How Do Galaxies Keep Growing? A Closer Look at Gas Accretion in NGC 99

Where does a galaxy get the fuel to form stars? Why do some regions look chemically “off” from the rest of the galaxy? Can we catch galaxies in the act of feeding themselves?

These big-picture questions lie at the heart of understanding how galaxies evolve across cosmic time. A galaxy is not a static system. On cosmic timescales, it evolves as it interacts with itself and the surrounding environment. Gas flows in from the surrounding cosmic web, and material is blown out by stellar explosions and supermassive black holes within the galactic disk. This constant exchange of material, called the baryon cycle, determines how galaxies grow, form stars, and enrich their interstellar medium with metals (elements heavier than helium).

In my recent study of the nearby spiral galaxy NGC 99, part of the DIISC (Deciphering the Interplay between the Interstellar medium, Stars, and the Circumgalactic medium) survey, I found intriguing evidence for gas inflow using a combination of optical, radio, and ultraviolet observations. By analyzing small pockets of star formation scattered throughout the galaxy, I uncovered chemical signatures that suggest NGC 99 is actively accreting low-metallicity gas from its surroundings, which could be a key mechanism in sustaining its star formation.

Why NGC 99?

NGC 99 is a nearby star-forming spiral galaxy, about 260 million light-years away. It’s an ideal target for a detailed, multiwavelength study because it has:

• Strong, extended neutral hydrogen (HI) detected in radio,

• Deep optical imaging and spectroscopy,

• A bright and nearby background quasar, which lets us probe its gaseous halo (the circumgalactic medium, or CGM),

• A face-on orientation

To map the galaxy’s chemical structure, we used the Binospec spectrograph on the 6.5-meter MMT Observatory (located near Tucson, AZ) to obtain optical spectra of 26 star-forming regions (H II regions) across the disk. We also analyzed HI imaging from the Very Large Array (VLA), narrowband Hα imaging, and UV absorption spectra from the Hubble Space Telescope’s (HST's) Cosmic Origin Spectrograph (COS) instrument.

Chemical Anomalies

The gas-phase metallicity of a galaxy can be an indicator of recent gas flows. Since most metals are forged in stars, regions that appear unexpectedly metal-poor—so-called Anomalously Low Metallicity (ALM) regions—can be clues to recent inflows of pristine (low metallicity) or nearly pristine gas. In this study, we identified two such ALM regions in NGC 99’s disk.

Here’s what made them stand out:

• Their oxygen abundances were ~0.16 dex lower than nearby regions at the same galactocentric radius.

• They showed velocity offsets of >35 km/s between the ionized gas (Hα) and the neutral gas (HI), suggesting dynamical disturbance.

• Morphologically, they blended into the disk and spiral arms—no signs of unusual structure.

When these regions were removed from our radial metallicity gradient fit, the slope steepened, reinforcing their anomalous nature.

Investigating the Source of Pristine Gas

The fresh gas that lowered the metallicity of the two ALM HII regions could have originated from two possible locations: the center of the galaxy (via galactic fountaining) or NGC 99's CGM (via cool streams of gas). Considering that gas from galactic fountains is typically higher in metallicity, we theorize that the ALM HII region were created from CGM gas. To investigate whether the kinematics of the CGM could support this type of accretion, we measure the characteristics of CGM gas via the absorption of Lyα light from a background quasar located 159 kpc away in projection.

• We detected four absorption components of Lyα.

• The most dominant of these components is corotating with the extended HI gas of the galaxy, meaning the CGM and disk gas is rotating in the same direction as well as approximately the same velocity, despite the large distance between them.

  
  

The kinematics of the NGC 99's CGM suggest two possible mechanisms for the gas to enter the galaxy disk: (1) along the extended disk or (2) through the halo vertically to the disk. In the first case, the fresh CGM gas may radially flow through the extended disk without much metal mixing occurring due to the low HI gas dispersion. For the second case, the CGM gas may infall onto the disk much like how high-velocity clouds (HVCs) do in the Milky Way. Previous studies have found that HVCs do indeed support star-formation in the Milky Way. We estimated the mass of accreted gas needed to lower the metallicity of the two ALM regions is on the order of 10⁶ solar masses, which is comparable to the mass of HVCs.