JO171

JO171
File:Excerpts From DECam’s Deep View of Abell 3667 (noirlab2524b).jpg JO171 appears in the upper-left panel. Its detached stellar ring and one-sided stripped material are visible in deep DECam imaging.
Observation data (J2000 epoch)
Constellation	Pavo
Right ascension	20h 10m 14.69s
Declination	−56° 38′ 30.1″
Redshift	0.052529
Group or cluster	Abell 3667
Characteristics
Type	Hoag-type ring galaxy; jellyfish galaxy
Mass	3.39 × 1010 M☉ M☉
Notable features	Counter-rotating ring undergoing ram-pressure stripping
Other designations
WINGS J201014.69−563830.1

JO171, also designated WINGS J201014.69−563830.1, is a ring galaxy and jellyfish galaxy in the merging galaxy cluster Abell 3667. It consists of an old, nearly spherical central stellar component surrounded by a detached ring of younger stars and gas. The ring extends from approximately 6.5 to 20 kiloparsecs (21,000 to 65,000 light-years) from the center and rotates in the opposite direction to the central spheroid.

The galaxy resembles Hoag's Object, although the two systems occupy different environments. Hoag's Object is comparatively isolated, whereas JO171 is moving through the dense intracluster medium of Abell 3667. Ram pressure has removed gas from one side of JO171's ring, producing ionized-gas tails and suppressing star formation in the stripped southern half. Star formation continues in the northern ring and in compact regions within the trailing material.

The ring was probably assembled through external gas accretion before JO171 entered the cluster, possibly while the galaxy was associated with a large-scale filament. Its present appearance records two separate evolutionary processes: the earlier formation of the counter-rotating ring and its more recent removal by ram pressure. JO171 was described as the first observed ring galaxy caught undergoing active ram-pressure stripping.

Identification and location

JO171 lies in the southern constellation Pavo at a spectroscopic redshift of 0.052529. Its WINGS catalogue designation, WINGS J201014.69−563830.1, records its approximate J2000 position:

$\alpha =20^{\mathrm{h}}10^{\mathrm{m}}14.69^{\mathrm{s}},$

$\delta =-56^{\circ }38^{\prime }30.1^{″}.$

At the galaxy's redshift, an angular distance of one arcsecond corresponds to approximately 1.01 kpc under the cosmological parameters adopted for its detailed spectroscopic analysis.

JO171 is located at a projected distance of approximately 0.64 R₂₀₀ from the brightest cluster galaxy of Abell 3667. Its position and line-of-sight velocity are consistent with a relatively recent infall into the cluster rather than a galaxy that has remained in the central region for several orbital periods.

Selected properties of JO171

Property	Value
Constellation	Pavo
Host cluster	Abell 3667
Spectroscopic redshift	0.052529
Stellar mass	$(3.39\pm 0.50)\times 10^{10}{M}_{\odot }$
Central-component effective radius	$1.39\pm 0.06$ kpc
Central-component Sérsic index	$2.96\pm 0.10$
Inner radius of ring	Approximately 6.5 kpc
Outer radius of ring	Approximately 20 kpc
Maximum projected ring rotation	Approximately 100 km/s
Maximum projected spheroid rotation	Approximately 60 km/s
Gas-phase oxygen abundance	$12+{\log }_{10}(\mathrm{O}/\mathrm{H})\approx 8.87$
Projected cluster-centric distance	Approximately 0.64 R200

Structure

JO171 has three principal stellar regions: a compact central spheroid, a low-surface-brightness gap, and an extended outer ring. The ring is incomplete in emission-line and ultraviolet observations because star formation has ceased over much of its southern half.

File:Hoag's object.jpg

Hoag's Object, the prototype of a class of ring galaxies with an old central component separated from a young outer ring. JO171 has a similar structure but is being altered by the environment of Abell 3667.

