Abstract
James Webb Space Telescope (JWST) Mid-Infrared Instrument (MIRI) images of the luminous infrared (IR) galaxy VV 114 are presented. This redshift ∼0.020 merger has a western component (VV 114W) rich in optical star clusters and an eastern component (VV 114E) hosting a luminous mid-IR nucleus hidden at UV and optical wavelengths by dust lanes. With MIRI, the VV 114E nucleus resolves primarily into bright NE and SW cores separated by 630 pc. This nucleus comprises 45% of the 15
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1. Introduction
The LIR[8 − 100034) Keck MIRLIN observations detected only the VV 114E nucleus at 12.5
Figure 1. (a) False-color optical image of VV 114 constructed with HST WFC3/UVIS F330W (0.3
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Standard image High-resolution imageFigure 2. A comparison of the JWST MIRI 5.6, 7.7, and 15 7 resolution), and ALMA CO(2-1) emission (
5: green contours in (i)). The axes are in degrees of R.A. and decl. The HST and JWST images are displayed in a logarithmic stretch to highlight both high and low surface brightness emission. The Spitzer and VLA images are displayed in a linear stretch. The eastern dust lanes visible in the 0.4
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Standard image High-resolution imageIn this Letter, JWST Early Release Science imaging observations of VV 114 with the Mid-Infrared Instrument (MIRI) are presented. With a spatial resolution of 018–0
48) over the wavelength range 5.6–15
2. Observations and Reduction
Observations of VV 114 were obtained with the MIRI instrument (Bouchet et al. 2015; Rieke et al. 2015) on JWST on 2022 July 2 (Program ID 1328; co-PIs Armus and Evans). Observations were taken with the F560W (3 × 56
3 = 512 × 512 pixels) centered to image V114E and VV 114W and the SUB128 Subarray mode (SUB128: 14
1 × 14
1 = 128 × 136 pixels) centered at the peak of the mid-IR emission in VV 114E. The SUB128 Subarray mode observations use shorter exposures designed to avoid saturation from the brightest regions. A three-point dither pattern was used for each Subarray mode per filter, and the total exposure times in F560W, F770W, and F1500W with the BRIGHTSKY Subarray mode are 348, 114, and 114 s, respectively, and are 46, 48, and 48 s, respectively, for the SUB128 Subarray mode. The images presented here are Stage 3 products produced through the standard JWST Science Calibration Pipeline using the software version (CRDS_VER) 11.16.3. Finally, World Coordinate System (WCS) corrections to images were derived by spatially correlating sources identified in the MIRI and WCS-corrected JWST NIRCAM images (see S. T. Linden et al. 2022, in preparation).
3. Photometry
In order to derive photometry for sources in the MIRI images, the IDL routine FIND was run with a 2548) on the F1500W MIRI image to identify unresolved sources. Photometry was then measured for the sources in each image in 5 pixel ( = 0
55) radius apertures and 7–9 pixel ( = 0
77–0
99) radius annuli for local background subtraction (where the background is measured via the mode of the pixels in each annulus) using the IDL routine APER. For the NE and SW cores of VV 114E, an 8 pixel ( = 0
88) radius aperture and a 9–11 ( = 0
99–1
1) pixel radius annuli for local background subtraction were adopted. Uncertainties were measured by varying the radius of the annuli over which the background estimates were measured. Aperture corrections were calculated using the encircled aperture-to-total energy for synthetic point-spread functions provided on the STScI website.
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The resultant photometric estimates are provided in Table 1. For measurements of the total emission in VV 114, the IPAC package SKYVIEW v3.8 was utilized. Measurements were done with a circular
radius with background subtraction.
