NGC 1417 Group
A Spiral-Led Galaxy Association in the Eridanus Region
Quick Reader
| Attribute | Details |
|---|---|
| Name | NGC 1417 Group |
| Type | Galaxy group |
| Location | Border of Eridanus and Taurus constellations, southern sky |
| Distance from Earth | ~80 million light‑years (~25 Mpc) |
| Dominant Galaxy | NGC 1417 – intermediate spiral |
| Other Members | NGC 1415, NGC 1416, NGC 1393, ESO 548‑G 061, and several dwarfs |
| Group Type | Moderately rich, spiral‑dominated, filament‑linked |
| Supercluster Association | Possibly part of the Eridanus–Fornax filament |
| Scientific Importance | Useful for studying group‑scale spiral evolution and galaxy dynamics in southern filaments |
| Observation | Galaxies visible in mid‑size telescopes; well studied in optical and HI surveys |
Introduction – A Spiraled Gathering in the Southern Filamentary Web
The NGC 1417 Group is a moderately populated galaxy association located in the southern sky, near the edge of the Eridanus constellation. Sitting at a distance of approximately 80 million light-years, the group centers on the luminous spiral galaxy NGC 1417, and includes a blend of spirals, lenticulars, and dwarfs.
Though not as massive as the Eridanus Cluster or as dense as Fornax, this group:
Represents a transition zone in the cosmic web
Likely sits along a filament connecting Eridanus and Fornax clusters
Offers insights into spiral galaxy dynamics in intermediate-density environments
Its structure, dynamics, and mix of galaxy types make it ideal for studying how spiral galaxies evolve in groups before entering denser clusters.
Group Structure and Composition
The NGC 1417 Group appears mildly concentrated, with NGC 1417 near the gravitational center. Unlike cluster-dominated systems, this group shows:
No dominant elliptical
No hot X-ray gas halo
A significant population of star-forming spirals
Key Member Galaxies
| Galaxy | Type | Notes |
|---|---|---|
| NGC 1417 | Intermediate spiral (SABb) | Star-forming disk, large spiral arms, possible bar |
| NGC 1415 | Lenticular (S0) | Compact core, signs of passive evolution |
| NGC 1416 | Spiral or transition-type | Slight HI deficiency, dusty features |
| NGC 1393 | Lenticular | Smooth halo, no recent star formation |
| ESO 548‑G 061 | Irregular/low surface brightness | Dwarf group member |
| PGC 13505 and others | Dwarf ellipticals | Satellites or loosely bound members |
These galaxies are separated by 0.5 to 2 million light‑years, with low velocity dispersion and signs of mild interactions, suggesting an early-to-mid evolutionary state.
Group Characteristics
| Parameter | Approximate Value |
|---|---|
| Distance | ~80 Mly |
| Velocity Dispersion | ~150–200 km/s |
| Member Count | 8–12 major galaxies + dwarfs |
| Galaxy Type Mix | ~60% spirals, 30% lenticulars, 10% irregulars |
| Binding State | Weak to moderate; may evolve into compact group or fall toward Eridanus Cluster |
Environmental Role in the Cosmic Web
The group likely traces part of the southern filament network, acting as a node between the Eridanus and Fornax clusters. Cosmicflows models and redshift maps suggest:
Nearby galaxies flow southwest toward Fornax
Others are part of the Eridanus filamentary web
The group sits near the Taurus–Eridanus Void, defining its edge
This positioning provides important clues about:
Gas accretion from filaments
Mild gravitational shear on disks
Environmental pre-processing before cluster infall
Star Formation, Gas Dynamics, and HI Structure
The NGC 1417 Group, unlike high-density clusters, presents a favorable environment for sustained star formation and cold gas retention. Most of its member galaxies — especially spirals like NGC 1417 and NGC 1416 — exhibit extended HI disks, active star-forming regions, and regular rotation curves, suggesting minimal environmental disruption.
Star Formation Profiles
| Galaxy | Star Formation Activity | Notes |
|---|---|---|
| NGC 1417 | Moderate to strong | UV and Hα emission in arms; no signs of quenching |
| NGC 1416 | Patchy/moderate | Outer disk shows localized star formation |
| NGC 1415 | Weak | Mostly passive; some residual activity in outer shell |
| NGC 1393 | None | Fully quenched lenticular |
| ESO 548‑G 061 | Variable | Low-level SF; HI-rich irregular |
| Dwarf members | Weak to intermittent | Possibly regulated by tidal influence or accretion |
GALEX ultraviolet imaging confirms that NGC 1417 is the most active member, with clear blue spiral arms and moderate IR emission in Spitzer/WISE mid-infrared bands.