Central spheroid

The central component is round and dominated by an old stellar population. Its light distribution is described by a Sérsic profile,

$I(R)=I_e\exp \{-b_n[{\left(\frac{R}{R_e}\right)}^{1/n}-1]\},$

where:

• $I(R)$ is the surface brightness at radius $R$;
• $I_e$ is the surface brightness at the effective radius;
• $R_e$ is the effective radius;
• $n$ is the Sérsic index; and
• $b_n$ is a coefficient determined by $n$.

For JO171, the fitted Sérsic index is $n=2.96\pm 0.10$, and the effective radius is $R_e=1.39\pm 0.06$ kpc. The spheroid is defined observationally as the region within approximately 3 kpc of the center.

Its surface brightness, radius, and stellar velocity dispersion place it close to the fundamental plane occupied by elliptical galaxies and classical bulges. Its low ellipticity and comparatively high Sérsic index are inconsistent with a disk-like pseudobulge.

The central spheroid contains approximately 56 percent of the galaxy's stellar mass. Most of its stars are older than those in the surrounding ring.

Stellar ring

The detached ring begins approximately 6.5 kpc from the center and extends to a radius of about 20 kpc. Its diameter is therefore close to 40 kiloparsecs (130,000 light-years).

The ring is most prominent at shorter optical and ultraviolet wavelengths because its light is dominated by younger stars. Its shape is not perfectly circular and includes spiral-like segments. No stellar bar has been detected between the spheroid and the ring.

Approximately 32 percent of the galaxy's stellar mass lies in the ring. A further 4 percent lies within the gap between the spheroid and ring, while about 8 percent is associated with external regions and stripped tails.

Approximate distribution of stellar mass

Component	Fraction of total stellar mass	Dominant characteristics
Central spheroid	56%	Old, compact and nearly round
Detached ring	32%	Younger, gas-rich and counter-rotating
Gap	4%	Low surface brightness
External and tail regions	8%	Recently formed or displaced material

The separation between the central component and ring gives JO171 a Hoag-like appearance. The asymmetry of its gas and recent star formation, however, reflects ongoing environmental processing rather than the regular structure of an undisturbed ring galaxy.

Kinematics

Spectroscopy separates the motions of the ionized gas from those of the stars. The gas and stars in the ring share the same principal direction of rotation and reach projected velocities of approximately 100 km/s.

The central spheroid rotates in the opposite direction, with a maximum projected velocity of approximately 60 km/s. Its rotation axis is misaligned from that of the ring by about 37°.

The opposing angular momenta can be represented by

${\overrightarrow{L}}_{\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{g}}\mathit{\cdot }{\overrightarrow{L}}_{\mathrm{s}\mathrm{p}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{o}\mathrm{i}\mathrm{d}}<0,$

where $\overrightarrow{L}$ denotes angular momentum. This counter-rotation indicates that the ring did not form solely from material that originally shared the spheroid's rotation.

The ionized-gas velocity field remains ordered through much of the ring, but it becomes increasingly disturbed within the stripped extensions. The tails are projected mainly along the plane of the sky, suggesting that much of JO171's motion through the cluster is transverse to the observer's line of sight.

Stellar populations

The stellar population contains several age components. Approximately 65.3 percent of its present stellar mass formed more than 5.7 billion years ago. Intermediate-age populations account for about 26.0 percent, stars formed between approximately 20 and 570 million years ago account for 8.2 percent, and populations associated with ongoing star formation account for about 0.5 percent.

Approximate stellar mass formed in different age intervals

Stellar age	Fraction of present stellar mass
Older than 5.7 billion years	65.3%
0.57–5.7 billion years	26.0%
20–570 million years	8.2%
Younger than approximately 20 million years	0.5%

About 91 percent of the galaxy's stellar mass was already present 600 million years ago. The spheroid formed earlier than the ring: approximately 85 percent of the spheroid's current stellar mass was in place six billion years ago, compared with about 45 percent of the ring's mass.

