[Paper Review] History and destiny of an emerging early-type galaxy: New IFU insights on the major-merger remnant NGC7252

John R. Weaver, B. Husemann+8 more|arXiv (Cornell University)|Jan 29, 2018

Galaxies: Formation, Evolution, Phenomena135 references14 citations

TL;DR

This study uses VLT-VIMOS IFU spectroscopy to analyze NGC 7252, a nearby major-merger remnant ~1 Gyr post-collision, revealing a rapidly re-formed, co-rotating central gas disc with ongoing star formation (SFR = 2.2 ± 0.6 M☉ yr⁻¹) and complex stellar kinematics indicating a polar merger configuration. Despite its elliptical-like morphology, the galaxy remains blue and actively forming stars, implying a prolonged transition to quiescence lasting several billion years.

ABSTRACT

NGC7252, which is one of the nearest major-merger galaxy remnants, is an ideal laboratory to study the processes inherent to the transformation of disc galaxies to ellipticals as observed about ~1Gyr after the collision. We obtained wide-field IFU spectroscopy with the VLT-VIMOS integral-field spectrograph covering the central 50"x50" of NGC7252 to map the stellar and ionised gas kinematics, and the distribution and conditions of the ionised gas, revealing the extent of ongoing star formation and recent star formation history. We find that the inner gas disc is not counter-rotating with respect to the stars and that the stellar kinematics appear complex with a clear indication of a prolate-like rotation component suggesting a polar merger configuration. The ongoing star formation rate is 2.2+-0.6 M_sun/yr and implies a typical depletion time of 2Gyr given the molecular gas content. Furthermore, the spatially-resolved star formation history suggests a slight radial dependence, moving outwards at later times. We confirm a large AGN-ionised gas cloud previously discovered 5kpc south of the nucleus, and find higher ionisation also at the galaxy centre relative to the surrounding gas disc. Although the higher ionisation towards the centre is potentially degenerate within the central star forming ring, it may be associated with a low-luminosity AGN. Although NGC7252 has been classified as post-starburst galaxy at the centre, the elliptical-like major-merger remnant still appears very active. A central kpc-scale gas disc has presumably re-formed quickly within the last 100Myr after final coalescence. The disc features ongoing star formation, implying Gyr long timescale to reach the red sequence through gas consumption alone. While NGC7252 is useful to probe the transformation from discs to ellipticals, it is not well-suited to study the transformation from blue to red at this point.

Motivation & Objective

• To investigate the kinematic and star formation properties of NGC 7252, a nearby major-merger remnant, to understand the transformation of disc galaxies into ellipticals.
• To resolve conflicting prior claims about counter-rotating gas discs and stellar kinematics in NGC 7252.
• To assess the role of AGN feedback and gas consumption timescales in the galaxy's evolution toward quiescence.
• To determine whether NGC 7252 is representative of the blue-to-red sequence transition or remains an outlier in its evolutionary phase.

Proposed method

• Acquired wide-field integral-field unit (IFU) spectroscopy with the VLT-VIMOS instrument, covering 50″ × 50″ centered on NGC 7252.
• Mapped stellar and ionized gas kinematics using emission and absorption line profiles, including Hα and [O III] for star formation rate (SFR) estimation.
• Derived spatially resolved star formation histories (SFH) using dust-corrected Hα luminosities and stellar population synthesis modeling.
• Measured molecular gas depletion time using the central molecular gas mass and SFR, assuming constant depletion timescale.
• Estimated AGN bolometric luminosity from [O III] luminosity to assess low-luminosity AGN activity.
• Analyzed stellar kinematics for signatures of prolate-like rotation to infer merger geometry (e.g., polar merger).

Experimental results

Research questions

• RQ1 Does the central gas disc in NGC 7252 exhibit counter-rotation relative to the stars, as previously reported?
• RQ2 What is the spatially resolved star formation history of NGC 7252, and does it show radial dependence?
• RQ3 What is the current star formation rate and molecular gas depletion timescale in the central region?
• RQ4 Is there evidence for a low-luminosity AGN at the galaxy center, and how does it affect ionization conditions?
• RQ5 To what extent is NGC 7252 representative of the transition from blue, star-forming galaxies to red, quiescent ellipticals?

Key findings

• The central gas disc in NGC 7252 is not counter-rotating; instead, it co-rotates with the stars, contradicting earlier claims.
• Stellar kinematics reveal both oblate- and prolate-like rotation components, suggesting a polar merger configuration or radial orbits with misaligned angular momentum.
• The ongoing star formation rate is 2.2 ± 0.6 M☉ yr⁻¹ within the central 7 kpc, with a molecular gas depletion time of approximately 2 Gyr.
• The star formation history shows elevated activity extending to ~1 Gyr ago, indicating a complex, radially dependent evolution beyond the first passage time.
• A high-ionization gas cloud is confirmed ~5 kpc south of the nucleus, and enhanced ionization at the center suggests a low-luminosity AGN with L_bol < 2.4 × 10⁴¹ erg s⁻¹.
• Despite its early-type morphology, NGC 7252 remains in the blue cloud with a slow transition to the red sequence, requiring ~5 Gyr to reach quiescence via gas consumption alone.