Lenticular galaxy

• 1. Introduction

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  Lenticular galaxies occupy a strange middle ground in the universe. Designated S0, they carry a disc like a spiral galaxy but show none of the sweeping arms that make spirals so visually striking. Their stars are old, their gas is mostly gone, and the formation of new stars has slowed to nearly nothing. What created them? Were they once spirals that burned through their fuel? Products of violent collisions between galaxies? Or did they grow their discs piece by piece, drawing in gas and smaller galaxies over cosmic time? Those questions have no single agreed answer, and that ambiguity is precisely what makes lenticular galaxies one of the least understood classes of objects in the sky.

• 2. Shape and Structure

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  The axial ratio distribution of lenticular galaxies rises steadily between 0.25 and 0.85, a pattern quite different from spiral galaxies, whose distribution is essentially flat across that same range. That rise signals something important: lenticulars are dominated by a large, spherical central bulge. Picture two disc galaxies seen edge-on, one with a prominent bulge and one without. The bulge-bearing galaxy will appear thicker relative to its width, pushing its axial ratio higher. A sample weighted toward such bulge-heavy galaxies will naturally cluster at those larger ratios.

  The disc itself is typically featureless, which blocks the classification methods used for spirals. The bulge is spherical, which makes elliptical-galaxy labels equally unsuitable. Astronomers therefore sort lenticulars by two other traits: how much dust the disc holds and whether a central bar is present. Galaxies without a bar fall into classes S01, S02, and S03, with the subscript indicating increasing dust absorption. Their barred equivalents run from SB01 through SB03.

  The surface brightness profile of a lenticular galaxy is usually described by combining a Sersic model for the spheroidal bulge with an exponentially declining profile for the disc. The bulge carries a Sersic index typically ranging from n = 1 to 4, steeper than the disc. A third component is sometimes added to account for a bar. Occasionally the profiles show a truncation at roughly four disc scalelengths, a feature also seen in some spiral galaxies.

• 3. Bars and Box-Shaped Bulges

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  NGC 1460 has one of the largest bars seen among lenticular galaxies. In the SB0 classification scheme, SB01 galaxies show only a slight brightening along opposite sides of the central bulge, but by SB03 the bar is well defined and extends through the transition zone between the bulge and the disc, as NGC 1460 illustrates.

  NGC 1375 and NGC 1175 take lenticular structure in a different direction. Both carry what are called box-shaped bulges and are classified as SB0 peculiar. Box-shaped bulges appear mainly in edge-on spiral galaxies; seeing them in lenticulars is rare. Despite their visual interest, the properties of bars in lenticular galaxies have not been researched in great detail, and understanding both those properties and the mechanism that forms bars could help clarify how lenticular galaxies came to be.

• 4. Stellar Content and Clusters

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  All of the stars in a lenticular galaxy are thought to be older than about a billion years. That age threshold aligns with their offset from the Tully-Fisher relation, which plots the rotational velocity of a disc galaxy against its luminosity. Older, redder stars dominate, which shifts the luminosity offset by roughly delta-I of approximately 1.5 magnitudes compared to spiral galaxies at the same rotational speed.

  Globular clusters, the dense spherical groupings of ancient stars, appear more often in lenticular galaxies than in spiral galaxies of similar mass and luminosity. Molecular gas is largely absent, which is why almost no new stars are forming. Hydrogen alpha emission and 21-centimetre emission, both reliable signs of cool star-forming gas in normal disc galaxies, are essentially undetectable. Dust, however, can still be present in the disc, setting lenticulars apart from elliptical galaxies, which have swept theirs away.

• 5. Measuring Motion

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  Separating a lenticular galaxy from an elliptical galaxy by their motion requires measuring three quantities: velocity dispersion (sigma), rotational velocity (v), and ellipticity (epsilon). A rough working criterion puts elliptical galaxies at v/sigma below 0.5 for epsilon equal to 0.3. Lenticulars, with their rotating discs, sit at higher v/sigma ratios.

  Applying a single v/sigma value to an entire galaxy is complicated, because the ratio can change with the radius at which it is measured. The ES galaxies, which bridge ellipticals and S0 galaxies with their intermediate-scale discs, show a high v/sigma at intermediate radii that drops to a low value at large radii. That variation makes a single number unreliable.

  The standard tool for mapping disc rotation, hydrogen alpha or 21-centimetre emission lines, is not available for most lenticulars because the cool gas that produces those lines is missing. Astronomers fall back on stellar absorption lines, which are harder to interpret and yield less reliable rotational velocities. Inclination measurements add further difficulty, as does the way the bulge and disc overlap in projection, and the random motions of individual stars blur the true circular velocity.

• 6. Formation Theories

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  The faded-spiral scenario draws on several strands of evidence. Gas-poor, or anemic, spiral galaxies exist as a plausible intermediate stage. Moore and colleagues documented that tidal harassment, the gravitational tugging from nearby galaxies, could strip gas and suppress star formation in dense regions. If the spiral arms then dissolved, the remaining galaxy would resemble many lenticulars. The Tully-Fisher offset described above is the clearest supporting evidence, because it implies lenticulars once spun at spiral-galaxy speeds and have since faded.

  A 2012 paper building on a classification system first proposed by Canadian astronomer Sidney van den Bergh arranged lenticular and dwarf spheroidal galaxies into a sequence running S0a through S0b, S0c, and down to dSph. That sequence parallels the spiral Hubble sequence from Sa through Sb, Sc, and Im, reinforcing the genetic relationship between spirals and lenticulars.

  The merger hypothesis rests on different evidence. Burstein and Sandage showed that lenticular galaxies typically display surface brightness much greater than other spiral classes, and lenticulars also tend toward larger bulge-to-disc ratios than simple fading from a spiral would produce. Collisions between spirals would boost the total stellar mass and could explain why lenticulars host globular clusters at higher rates. A complication is that mergers struggle to reproduce the Tully-Fisher offset without assuming the progenitor spirals were very different from present-day spirals.

  A third path involves accretion. In this picture, a pre-existing spheroidal structure gradually builds a disc by pulling in gas and small satellite galaxies over time. This scenario was first proposed to explain how the compact massive spheroids seen at high redshift could grow into the equally compact massive bulges found in nearby massive lenticulars. Bigger lenticulars may have assembled first, when more gas was available, while lower-mass systems were slower to collect disc-building material. Within galaxy clusters, ram-pressure stripping actively removes gas and blocks further accretion, arresting disc growth. LEDA 2108986, an extremely isolated low-luminosity lenticular galaxy, poses a challenge for the ram-pressure scenario, because such stripping cannot account for its properties in the absence of a dense cluster environment.