Forming Galaxies Without Bulges


Pure disk galaxies — thin disk galaxies that don’t have a central bulge — are a puzzling presence in our universe. How were these galaxies able to escape the effects that normally generate bulges? A new study has examined the properties of pure disk galaxies over the last 8 billion years in an effort to learn more.

Challenging the Model

pure disk galaxy

Example of a pure disk galaxy at z=0.86. Its brightness profile, in which surface brightness is plotted against semi-major axis, exhibits a pure exponential decay from the center of the galaxy to its edge. [Sachdeva&Saha 2016]

According to the commonly accepted picture of galaxy formation, galaxies grow hierarchically via major and/or minor mergers. These mergers ultimately scramble the galaxies’ preexisting disks, thicken the disk structures, and cause the formation of classical bulges at the disk centers.

The fact that we also observe pure disk galaxies without central bulges challenges this picture. If all galaxy formation is driven by the hierarchical model, then how have pure disk galaxies manage to escape the effects of merger activity?

In a new study, Sonali Sachdeva and Kanak Saha (Inter-University Centre for Astronomy and Astrophysics, India) examine the population of pure disk galaxies out to a redshift of z~1. Their goal is to better understand the properties of this strange category of galaxies throughout the last 8 billion years.

Disks Past and Present

Sachdeva and Saha examine the light profiles of ~570 galaxies from the Hubble Deep Field and the Sloan Digital Sky Survey. They categorize as pure disk galaxies those that can be described well by a single exponential function from the center of the galaxy out to its outer edges. Galaxies requiring an additional functional component to model the excess light in the center are classified as galaxies with bulges.

Using this categorization, the authors find that 94 of the 570 galaxies are pure disk galaxies. When they bins these galaxies into three redshift bins between z~1 and z~0, pure disk galaxies account for 15–18% of the total galaxies in each bin. This tells us that the fraction of pure disk galaxies hasn’t altered much in the last 8 billion years.

B/T ratio

Distribution of the bulge-to-total light ratio for the pure disk galaxies (blue) and the other disk galaxies (red) in the sample. [Sachdeva&Saha 2016]

Non-Merger Growth

Further examining the brightness profiles for these pure disk galaxies, Sachdeva and Saha find that both the average central surface brightness and the average scale length are the same across different redshift bins — which means that gas isn’t being fed into the interior parts of these galaxies over time. Yet in spite of this, the total stellar mass and the size of these galaxies grows substantially from z~1 to the present day — by 40% and 60%, respectively.

How could these galaxies be growing without changing their profiles (as would happen if their growth were caused by mergers)? The authors propose that these galaxies may be isolated and protected from mergers, and they grow through smooth accretion via cosmic filaments of cold gas onto their outskirts. Additional study of this unique category of galaxies may provide further insight into different mechanisms of galaxy evolution.


Sonali Sachdeva and Kanak Saha 2016 ApJ 820 L4. doi:10.3847/2041-8205/820/1/L4