By 600 million years ago, approximately 99 percent of the spheroid and 81 percent of the ring had formed. The ring subsequently produced about one-fifth of its present stellar mass during the most recent 600 million years.

Star formation intensified in the ring between approximately 20 and 570 million years ago. The timing is compatible with compression and redistribution of gas during the galaxy's approach to Abell 3667, although the exact onset of stripping cannot be determined from stellar ages alone.

Star formation

Current star formation is concentrated in the northern half of the ring and in knots embedded within stripped material. The southern half is comparatively deficient in both ionized gas and young stellar populations.

The unequal distribution indicates that star formation was not suppressed everywhere at the same time. Gas on the exposed side was removed or displaced, while denser material on the opposite side remained capable of forming stars.

Far-ultraviolet imaging obtained with the AstroSat Ultraviolet Imaging Telescope detected significant emission in the tails of JO171. The ultraviolet sources align with clumpy Hα-emitting regions and trace stars formed during the most recent tens to hundreds of millions of years.

The combination of Hα and far-ultraviolet emission distinguishes several timescales:

• Hα primarily traces massive stars with lifetimes of roughly ten million years;
• far-ultraviolet light remains visible for longer after a star-forming episode; and
• optical stellar-population fitting traces both recent and ancient episodes.

These measurements show that some stripped gas continues to form stars after leaving the main ring, while diffuse ionized emission without a corresponding ultraviolet source may be powered partly by shocks, conduction, or other non-stellar processes.

Ionized gas

The ionized interstellar medium is traced by emission lines including Hα, Hβ, [O III], [N II], and [S II]. Strong emission is visible in the northern part of the ring and in one-sided tentacles extending away from the galaxy.

The gas tails point broadly away from the center of Abell 3667. Their orientation is consistent with ram pressure acting as JO171 moves through the cluster atmosphere.

Emission-line ratios can be used to distinguish gas ionized by young stars from gas affected by shocks or an active nucleus. Star-forming regions in JO171 are concentrated in discrete knots, while more diffuse emission occupies parts of the tails.

The Hα luminosity of a star-forming region is related to the rate of formation of massive stars. In simplified form,

$\mathrm{S}\mathrm{F}\mathrm{R}=C_{\mathrm{H}\alpha }{L}_{\mathrm{H}\alpha },$

where $L_{\mathrm{H}\alpha }$ is the extinction-corrected Hα luminosity and $C_{\mathrm{H}\alpha }$ depends on the adopted initial mass function and stellar-population model.

Gas-phase metallicity

The chemical abundance of JO171's ionized gas was measured through nebular emission-line ratios. Its median oxygen abundance is

$12+{\log }_{10}\left(\frac{\mathrm{O}}{\mathrm{H}}\right)\approx 8.87.$

This is slightly below the value of approximately 9.06 expected from a local mass–metallicity relation for a galaxy of its stellar mass, although the numerical comparison depends on the abundance calibration.

No strong, organized radial metallicity gradient was detected across the remaining ionized gas. Mixing associated with external gas accretion, radial transport, and stripping may have reduced an earlier gradient.

A metallicity difference in dex corresponds to a logarithmic abundance ratio:

$\mathrm{\Delta }Z={\log }_{10}\left[\frac{(\mathrm{O}/\mathrm{H}{)}_1}{(\mathrm{O}/\mathrm{H}{)}_2}\right].$

The approximate 0.19-dex difference between the measured abundance and the comparison value corresponds to an oxygen abundance about 35 percent lower than the comparison expectation.

The absence of an extremely metal-poor ring indicates that its gas has undergone substantial chemical enrichment. It does not exclude an external origin because accreted gas may have been enriched before entering the galaxy or mixed with material already present in the spheroid.

