Research Interests
Galactic nuclei: dust structure & evolution, accretion & jet structure, spectral energy distributions, gas kinematics & black hole masses
Infrared observations: high-spatial resolution, mid-infrared, ground-based methods, dust & PAH emission
My research is focused on the study of the nuclei of galaxies which all presumably harbor supermassive black holes (SMBH). It is believed that these black holes grow and evolve during several phases of nuclear activity. During those phases large amounts of emission are produced over the whole wavelength range by the innermost material while it is accreted onto the SMBH. To explain the observed spectral energy distribution (SED) two key components are necessary in the so-called unification scheme, the accretion disk and a surrounding dusty obscurer, which is commonly called the torus. The resulting inclination-dependent obscuration can explain in principle all observed subclasses of these active galactic nuclei (AGN). However, the physical nature and morphology of this obscurer is still poorly understood. It is unknown how it is formed, what maintains its vertical thickness and under which circumstances it appears and disappears. Although theoretical studies have looked into these issues, observational constraints are still lacking.
AGN-heated dust can be well-observed in the mid-infrared (MIR) where its thermal emission is dominating. For this, high spatial resolution is key in order to separate the nuclear emission from the circum-nuclear emission, which is caused by star formation heated dust, which is why I focus on instruments like VLT/VISIR for my research.
Past & current research
MIR properties of nearby low-luminosity AGN (LLAGN):
LLAGN are the most common among active galaxies. They represent the initial and final stages of activity, being perfect candidates to investigate the formation and fading of the putative torus. Our VISIR narrow-band imaging programme showed that all observed nearby LLAGN exhibit sub-arcsec point-like nuclear MIR emission and fall onto the MIR-X-ray correlation (Asmus et al. 2011). Furthermore, the possible star formation contribution was constrained to be minor and the MIR-X-ray luminosity ratio found to be independent of the accretion rate over ~5 orders of magnitude. Thus, these results show that the dusty torus might exist in the observed LLAGN, i.e., the unification scheme might still hold for them. However, only few LLAGN are observable in the MIR due to the limited sensitivity from the ground. To maximize the sample, I extended our investigation onto the Northern hemisphere with the the Palomar LLAGN sample using Gemini/Michelle (included in SASMIRALA below).
The SubArcSecond Mid-InfraRed Atlas of Local AGN (SASMIRALA):
The subarcsecond MIR atlas of local AGN is a collection of all available N- and Q-band images obtained at ground-based 8-meter class telescopes with public archives (Gemini/Michelle, Gemini/T-ReCS, Subaru/COMICS, and VLT/VISIR). It includes in total 895 images, corresponding to 253 local AGN with a median redshift of 0.016 (Asmus et al. 2014) and is accessible in the Virtual Observatory. The atlas contains the uniformly processed and calibrated images and nuclear photometry obtained through Gauss and PSF fitting for all objects and filters. This also includes measurements of the nuclear extensions. In comparison to the arcsecond-scale MIR emission as probed by Spitzer, the continuum emission is much lower on subarcsecond scales in many cases. The silicate feature strength is similar on both scales and generally appears in emission (absorption) in type I (II) AGN. However, the polycyclic aromatic hydrocarbon emission appears weaker or absent on subarcsecond scales. The differences of the MIR SEDs on both scales are particularly large for AGN/starburst composites and close-by (and weak) AGN. The nucleus dominates over the total emission of the galaxy only at luminosities ≳1044 erg s-1. The AGN MIR atlas is well suited not only for detailed investigation of individual sources but also for statistical studies of AGN unification.
The mid-infrared -- X-ray correlation for AGN:
We used SASMIRALA to re-investigate the well-known correlation between the AGN emission at MIR and X-ray wavelengths (Asmus et al. 2015). This correlation is surprising for many reasons: 1. It extends over the full range of probed luminosities with a slope of unity not showing any change of slope as expected for example by the receding torus scenario.
2. It is very tight with a scatter of two, applicable to all AGN types (except maybe narrow-line Seyfert 1s), which implies very small differences in the emission of the different AGN types, in particular unobscured versus obscured AGN. Canonical torus models predict larger differences between these two.
