Lenticular galaxy
The Spindle Galaxy NGC 5866 sits in the constellation Draco and displays a visible disk component alongside a prominent bulge. This structure places it between an elliptical galaxy and a spiral galaxy within the Hubble classification scheme. Astronomers denote this type as S0 to distinguish it from the E class of ellipticals or the Sa, Sb, Sc classes of spirals. The defining feature is the absence of large-scale spiral arms despite retaining a disc shape. Lenticular galaxies often exhibit higher bulge-to-disk ratios than typical spirals do. They may show a central bar but lack the canonical arm structure found in late-type galaxies. The distribution of axial ratios for these systems rises steadily between 0.25 and 0.85. In contrast, the distribution for spiral galaxies remains essentially flat across that same range. Larger axial ratios occur when observing face-on disks or spheroidal bulge-dominated samples. A sample with prominent spheroidal components naturally contains more galaxies at larger axial ratios. This fact implies lenticulars are dominated by a central bulge component rather than just a thin disc.
All stars within these systems are thought to be older than about one billion years. Their stellar populations consist predominantly of redder aging stars similar to those found in elliptical galaxies. Globular clusters appear more frequently in lenticular galaxies than in spiral galaxies of comparable mass and luminosity. These objects possess little to no molecular gas which prevents significant hydrogen alpha emission. They also lack 21-cm emission lines typically used to trace cool gas in other galaxy types. Despite this deficiency they may still retain significant amounts of dust within their disks. The kinematics of disk galaxies usually rely on H-alpha or 21-cm emission lines. Lenticular galaxies generally do not contain these emission lines due to their gas-poor nature. Consequently astronomers must derive rough mass estimates from stellar absorption lines instead. Stellar absorption lines prove less reliable than emission line measurements for determining rotation speeds. Determining accurate rotational velocities becomes difficult because of projection effects in the bulge-disk interface region. Random motions of stars further affect true rotational velocity calculations. These combined factors make kinematic measurements considerably harder compared to normal disk galaxies.
The absence of gas and presence of dust suggest some lenticulars originate from faded spiral galaxies. Tidal harassment by nearby galaxies could aid this process in dense regions like the Virgo Cluster. A 2012 paper proposed a new classification system first suggested by Canadian astronomer Sidney van den Bergh. This S0a-S0b-S0c-dSph sequence parallels the Hubble sequence for spirals and irregulars. Some lenticular galaxies are more luminous than typical spirals which suggests they are not merely faded remnants. Galaxy mergers might increase total stellar mass while creating a disk-like arm-less appearance. Advanced models including both a general Sérsic profile and bar indicate smaller bulges. Mergers cannot account for the offset from the Tully-Fisher relation without assuming merged galaxies were quite different. Disk growth via accretion of gas and small galaxies around pre-existing spheroidal structures offers another explanation. In a downsizing scenario bigger lenticular galaxies may have been built first in a younger universe. Lower-mass galaxies might be slower to attract their disk-building material as seen with LEDA 2108986. Ram-pressure stripping within galaxy clusters removes gas and prevents further disk development.
NGC 1375 and NGC 1175 serve as examples of lenticular galaxies featuring box-shaped bulges. These objects are classified as SB0 pec due to their peculiar structural characteristics. Box-shaped bulges appear mostly in edge-on spiral galaxies but rarely in lenticular types. Like spiral galaxies these systems can possess a central bar structure. The classification system for normal lenticulars depends on dust content rather than bar prominence. Barred lenticular galaxies are instead classified by how prominent that central bar appears. SB01 galaxies display the least defined bar structure with only slightly enhanced surface brightness along opposite sides. The prominence of the bar increases with index number up to SB03 classifications. NGC 1460 represents one of the largest bars seen among all lenticular galaxies. It extends through the transition region between the bulge and disk components. Unfortunately properties of bars in lenticular galaxies have not been researched in great detail. Understanding these properties would help clarify the formation or evolution history of these systems. Surface brightness profiles are well described by the sum of a Sérsic model plus an exponentially declining model. A third component often accounts for the presence of a bar within the galaxy.