Table 1. MIRI Photometry of VV 114E Cores and Compact Star-forming Knots
ID | R.A. (J2000.0) | Decl. (J2000.0) | F560W | F770W | F1500W |
---|---|---|---|---|---|
(deg) | (deg) | (mJy) | (mJy) | (mJy) | |
(NE) 1 | 16.948352 | −17.506821 | 19 ± 7 | 90 ± 2 | 190 ± 6 |
(SW) 2 | 16.947965 | −17.507073 | 73 ± 10 | 160 ± 8 | 190 ± 6 |
3 | 16.942842 | −17.507308 | 0.17 ± 0.001 | 0.85 ± 0.01 | 2.5 ± 0.3 |
4 | 16.942898 | −17.508056 | 0.24 ± 0.006 | 1.1 ± 0.05 | 4.5 ± 0.2 |
5 | 16.942909 | −17.507061 | 0.045 ± 0.005 | 0.22 ± 0.01 | 0.88 ± 0.16 |
6 | 16.9433 | −17.508064 | 0.066 ± 0.002 | <0.29 | 1.1 ± 0.2 |
7 | 16.9434 | −17.508269 | 0.14 ± 0.02 | 0.82 ± 0.04 | 1.8 ± 0.1 |
8 | 16.943513 | −17.507683 | <0.051 | <0.71 | 0.49 ± 0.02 |
9 | 16.943553 | −17.507894 | <0.044 | <0.25 | 1.1 ± 0.1 |
10 | 16.943728 | −17.508955 | 0.084 ± 0.03 | 0.48 ± 0.13 | 0.70 ± 0.13 |
11 | 16.944169 | −17.506264 | 0.34 ± 0.02 | 2.4 ± 0.05 | 6.2 ± 0.3 |
12 | 16.944281 | −17.507305 | 0.46 ± 0.007 | 1.8 ± 0.1 | 6.0 ± 0.3 |
13 | 16.944435 | −17.506723 | <0.051 | <0.16 | 0.43 ± 0.12 |
14 | 16.944479 | −17.506443 | 0.10 ± 0.05 | 0.61 ± 0.30 | 1.0 ± 0.3 |
15 | 16.944516 | −17.505324 | 0.12 ± 0.006 | 0.63 ± 0.04 | 0.93 ± 0.10 |
16 | 16.944565 | −17.505749 | 0.62 ± 0.13 | 3.6 ± 1.2 | 5.6 ± 0.6 |
17 | 16.944635 | −17.507619 | 1.0 ± 0.08 | 5.3 ± 0.4 | 14 ± 1.0 |
18 | 16.944663 | −17.507867 | 0.73 ± 0.04 | 3.1 ± 0.2 | 18 ± 0.2 |
19 | 16.944907 | −17.508971 | 0.18 ± 0.01 | 1.0 ± 0.03 | 2.3 ± 0.1 |
20 | 16.944914 | −17.507632 | 0.63 ± 0.14 | 3.6 ± 1.0 | 8.0 ± 0.3 |
21 | 16.944943 | −17.506742 | 0.22 ± 0.02 | 1.7 ± 0.1 | 2.5 ± 0.1 |
22 | 16.944956 | −17.506576 | 0.270 ± 0.03 | 1.7 ± 0.1 | 3.7 ± 0.3 |
23 | 16.945271 | −17.507489 | 0.12 ± 0.02 | 0.70 ± 0.22 | 2.6 ± 0.2 |
24 | 16.945378 | −17.508882 | 0.15 ± 0.01 | 0.80 ± 0.07 | 0.8 ± 0.1 |
25 | 16.945624 | −17.507612 | 0.20 ± 0.04 | <1.1 | 1.4 ± 0.1 |
26 | 16.945726 | −17.508725 | 0.14 ± 0.001 | 0.74 ± 0.007 | 0.93 ± 0.04 |
27 | 16.945768 | −17.507399 | 0.55 ± 0.02 | 2.8 ± 0.04 | 4.3 ± 0.1 |
28 | 16.946031 | −17.508540 | 0.037 ± 0.005 | 0.24 ± 0.05 | 0.80 ± 0.05 |
29 | 16.946061 | −17.506651 | 0.20 ± 0.05 | 1.4 ± 0.2 | 3.9 ± 0.1 |
30 | 16.946127 | −17.507456 | 0.77 ± 0.12 | 4.1 ± 0.4 | 9.2 ± 1.4 |
31 | 16.946162 | −17.508359 | 0.036 ± 0.01 | <0.44 | 1.2 ± 0.1 |
32 | 16.946188 | −17.506176 | 0.094 ± 0.001 | 0.62 ± 0.03 | 1.7 ± 0.1 |
33 | 16.946347 | −17.506839 | 0.53 ± 0.05 | 2.4 ± 0.3 | 5.0 ± 0.3 |
34 | 16.946579 | −17.504668 | 0.066 ± 0.006 | 0.38 ± 0.02 | 0.63 ± 0.04 |
35 | 16.946696 | −17.507147 | 0.48 ± 0.01 | 2.6 ± 0.5 | 2.0 ± 0.3 |
36 | 16.946775 | −17.506376 | 0.38 ± 0.02 | 2.5 ± 0.04 | 4.8 ± 0.02 |
37 | 16.947041 | −17.507356 | 1.09 ± 0.1 | 5.2 ± 0.8 | 13 ± 1 |
38 | 16.947402 | −17.507038 | ⋯ | 4.5 ± 0.1 | 6.1 ± 0.7 |
39 | 16.947425 | −17.507268 | 1.8 ± 0.4 | 9.4 ± 1.9 | 19 ± 2 |
40 | 16.948176 | −17.511765 | 0.047 ± 0.0004 | 0.25 ± 0.004 | 0.47 ± 0.02 |
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4. Results
4.1. VV 114E Nucleus and Star-forming Regions
Figure 1 is a comparison of false-color Hubble Space Telescope (HST) optical and JWST images of VV 114. Figure 2 is a comparison of the MIRI images to the HST 0.4 6), consistent with the Keck 12.5
45 (=180 pc: Figure 3(a)), and the southern component has a tail of emission that extends to the SW. (For the photometric results, the southern component is treated as part of the SW core). In the VLA 33 GHz image, the SW core is resolved into four components, and thus the tail of emission in the 7.7
