The effects of bar strength and kinematics on galaxy evolution II: The global and local impacts of slow-strong bars

Abstract

There is now clear evidence, from a variety of studies, that galactic bars contribute to and/or accelerate processes which quench galaxies. However, bars have a variety of strengths and pattern speeds, and previous work has suggested that slow and strong bars impact their hosts the most. In this paper, we continue to investigate the impact of bar strength and bar speed on host galaxy evolution in a sample of barred galaxies identified via classifications from Galaxy Zoo. We perform a comprehensive assessment of star-formation tracers spanning a variety of timescales, based on spatially resolved spectroscopic information from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey. Specifically, we examine the radial distributions of EW[Halpha], HdeltaA, Hbeta, and Dn4000; spectral data that trace star-formation on current, intermediate, and much longer timescales. We investigate how these star-formation tracers vary with respect to each other in diagnostic evolutionary planes for eight categories of barred galaxies (combinations of star forming or quenching; strong and weak; fast and slow). We continue to find that slow-strong bars drive the quenching of their hosts the most by triggering active star-formation throughout the barred region; however, we note some additional complexity: we observe that stronger bars boost star-formation at the bar centre while slower bars have increased star-formation along the bar. This work adds to the growing evidence that galactic bars have both global and local impacts on their host galaxies.

Figures (8)

• Figure 1: Fraction of galaxies that are quenching in each bar-type of the TW sample (peach line) and of the total TW sample shown for comparison (gray dashed line). We note that the quenching fractions by bar type are not statistically different than that of the entire population within the Poisson errors for this sample size (which are presented in the Section \ref{['sec:results']}), but the trends suggest that more galaxies with strong bars are quenching than those with weak bars, and more with fast bars than slow.
• Figure 2: An illustration the process of constructing radial profiles, similar to that shown in Figure 2 in geronthesis. The left panel shows the EW[H$\alpha$] distribution with a 3"x30" aperture aligned with the position angle of the bar for the MaNGA galaxy, plate-ifu=12622-9101geronthesis. The middle panel shows EW[H$\alpha$] plotted against the distance from the galaxy center normalized to the bar radius for the region with spaxels whose centers lie within the aperture. A median profile for this galaxy (shown in black) is obtained by taking the median EW[H$\alpha$] within bins of size 0.15 $R_{\rm bar}$. This differs slightly from that shown in geronthesis due to slightly different choices about data corrections. The rightmost panel shows the total median profile obtained by taking the median of all individual profiles within the strongly barred subset of the TW sample (113 galaxies), with the shaded orange regions spanning the 33rd and 66th percentile. We show the contributing individual median profiles in light gray, with the profile of the example MaNGA galaxy shown in a thicker gray line. This plot is for the TW sample that we solely use in this work geronthesis, whereas Figure 2 in geronthesis shows this for the DESI-MaNGA sample geronthesis.
• Figure 3: Radial profiles of EW[H$\alpha$], H$\delta_\mathrm{A}$, H$\beta$, and Dn4000 for star forming galaxies (left panels) and quenching galaxies (right panels) sub-divided by both bar strength and speed (main panels). Shaded regions show the 33rd-66th percentile and the points are median values within bins of size 0.15$R_{\rm bar}$. Smaller sub-panels beneath each main panel show the difference between the profiles of bars of the same strength but of different speeds in the corresponding main panel (i.e. Slow/strong--Fast/strong and Slow/weak--Fast/weak, represented by the acronyms SSG, FSG, SW, and FW respectively). Where the residuals between the profile pairs are statistically significant ($>3 \sigma$) using an Anderson-Darling test, points are colored red. Slow-strong bars in SF host galaxies have higher sSFRs, younger stellar populations, and more recent/recently completed SF within the barred region.
• Figure 4: Median values (circular/triangular colored markers) of H$\alpha$, H$\beta$, and H$\delta_\mathrm{A}$ against those of Dn4000 in bins of size 0.3$R_{\rm bar}$ (hereafter referred to as quenching tracks) for all eight subsets of the TW sample. Markers are colored by radius, with dark blue representing the galaxy centre and light orange/pink the galaxy outskirts. The bar-end region (out to $\sim 1.35R_{\rm bar}\xspace$) corresponds to a green color and is enlarged and outlined in black for emphasis. Each panel shows two quenching tracks for each bar group specified by bar strength and speed: the tracks for bars in SF and quenching host galaxies, with the corresponding median values shown in filled and unfilled markers respectively. Panels are arranged left-to-right by bar strength, with strong bars presented in the left columns and weak bars in the right. Bar speed is indicated by the marker shape, with fast bars shown in triangles and slow bars in circles. Light gray background points show the 2D distribution of measurements of each star-formation tracer ( EW[H$\alpha$], H$\beta$, and H$\delta_\mathrm{A}$) within 1 $R_e$ against those of Dn4000 for all 10,0001 MaNGA galaxies Westfall2019. Note that the background distribution of each star-formation tracer against Dn4000 is the same across panels in the same row. We observe that slow and strong bars show the most efficient evolution within the barred region, as star-formation is dramatically enhanced in SF host galaxies and effectively suppressed in quenching hosts.
• Figure 5: Residuals of quenching tracks in regions aligned with and perpendicular to the bar (i.e. the difference of the tracks in regions with the aperture placed on the bar and off the bar, as shown in the upper right inset figure) for all eight subsets of the TW sample. Panels and markers follow the same formatting as Figure \ref{['fig:quenching_tracks']}; however, the average values within the central bin ($\sim$0.15$R_{\rm bar}$) are omitted as the tracks overlap in the central region. Gray shaded regions represent the median Poisson errors on the radial distributions of each star-formation tracers over all eight bar groups, applied to a reference point around zero (so points within the gray shaded regions indicate no difference between the two alignments to 1$\sigma$). We see that, with the exception of slow-weak bars, the enhancement of star-formation at the ends of all bars in SF hosts is a local impact, but is especially notable in slow-strong bars.