Star formation outside galaxies undergoing gravitational and hydrodynamic interactions: Dust attenuation and the star formation rate

This study investigates dust attenuation and star formation rates in extragalactic structures formed by different interaction mechanisms, finding that despite their distinct origins, ram-pressure stripped tails and gravitationally induced rings or tidal tails exhibit notably similar dust content and star formation activity.

The Gist

Imagine galaxies as giant, swirling cities of stars, gas, and dust. Usually, these cities are quiet and stable. But sometimes, they get into trouble. They either crash into each other (a gravitational interaction) or they get blown through a cosmic wind tunnel (a hydrodynamic interaction).

This paper is like a detective story where astronomers investigate what happens to the "debris" left behind after these cosmic accidents. Specifically, they are looking at two things in the wreckage: dust (which acts like a thick fog) and new baby stars (which are the bright lights trying to shine through that fog).

Here is the breakdown of the study using simple analogies:

1. The Two Types of Cosmic Accidents

The researchers looked at four different "crime scenes" where galaxies were being disturbed:

• The "Jellyfish" Galaxies (JO201 and JW100): Imagine a jellyfish swimming through water. As it moves, the water pushes against it, stripping away its tentacles. In space, when a galaxy moves too fast through a dense cluster of other galaxies, the "wind" of hot gas (called the Intra-Cluster Medium) rips the galaxy's gas and dust out, creating long, trailing tails. These look like jellyfish tentacles. This is a hydrodynamic interaction (wind/water).
• The "Crash" Galaxies (NGC 5291 and NGC 7252): Imagine two cars crashing head-on or swinging around each other. The force of the crash flings debris (stars, gas, and dust) far out into space, creating rings or long tails. This is a gravitational interaction (gravity).

2. The Mystery: The "Fog" and the "Lights"

The scientists wanted to know: Does the type of accident matter for how much dust is in the debris and how many new stars form there?

• The Dust (The Fog): Dust is tricky. It blocks the light from new stars. If you just look at the light, you might think there are fewer stars than there actually are because the dust is hiding them. It's like trying to count people at a party through a thick, smoky room.
• The Stars (The Lights): New stars are bright and hot. They are the "lights" the astronomers are trying to count.

3. The Investigation

The team used a special space telescope (AstroSat) that can see ultraviolet light (a type of light that is great at spotting young, hot stars but is easily blocked by dust).

They looked at the "tentacles" of the Jellyfish galaxies and the "rings/tails" of the Crash galaxies. They measured:

1. How much dust was there? (By looking at how much the light was reddened, similar to how a sunset looks redder when there is more dust in the air).
2. How many stars were actually forming? (By correcting for the dust "fog" to get the true number).

4. The Big Surprise

The researchers expected the two types of accidents to produce very different results. They thought the "wind" (Jellyfish) might strip away dust differently than the "crash" (Gravitational).

But they found that the results were surprisingly similar!

• The Dust: The amount of dust in the tails of the Jellyfish galaxies was comparable to the dust in the rings and tails of the Crash galaxies.
• The Star Formation: The rate at which new stars were being born in these debris fields was also very similar across all four cases.

The Analogy:
Think of it like two different ways of making a mess in a kitchen.

• Scenario A: You blow a fan at a pile of flour (Hydrodynamic/Wind).
• Scenario B: You knock a bowl of flour off the table (Gravitational/Crash).

You might expect the flour to land differently in each case. But this study found that, surprisingly, the flour (dust) and the crumbs (stars) ended up in very similar patterns and quantities in both scenarios.

5. Why Does This Matter?

This is a big deal because it tells us that nature is consistent. Whether a galaxy is being battered by wind or smashed by gravity, the physics of how gas turns into stars and how dust behaves in the debris is remarkably the same.

It also solved a puzzle: In the "Jellyfish" tails, some parts had very little dust (like the outer tips of the tentacles), while parts closer to the galaxy had thick dust. This suggests that the "wind" strips the lighter gas first, leaving the heavier dust behind closer to the galaxy, or that the dust gets destroyed as it travels further out into the harsh environment.

The Bottom Line

Galaxies are messy, but their messiness follows rules. Whether they are being pushed by a cosmic wind or pulled apart by gravity, the "debris" they leave behind creates new stars at similar rates and contains similar amounts of dust. It's as if the universe has a standard recipe for making new stars out of cosmic wreckage, regardless of how the wreckage was created.

Technical Summary

Here is a detailed technical summary of the research paper "Star formation outside galaxies undergoing gravitational and hydrodynamic interactions: Dust attenuation and the star formation rate."

1. Problem Statement

Galaxies undergo perturbations from two primary mechanisms: gravitational interactions (e.g., mergers, tidal forces) and hydrodynamic interactions (e.g., ram-pressure stripping in dense cluster environments). Both mechanisms can strip gas and dust from galaxy disks, creating extended extragalactic structures such as tidal tails, collisional rings, and ram-pressure stripped tails. Within these structures, in situ star formation often occurs.

However, a critical gap exists in understanding how these different perturbation mechanisms influence the dust content and star formation rates (SFR) in these external structures. Dust significantly obscures ultraviolet (UV) and optical emission from young stars, leading to potential underestimations of SFR if not properly corrected. While previous studies have examined these phenomena individually, there is a lack of comparative analysis between dust attenuation and SFR in structures formed by hydrodynamic processes versus those formed by gravitational interactions.

2. Methodology

The study employs a comparative approach using high-resolution ultraviolet imaging data from the Ultraviolet Imaging Telescope (UVIT) onboard the Indian space observatory AstroSat.

