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Register a new userA Normal Stellar Disk in the Galaxy Malin 1
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Authors
A.J. Barth
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Since its discovery, Malin 1 has been considered the prototype and most extreme example of the class of giant low surface brightness disk galaxies. Examination of an archival Hubble Space Telescope I-band image reveals that Malin 1 contains a normal stellar disk that was not previously recognized, having a central I-band surface brightness of mu_0 = 20.1 mag arcsec^-2 and a scale length of 4.8 kpc. Out to a radius of ~10 kpc, the structure of Malin 1 is that of a typical SB0/a galaxy. The remarkably extended, faint outer structure detected out to r~100 kpc appears to be a photometrically distinct component and not a simple extension of the inner disk. In terms of its disk scale length and central surface brightness, Malin 1 was originally found to be a very remote outlier relative to all other known disk galaxies. The presence of a disk of normal size and surface brightness in Malin 1 suggests that such extreme outliers in disk properties probably do not exist, but underscores the importance of the extended outer disk regions for a full understanding of the structure and formation of spiral galaxies.
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Low surface brightness galaxies (LSBGs) represent a significant percentage of local galaxies but their formation and evolution remain elusive. They may hold crucial information for our understanding of many key issues (i.e., census of baryonic and dark matter, star formation in the low density regime, mass function). The most massive examples - the so called giant LSBGs - can be as massive as the Milky Way, but with this mass being distributed in a much larger disk. Malin 1 is an iconic giant LSBG, perhaps the largest disk galaxy known. We attempt to bring new insights on its structure and evolution on the basis of new images covering a wide range in wavelength. We have computed surface brightness profiles (and average surface brightnesses in 16 regions of interest), in six photometric bands (FUV, NUV, u, g, i, z). We compared these data to various models, testing a variety of assumptions concerning the formation and evolution of Malin 1. We find that the surface brightness and color profiles can be reproduced by a long and quiet star-formation history due to the low surface density; no significant event, such as a collision, is necessary. Such quiet star formation across the giant disk is obtained in a disk model calibrated for the Milky Way, but with an angular momentum approximately 20 times larger. Signs of small variations of the star-formation history are indicated by the diversity of ages found when different regions within the galaxy are intercompared.For the first time, panchromatic images of Malin 1 are used to constrain the stellar populations and the history of this iconic example among giant LSBGs. Based on our model, the extreme disk of Malin 1 is found to have a long history of relatively low star formation (about 2 Msun/yr). Our model allows us to make predictions on its stellar mass and metallicity.
Our R-band data show that the optical light from Malin 1 corresponds well with the >2-arcmin extent of the galaxys HI content and continues well beyond previously published V-band optical light radial profiles. Analysis of our image yields improved understanding of the galaxys properties. We measure ellipticity of 0.20 +/- 0.03, implying inclination of 38 +/- 3 degrees, and we trace the radial profile to 77 arcsec. A single dusty spiral arm is also weakly discernable, and is consistent with the rotation direction of the HI and spiral structure of the inner disk. Possible scenarios for the origin of the spiral structure are discussed.
Our goal is to see if there is molecular gas extending throughout the optical low surface brightness disk of the galaxy Malin 2. We used the heterodyne receiver array (HERA) mounted on the IRAM 30m telecope to make deep observations at the frequency of the CO(2--1) line at nine different positions of Malin~2. With a total observing time of 11 hours at a velocity resolution of 11 km/s we achieve a sensitivity level of ~1 mK. We detect CO(2-1) line emission from Malin~2. The line is detected in four of the nine HERA beams; a fifth beam shows a marginal detection. These results not only confirm that there is molecular gas in the disk of Malin 2, but they also show that it is spread throughout the inner 34~kpc radius as sampled by the observations of the galaxy disk. The mean molecular gas surface density in the disk is $1.1pm0.2~M_{odot}~pc^{-2}$ and the molecular gas mass lies between the limits $4.9times10^{8}$ to $8.3times10^{8}~M_{odot}$. The observed velocity dispersion of the molecular gas is higher ($sim 13$,km,s$^{-1}$) than in star forming galactic disks. This could explain the disk stability and its low star formation activity.
The formation scenario for giant low surface brightness (gLSB) galaxies with discs as large as 100 kpc still remains unclear. These stellar systems are rare and very hard to observe, therefore a detailed insight on every additional object helps to understand their nature. Here we present a detailed observational study of the gLSB UGC 1922 performed using deep optical imaging and spectroscopic observations combined with archival ultraviolet data. We derived spatially resolved properties of stellar and ionized gas kinematics and characteristics of stellar populations and interstellar medium. We reveal the presence of a kinematically decoupled central component, which counter rotates with respect to the main disc of UGC 1922. The radial metallicity gradient of the ionised gas is in agreement with that found for moderate-size LSB galaxies. At the same time, a slowly rotating and dynamically hot central region of the galaxy hosts a large number of old metal-rich stars, which creates an appearance of a giant elliptical galaxy, that grew an enormous star forming disc. We reproduce most of the observed features of UGC 1922 in N-body/hydrodynamical simulations of an in-plane merger of giant Sa and Sd galaxies. We also discuss alternative formation scenarios of this unusual system.
Malin 1, being a class of giant low surface galaxies, continues to surprise us even today. The HST/F814W observation has shown that the central region of Malin 1 is more like a normal SB0/a galaxy, while the rest of the disk has the characteristic of a low surface brightness system. The AstroSat/UVIT observations suggest scattered recent star formation activity all over the disk, especially along the spiral arms. The central 9 ($sim 14$ kpc) region, similar to the size of the Milky Ways stellar disk, has a number of far-UV clumps - indicating recent star-formation activity. The high resolution UVIT/F154W image reveals far-UV emission within the bar region ($sim 4$ kpc) - suggesting the presence of hot, young stars in the bar. These young stars from the bar region are perhaps responsible for producing the strong emission lines such as H$alpha$, [OII] seen in the SDSS spectra. Malin 1B, a dwarf early-type galaxy, is interacting with the central region and probably responsible for inducing the recent star-formation activity in this galaxy.
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