Constraining star formation in M87 using deep HST UV data

TL;DR

This study leverages the deepest HST F275W UV data to constrain the star formation in M87, a nearby cool-core giant elliptical, by identifying faint UV point sources and UV filaments in the central region. Using diffuse light subtraction, SExtractor detection, and SLUG-based stochastic population modeling, the authors find a time-averaged SFR of about $1.8\times10^{-5}$ M$_\odot$ yr$^{-1}$ with a dominant burst at $\sim125$ Myr ago that formed roughly $10^3$ M$_\odot$ of stars, and little evidence for ongoing star formation. The UV filaments co-spatial with H$\alpha$ are powered by metal-line cooling from collisional ionization, not by young stars, suggesting ongoing low-level precipitation of the ICM despite efficient AGN quenching. The results imply that AGN feedback has suppressed star formation in M87 for at least ~200 Myr, though episodic, low-level star formation may be intermittently triggered by earlier AGN outbursts, contributing to the understanding of the feedback–cooling cycle in massive ellipticals and their central black holes.

Abstract

We analyzed the deepest Hubble Space Telescope (HST) F275W ultraviolet (UV) imaging of M87 to obtain the most robust constraints on its star formation rate (SFR) and star formation history (SFH). After removing the galaxy continuum and globular clusters, we detected an excess of UV point sources near the center. By comparing their colors to young stellar source (YSS) colors generated by stochastically simulated star formation (SF) for various SFRs and SFHs, we ruled out their origin as a UV-upturn population and identified them as YSS. We found an extremely low SFR of $\sim 2\times10^{-5}$ M$_\odot$ yr$^{-1}$ in M87, with evidence of a weak starburst $\sim$125 Myr ago that formed $\sim 1000$ M$_\odot$ of stars. Unlike other cool-core clusters where SF is stronger and directly linked to cooling gas, we found no spatial correlation between YSS and H$α$ filaments. Comparing SF activity with M87's AGN outburst history suggests that recent AGN feedback events ($\lesssim$12 Myr ago) neither triggered nor were associated with any detectable SF, however, earlier outbursts may have triggered weak starbursts. We detected UV filaments co-spatial with H$α$ filaments with similar lengths and widths, though they are obscured by dust near the center. These filaments are likely powered by metal-line emission from collisional ionization, suggesting ongoing low-level precipitation of the intracluster medium. Our results indicate that AGN feedback has quenched SF significantly in M87 for at least 200 Myr, even though some precipitation persists. Additionally, we identified a hotspot created by the counterjet, with the spectral index also constrained.

Figures (7)

• Figure 1: Left panel: Composite RGB image of M87 (blue: WFC3 F275W, green: F606W, red: ACS F814W). Right panel: The same image after subtracting the diffuse galactic emission and further smoothed using a Gaussian kernel with a standard deviation of 4 pixels. H$\alpha$ emission from HST ACS F660N image is shown in Green. This image highlights compact sources and H$\alpha$ filaments otherwise obscured by the underlying diffuse light. A few blue spiral galaxies are visible in the outskirts of the image, whereas most of the yellow point sources are M87 globular clusters. North is up and East is to the left in both panels.
• Figure 2: Top Left: Color-magnitude relation of all point sources detected in HST WFC3 F275W and F606W images of M87, showing a clear separation into two distinct groups. The black dashed lines denote the boundary separating these groups. The red lines show the completeness percentage. Top Right: Color-magnitude diagram for F275W and F814W band magnitudes. Group 1 points defined in the left panel are shown in Magenta color. The two clusters are separated in this CDM as well, however, the separation between the two groups is smaller. Bottom: The F275W residual image after subtracting galaxy continuum emission shows the detected UV sources and filaments. The Magenta and Cyan crosses are "Group 1" and "Group 2" sources, respectively.
• Figure 3: Left Panel: The number density of point sources in HST F275W images of M87 and HDUV fields per 0.5 mag per square degree plotted for comparison. Right panel: Radial distribution of point sources in M87 F275W image from the center of the galaxy. The purple dash-dotted lines show the radial profile of Group 2 sources in different magnitude bins.
• Figure 4: The figure shows the 2D posterior probability density distribution of cluster age and mass, with density indicated by the colorbar. To the top and right of the main panel, we show the marginalized PDFs of cluster mass and age, respectively. The dashed red lines mark the times of past AGN outbursts inferred from X-ray cavity ages and radio lobes.
• Figure 5: Residuals in the WFC3 F275W image (left) after subtracting a galaxy continuum model and the ACS F660N image containing the H$\alpha$+[N ii] emission (right). The cyan contours in both images represent Very Large Array (VLA) 1.4 GHz radio emission at levels of 0.12, 0.19, 0.59 and 2.84 Jy/beam. The inset image in the right panel provides a zoomed-in view of the nucleus in F660N, highlighting H$\alpha$+[N ii] filaments overlapping with dust absorption contours (green). The magenta arrows indicate UV and optical filaments, while the cyan arrow shows the hotspot created by the counterjet, which is detected in the F275W image but not in F660N. North is up in both images.