Figure 3. A close-up comparison of the VV 114E nucleus at 7.7 2: Song et al. 2022). In the JWST image, the nucleus is separated into an NE and SW core. The SW core is further resolved into two components, with the most southern component (SW-S knot) having a tail of emission extending in the SW direction. In the VLA image, the SW core is resolved into four components.
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Standard image High-resolution imageThe extended mid-IR emission observed with IRAC and MIPS (Figures 2(b), (c)) west of the VV 114E nucleus is partially resolved by MIRI into ∼40 compact knots. These knots collectively comprise ∼20% of the light at 15
The 15
In order to better interpret the nature of the detected sources, their mid-IR colors were compared against those of the LIRGs in GOALS for which Spitzer IRS observations are available (Stierwalt et al. 2013). The IRS spectra were convolved with the MIRI filter transmission functions in order to derive the flux density in each filter. The results are illustrated in Figure 4. In the plot, the flux density ratio F770W/F560W is effectively a measure of the excess in the F770W filter due to PAH emission, and F1500W/F560W is a measure of the mid-IR spectral slope (note that the 6.2
Figure 4. A F770W/F560W vs. F1500W/F560W flux density ratio plot of the VV 114E cores (stars) and the mid-IR knots (filled black circles) in VV 114. The diffuse component of VV 114 is plotted as a "+" symbol. Also plotted are LIRGs in the GOALS sample with ratios derived from Spitzer IRS data. The LIRGs are color coded by their 6.2
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Standard image High-resolution imageIn Figure 4, the mid-IR colors of the mid-IR knots are shown. If the assumption is made that MIRI sources and GOALS LIRGs occupying the same area of the plot have nearly identical mid-IR spectra, the majority of the knots have (1) red dust continuum slopes (F1500W/F560W) and (2) strong PAH emission. The colors are thus consistent with highly reddened star-forming regions.
The VV 114E cores are also plotted in Figure 4. The NE core has star formation–like mid-IR colors, whereas the SW core has a bluer dust continuum and a low PAH equivalent width based on the relatively low F770W/F560W.
The mid-IR colors of the total VV 114 emission agree well with colors derived from Spitzer IRS. Further, the total MIRI and Spitzer IRAC measurements of VV 114 (-radius aperture) in similar wavelength bands agree to within 9%–20%—i.e., F560W (0.21 Jy) versus IRAC 5.8
4.2. Diffuse PAH Emission
The JWST MIRI images also show that VV 114 contains significant diffuse emission. This emission is most notable at 7.7
5. Discussion
The JWST images in Figures 1 and 2 are clearly illustrative of how the vast improvement in both the spatial resolution and sensitivity over space- and ground-based telescopes has enabled the clearest mid-IR view to date of this dust-obscured galaxy merger. The VV 114E nucleus is resolved into two primary cores, with the SW core further resolved into two components. As summarized in Section 4.1, the mid-IR colors of VV 114E NE are consistent with a reddened star-forming region. It has been suggested based on the measured high HCN/HCO+ submillimeter line ratio of the NE core (Iono et al. 2013) that it is the location of the putative AGN hinted at by mid-IR (Le Floc'h et al. 2002) and X-ray (Grimes et al. 2006) observations. The AGN would have to be optically thick at mid-IR wavelengths to be consistent with the present data. In contrast, the SW core has colors consistent with a low PAH equivalent width source and thus may be a more likely AGN candidate. Spatially resolved JWST mid-IR spectra of VV 114E will be analyzed in a companion paper (J. A. Rich et al. 2022, in preparation).