Ram-pressure stripping

JO171 is moving through the hot intracluster gas of Abell 3667. The external medium exerts ram pressure on the galaxy's interstellar gas:

$P_{\mathrm{r}\mathrm{a}\mathrm{m}}={\rho }_{\mathrm{I}\mathrm{C}\mathrm{M}}{v}_{\mathrm{r}\mathrm{e}\mathrm{l}}^2,$

where:

• $P_{\mathrm{r}\mathrm{a}\mathrm{m}}$ is the ram pressure;
• ${\rho }_{\mathrm{I}\mathrm{C}\mathrm{M}}$ is the density of the intracluster medium; and
• $v_{\mathrm{r}\mathrm{e}\mathrm{l}}$ is JO171's velocity relative to that medium.

Gas becomes vulnerable to stripping when the external pressure exceeds the galaxy's gravitational restoring pressure. For a simplified stellar and gas disk, the Gunn–Gott criterion is

${\rho }_{\mathrm{I}\mathrm{C}\mathrm{M}}{v}_{\mathrm{r}\mathrm{e}\mathrm{l}}^2\gtrsim 2\pi G{\mathrm{\Sigma }}_{\star }{\mathrm{\Sigma }}_{\mathrm{g}\mathrm{a}\mathrm{s}},$

where ${\mathrm{\Sigma }}_{\star }$ and ${\mathrm{\Sigma }}_{\mathrm{g}\mathrm{a}\mathrm{s}}$ are the local stellar and gas surface densities.

JO171 differs from an ordinary spiral galaxy because much of its cold gas lies in a detached ring. The ring is less strongly anchored than gas embedded within a continuous massive stellar disk. A stripping calculation based on a Milky Way-like disk therefore underestimates the amount of gas that can be removed from JO171.

The restoring pressure at a radius $R$ can be written more generally as

$P_{\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{v}}(R)={\mathrm{\Sigma }}_{\mathrm{g}\mathrm{a}\mathrm{s}}(R)\underset{z}{\max }\left|\frac{\partial \mathrm{\Phi }(R,z)}{\partial z}\right|,$

where $\mathrm{\Phi }$ is the gravitational potential and $z$ is height above the local plane.

Because the ring has a lower stellar surface density than the spheroid, the quantity $P_{\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{v}}$ declines sharply outside the central region. Ram pressure can therefore remove ring gas without significantly disturbing the central stellar structure.

Observable effects

Ram pressure has produced several linked features:

• ionized-gas tails extending northward;
• a strong asymmetry between the northern and southern ring;
• little current star formation in the stripped southern half;
• star-forming knots in retained and displaced gas;
• gas velocities that become disturbed outside the stellar ring; and
• an otherwise regular central stellar component.

Stars already formed in the ring are not removed directly by hydrodynamic pressure. Gas and young stars can therefore display a much stronger asymmetry than the older stellar population.

The stripped material gives JO171 its jellyfish morphology. Unlike tidal tails, which commonly contain both gas and old stars, ram-pressure tails are dominated by gaseous material and stars formed from that gas after displacement.

Abell 3667 environment

File:DECam’s Deep View of Abell 3667 (noirlab2524a).jpg

Deep DECam view of Abell 3667, the massive merging cluster containing JO171. The image also records diffuse intracluster light, foreground Galactic cirrus, and numerous cluster galaxies.

Abell 3667 is a massive, dynamically disturbed galaxy cluster at a redshift of approximately 0.0553. It contains two major substructures associated with a merger that occurred roughly one billion years ago.

Spectroscopic observations identified hundreds of cluster members and measured a line-of-sight velocity dispersion of approximately 1056±38 km/s. The high velocity dispersion and dense intracluster medium provide conditions capable of stripping gas from infalling galaxies.

The cluster's approximate virial mass is

$M_{200}\approx 1.7\times 10^{15}{M}_{\odot }.$

The quantity $M_{200}$ represents the mass enclosed within the radius at which the mean density is 200 times the critical density of the universe.

Abell 3667 also contains a prominent X-ray cold front, a radio halo, and two large radio relics produced by shocks and turbulence associated with the cluster merger.