Determining the distinction between elliptical and lenticular galaxies relies heavily on velocity dispersion measurements. Astronomers examine the v over sigma ratio for a fixed epsilon value to differentiate classes. Elliptical galaxies typically show v over sigma values less than 0.5 when epsilon equals 0.3. Lenticular galaxies possess much larger ratios due to their non-negligible rotational velocities from the disk component. This approach using a single ratio proves problematic because it depends on the radius measured. ES galaxies bridging E and S0 types exhibit high ratios at intermediate radii that drop at large radii. Kinematic information and rough mass estimates come from stellar absorption lines which lack emission line reliability. There is considerable difficulty deriving accurate rotational velocities for lenticular galaxies. Inclination measurements prove difficult while projection effects complicate the bulge-disk interface region analysis. Random motions of stars further affect true rotational velocity calculations. These combined factors make kinematic measurements considerably harder compared to normal disk galaxies. The offset Tully-Fisher relation shows best-fit lines for spirals and lenticulars follow the same slope but differ by approximately 1.5 in luminosity.
The Cartwheel Galaxy lies about 500 million light-years away within the constellation Sculptor. NGC 4608 sits roughly 56 million light years away in Virgo and features a barred structure. NGC 1533 serves as a prototypical lenticular galaxy located in the constellation Dorado. NGC 2787 displays visible dust absorption despite being classified as an S0 galaxy. NGC 3115 and NGC 3632 represent additional cataloged examples of this morphological type. Many members reside within major clusters such as the Virgo Cluster or Fornax Cluster. NGC 1387 contains a large nuclear ring and belongs to the Fornax Cluster population. Lenticular galaxies are often considered poorly understood transition states between spiral and elliptical types. Their intermediate placement on the Hubble sequence results from having both prominent disk and bulge components. The disk component is usually featureless which precludes classification systems similar to spiral galaxies. Elliptical galaxy classifications also prove unsuitable because their bulge component is usually spherical. These objects share spectral features and scaling relations with elliptical galaxies while retaining disc properties.
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Common questions
What is the Spindle Galaxy NGC 5866 and how does it fit into galaxy classification?
The Spindle Galaxy NGC 5866 sits in the constellation Draco and displays a visible disk component alongside a prominent bulge. This structure places it between an elliptical galaxy and a spiral galaxy within the Hubble classification scheme.
How do astronomers distinguish lenticular galaxies from elliptical galaxies using velocity dispersion measurements?
Determining the distinction between elliptical and lenticular galaxies relies heavily on velocity dispersion measurements. Lenticular galaxies possess much larger v over sigma ratios due to their non-negligible rotational velocities from the disk component compared to elliptical galaxies which typically show values less than 0.5 when epsilon equals 0.3.
Which specific lenticular galaxies feature box-shaped bulges and what makes them unique?
NGC 1375 and NGC 1175 serve as examples of lenticular galaxies featuring box-shaped bulges. These objects are classified as SB0 pec due to their peculiar structural characteristics that appear mostly in edge-on spiral galaxies but rarely in lenticular types.
What is the distance and location of the Cartwheel Galaxy relative to other lenticular galaxies like NGC 4608?
The Cartwheel Galaxy lies about 500 million light-years away within the constellation Sculptor while NGC 4608 sits roughly 56 million light years away in Virgo and features a barred structure. Many members reside within major clusters such as the Virgo Cluster or Fornax Cluster including NGC 1533 located in the constellation Dorado.
Why do lenticular galaxies lack emission lines used for kinematic measurements in normal disk galaxies?
These objects possess little to no molecular gas which prevents significant hydrogen alpha emission and they also lack 21-cm emission lines typically used to trace cool gas in other galaxy types. Consequently astronomers must derive rough mass estimates from stellar absorption lines instead because these systems generally do not contain the emission lines found in normal disk galaxies.