Skip to main content

Galaxy Sizes as Tracers of Dark Matter

Periodic Reporting for period 1 - GALSIZE (Galaxy Sizes as Tracers of Dark Matter)

Reporting period: 2020-09-01 to 2022-08-31

The project Galaxy Sizes as Tracers of Dark Matter (GALSIZE) is aimed at studying the mass distribution of galaxies using different tracers, scaling relations, and simulations. The main scientific objectives of the project GALSIZE are to 1) establish a new paradigm of galaxy data-model comparisons, developing a definitive theory of disk formation based on galaxy sizes; 2) characterize the nature of, and coupling between, dark and luminous matter in galaxies, with the aid of scaling relations, mass tracers, and numerical simulations; and 3) expand model inferences into the uncharted realm of low-mass galaxies. These objectives are tackled with the largest set of structural parameters derived from the best optical, near-infrared, Hα, and ultraviolet imaging.

In the LamdaCDM cosmological paradigm, galaxies are formed from the cooling and condensation of gas in the centre of dark matter halos. In that context, an important challenge is to link the properties of present-day galaxies to their parent host halos with the aid of observations and cosmological simulations. When matched together, galaxy properties have often revealed fundamental laws of nature resulting from the physical causation between at least two independent observables. These properties typically involve sizes, luminosities, and masses. Size measurements have been notoriously heterogeneous. It has been shown that suitably chosen radii may yield the tightest galaxy scaling relations. Physically motivated sizes may herald a new paradigm in galaxy formation studies.

We propose to capitalize on these recent developments to establish a firm connection between star formation, accretion models, scaling relations, and the distribution and nature of dark matter in galaxies of different masses (from dwarfs to giants). Our sample will significantly improve upon that of previous studies, thus enabling a first detailed characterization of the link between star formation thresholds and disk properties.
We have assembled the largest publicly available compilation of photometric data to probe star formation (SF) in local galaxies (distances < 40 Mpc), comprising > 850 ultraviolet (UV) and Hα images (Díaz-García et al. 2020).

We have released the early-type galaxy (ETG) extension of the Spitzer Survey of Stellar Structure in Galaxies (S4G), that is, 3.6- and 4.5-micron images for 460 gas-poor galaxies obtained with the Spitzer space telescope (Watkins et al. 2022). We are also publishing the associated surface photometry together with galaxy structural parameters, including sizes. We derived the following scaling relations: mass–size, mass–concentration, mass–surface brightness, and mass–color. The relations between mass and isophotal radius and between mass and surface brightness within 1 kpc, in particular, show notably small scatter. The former provides important constraints on the limits of size growth in galaxies, possibly related to star formation thresholds.

We are completing the S4G survey with the i-band imaging of 211 late-type galaxies with the Liverpool Telescope and the New Technology Telescope. The program is 100% complete. The observed objects are relatively gas-poor galaxies and represent an important subclass for studies of galaxy quenching.

We have probed the emission from giant molecular clouds (GMCs) in the inner parts of 12 nearby galaxies using the IRAM-30m radiotelescope (Díaz-García et al. 2021). We have traced the carbon monoxide (CO) emission and used it to estimate molecular gas masses. These are needed to study fundamental scaling relations that link cold gas and star formation. We have publicly released these calibrated CO data. Both environmental and internal quenching are found to be responsible for the lowest reported star formation efficiencies.

The most physically motivated proxy of disk size may be based on the detection of a gas density threshold. That is, the radius where a critical gas surface density is reached, beyond which gas is no longer transformed into stars. We looked for truncations, a sharp drop-off, in UV light profiles that could be linked to gas density thresholds for SF. We discovered a very interesting case: UGC 7321; this is a diffuse, isolated, bulgeless, ultra-thin galaxy which is thought to be under-evolved both dynamically and in star formation. We reported the discovery of a truncation at and above its mid-plane that is linked to a SF threshold. This poses an important constraint on physically-motivated disk size measurements among low-mass galaxies. As UGC 7321 has no strong signs of accretion, our findings are consistent with its thick disk and truncations being formed via internal mechanisms alone.

We have investigated the origin of the extraplanar diffuse ionized gas (eDIG) in five nearby low-mass edge-on disk galaxies. We acquired Multi Unit Spectroscopic Explorer (MUSE) integral field spectroscopy and deep narrow-band Hα (Rautio et al. 2022). Our results support a surprisingly composite image of ionization mechanisms and a multifarious origin for the eDIG.

We have used the MEGARA integral-field-unit (IFU) instrument at the Gran Telescopio Canarias (GTC; largest optical telescope in the world) for the acquisition of kinematics of low-mass galaxies in the Virgo cluster. We got spectra for one dwarf (VCC1614). We are pushing to resolution limits hardy ever achieved by previous IFU surveys.

We used a sample of catalogued interacting and non-interacting galaxies in the S4G survey and calculated depletion times and gas fractions (Díaz-García & Knapen 2020). We conclude that galaxy mergers cause a moderate enhancement of the star formation efficiency.

We have used S4G imaging from the 140 edge-on local galaxies to measure their intrinsic thickness. We have reassessed the relationship between the scale-height and the scale-length of disks as a function of morphological type. This valuable relation allows to obtain a first-order estimate of the thickness of nearly face-on galaxies.

Scientific results have been presented in 7 international virtual conferences (e.g. EWASS, AAS), leading to the publication of four conference proceedings. Also, we have published four papers, and submitted another two, in Astronomy & Astrophysics (A&A). The standards of open science were fulfilled by uploading to the arXiv repository (astro-ph) the articles with our scientific results.
This project has enhanced our knowledge on how galaxies form and evolve by using state-of-the-art data (images and spectra) and technological facilities (telescopes). We have expanded the analysis of galaxy structural parameters using advanced methodological approaches, computer techniques, and unprecedentedly large datasets.

A revolution in our understanding of the Universe is anticipated with NASA’s JWST (just launched into space) and LSST (on Earth). These, combined with HI (e.g. SKA) and IFU surveys (e.g. WEAVE), will open a window for exploratory studies of galaxy formation. Research lines opened in this MSCA will be further expanded using the aforementioned surveys. Also, we have released important comparison galaxy samples for any future high-redshift (e.g. EUCLID) or upcoming all-sky 21 cm HI line surveys.
1-D model of the 3.6 microns surface brightness profiles of UGC 7321 in edge-on and face-on views.
Axial surface brightness profiles of UGC 7321 at 3.6 microns, grz, and near-UV.
Images of the low-mass ultra-thin galaxy UGC 7321 in 3.6 microns, grz, near- and far-ultraviolet.