File:Radio relics in Abell 3667.png

Composite view of Abell 3667. White traces X-ray emission from the hot intracluster medium, while orange shows MeerKAT radio emission from the cluster's relics and radio galaxies.

JO171 lies near a region affected by the cluster merger and its associated disturbed intracluster medium. Local density, pressure, and velocity structure may differ substantially from a smooth spherical cluster model.

A standard stripping model for a regular disk predicts only modest gas loss at JO171's projected location. Its pronounced tails can be explained by the weak gravitational binding of its ring, projection effects, an irregular intracluster medium, or a combination of these factors.

The cluster environment affects only the galaxy's recent history. The counter-rotating ring predates JO171's entry into Abell 3667 and therefore requires a separate formation mechanism.

Formation of the ring

Several mechanisms can produce ring structures in galaxies:

1. orbital resonances associated with a stellar bar;
2. an approximately head-on collision with another galaxy;
3. tidal accretion or a minor merger; and
4. the gradual accretion of gas with angular momentum different from that of the original galaxy.

The absence of a bar makes a resonance ring unlikely. A classical collisional ring is also disfavored because the ring and spheroid counter-rotate and no suitable intruding galaxy has been identified.

A major merger would be expected to disrupt the spheroid more strongly and produce a less regular central component. The old spheroid, detached ring, and opposing rotation are more consistent with prolonged external accretion of gas whose angular momentum differed from that of the original galaxy.

A retrograde minor merger with a mass ratio between approximately 1:2 and 1:5 cannot be excluded completely. Such an event would need to preserve the compact spheroid while placing a substantial quantity of gas and stars into a broad, counter-rotating ring.

The favored interpretation is the acquisition of gas from a cosmic filament before cluster infall. The gas settled at large radius, formed stars, and developed the detached ring. JO171 later entered Abell 3667, where the cluster atmosphere began removing the ring gas.

The galaxy's evolution can therefore be divided into three broad stages:

Stage	Principal process	Result
Early evolution	Formation of the central spheroid	Compact, old stellar component
Pre-cluster evolution	External gas accretion	Young, counter-rotating ring
Cluster infall	Ram-pressure stripping	Gas tails, asymmetric star formation and partial quenching

Comparison with Hoag's Object

JO171 belongs to the small group of galaxies whose structure resembles Hoag's Object: an old central component surrounded by a detached ring of younger stars.

Comparison between JO171 and Hoag's Object

Property	JO171	Hoag's Object
Environment	Massive merging cluster	Comparatively isolated
Central component	Old, round spheroid	Old, round spheroid
Ring radius	Approximately 6.5–20 kpc	Approximately 12–25 kpc
Approximate spheroid B-band magnitude	−20.8	−20.7
Central ellipticity	Approximately 0.05	Approximately 0.03
Counter-rotation	Ring counter-rotates relative to spheroid	No equivalent cluster-stripping configuration
Gas stripping	Active and strongly asymmetric	No comparable ram-pressure tails
Current star formation	Concentrated in northern ring and tails	Distributed around the outer ring

The similar central luminosities and ring dimensions indicate that JO171 is structurally comparable to Hoag's Object rather than an ordinary barred-ring galaxy. Its cluster environment has subsequently made the ring incomplete and asymmetric.

The comparison also shows that visual morphology alone does not determine a galaxy's history. Similar ring structures can be followed by different evolutionary paths depending on gas supply and environment.

Observations

JO171 was included in the GAs Stripping Phenomena in galaxies with MUSE program, known as GASP. The survey used integral-field spectroscopy to examine gas removal and star formation in galaxies across a range of environments.

File:The MUSE instrument on the VLT.jpg

The Multi Unit Spectroscopic Explorer attached to the Very Large Telescope. MUSE observations provided spatially resolved spectra of JO171's stars and ionized gas.

JO171 was observed with the Multi Unit Spectroscopic Explorer on the Very Large Telescope on 13 May 2016. The observation consisted of four exposures totaling 2,700 seconds. Small spatial offsets and rotations between exposures reduced instrumental artifacts.