HI Mapping and Cold Gas Trends
Observations from HIPASS and ATCA indicate that:
NGC 1417 has a large HI envelope, slightly lopsided, possibly from past accretion
NGC 1416 shows mild HI asymmetry, but no clear signs of stripping
Dwarfs and irregulars retain high HI-to-stellar mass ratios, consistent with slow environmental processing
No HI bridges or tails currently detected — indicating little recent interaction
📌 Insight: The group is in a pre-interaction phase, where galaxies are evolving largely independently, possibly accreting gas from the surrounding filament.
Secular Evolution in the Group
Due to the group’s low density and lack of X-ray hot gas, members undergo secular evolution, shaped by:
Internal bar dynamics
Minor companions
Weak gravitational tides
This allows studies of:
Bar-driven inflow
Spiral density wave behavior
Passive transformation of S0 galaxies
Comparison with Eridanus and Fornax Cluster Environments
| Property | NGC 1417 Group | Eridanus Group | Fornax Cluster |
|---|---|---|---|
| Galaxy Types | Spiral-rich | Mixed | Early-type dominated |
| Gas Content | HI-rich | Moderate | Mostly stripped |
| Velocity Dispersion | ~150–200 km/s | ~240–300 km/s | ~370–400 km/s |
| Environment | Loose, filamentary | Compact group | Cluster core |
| X-ray Emission | None | Weak, localized | Strong central X-ray halo |
📌 Conclusion: NGC 1417 Group lies between field and group environments, offering a clean contrast to the denser and more processed galaxies in Eridanus and Fornax.
Group Assembly History and Future Evolution
Peculiar velocity models and redshift mapping suggest the NGC 1417 Group is:
Still assembling, possibly from smaller subgroups along a filament
Moving southward toward the gravitational basin of Fornax or Eridanus
Likely to remain a loose group, unless accreted by a larger structure in the next few Gyr
Dynamically, it resembles the early stages of compact group formation, and may eventually:
Merge its inner galaxies
Trigger tidal transformation
Build up a hot intragroup medium
But for now, it remains a quiet zone for spiral survival and gas-driven evolution.
Cosmological Significance – A Group Evolving in a Quiet Lane
The NGC 1417 Group may not be a massive cluster or gravitational focal point, but it provides crucial insight into how galaxies evolve in mildly bound, spiral-dominated environments that are still embedded within the cosmic web.
Its significance lies in:
Serving as a laboratory for secular evolution
Demonstrating how filament-fed spirals retain gas and structure
Acting as a transition node between loose field galaxies and denser, cluster-bound systems
Helping refine our understanding of galaxy pre-processing and gas retention in group-scale halos
Frequently Asked Questions (FAQ)
Q: What is the NGC 1417 Group?
A: A moderately populated galaxy group located ~80 million light-years away, centered around the spiral galaxy NGC 1417. It is located near the Eridanus–Fornax filament in the southern sky.
Q: What makes this group unique?
A: Unlike many groups dominated by ellipticals or lenticulars, the NGC 1417 Group is:
Spiral-rich
Gas-retaining
Evolving in a low-density, non-hostile environment
Q: Is there any hot gas or X-ray halo?
A: No strong X-ray emission has been detected, indicating the group:
Is not virialized
Lacks a dense intragroup medium
Is still in a formative stage
Q: Are the galaxies interacting?
A: There is no evidence of major ongoing interactions. Most members, including NGC 1417, show regular disk structure, suggesting a dynamically quiet phase.
Q: Is the group gravitationally bound?
A: It appears to be partially bound, with low velocity dispersion. It is likely stable at the core, but some outer galaxies may be filament-linked or transient.
Comparison with Other Spiral-Led Groups
| Group | Dominant Galaxy | Environment | Evolution Stage | Interaction Signs |
|---|---|---|---|---|
| NGC 1417 Group | NGC 1417 | Loose, filamentary | Pre-processed | Mild or none |
| Canes II Group | NGC 4244 | Local Volume | Loose, evolving | None |
| M101 Group | M101 | Low-density sheet | Spiral-rich, disturbed | Moderate |
| M81 Group | M81 | Compact group | Dynamically evolved | Strong |
| NGC 2997 Group | NGC 2997 | Southern field | Field-dominated | Mild |
📌 NGC 1417 Group stands out as a southern analog to Canes II and M101 Group — excellent for studying internal disk dynamics and star formation in spiral galaxies unshaped by dense environments.
Final Thoughts – A Group Preserving Spiral Integrity
The NGC 1417 Group is a reminder that not all galaxies are shaped by trauma. Some evolve quietly—gathering gas, forming stars, and slowly transforming under the influence of their own bars, disks, and dark matter halos.
For astronomers, it offers:
A snapshot of pre-cluster life
A way to explore spiral disk evolution in calm settings
A southern-sky reference group for comparative studies
For cosmology, it’s a building block in the cosmic web — modest in mass, but critical in linking filaments, clusters, and voids across the evolving universe.
