Studying Malin 1 and giant Low Surface Brightness Galaxies, today and in the future.

Samuel Boissier 1 ,  Junais 1

  • 1 LAM (Laboratoire d'Astrophysique de Marseille), Marseille

Abstract

Malin 1 has regained a lot of interest in the recent years (Galaz et al. 2015, Boissier et al. 2016, Junais et al. 2020). It is the archetype of giant low surface brightness galaxies, galaxies with central surface brightness much below the Freeman value (mu0B=21/65 mag/arcsec**2), but extremely massive (above 10**10 solar masses), with a huge gaseous disk supporting star formation around an early-type central component. About ten such galaxies are clearly identified in the nearby universe, focusing on galaxies with HI measurements and UV data. These galaxies may also be related to XUV galaxies found by GALEX (Thilker et al. 2005, Gil de Paz 2005) in which extended star forming disks surround the "usual" components of galaxies. The formation and origin of these galaxies is still debated, and spectral data can bring us insight on this subject (Saburova et al. 2020).

We will discuss these possibilities focusing on Malin 1, for which recent spectroscopic data were recently analysed in Junais et al. (2020), showing a steeply rising rotation curve up to several hundred km/s and pointing to the presence of a large DM halo, but also complex kinematics in the central part of the galaxy that will need further study. We also will show SFR measurements from the H-alpha emission line in some regions of the galaxy.

Future data will allow us to get even more insight on the properties of Malin 1 and other giant low surface brightness galaxies. I will mention especially expected data with MUSE for Malin-1, but those galaxies are also a good science target for futures IFUs such as blue-MUSE. At higher redshifts, the occurrence rate of such galaxies remains a mystery. However, we will suggest that SKA promises to change this picture, since the large gaseous content of giant low surface brightness galaxies make them a potential discovery niche in SKA surveys.

Malin 1: Lord of the Giant Low Surface Brightness Galaxies

Malin 1 is a massive, large (over 200 kpc in diameter) galaxy at only a few hundred megaparsec from us. However it was unoticed until it was imaged by David Malin (left). On the right, on the same scale, we show a CFHT MegaCam image from the NGVS survey (Boissier et al. 2016), in which the huge disk really reveal itself owing to the depth of the data.

What made this galaxy so hard to detect is the incredible low surface brightness of its giant disk : the extrapolated central surface brightness of the disk is fainter than 25 mag arcsec-2.

Many other Low Surface Brightness (LSB) galaxies are known, but Malin 1 is the most striking example of the Giant LSBs (see below).

Among LSB galaxies, Sprayberry et al. (1995) defined "Giant LSBs" by using a "diffuseness" criterium (based on scale-length and surface-brightness). Giant LSBs are massive galaxies, similar to regular spiral galaxies, that are however much more diffuse than them. Malin 1 is the most extreme in this class.

Other new definitions for selecting LSB, "unusual" galaxies have been proposed recently. They include the "Ultra Diffuse Galaxies definition (UDGs, Van Dokkum et al. 2015, koda et al. 2015) indicated as the red box in this diagram (assuming a disk geometry to convert effective radius to scalelength to use the same diagram). Lim et al. (2020) also proposed to define unusual galaxies based on their distance from normal scaling relationships.

Please also listen to Junais' talk for more on a sample of diffuse galaxies/UDGs/outliers in the Virgo cluster.

The figure is based on Hagen et al. (2016) who showed that we still find new analogues to Malin 1.

Recent results : a 200 kpc wide star forming disk, and a crazy rotation curve

Here, we present some recent results, taken mostly from Junais et al. (2020).

The results are based on spectra obtained in 2016 and 2019 in long-slit spectroscopy from Magelan Telescope. The data were obtained for 4 different slit positions on the sky. We refer to the paper for the details, and present below two highlights from this paper: one on star formation rate density in a few regions, and a second one on new constraints on the rotation curve of Malin 1.

1 - Star formation in a giant disk

This diagram shows the star formation rate surface density determined from spectroscopy in a few regions of Malin 1 in Junais et al. (2020).

The inner part of Malin 1 behaves like a regular early-type massive galaxy.

The Star Formation Rate in the outer regions is weak (it is undetected with these Magelan spectra in many regions that were however seen in UV). However, it is detected  in one very distant region at a very low level, expected in e.g. XUV galaxies.

2- A crazy rotation curve

Dynamic information can help us in understanding the origin of Giant LSB galaxies (e.g. Saburova et al. 2021).

The spectra analysed in Junais et al. (2020) allow us to show that the inner rise of the rotation curvey is very steep, with values as high than 400 km/s at a few kpc, before joining the 200 km/s plateau found in HI by Lelli et al. (2010).

A mass model of this rotation curve indicates that the central part of the galaxy is probably dominated by baryons, but Dark Matter in large amounts in still needed for the outer plateau. The very high velocities could indicate asymetric motions. Better data are necessary to better understand what is happening in the center of the galaxy.

The Future

What is the future for studying Malin 1 and Malin 1-like galaxies ?

  • We are pleased to announce that MUSE observations of Malin 1 were finally accepted and partially done (PI Gazpar Galaz). These new data will allow us to demonstrate the presence of ionised gaz in the star forming disk, and to get a new insight in the dynamics of the system.
  • Moderns and future IFUs (BlueMUSE) could bring us even more data on the dynamics of giant LSBs, and similar systems like XUV galaxies.
  • SKA 1 will detect Malin 1-like galaxies, if they exist, up to redshift 1, owing to their large HI mass  (Acero et al., 2017), even if their surface brightness would make easy to miss in optical surveys.
  • Many ongoing surveys are extremely sensible to the LSB regime, and will cover an unprecedented area of the sky, with the potential to find more of these fantastic galaxies (LSST, DES, Euclid, CFIS-UNION). If we find many Malin-1 like galaxies, we will be able to study the GiantESTs LSB in a statistical way. If not, we will have to understand what makes Malin 1 so unique.