MUSE records a spectrum at each position across a one-arcminute field. The JO171 data cube contained approximately 90,000 spatial elements and covered wavelengths from about 4,500 to 9,300 ångströms. Its spatial sampling was 0.2 arcsecond per pixel, and its spectral resolution was approximately 2.6 ångströms.

The spectra were used to map:

• stellar and ionized-gas velocity;
• stellar age and mass;
• star formation rate;
• dust extinction;
• gas-phase metallicity; and
• the dominant source of gas ionization.

Broadband and Hα images were extracted from the same spectroscopic data cube. Optical photometry from the WINGS and OmegaWINGS surveys provided additional information about the galaxy and its cluster environment.

Far-ultraviolet observations later identified young stellar populations in the tails and confirmed that some displaced gas remains capable of forming stars.

Deep DECam imaging released in 2025 showed JO171 within the larger structure of Abell 3667 and resolved its one-sided jellyfish morphology over a wider field.

Scientific importance

JO171 connects the study of ring-galaxy formation with environmental galaxy evolution. Its structure preserves evidence of external gas acquisition, while its tails record the later interaction with a cluster atmosphere.

The counter-rotation provides direct evidence that the ring and spheroid did not form from a single, undisturbed rotating component. The old central population and younger ring constrain the timing of their assembly, while metallicity limits how chemically primitive the acquired gas could have been.

Its detached ring also provides a test of ram-pressure theory outside the usual geometry of a spiral disk. Gas in the ring has less stellar mass above and below it than gas at the same radius in a continuous disk, reducing the gravitational restoring force.

JO171 demonstrates that ram pressure can transform an unusual galaxy without erasing evidence of its earlier formation. The central spheroid, counter-rotating stellar ring, quenched southern region, and star-forming tails preserve distinct stages of that evolution.

The galaxy is also relevant to studies of intracluster star formation. Stars formed in the tails may eventually separate dynamically from JO171 and contribute to the diffuse stellar population of Abell 3667.

See also

• Abell 3667
• Galaxy formation and evolution
• Gunn–Gott criterion
• Hoag's Object
• Intracluster medium
• Jellyfish galaxy
• Mass–metallicity relation
• Ram pressure
• Ram-pressure stripping
• Ring galaxy

Further reading

• Jaffé, Yara L.; Poggianti, Bianca M.; Moretti, Alessia; Gullieuszik, Marco; et al. (June 2018). "GASP. IX. Jellyfish galaxies in phase-space: An orbital study of intense ram-pressure stripping in clusters". Monthly Notices of the Royal Astronomical Society. 476 (4): 4753–4764. arXiv:1802.07297. Bibcode:2018MNRAS.476.4753J. doi:10.1093/mnras/sty500.

• Vulcani, Benedetta; Poggianti, Bianca M.; Gullieuszik, Marco; Moretti, Alessia; et al. (February 2019). "Enhanced star formation in both disks and ram-pressure-stripped tails of GASP jellyfish galaxies". Monthly Notices of the Royal Astronomical Society. 487 (2): 2278–2294. arXiv:1810.05164. Bibcode:2019MNRAS.487.2278V. doi:10.1093/mnras/stz1399.

• Gullieuszik, Marco; Poggianti, Bianca M.; McGee, Sean L.; Moretti, Alessia; et al. (January 2021). "GASP. XXI. Star formation rates in the tails of galaxies undergoing ram-pressure stripping". The Astrophysical Journal. 909 (1). arXiv:2006.16032. Bibcode:2021ApJ...909...22G. doi:10.3847/1538-4357/abda37.

External links

• Detailed MUSE study of JO171 at the Astrophysics Data System
• GASP study of JO171 at arXiv
• Deep DECam images of JO171 and Abell 3667 at NSF NOIRLab
• GASP survey website at the Astronomical Observatory of Padova

Category:Ring galaxies Category:Peculiar galaxies Category:Pavo (constellation)

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