Frits Sweijen

PhD Candidate @ Leiden Observatory

image

Hi, I'm Frits. Currently I'm a PhD candidate at Leiden Observatory, in the Netherlands and software developer at ASTRON. My research project focusses on widefield sub-arcsecond imaging at low frequencies. This is done using the LOFAR telescope, with station in the Netherlands and all across Europe. Besides my research I have a strong interest in HPC/HTC computing and software development. This is, for example, reflected in my efforts of maintaining the LOFAR software at Leiden and packaging our software required for processing data, in Singularity containers being used both on our local compute nodes and on large compute facilities such as those at SURF. Outside of research I also occasionally engage in outreach from time to time, for example giving public talks, guest lectures at secondary schools, or writing articles about research topics.


Visualisations

LoTSS DR2 3D locations of galaxies

The LOFAR Two-metre Sky Survey (LoTSS) has released its second data release (DR2), containing over 4 million radio sources. Many of these are galaxies harbouring an active super-massive black hole in their core. Positions of approximately 1.6 million radio sources in LoTSS DR2, for which distances are available, are shown here. Read more in the official press release

Research Experience

Piercing the dusty veil of hyper-luminous infrared galaxies: Sub-arcsecond 144 MHz ILT observations of HLIRGs in the Lockman Hole

F. Sweijen, Y. Lyu, L. Wang, F. Gao, H. J. A. Röttgering, R. J. van Weeren, L. K. Morabito, P. N. Best, K. Małek, W. Williams, I. Prandoni, M. Bonato, M. Bondi

Hyper-luminous infrared galaxies (HLIRGs) are among the most extreme systems in the Universe. With infrared (IR) luminosities exceeding 10^13 L⊙ they can have IR-derived star formation rates (SFRs) exceeding 103 M⊙ yr−1. Theoretical models have a hard time reproducing the observed number densities of such extreme star-forming systems. It is known that at least part of the population harbours active galactic nuclei (AGNs), but their prevalence and relative contribution to the IR output is still debated. Assessing this is further complicated by the heavy dust obscuration. Starting from a highly complete Herschel-selected sample, we made use of recent wide-field sub-arcsecond 144 MHz International LOFAR Telescope (ILT) observations of the Lockman Hole field to probe AGN activity in HLIRGs in a dust-unobscured way. AGN presence was deduced through means of the brightness temperature (Tb). 98% of our detected sources are found to likely host radio-AGNs, raising the number of identified AGNs among the HLIRG population in this field from 16% to 32%. This increased number of AGNs is not sufficient to bring observations and predictions of HLIRG number densities in agreement, however.

Deep sub-arcsecond wide-field imaging of the Lockman Hole field at 144 MHz

F. Sweijen, R. J. van Weeren, H. J. A. Röttgering, L. K. Morabito, N. Jackson, A. R. Offringa, S. van der Tol, B. Veenboer, J. B. R. Oonk, P. N. Best, M. Bondi, T. W. Shimwell, C. Tasse, A. P. Thomson

Recent observations of the radio sky show that the vast majority of sources detected at 144 MHz are unresolved at the typical resolution of a few arcseconds1, demonstrating the need for sub-arcsecond-resolution surveys to make detailed studies. At low radio frequencies, high spatial resolution is challenged by the ionosphere and by the propagation delay of radio waves that it induces2. If not adequately corrected for, this blurs the images to arcsecond or even arcminute scales. In addition, the required image size to map the degree-scale field of view of low-frequency radio telescopes at sub-arcsecond resolution is far greater than what typical software and hardware are currently capable of handling. Here we present deep degree-scale sub-arcsecond imaging at low radio frequencies. We derive ionospheric corrections in 44 directions on individual sources with compact sub-arcsecond structures. This has yielded a sensitive 6.6 deg2 144 MHz map with a resolution of 0.38″ × 0.30″ and a sensitivity of 25 μJy per beam, near the phase centre. This will allow mapping of the entire northern low-frequency sky at sub-arcsecond resolution. Read more about this at Nature's community blog.

High-resolution international LOFAR observations of 4C 43.15 - Spectral ages and injection indices in a high-z radio galaxy

F. Sweijen, L. K. Morabito, J. Harwood, R. J. van Weeren, H. J. A. Röttgering, J. R. Callingham, N. Jackson, G. Miley, J. Moldon

It has long been known that radio sources with the steepest spectra are preferentially associated with the most distant galaxies, the α − z relation, but the reason for this relation is an open question. The spatial distribution of spectra in high-z radio sources can be used to study this relation, and low-frequency observations are particularly important in understanding the particle acceleration and injection mechanisms. However, the small angular sizes of high-z sources together with the inherently low resolution of low- frequency radio telescopes until now has prevented high angular resolution low-frequency observations of distant objects. Here we present subarcsecond observations of a z = 2.4 radio galaxy at frequencies between 121 MHz and 166 MHz. We measure the spatial distribution of spectra, and discuss the implications for models of the α − z relation. We targeted 4C 43.15 with the High Band Antennas (HBAs) of the International LOFAR Telescope (ILT) with a range of baselines up to 1300 km. At the central frequency of 143 MHz we achieve an angular resolution of ∼ 0.3′′. By complementing our data with archival Very Large Array (VLA) data we study the spectral index distribution across 4C 43.15 between 55 MHz and 8.4 GHz at resolutions of 0.4′′ and 0.9′′. With a magnetic field strength of B = 5.2 nT and fitted injection indices of αinj,south = −0.8 and αinj,south = −0.6, fitting a Tribble spectral ageing model results in a spectral age of τspec = 1.1 ±0.1 Myr. We conclude that our data on 4C 43.15 indicates that inverse Compton losses could become comparable to or exceed synchrotron losses at higher redshifts and that inverse Compton losses could be a viable explanation for the α−z relation. Statistical studies of these objects will become possible in the future when wide-area subarcsecond surveys start.

