Molecular Gas Reveals a Hidden Nucleus Shining Unevenly

NGC 3256 is a merger of gas-rich galaxies about 40 megaparsecs, or 130 million light-years, away. Its southern nucleus contains a heavily obscured, relatively low-luminosity active galactic nucleus (AGN): a compact energy source associated with matter accreting onto a supermassive black hole. A fast molecular outflow extends in two opposite directions from the region.

In a study accepted for publication in Astronomy & Astrophysics, Miguel Pereira-Santaella and colleagues report that the outflow sees a different infrared source from the one astronomers see along Earth’s line of sight. Their analysis suggests that radiation near a wavelength of 4.7 micrometers is stronger along the outflow than toward us. They propose that an opening in the surrounding dusty material channels that light outward.

Reading scattered light

The team reanalyzed observations from the James Webb Space Telescope’s Near-Infrared Spectrograph, or NIRSpec, together with carbon monoxide data from the Atacama Large Millimeter/submillimeter Array (ALMA). NIRSpec provides spectra across the source, allowing the researchers to compare separate regions rather than blend the nucleus and its surroundings into one measurement.

The near-infrared observations show a band of carbon monoxide (CO) emission extending roughly 300 parsecs, or nearly 1,000 light-years, from the nucleus along the base of the outflow. Infrared photons from warm dust excite the molecules; the molecules then re-emit photons in a set of spectral lines as their vibrational and rotational states change. In effect, the CO acts as a tracer of the radiation reaching the gas.

The researchers extracted spectra from 13 regions, each about 80 parsecs across, mostly along the outflow. They removed the contribution from the extended wings of the bright nuclear image and accounted for nearby spectral features from other species. Their model applies to regions where the CO band appears in emission, not to the nuclear absorption or the mixed absorption-and-emission pattern seen in parts of the disk.

To interpret the lines, the team simulated photons scattering through idealized, uniformly dense spherical clouds illuminated from one side. A grid of 6,250 models explored different gas temperatures, densities and CO columns. The calculations treated the opacity of individual lines and the direction in which photons escaped; the ALMA CO line helped estimate the number of molecules. Comparing predicted and observed line strengths then yielded an estimate of the infrared flux illuminating each region.

A brighter view along the outflow

For most regions, the inferred nuclear brightness at 4.7 micrometers was about two to four times the value inferred from the spectrum seen from Earth. Three regions farther from the outflow axis were consistent with our view, while another off-axis region required stronger illumination. The authors suggest that an uneven, possibly clumpy distribution of obscuring material could help explain that exception.

The fitted gas temperatures also decline with distance: from roughly 40 kelvin near 70 parsecs to about 20 kelvin at distances approaching 300 parsecs. The gas density, however, was not well constrained by these data.

The inferred radiation field also falls more slowly with distance along the outflow than the inverse-square decrease expected from a simple point source. The authors find that a collimated beam can reproduce this trend. Using the observed width of the outflow to estimate a half-opening angle of roughly 15–20 degrees, their geometric model gives an opening about 15 ± 5 parsecs across. They compare that with an estimated 25 ± 5 parsecs for the obscuring envelope. These are model-dependent sizes, not direct resolved images of an escape channel.

Collimation is not the only possible explanation. The paper notes that a roughly twofold decline in nuclear luminosity over about 1,000 years could also produce a distance-dependent signal because light takes time to reach the gas. The authors favor the channel interpretation partly because a simple isotropic luminosity decline would also brighten the inferred illumination of the off-axis regions, which generally is not observed.

A nucleus between categories

The absence of extended high-ionization emission suggests to the authors that obscuring material covers much of the AGN’s sky, limiting the escape of energetic radiation. Yet their estimated hydrogen column density, approximately 10²²–10²³ atoms per square centimeter, is closer to that of a typical obscured Type 2 AGN than to the much denser compact obscured nuclei found in some infrared-bright galaxies. The paper therefore describes NGC 3256 S as sharing characteristics of both.

Blueshifted CO absorption toward the nucleus suggests that its obscuring envelope is expanding. Together with the outflow and inferred illumination pattern, this is consistent with the AGN beginning to clear its surroundings, but it does not establish how the channel formed: an outflow, a possible radio jet or a preexisting structure could all play a role.

The researchers also propose using the ratio of the infrared CO band to the millimeter CO line to estimate hidden core luminosity in similar systems. That calibration comes from this one outflow and depends on gas conditions and geometry; it is a potential tool, not yet a universal measurement method.