roughly $10^9{M}_{\odot }$ in stars. An extended dark matter halo reaches roughly $200\mathrm{kpc}$ and dominates the total mass, of order $10^{12}{M}_{\odot }$. These are approximate component scales; the gaseous galactic halo also extends beyond the stellar galactic disk.

The Milky Way has a thin stellar galactic disk, radius roughly $15$–$20\mathrm{kpc}$ and scale height about $0.3\mathrm{kpc}$, containing gas, dust, young and old stars, spiral arms and most current star formation. Its stellar mass is a few $10^{10}{M}_{\odot }$. A thicker, older galactic disk has scale height around $1\mathrm{kpc}$ and stellar mass of order $10^{10}{M}_{\odot }$, with little cold gas.

The central galactic bulge and bar extend a few thousand parsecs and contain about $10^{10}{M}_{\odot }$, largely older stars. The central supermassive black hole has mass about $4\times 10^6{M}_{\odot }$ and is negligible in the total galactic budget. A diffuse old metal-poor stellar halo, extending tens to over $100\mathrm{kpc}$, contains globular clusters and roughly $10^9{M}_{\odot }$ in stars. An extended dark matter halo reaches roughly $200\mathrm{kpc}$ and dominates the total mass, of order $10^{12}{M}_{\odot }$. These are approximate component scales; the gaseous galactic halo also extends beyond the stellar galactic disk.

Edge-on, the bright thin galactic disk forms a narrow band, with a central bulge protruding above and below it and a thinner gas-and-dust lane along the midplane. Young stars and H II regions occupy a vertical scale of a few tenths of a kiloparsec or less; a broader old thick disk has scale of order a kiloparsec. These are approximate scale heights, not sharp boundaries. The old stellar halo and globular clusters form a much larger, low-surface-brightness spheroidal component, extending to many tens of kiloparsecs or more. The dark matter halo is inferred dynamically rather than directly visible in such a sketch. The preceding figure is a structural diagram, not an external photograph or a reconstruction of a particular survey.

A stellar halo is the extended, approximately spheroidal stellar population surrounding a galaxy. It differs from the total dark matter halo. If its outer luminosity density is $r^{-p}$, its projected light follows the projection of a spherical power-law emissivity and scales as $R^{1-p}$ away from breaks and outer truncation.