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| Main Authors: | , , , , , , , , , , , , |
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| Format: | Preprint |
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2023
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| Online Access: | https://arxiv.org/abs/2312.03065 |
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| _version_ | 1866909369628622848 |
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| author | Killi, Meghana Watson, Darach Brammer, Gabriel McPartland, Conor Antwi-Danso, Jacqueline Newshore, Rosa Coe, Dan Allen, Natalie Fynbo, Johan P. U. Gould, Katriona Heintz, Kasper E. Rusakov, Vadim Vejlgaard, Simone |
| author_facet | Killi, Meghana Watson, Darach Brammer, Gabriel McPartland, Conor Antwi-Danso, Jacqueline Newshore, Rosa Coe, Dan Allen, Natalie Fynbo, Johan P. U. Gould, Katriona Heintz, Kasper E. Rusakov, Vadim Vejlgaard, Simone |
| contents | JWST has revealed a class of numerous, extremely compact sources, with rest-frame red optical/near-infrared (NIR) and blue ultraviolet (UV) colours, nicknamed "little red dots". We present one of the highest signal-to-noise ratio JWST NIRSpec/PRISM spectra of a little red dot, J0647_1045 at $z = 4.5321 \pm 0.0001$, and examine its NIRCam morphology, to differentiate the origin of the UV and optical/NIR emission, and elucidate the nature of the little red dot phenomenon. J0647_1045 is unresolved ($r_e < 0.17$ kpc) in the three NIRCam long-wavelength filters, but significantly extended ($r_e = 0.45 \pm 0.06$ kpc) in the three short-wavelength filters, indicating a red compact source in a blue star-forming galaxy. The spectral continuum shows a clear change in slope, from blue in the optical/UV, to red in the restframe optical/NIR, consistent with two distinct components, fit by power-laws with different attenuation: $A_V = 0.54 \pm 0.01$ (UV) and $A_V = 5.7 \pm 0.2$ (optical/NIR). Fitting the H$α$ line requires both broad (full width at half-maximum $\sim 4300 \pm 300 km s^{-1}$) and narrow components, but none of the other emission lines, including H$β$, show evidence of broadness. We calculate $A_V = 1.1 \pm 0.2$ from the Balmer decrement using narrow H$α$ and H$β$, and $A_V > 4.1 \pm 0.2$ from broad H$α$ and upper limit on broad H$β$, consistent with the blue and red continuum attenuation respectively. Based on single-epoch H$α$ linewidth, the mass of the central black hole is $8 \pm 1 \times 10^8 M_\odot$. Our findings are consistent with a multi-component model, where the optical/NIR and broad lines arise from a highly obscured, spatially unresolved region, likely a relatively massive active galactic nucleus, while the less obscured UV continuum and narrow lines arise, at least partly, from a small but spatially resolved star-forming host galaxy. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2312_03065 |
| institution | arXiv |
| publishDate | 2023 |
| record_format | arxiv |
| spellingShingle | Deciphering the JWST spectrum of a 'little red dot' at $z \sim 4.53$: An obscured AGN and its star-forming host Killi, Meghana Watson, Darach Brammer, Gabriel McPartland, Conor Antwi-Danso, Jacqueline Newshore, Rosa Coe, Dan Allen, Natalie Fynbo, Johan P. U. Gould, Katriona Heintz, Kasper E. Rusakov, Vadim Vejlgaard, Simone Astrophysics of Galaxies JWST has revealed a class of numerous, extremely compact sources, with rest-frame red optical/near-infrared (NIR) and blue ultraviolet (UV) colours, nicknamed "little red dots". We present one of the highest signal-to-noise ratio JWST NIRSpec/PRISM spectra of a little red dot, J0647_1045 at $z = 4.5321 \pm 0.0001$, and examine its NIRCam morphology, to differentiate the origin of the UV and optical/NIR emission, and elucidate the nature of the little red dot phenomenon. J0647_1045 is unresolved ($r_e < 0.17$ kpc) in the three NIRCam long-wavelength filters, but significantly extended ($r_e = 0.45 \pm 0.06$ kpc) in the three short-wavelength filters, indicating a red compact source in a blue star-forming galaxy. The spectral continuum shows a clear change in slope, from blue in the optical/UV, to red in the restframe optical/NIR, consistent with two distinct components, fit by power-laws with different attenuation: $A_V = 0.54 \pm 0.01$ (UV) and $A_V = 5.7 \pm 0.2$ (optical/NIR). Fitting the H$α$ line requires both broad (full width at half-maximum $\sim 4300 \pm 300 km s^{-1}$) and narrow components, but none of the other emission lines, including H$β$, show evidence of broadness. We calculate $A_V = 1.1 \pm 0.2$ from the Balmer decrement using narrow H$α$ and H$β$, and $A_V > 4.1 \pm 0.2$ from broad H$α$ and upper limit on broad H$β$, consistent with the blue and red continuum attenuation respectively. Based on single-epoch H$α$ linewidth, the mass of the central black hole is $8 \pm 1 \times 10^8 M_\odot$. Our findings are consistent with a multi-component model, where the optical/NIR and broad lines arise from a highly obscured, spatially unresolved region, likely a relatively massive active galactic nucleus, while the less obscured UV continuum and narrow lines arise, at least partly, from a small but spatially resolved star-forming host galaxy. |
| title | Deciphering the JWST spectrum of a 'little red dot' at $z \sim 4.53$: An obscured AGN and its star-forming host |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2312.03065 |