[Paper Review] An H-alpha survey aiming at the detection of extraplanar diffuse ionized gas in halos of edge-on spiral galaxies. I. How common are gaseous halos among non-starburst galaxies?

Joern Rossa, R.‐J. Dettmar|ArXiv.org|Apr 24, 2003

Galaxies: Formation, Evolution, Phenomena41 references65 citations

TL;DR

This study presents an H𝛼 survey of 74 edge-on spiral galaxies to determine the prevalence of extraplanar diffuse ionized gas (eDIG) in non-starburst galaxies. Using H𝛼 imaging, it finds that eDIG halos are common in galaxies with sufficient star formation activity, establishing a threshold SFR surface density of (3.2±0.5)×10⁴⁰ erg s⁻¹ kpc⁻² for detectable eDIG, confirming a direct link between disk star formation and halo ionization.

ABSTRACT

In a series of two papers we present results of a new H-alpha imaging survey, aiming at the detection of extraplanar diffuse ionized gas in halos of late-type spiral galaxies. We have investigated a sample of 74 nearby edge-on spirals, covering the northern and southern hemisphere. In 30 galaxies we detected extraplanar diffuse emission at mean distances of |z|~1-2 kpc. Individual filaments can be traced out to |z|<= 6 kpc in a few cases. We find a good correlation between the FIR flux ratio (S_60/S_100) and the SFR per unit area (L_FIR/D^2_25), based on the detections/non-detections. This is actually valid for starburst, normal and for quiescent galaxies. A minimal SFR per unit area for the lowest S_60/S_100 values, at which extended emission has been detected, was derived, which amounts to (dE/dt)_A25^thres = (3.2+-0.5)x10E+40 erg/s/kpc^2. There are galaxies where extraplanar emission was detected at smaller values of L_FIR/D^2_25, however, only in combination with a significantly enhanced dust temperature. The results corroborate the general view that the gaseous halos are a direct consequence of SF activity in the underlying galactic disk.

Motivation & Objective

• To determine how common extraplanar diffuse ionized gas (eDIG) halos are in non-starburst, edge-on spiral galaxies.
• To investigate the correlation between star formation activity in the galactic disk and the presence of eDIG in the halo.
• To establish a quantitative threshold in star formation rate surface density (SFR per unit area) for the detection of eDIG.
• To assess whether dust temperature enhancements can lower the detectability threshold for eDIG in low-SFR galaxies.
• To provide an unbiased sample covering a broad range of FIR luminosities and Hubble types, including early- and late-type spirals.

Proposed method

• Conducted deep H𝛼 narrowband imaging on 74 nearby edge-on spiral galaxies using ESO telescopes in Chile and Calar Alto.
• Measured the H𝛼 flux and surface brightness at vertical distances |z| up to 6 kpc from the disk plane to detect extended ionized emission.
• Calculated the star formation rate (SFR) per unit area using the FIR luminosity (L_FIR) and the galaxy's 25th magnitude isophotal diameter (D_25).
• Computed the FIR flux ratio S_60/S_100 as an indicator of dust temperature and star formation efficiency.
• Defined a threshold SFR surface density (L_FIR/D²_25) below which eDIG is not detected, using statistical analysis of detections and non-detections.
• Compared results with existing samples, including starbursts, to validate the threshold and assess its generality.

Experimental results

Research questions

• RQ1 What fraction of non-starburst, edge-on spiral galaxies host detectable extraplanar diffuse ionized gas (eDIG) halos?
• RQ2 Is there a minimum star formation rate surface density (SFR per unit area) below which eDIG is not detectable in the halo?
• RQ3 How does the FIR flux ratio S_60/S_100 correlate with the SFR per unit area in galaxies with and without eDIG?
• RQ4 Can enhanced dust temperature in low-SFR galaxies allow for eDIG detection below the nominal threshold?
• RQ5 Is the disk-halo connection via star formation activity consistent across galaxies of varying Hubble types and FIR luminosities?

Key findings

• Extraplanar diffuse ionized gas (eDIG) was detected in 30 out of 74 edge-on spiral galaxies at vertical distances |z| ≈ 1–2 kpc.
• In a few cases, eDIG filaments extend up to |z| ≤ 6 kpc, indicating significant energy and mass transport into the halo.
• A threshold SFR surface density of (3.2±0.5)×10⁴⁰ erg s⁻¹ kpc⁻² was derived below which eDIG is not detected in the sample.
• Galaxies with eDIG detection at SFR surface densities below this threshold were found only when dust temperature was significantly enhanced.
• A strong correlation was found between the FIR flux ratio S_60/S_100 and the SFR per unit area (L_FIR/D²_25), valid across starburst, normal, and quiescent galaxies.
• The results support the hypothesis that eDIG halos are a direct consequence of star formation activity in the galactic disk, with energy and momentum from supernovae and stellar winds driving the ionized outflows.