Identifying and distinguishing quenching galaxies with spatially resolved star formation in TNG50

Abstract

Using the TNG50 simulation, we determine observationally motivated metrics that can distinguish quenching galaxies from star forming galaxies for $M_{*} \geqslant 10^{9.5}~M_{\odot}$, based on the spatial distribution of their stellar populations. Quenching galaxies are not fully quenched but have low levels of ongoing star formation that decreases over time. The morphological metrics consider the concentration of star formation, size of the star forming disk, and characteristic radii that trace sharp truncations of star formation. These metrics can separate simulated quenching galaxies based on morphology into populations where star formation is suppressed inside-out and outside-in. Inside-out quenched galaxies are more likely to be the most massive galaxy within their halo in the field, while outside-in quenched galaxies are satellites residing in dense environments and begin quenching ${\sim} 1~\text{Gyr}$ after being accreted. Outside-in quenched galaxies typically take ${\sim} 1.5~\text{Gyr}$ to quench, and inside-out quenched galaxies can take up to ${\sim} 3.5~\text{Gyr}$, where the duration of quenching is a function of stellar mass. We find that each population of quenched galaxy experiences evolution of their morphological metrics, where the different quenched populations reside in unique locations in parameter space. Galaxies in the later stages of quenching are more easily distinguished than those in the early stages, when compared to star forming galaxies. In addition, inside-out quenched galaxies can be distinguished compared to outside-in quenched galaxies, and the progress through the quenching episode can be estimated for both populations. These results have broad implications for distinguishing quenching galaxies in large galaxy surveys.

Figures (16)

• Figure 1: Top: An SFH for a normal star forming galaxy ($M_{*} = 10^{10.72}~M_{\odot}$ at $z = 0$; dashed black line), shown as a function of time (age of the Universe). Additionally shown are the ${\pm} 2 \sigma$ limits from the median SFH of SFMS galaxies in a similar stellar mass range (${\pm} 0.1~\text{dex}$) at every snapshot (dotted--dashed gray lines). Due to the slightly stochastic nature of SFHs over short timescales, we opt to smooth our SFHs with a Gaussian filter, where the smoothing kernel is one snapshot. Bottom: An SFH for a quenched galaxy ($M_{*} = 10^{10.21}~M_{\odot}$ at $z = 0$; solid black line), where the primary quenching event is highlighted using a vertical colored band. Also shown are the corresponding percentiles (gray dotted--dashed lines) for SFMS galaxies with a similar stellar mass (${\pm} 0.1~\text{dex}$) at every snapshot. Once the SFH for the displayed quenched galaxy drops below the 2.5th percentile, it remains below this threshold, thereby defining this galaxy as quenched. The primary quenching episode for this galaxy lasts ${<} 2~\text{Gyr}$, and by $z = 0$ the galaxy has been quenched for ${>} 7~\text{Gyr}$. The subhalo ID for this quenched galaxy is 198186 at $z = 0$.
• Figure 2: Star formation rate as a function of stellar mass for the fiducial sample (1666 galaxies with $M_{*} \geqslant 10^{9.5}~M_{\odot}$; see Section \ref{['subsubsec:sample']}) at $z = 0$. Galaxies that populate the SFMS are shown in blue. Galaxies with low levels of star formation ($\text{SFR} \approx 0$) have their SFR set to a small, random nonzero value between ${\sim} 10^{-3}~M_{\odot}~\text{yr}^{-1}$ and ${\sim} 10^{-4}~M_{\odot}~\text{yr}^{-1}$ for visualization purposes. Galaxies that satisfy our quenched definition (see Section \ref{['subsubsec:quenched']}) are shown in red and do not necessarily have $\text{SFR} = 0$, while galaxies that do not populate the SFMS (as defined in Section \ref{['subsubsec:sfms']}) and that are not quenched are shown in black. These galaxies reside below the SFMS.
• Figure 3: The galaxy stellar mass function of our 361 quenched galaxies (black dots; $z = 0$), shown alongside two fitted galaxy stellar mass functions for local ($0.004 < z < 0.08$) red galaxies baldry2004 in SDSS york2000, and local ($z < 0.06$) red galaxies baldry2012 in GAMA driver2011. baldry2004 and baldry2012 define red galaxies based on location in the u---r color--magnitude diagram. We additionally show a stellar mass function for TNG50 galaxies based on a cut in sSFR using $\log{(\text{sSFR}/\text{yr}^{-1})} < -10.5$ (open circles).
• Figure 4: Top: projections of an $M_{*} = 10^{10.21}~M_{\odot}$ (at $z = 0$) quenched galaxy (the same galaxy as shown in the bottom panel of Figure \ref{['fig:SFHs']}) at three points during its primary quenching episode. From left to right, the time series shows the snapshot at the onset of quenching, ${\sim}$38% through the quenching episode, and ${\sim}$75% through the quenching episode. The postage stamp cutout images show spatial sSFR values, while the green contours trace the stellar mass distribution, and the white dotted circles describe multiples of the effective radius $R_{\text{e}}$. The time from the leftmost panel to the middle panel is $615~\text{Myr}$, while the rightmost panel is an additional $642~\text{Myr}$ after the middle panel. These projections highlight the evolution of the size of the star forming disk relative to the stellar disk, as the star forming disk shows significant outer truncation with increasing time, while the stellar disk remains largely unchanged, in fact shrinking by ${\sim}$10%. These panels also display the concentration of star formation. Bottom: sSFR radial profiles for snapshots that correspond to the top panels (red points; bin width of $0.25~R_{\text{e}}$). The median sSFR profile for SFMS galaxies at a similar stellar mass is shown with a dotted black line, with the ${\pm} 1 \sigma$ interval shown as a light gray band. The rightmost panel highlights $R_{\text{outer}}$ (vertical dotted line), as there is a clear discontinuity in the sSFR profile for this quenched galaxy. In this case, $R_{\text{outer}}/R_{\text{e}} \approx 2.3$.
• Figure 5: Evolution of the morphological metrics for the same galaxy as in the bottom panel of Figure \ref{['fig:SFHs']} and in Figure \ref{['fig:postage_stamps_and_profiles']}, where the vertical axis on the left records the values for $C_{\text{SF}}$ (solid black line) and $R_{\text{SF}}$ (dashed black line), while the vertical axis on the right records the values for $R_{\text{inner}}/R_{\text{e}}$ (dotted red line) and $R_{\text{outer}}/R_{\text{e}}$ (dotted--dashed red line). The extent of the x-axis reflects the primary quenching episode for this galaxy, which lasts ${<} 2~\text{Gyr}$, and is the same as the colored band seen in the bottom panel of Figure \ref{['fig:SFHs']}. This galaxy's morphological evolution is consistent with an outside-in signature, seen through the decrease of $R_{\text{SF}}$ and $R_{\text{outer}}$, along with $C_{\text{SF}}$ modestly increasing. This galaxy shows no evolution for $R_{\text{inner}}$, a feature commonly noticed for galaxies in the outside-in class in our sample. $R_{\text{e}}$ shrinks by ${\sim}$10% for this galaxy through the quenching episode.