[Paper Review] Modeling the spectral energy distribution of galaxies. II. Disk opacity and star formation in 5 edge-on spirals

A. Misiriotis, C. C. Popescu+2 more|ArXiv.org|Apr 20, 2001

Galaxies: Formation, Evolution, Phenomena1 references71 citations

TL;DR

This study models the spectral energy distributions (SEDs) of five edge-on spiral galaxies from ultraviolet to submillimeter wavelengths using radiative transfer and dust emission modeling. It finds that young stellar populations dominate dust heating (contributing up to 60%), with star formation rates (SFRs) normalized to disk area ranging from $7 \times 10^{-4}$ to $2 \times 10^{-2}\,{\rm M}_{\odot}\,{\rm yr}^{-1}\,{\rm kpc}^{-2}$, consistent with face-on galaxy trends and the Kennicutt-Schmidt law.

ABSTRACT

Using tools previously described and applied to the prototype galaxy NGC 891, we model the optical to far-infrared spectral energy distributions (SED) of four additional edge-on spiral galaxies, namely NGC 5907, NGC 4013, UGC 1082 and UGC 2048. Comparing the model predictions with IRAS and, where available, sub-millimeter and millimeter observations, we determine the respective roles of the old and young stellar populations in grain heating. In all cases, the young population dominates, with the contribution of the old stellar population being at most 40%, as previously found for NGC 891. After normalization to the disk area, the massive star-formation rate (SFR) derived using our SED modeling technique, which is primarily sensitive to the non-ionizing ultraviolet output from the young stellar population, lies in the range 7e-4 - 2e-2 M_sun * yr^-1 * kpc^-2. This is consistent with normalized SFRs derived for face-on galaxies of comparable surface gas densities from H_alpha observations. Though the most active star-forming galaxy of the five in absolute terms, NGC 891 is not an exceptional system in terms of its surface density in SFR.

Motivation & Objective

• To extend SED modeling of edge-on spiral galaxies beyond the prototype NGC 891 to a broader sample.
• To determine the relative contributions of old and young stellar populations to dust heating in galactic disks.
• To derive and validate star formation rate (SFR) surface densities using SED modeling that is sensitive to non-ionizing UV emission.
• To test whether NGC 891 is exceptional in its star formation activity compared to other normal spiral galaxies.
• To compare SED-derived SFRs with Hα-based SFRs and assess consistency with the Kennicutt-Schmidt law.

Proposed method

• Applied a radiative transfer model to edge-on galaxies, assuming a 'sandwich' geometry of stars and dust with distinct scale heights.
• Used multi-wavelength observations from IRAS, submillimeter, and millimeter facilities to constrain the SED from UV to far-IR.
• Modeled dust emission using a grain size distribution and stochastic heating of small grains to reproduce the FIR/submm SED shape.
• Calibrated the SED model using the non-ionizing UV output from young stellar populations as the primary SED constraint.
• Computed disk-averaged SFR surface densities by normalizing total SFRs to the disk area defined by 3 times the stellar scale length.
• Validated results using a two-dust-disk model and compared with Hα-derived SFRs and the Kennicutt-Schmidt law.

Experimental results

Research questions

• RQ1 What is the relative contribution of old versus young stellar populations to dust heating in edge-on spiral galaxies?
• RQ2 How do SED-derived star formation rate surface densities compare with those from Hα observations in edge-on systems?
• RQ3 Is NGC 891 an outlier in terms of star formation surface density among normal spiral galaxies?
• RQ4 To what extent does the SED modeling based on non-ionizing UV emission reproduce the observed FIR and submillimeter fluxes?
• RQ5 Do the derived SFRs for edge-on spirals follow the Kennicutt-Schmidt law for face-on disk galaxies?

Key findings

• In all five edge-on spirals, the young stellar population dominates dust heating, contributing at least 60% to the total energy input.
• The old stellar population contributes at most 40% to dust heating, confirming earlier findings for NGC 891.
• The SFR surface density derived from SED modeling ranges from $7 \times 10^{-4}$ to $2 \times 10^{-2}\,{\rm M}_{\odot}\,{\rm yr}^{-1}\,{\rm kpc}^{-2}$, with NGC 4013 being the most active in absolute terms.
• The SED-derived SFRs are consistent with Hα-based SFRs when corrected for extinction, supporting the reliability of the method.
• NGC 891 is not exceptional in SFR surface density compared to other normal spiral galaxies, indicating that its extended ionized and hot gas halos are not due to unusually high star formation.
• The SFRs derived from the standard SED model are consistent with those from the more complex two-dust-disk model, validating the method's robustness.