• Sample Selection: The authors selected four targets representing two distinct interaction types:
  • Hydrodynamic Interactions (Ram-Pressure Stripping): Jellyfish galaxies JO201 (in Abell 85) and JW100 (in Abell 2626). These galaxies are undergoing extreme stripping, forming tails of gas and dust.
  • Gravitational Interactions: The NGC 5291 system (a collisional ring formed by a high-speed head-on collision) and the NGC 7252 system (a post-merger system with prominent tidal tails).
• Data Processing:
  • Imaging: Utilized FUV (1481 Å) and NUV (2418 Å) filters from UVIT, along with optical $r$-band data from the DECaLS survey.
  • Source Extraction: Used the ProFound package to identify and segment star-forming (SF) knots in the stripped tails and rings. AGN flux was removed prior to extraction.
  • Extinction Correction: Corrected for foreground Galactic extinction using standard laws (Cardelli et al. 1989; O'Donnell 1994).
• Analysis Techniques:
  • Dust Attenuation: Calculated using the UV continuum slope ($\beta$) derived from the FUV–NUV color. The internal attenuation ($A_{FUV}$) was estimated using the Meurer et al. (1999) relation: $A_{FUV} = 4.43 + 1.99\beta$.
  • Star Formation Rate: Derived from dust-corrected FUV luminosities assuming a constant SFR over $10^8$ years and a Salpeter Initial Mass Function (IMF).
  • Comparison: The study compares the dust attenuation ($A_{FUV}$) and dust-corrected SFR surface densities ($\Sigma_{SFR}$) of SF knots across all four systems.

3. Key Contributions

• New Analysis of Jellyfish Galaxies: The paper presents new, high-resolution results for JO201 and JW100, specifically deriving dust attenuation and SFR for resolved SF knots in their stripped tails using the UV slope method.
• Unified Methodology: By applying the same $\beta$-slope dust correction method to both hydrodynamic (jellyfish) and gravitational (NGC 5291/7252) systems, the study ensures a consistent basis for comparison, overcoming previous inconsistencies where different correction methods (e.g., Balmer decrement vs. UV slope) were used.
• Characterization of Dust Distribution: The study maps the spatial distribution of dust attenuation within the tails, revealing gradients and correlations with distance from the host galaxy disk.

4. Key Results

• Dust Attenuation ($A_{FUV}$):
  • Jellyfish Tails (JO201 & JW100): Exhibit a wide range of attenuation values.
    • JO201 tails: Range 0 – 2.35 mag (Median: 0.87 mag).
    • JW100 tails: Range 0.44 – 1.36 mag (Median: 0.93 mag).
    • The tails contain two distinct populations: "dust-free" knots (similar to NGC 5291) located far from the disk, and "dusty" knots (similar to NGC 7252) concentrated closer to the disk.
  • NGC 5291 Ring: Shows the lowest attenuation spread (0 – 0.87 mag), with all knots having attenuation $\le$ the median of the jellyfish tails. This suggests very young, dust-poor stellar populations.
  • NGC 7252 Tails: Exhibit high attenuation (1.12 – 2.33 mag), with all knots exceeding the median attenuation of the jellyfish tails.
• Star Formation Rates (SFR):
  • Correction Impact: Correcting for internal dust attenuation increased the integrated SFR in the jellyfish tails by a factor of $\sim$2 (e.g., JO201 tail SFR increased from 4.6 to 10.3 $M_\odot$/yr).
  • SFR Surface Density ($\Sigma_{SFR}$): The dust-corrected SFR densities of knots in the jellyfish tails are comparable to those in the NGC 5291 ring and NGC 7252 tails.
    • Mean $\Sigma_{SFR}$ for NGC 5291 ring: 0.0034 $M_\odot$/yr/kpc$^2$.
    • Mean $\Sigma_{SFR}$ for JO201 tails: 0.0034 $M_\odot$/yr/kpc$^2$.
    • Mean $\Sigma_{SFR}$ for NGC 7252 tails: 0.0043 $M_\odot$/yr/kpc$^2$.
    • Mean $\Sigma_{SFR}$ for JW100 tails: 0.0044 $M_\odot$/yr/kpc$^2$.
• Spatial Gradients:
  • In ram-pressure stripped tails (JO201, JW100), SFR density and dust attenuation generally decrease with distance from the galaxy disk. This is attributed to the "outside-in" stripping process where diffuse gas is stripped first, while denser molecular gas (and associated dust) remains closer to the disk.
  • In contrast, the NGC 5291 collisional ring shows no clear gradient, likely due to the nature of the high-speed head-on collision which does not involve differential stripping.
• Dust Obscuration Fraction: The fraction of obscured star formation ($f_{obscured}$) ranges from 0.34 (NGC 5291) to 0.76 (NGC 7252), indicating that >30% of star formation in these external structures is hidden by dust.

5. Significance and Conclusion

The study concludes that despite the fundamentally different formation scenarios—hydrodynamic ram-pressure stripping versus gravitational tidal interactions—the resulting dust content and star formation activity in the extragalactic debris are notably similar.

• Physical Insight: The similarity in SFR densities suggests that the efficiency of converting gas into stars in these extreme environments is comparable, regardless of the triggering mechanism.
• Dust Dynamics: The findings highlight that dust is effectively stripped along with gas in ram-pressure events, forming coherent structures. However, the distribution is inhomogeneous, with dust surviving better in denser clumps closer to the galaxy and being more susceptible to destruction (sputtering) or stripping in the diffuse outer tails.
• Evolutionary Context: The results enhance models of galactic evolution by demonstrating that both gravitational and hydrodynamic perturbations can trigger intense, dust-obscured star formation outside galaxy disks. This implies that a significant portion of star formation in the universe, particularly in cluster environments, may occur in these extended, often dust-obscured regions, necessitating UV and IR observations for accurate census.