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My brownbag presentation on dwarf galaxies ✨

Let’s look at this beautiful picture of a spiral galaxy. This is a Hubble image of the Messier 101 (M101), also called the Pinwheel Galaxy. The Pinwheel Galaxy is a classic spiral galaxy that is nearly twice the size of the Milky Way and is located in the constellation Ursa Major. It is estimated to contain at least one trillion stars. It looks breathtaking, doesn't it?

![](/images/921px-M101_hires_STScI-PRC2006-10a%20\(1\).jpg)<sub>The Pinwheel Galaxy. Source: European Space Agency & NASA.</sub>

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However, these are not the kinds of galaxies our research group studies.

Below is NGC 5477, an irregular galaxy with no obvious structure but with visible signs of ongoing starbirth within the clouds of glowing hydrogen gas. NGC 5477 is actually a dwarf galaxy in the neighborhood of the Pinwheel Galaxy. This is what many dwarf galaxies look like, often appearing very diffuse and featureless.

![](/images/dwarf-galaxy-picture.jpg)<sub>An archetypal dwarf galaxy. Source: ESA/Hubble & NASA.</sub>

> So, what is a dwarf galaxy?

**A galaxy is a dwarf galaxy if it has baryonic mass < 10<sup>9</sup> solar masses**. Dwarf galaxies are the least massive, most abundant, and most widely distributed type of galaxies. Hence, they are key to testing theories in galaxy and universe evolution. Given their low metallicity, high gas content and ongoing star formation, these objects are supposed to resemble the first galaxies that formed at the earliest epochs, and may represent a window on the distant, early Universe.

The number of intriguing questions related to the extreme nature of dwarf population keeps increasing, as we obtain a more detailed view of their properties and we find puzzling discrepancies with theoretical expectations. The next decade promises to be full of discoveries, with missions and surveys such as the James Webb Space Telescope, the Vera Rubin Observatory, the Extremely Large Telescope, and the Nancy Roman Space Telescope, featuring state-of-the-art wide-field optical and infrared imagers combined with high-resolution spectrographs.


Intro to Dwarf Galaxies

*This is a short excerpt from my manuscript draft.*

<div style="text-align: center">![](/images/star-forming-region.jpg)<sub>Figure 1: The star-forming nebula W51 is one of the largest "star factories" in the Milky Way galaxy. (Source: NASA/JPL-Caltech).</sub></div>

Clusters of young stars are associated with dense interstellar matter from which they formed. Many of these stars are **massive OB types**, their hot temperature immediately **photoionizing nearby hydrogen gas**.  The amount of ionizing photons emitted by OB stars and absorbed locally is traced by the **Hα luminosity**, which is the most prominent in the Balmer series and is a well-calibrated tracer of the ionized gas in star-forming regions. While UV traces the longer-lived O, B, and A stars, Hα primarily traces O and B stars. Hα is then used to measure SFR over the lifetimes of these stars (<20 Myr), both locally and out to z\~2.  Since Hα is directly proportional to the total ionizing flux in star-forming regions, several studies have relied on Hα measurements to study star formation properties.

As with UV-optical emissions, an important source of uncertainty in using Hα as SFR tracer is **dust extinction**. The intensities of the hydrogen lines grow fainter with absorption, whose effects are more pronounced towards the UV range. To correct for this, the Hα/Hβ line-flux ratio, hereafter the **Balmer Decrement**, is measured and compared to the intrinsic value that is based on local conditions only and that has a theoretical value of 2.86.

<div style="text-align: center">![](/images/h-alpha-spec.jpg)<sub>Figure 2: Hα region of a dwarf galaxy.</sub></div>

References:

*   Berman, Louis, and R. J. Trumpler. 1936. “The Effect of Space Reddening on the Balmer Decrement in Planetary Nebulæ.” *Monthly Notices of the Royal Astronomical Society* 96 (9): 890–99. <https://doi.org/10.1093/mnras/96.9.890>.

*   Geha, Marla, Yao-Yuan Mao, Risa H. Wechsler, Yasmeen Asali, Erin Kado-Fong, Nitya Kallivayalil, Ethan O. Nadler, et al. 2024. “The SAGA Survey. IV. The Star Formation Properties of 101 Satellite Systems around Milky Way-Mass Galaxies.” arXiv. <https://doi.org/10.48550/arXiv.2404.14499>.

*   Kennicutt Jr., Robert C. 1998. “STAR FORMATION IN GALAXIES ALONG THE HUBBLE SEQUENCE.” *Annual Review of Astronomy and Astrophysics* 36 (Volume 36, 1998): 189–231. <https://doi.org/10.1146/annurev.astro.36.1.189>.

*   Osterbrock, Donald E. Astrophysics of Gaseous Nebulae and Active Galactic Nuclei. University Science Books, 1989.

*   Shivaei, Irene, Naveen A. Reddy, Charles C. Steidel, and Alice E. Shapley. 2015. “INVESTIGATING Hα, UV, AND IR STAR-FORMATION RATE DIAGNOSTICS FOR A LARGE SAMPLE OF z ∼ 2 GALAXIES.” *The Astrophysical Journal* 804 (2): 149. <https://doi.org/10.1088/0004-637X/804/2/149>.



H-alpha as a star formation indicator

Welcome to my life as a student researcher in astrophysics!! 🤩🫶

Hello! It's me, Lourdes, and I work on dwarf galaxies under the Data & Computation Research Group in the National Institute of Physics, University of the Philippines Diliman. My work involves a lot of data and programming 👩‍💻 —fitting spectra and characterizing environments of dwarf galaxies. As a regular student with a heavy academic load though, I can only work on my research outside of class hours, sometimes running late into the night. Despite the late hours, I actually enjoy coding in my room. Plus, I’ve found a new best friend: coffee ☕️🤪

![](/images/coding.jpg)<sub>Plotting dwarf galaxy spectra.</sub>

![](/images/coffee.jpg)<sub>Vitamin C(offee).</sub>

I meet with my research adviser, Dr. Reinabelle Reyes, during our regular Research Meetings (RMs). Here's a picture of our small (dare I say, dwarf? 👀) dwarf galaxy group, complete for the first time earlier this semester. Our group collaborates with Dr. Mia de los Reyes from Amherst College to study some properties of dwarf galaxies in the DESI survey.

![](/images/7.jpg)<sub>Group RM</sub>

I’m currently drafting my thesis manuscript, so the Review of Related Literature (RRL) is worth mentioning. Not exactly my favorite part, but it's part of the process. I can finally relate to reading long papers just to write a single sentence XD.

![](/images/research-rrl.jpg)<sub>Reading 💫⭐️🔭-astro books to reading 💻📊🌌-astro papers pipeline.</sub>

Anyway, here’s me matching my mediumvioletred spectra. So pink, so me 🤩.

![](/images/pink-starlourds.jpg)<sub>Pink starlourds</sub>

![](/images/dg-spectra.jpg)<sub>Pink dwarf galaxy spectra.</sub>

-xoxo, Lourdes ★



Our Dwarf Galaxy Research Group!

Intro to Dwarf Galaxies

H-alpha as a star formation indicator

My Research Life

Data & Computation Research Group