This poster is the 44th in my series of compilation posters.  All of these posters can be seen in my Astrobin Poster Collection.

This poster is a collection of galaxy bulges.  Not all bulges are created equal.  Scientist now believe that there are three types of bulges – one created externally by mergers, one created internally by slow gas transport via the bar from the disk to the bulge, and one not created at all:

Classical Bulges

These are called Classical because years ago this is how it was believed that most bulges were formed.   Similar to the formation of elliptical galaxies, classical bulges are the result of galaxy mergers.  Consequently, classical bulges resemble small elliptical galaxies – a spherical shape which outshines the rest of the disk.  The bulge often appears as a large central glow.  Interestingly, this is now the smallest category of the 3 galaxy bulge types. 

Pseudobulges

Pseudobulges, also called disk-type bulges, are recognized by their distinctive X-pattern in the edge-on view.  This pattern is also often described as peanut-shaped or Figure 8 shaped.  The pattern is due to the inherent vertical instability of the bar and the resulting periodic orbits of stars in the bar region.

Viewed face-on, this pattern is seen as a barlens.  A barlens is a distinctive shape of a bright oval lens superimposed on a long, narrow, bright bar.  The oval is long in comparison to a standard core – almost half the length of the bar.  A barlens is hard to describe – the classic pseudobulge barlens shape is best seen in the face-on barlens galaxy NGC 4314.  This galaxy also has a colorful small nuclear core.

It is believed that these bulges are formed by a secular (slow) process of gas transport from the disk to the bulge area.  All of these galaxy types have a bar, although often the bar is hard to see in the edge-on views of the poster.  The bar is believed to be the mechanism for creating the pseudo bulge.   These pseudobulges form the largest of the 3 bulge categories.

Pure Disk Bulges

Pure disk galaxies (PDG) have a flat pancake profile and little if any bulge.  Scientists believe that these galaxies have remained isolated and have not encountered another galaxy yet (avoiding a merger and the resulting classical bulge).  That makes sense.  But scientists are not sure why such a galaxy would not develop a bar and a pseudobulge through its own internal processes, like most other isolated galaxies.  

Perhaps the lack of a bulge in PDG galaxies is due to a paucity of gas in their outer disk.  Or maybe it is an unknown mechanism which is inhibiting bar formation. 

Even though PDGs have no bulge, they are still forming stars.  In fact, they seem to have more of a blue appearance than the galaxies in the other categories. 

Hybrid Galaxies

The 3 bulge types above are due to the impact of mergers and bars, or lack thereof.  It makes sense, then, that hybrid galaxies exist which have characteristics of multiple types.  For example, a classical bulge could form from a merger and be supplemented by a pseudobulge over time.  On the poster you will see a few of these hybrids, mainly classic bulges that also have a faint X-patterm (e.g., NGC 7814) and pure disk bulges that have a slight bulge (e.g., NGC 5907). 

Acknowledgement

The study of bulges has been led in the past few decades by such prominent professors as Buta, Kormendy, Kennicutt, Fisher, Athanassoula, and others.  Information from many of their papers has been synthesized for this poster. 

The galaxy bulge continues to be an interesting and intense area of study and I am sure that we will know much more about them and their galaxies in the coming years.

If you would like to read more about any of these objects, each of the objects in the poster (along with many others which didn't make the poster cut)  has previously been uploaded and described individually on Astrobin. They all reside in these 3 Astrobin collections:
Classical Bulge
Pseudobulges
Pure Disk Bulges