3. Even radio-loud AGN seem to follow the correlation rather well, despite the expected additional contribution of jet synchrotron emission to the MIR. Looking very closely, it seems that the slope od this correlation is different for the radio-loud objects at least but with a normalisation such that the population overlap in the MIR--X-ray plane.
A possible explanation for at least the first two points, is given by the results from the next project:
The mid-infrared emission from polar dust:
Up to know the nuclear MIR emission has been associated directly with the dusty obscurer, and could well be fit with clumpy torus models. However, the structures at parsec sales resolved with MIR interferometry (Hönig et al. 2012, 2013, Tristram et al. 2014, and López-Gonzaga et al. 2014) show a surprising polar elongation which is somewhat at odds with the canonical clumpy torus.
This polar emission is also found on larger (10 to 100pc) scales as it turns out! We could show this statistically for the first time using the extended AGN in SASMIRALA, which have no significant star formation on nuclear scales (Asmus et al. 2016). The results indicate that the MIR emission of AGN is in fact dominated by dust on the edges of the ionisation cones (a dusty wind?) instead of a clumpy torus. This in turn implies that the real obscurer must be more compact and hotter than previously thought...
The subarcsecond MIR atlas of local AGN is a collection of all available N- and Q-band images obtained at ground-based 8-meter class telescopes with public archives (Gemini/Michelle, Gemini/T-ReCS, Subaru/COMICS, and VLT/VISIR). It includes in total 895 images, corresponding to 253 local AGN with a median redshift of 0.016 (Asmus et al. 2014) and is accessible in the Virtual Observatory. The atlas contains the uniformly processed and calibrated images and nuclear photometry obtained through Gauss and PSF fitting for all objects and filters. This also includes measurements of the nuclear extensions. In comparison to the arcsecond-scale MIR emission as probed by Spitzer, the continuum emission is much lower on subarcsecond scales in many cases. The silicate feature strength is similar on both scales and generally appears in emission (absorption) in type I (II) AGN. However, the polycyclic aromatic hydrocarbon emission appears weaker or absent on subarcsecond scales. The differences of the MIR SEDs on both scales are particularly large for AGN/starburst composites and close-by (and weak) AGN. The nucleus dominates over the total emission of the galaxy only at luminosities ≳1044 erg s-1. The AGN MIR atlas is well suited not only for detailed investigation of individual sources but also for statistical studies of AGN unification.
The mid-infrared -- X-ray correlation for AGN:
We used SASMIRALA to re-investigate the well-known correlation between the AGN emission at MIR and X-ray wavelengths (Asmus et al. 2015). This correlation is surprising for many reasons: 1. It extends over the full range of probed luminosities with a slope of unity not showing any change of slope as expected for example by the receding torus scenario.
2. It is very tight with a scatter of two, applicable to all AGN types (except maybe narrow-line Seyfert 1s), which implies very small differences in the emission of the different AGN types, in particular unobscured versus obscured AGN. Canonical torus models predict larger differences between these two.
3. Even radio-loud AGN seem to follow the correlation rather well, despite the expected additional contribution of jet synchrotron emission to the MIR. Looking very closely, it seems that the slope od this correlation is different for the radio-loud objects at least but with a normalisation such that the population overlap in the MIR--X-ray plane.
A possible explanation for at least the first two points, is given by the results from the next project:
The mid-infrared emission from polar dust:
Up to know the nuclear MIR emission has been associated directly with the dusty obscurer, and could well be fit with clumpy torus models. However, the structures at parsec sales resolved with MIR interferometry (Hönig et al. 2012, 2013, Tristram et al. 2014, and López-Gonzaga et al. 2014) show a surprising polar elongation which is somewhat at odds with the canonical clumpy torus.
This polar emission is also found on larger (10 to 100pc) scales as it turns out! We could show this statistically for the first time using the extended AGN in SASMIRALA, which have no significant star formation on nuclear scales (Asmus et al. 2016). The results indicate that the MIR emission of AGN is in fact dominated by dust on the edges of the ionisation cones (a dusty wind?) instead of a clumpy torus. This in turn implies that the real obscurer must be more compact and hotter than previously thought...