The LIR of the nuclear cores can be estimated by adopting the IR 8–1000 18 = 70 pc) can be used to derive a lower limit of the IR luminosity density of
Approximately 40 knots that are bright at mid-IR wavelengths are identified. These knots have mid-IR colors consistent with star-forming regions (Section 4.1) and are distributed in the overlap region and in VV 114W. At the resolution of the 15
The most striking feature in the MIRI images is the diffuse emission, especially the filamentary structure that covers most of the F770W image. This band contains the broad 7.7
These new JWST observations of VV 114 allow for the first time measurements of the properties of the most obscured IR-luminous star-forming regions and quantification of the contributions of diffuse emission to mid-IR light in the extreme environment of a luminous galaxy merger.
6. Summary
New JWST MIRI observations of VV 114 are presented. The following conclusions are reached:
- (1)The imbedded eastern nucleus of VV 114 is resolved into an NE and SW core. The mid-IR colors of these cores are consistent with a starburst (NE) and an AGN (SW). The lower limits of their estimated
Σ IR lie between the Orion core and the nuclei of Arp 220. - (2)Approximately 40 star-forming regions (knots) bright at mid-IR wavelengths are identified, 28% of which have no optical counterpart. Half of the knots with no optical counterpart lie in the overlap region. The knots have LIR ∼ 0.02–5 × 1010 L⊙, which translates into star formation rates of 0.02–6 M⊙ yr−1. These values are above the median LIR of comparably sized radio-identified star-forming regions in nearby, normal star-forming galaxies, but overlap the median LIR of radio-identified GOALS LIRGs.
- (3)Diffuse emission accounts for a significant fraction of the light in each MIRI image. In particular, filamentary emission is detected at 7.7
μ m that accounts for 60% of the light at this wavelength. Approximately 80% of this extended component is estimated to be PAH emission heated by the star-forming regions and underlying stellar population.
The authors thank the anonymous referee for comments that improved the overall quality of the manuscript. The authors also thank the members of the international MIRI Instrument Team for their work to bring the MIRI instrument to fruition. The research was supported by NASA grants JWST-ERS-01328, HST-GO-15472, and HST-GO-16914. Y.S. was funded in part by the NSF through the Grote Reber Fellowship Program administered by Associated Universities, Inc./National Radio Astronomy Observatory. V.U acknowledges funding support from NASA Astrophysics Data Analysis Program (ADAP) grant 80NSSC20K0450. K.I. acknowledges support by the Spanish MCIN under grant PID2019-105510GB-C33/AEI/10.13039/501100011033. S.A. gratefully acknowledges support from an ERC Advanced grant 789410, from the Swedish Research Council and from the Knut and Alice Wallenberg (KAW) Foundation. H.I. and T.B. acknowledge support from JSPS KAKENHI Grant Number JP21H01129 and the Ito Foundation for Promotion of Science. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The authors acknowledge the usage of the NASA/IPAC Infrared Science Archive, which is funded by the National Aeronautics and Space Administration and operated by the California Institute of Technology, and CARTA (Comrie et al. 2021).
Facilities: JWST - James Webb Space Telescope, HST - , Spitzer - , VLA - , ALMA - , MAST - , IRSA - , NASA/ADS - , IPAC/NED. -
Software: IDL Astronomy Library (Landsman 1993), Astropy (Price-Whelan et al. 2018), CARTA (Comrie et al. 2021), Cosmology Calculator (Wright 2006), skyview (https://www.ipac.caltech.edu/skyview/), synphot (STScI Development Team 2018).