PhD candidate

Leiden Observatory
2017 - Present

High resolution imaging at low frequencies

High resolution observations allow one to unmistakenly identify AGN activity, as strong compact emission and study extended sources in more detail, especially as their distance increases. LOFAR provides a unique view on these objects with its unprecedented sensitivity and resolution at the lowest radio frequencies. Key points during this research are:

  • Expanding LOFAR's high resolution imaging capabilities to widefield imaging.
  • Studying the 151 MHz radio sky at an unprecendented combination of depth, field of view and angular resolution.

MSc Thesis

Kapteyn Astronomical Institute
2016 - 2017

An Observational Study of the z~4 strongly lensed DSFG MM18423+5938

In this thesis we present the result of a study on the strongly gravitationally lensed dusty star-forming galaxy (DSFG) MM18423+5938. We use data from the HST at 1.1 µm to discuss the lens morphology, which is more complex than it appears to be. With the VLA, we use 5 GHz observations to add a second photometric data point to the synchrotron part of the spectrum. The system is doubly imaged in the radio continuum, with S_5GHz = 74 ± 19 mJy. We find a spectral index of α = -0.83 ± 0.34; consistent with other DSFG samples. The molecular gas content was studied through CO(1-0) and CO(2-1) emission using both previously published data and archival data. We find the CO(1-0) line intensity to be I_CO(1-0) = 0.5 ± 0.3 μ⁻¹ Jy km s⁻¹, with a line luminosity of L'_CO(1-0) = 3.1 ± 1.9 × 10¹¹ μ⁻¹ K km s⁻¹ pc², resulting in a molecular gas mass of M_gas = 2.5 ± 0.5 × 10¹¹ μ⁻¹ M_sol. For the CO(2-1) emission we find a lensed line intensity I_CO(2-1) = 2.8 ± 1.5\ μ⁻¹ Jy km s⁻¹, where μ is the magnification, resulting in a line luminosity of L'_CO(2-1) = 4.5 ± 0.8 × 10¹¹ μ⁻¹ K km s⁻¹ pc². We identify a possible double peak profile, which would be a hint of a rotating disk, but detailed dynamical modeling is required to test this. Splitting the line into a (relative) red, green and blue part shows different spatial structures, further hinting at a rotating structure. Finally 250 μm, 350 μm and 500 μm measurements from the Herschel Observatory were used to constrain the peak of the SED. We fitted an MBB and power law to the spectrum. From the SED, we then obtained T_d = 37.8^{+2.0}_{-1.9} K, β = 1.7 ± 0.2, L_FIR = 9.76 ± 0.30 × 10¹³ μ⁻¹L_sol, q = 2.59 ± 0.47 and SFR = 1.7 × 10^4 μ⁻¹ M_sol yr⁻¹. All of the parameters are consistent with other, unlensed DSFGs assuming a magnification μ ~ 12, where applicable.

BSc Project

Kapteyn Astronomical Institute
2014 - 2015

Computer Control of a Horn Antenna and Measuring the Sun at 11 GHz

This thesis presents the work I have done as part of a project to build a radio telescope to observe the CMB at 11GHz. It covers some basic radio astronomy theory and the weather conditions in Groningen. After this it focusses on controlling the telescope and its measuring equipment through a Raspberry Pi. It will conclude with observations of the Sun and a satellite to verify the beam size. This was found to be 12.07°±0.13° using the Sun and 12.61°±0.19° using a satellite,in agreement with expectations.

Outreach Experience

[Artikel] LOFAR brengt de hemel op de laagste radiofrequenties scherper dan ooit in beeld

Zenit
December 2021

Artikel over een verzameling LOFAR papers die tegelijkertijd gepubliceerd werden in Astronomy & Astrophysics.

[Artikel] Nieuw licht op hyperlumineuze sterrenstelsels

Zenit
December 2021

Artikel over een onderzoek geleid door L. Wang bij SRON, waarin geconstateerd werd dat er veel meer infrarood-heldere sterrenstelsels met enorme stervormingssnelheden zijn dat onze huidige theorieen kunnen verklaren.

My software

LOFAR SKSP Singularity recipes

https://tikk3r.github.io/flocs/

Singularity recipes maintained by me, packaging SKSP related software. It is aimed to provides an easy-to-deploy software solution to run e.g. the prefactor and lofar-vlbi pipelines.

An interactive potting utility to explore LOFAR calibration solutions in H5parm format. Can be used to explore solutions for single stations as a function of time and/or frequency.

A small Python module to easily retrieve cutouts from the Legacy survey. It is a wrapper around their URL for retrieving cutouts. Both standalone usage or importing as a module are supported.

Python module to retrieve cutouts from a variety of astronomical surveys. It is a wrapper around survey-specific service or SkyView. Both standalone usage or importing as a module are supported.