AstroSat UV Deep Field IV. An Extended UV disk around a massive spiral galaxy at z=0.67

TL;DR

This study demonstrates that extended UV (XUV) disks, signatures of inside-out disk growth, exist at intermediate redshift in a very massive spiral galaxy AUDFs_N02766. Using deep UVIT imaging (N242W) complemented by HST, VLT/ISAAC, and Spitzer data, the authors extract PSF-corrected rest-frame FUV and Ks radial profiles, revealing an XUV disk extending to ~63 kpc. The outer disk hosts clumpy UV structures with a total XUV SFR of about 2.8 M_sun/yr, while a significant inner-disk SFR persists; lopsidedness analyses yield an inferred gas accretion rate of ~11 M_sun/yr, capable of replenishing outer-disk gas. The results support ongoing inside-out growth and cold gas accretion at $z obreak ext{~}0.67$, providing a crucial link between local XUV populations and galaxy evolution at earlier epochs. The work also highlights challenges in UV-based SFR tracing due to PSF and dust effects, guiding future XUV studies with upcoming UV missions and wide-field surveys.

Abstract

Extended ultraviolet (XUV) emission in nearby disk galaxies supports the inside-out growth scenario through low-efficiency star formation in their outer regions. However, such detections have largely been limited to the local Universe (z ~ 0) due to the need for deep, high-resolution UV imaging. We report the detection of a clumpy XUV disk in a massive, isolated spiral galaxy $(log(M_*/M_\odot) \approx 11.04)$ at z=0.67, observed with AstroSat/UVIT. The intrinsic rest frame FUV surface brightness profile, corrected for the instrument PSF, shows a more extended disk than its optical and IR counterparts. The XUV disk reaches nearly twice the optical radius and includes a large UV-bright low surface brightness (LSB) region $(S_{LSB}/S_{K80}\approx9.3, μ_{FUV}-μ_K\approx 1)$, consistent with the Type II XUV definition. Additionally, the detection of UV clumps without optical counterparts supports a Type I classification, suggesting gravitational instabilities and recent star formation. These features point to recent cold gas accretion onto the outer disk. From the asymmetric light profile, we estimate a gas accretion rate of $\sim 11 M_\odot$ $yr^{-1}$, providing evidence of active disk growth at intermediate redshift.

Figures (12)

• Figure 1: (a) RGB image of AUDFs_N02766 using HST bands: F160W (red), F850LP (green), and F435W (blue). Nearby HST detections (HLF catalog) are marked with dashed ellipses; redshift types are colour-coded: cyan (spectroscopic), green (grism), orange (photometric), and gray (unknown). The magenta ellipse denotes the optical edge ($R_{major}\sim4.5\arcsec$); the pink ellipse outlines the XUV disk at SNR$>$3 ($R_{major}\sim9\arcsec$); the brown ellipse marks the SFR threshold ($\mu_{FUV}=27.25$). (b) Contrast map of AUDFs_N02766 in UVIT-N242W, obtained via an unsharp-masking procedure, showing HST counterparts of UV detections (black crosses), detected clumps (white circles, $r=1$ PSF FWHM) with red IDs, and an extended feature labelled E1. The bottom foreground galaxy ($z\sim0.075$), which is particularly bright in UV, has its boundaries marked with a black dashed ellipse. (c) HST-UV RGB image (F336W, F275W, F225W) showing the bulgeless UV disk and resolved clumps within the optical boundary (R$<4.5\arcsec$). (d) Surface brightness profiles in rest frame Ks(IRAC-1), rest frame FUV (UVIT-N242W), and (inset) rest frame 840nm (HST-F140W), corrected for cosmological dimming and extinction. PSF-convolved models (solid), intrinsic models (dashed), and sersic+ exponential for N242W (faint dashed) are shown. $K_{80}$ marks 80% light radius of rest frame Ks (IRAC-1) model; $R_{SFR}$ (brown arrow) marks $\mu_{FUV}=27.25$; magenta arrow shows optical edge at $4.5\arcsec$(31.5 kpc), and pink arrow marks the end of our XUV-detection boundary at $9\arcsec$ (63 kpc).
• Figure 2: The best-fit SED model (black) is shown alongside the observed fluxes (red points). The attenuated and unattenuated stellar continua are shown as blue and green dashed lines, respectively. The reduced $\chi^2$ for the fit is 1.2. The transmission curves (scaled) of the filters used in the fitting are overlaid as grey shaded regions.
• Figure 3: (a) False-color image of stacked HST bands (F125W, F140W, F160W) with 1$\sigma$ (black) and 3$\sigma$ (red) isophotal contours overlaid. A white slit is placed along the direction of observed lopsidedness. (b) Light profile of AUDFs_N02766 along the slit (blue markers) compared to the mean profile ($A_0$, black). The black horizontal dotted line represents the $1\sigma$ level shown in (a). (c) Radial variation of $A_1/A_0$. The mean $<A_1/A_0>$ is computed over the radial range highlighted in the blue region.
• Figure 4: A comparison of sSFR vs stellar mass of AUDFs_N02766 and all the detected XUV disks (L11:Lemoinas2011, T07:Thilker_2007ApJS..173..538T, T05: Thilker_2005ApJ...619L..79T, B22:2022NaturBorgohain, P21:DIISC_II) along with the population of galaxies at $z\approx 0$ shown in Black Salim_2016ApJS..227....2S.
• Figure 5: The optical spectrum of AUDFs_N02766 as observed by the VLT-Vimos survey Spectrum_VLT_2010AA...512A..12B