J/A+A/706/A85 Size-Shape Difference for 67 GASP galaxies (Lassen+, 2026)
Distinguishing ram pressure from gravitational interactions.
Applying the Size-Shape Difference method to real galaxies.
Lassen A.E., Smith R., Vulcani B., Tonnesen S., Calderon-Castillo P.,
Poggianti B.M., Fritz J., George K., Ignesti A., Jaffe Y., Marasco A.,
Matijevic L., Moretti A., Radovich M., Tomicic N.
<Astron. Astrophys. 706, A85 (2026)>
=2026A&A...706A..85L 2026A&A...706A..85L (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, optical ; Clusters, galaxy ; Interstellar medium ;
Morphology ; Spectroscopy ; Redshifts
Keywords: galaxies: evolution - galaxies: interactions - galaxies: spiral -
galaxies: star formation
Abstract:
In dense environments, mechanisms like ram pressure stripping (RPS)
and gravitational interactions can induce the formation of similar
morphological features in galaxies, distinguishable only through a
detailed study of the stellar properties. While RPS affects recently
formed stars through the displacement of the gas disk from which they
are formed, gravitational interactions perturb stars of all ages
rather similarly. We present the first observational test of the
Size-Shape Difference (SSD) measure, a novel approach, originally
designed and validated for simulated galaxies, that quantifies
morphological differences between young and intermediate-age stellar
populations to distinguish RPS from gravitationally interacting
galaxies. We analyze 67 galaxies from the GASP survey using spatially
resolved star formation history derived from the SINOPSIS spectral
fitting code. In our fiducial model, we compare stellar populations in
two age bins (t<20Myr and 20Myr≤t<570Myr) to calculate SSD values.
The sample includes confirmed cases of RPS with different stripping
intensities, as well as undisturbed and gravitationally interacting
galaxies. We find that the extreme cases of RPS show SSD values ∼3.5x
higher than undisturbed and gravitationally interacting galaxies
(56+24-15) as compared to 16+6-2) and 16+6-3,
respectively), confirming simulation predictions. This enhancement
reflects RPS-induced asymmetries: the youngest stars are either
compressed along the leading edge or displaced into extended tails of
cold gas from which they are formed (or both), while older populations
remain undisturbed. In contrast, gravitational interactions perturb
all stars uniformly, producing lower SSD values. SSD robustly
distinguishes strong RPS cases, even when different age bins are used.
This holds even without correcting for disk inclination, or when
single-band imaging are used to trace stellar distributions. This
makes SSD a promising tool to select RPS candidates for spectroscopic
follow-up in upcoming large-scale surveys.
Description:
Size-Shape Difference values for the 67 galaxies analyzed in the
paper. Additionally, this table contains the estimated SSD errors,
redshifts, galaxy coordinates, Jtype classifications as reported in
Poggianti et al. (2025A&A...699A.357P 2025A&A...699A.357P), and the galaxy class (control,
tidal, RPS) of each galaxy. The reported values in this table consider
the surface density SFR maps of each galaxy, derived from SINOPSIS,
with SFR below 20Myr used to trace the spatial distribution of the
youngest stellar populations and SFR averaged in the range
20Myr<t<570Myr to trace the distribution of the intermediate-age
stellar populations. Additionally, this considers a bin width of 20
degrees and the Lagrangian radii set of r75, r90, r95 and r99. For
more details, we refer the reader to the paper.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 110 67 SSD values of the 67 GASP galaxies derived
in the paper
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Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 12 A12 --- ID Galaxy ID
14- 15 I2 h RAh Right ascension (J2000)
17- 18 I2 min RAm Right ascension (J2000)
20- 25 F6.3 s RAs Right ascension (J2000)
27 A1 --- DE- Declination sign (J2000)
28- 29 I2 deg DEd Declination (J2000)
31- 32 I2 arcmin DEm Declination (J2000)
34- 38 F5.2 arcsec DEs Declination (J2000)
40- 47 F8.6 --- z Galaxy redshift
49- 53 F5.1 --- Jtype ? JType classification (1)
55- 73 F19.15 --- SSD Measured Size-Shape Difference
75- 93 F19.17 --- e_SSD Size-Shape Difference uncertainty.
95-110 A16 --- GClass Name of the group each galaxy has been assigned
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Note (1): JType classification scheme as follows:
0.0 = For JType=0 galaxies
0.3 = For JType=0.3 galaxies
1.0 = For JType=1 galaxies
2.0 = For JType=2 galaxies
3.0 = For JType=3 galaxies
-99.0 = When classification does not apply
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Acknowledgements:
Augusto E. Lassen, augusto.lassen(at)inaf.it
(End) Patricia Vannier [CDS] 08-Dec-2025