2018年IOAA数据分析第1题-产星星系中的尘埃与年轻恒星

(D1) Dust and Young Stars in Star-forming Galaxies             [75 points]

As a by-product of the star-forming process in a galaxy, interstellar dust can significantly absorb stellar light in ultraviolet (UV) and optical bands, and then re-emit in far-infrared (FIR), which corresponds to a wavelength range of 10-300 µm. 

1.1. In the UV spectrum of a galaxy, the major contribution is from the light of the young stellar population generated in recent star-formation processes, thus the UV luminosity can act as a reliable tracer of the starformation rate (SFR) of a galaxy. Since the observed UV luminosity is strongly affected by dust attenuation, extragalactic astronomers define an index called the UV continuum slope (β) to quantify the shape of the UV continuum:

𝑓 V=𝑄⋅𝜆Z

where 𝑓V is the monochromatic flux of the galaxy at a given wavelength 𝜆 (in the unit of  W m-3) and 𝑄 is a scaling constant. 

(D1.1.1) (6 points) AB magnitude is a specific magnitude system. The AB magnitude is defined as:

m_AB=−2.5log（ 𝑓_ν /3631Jy)

The AB magnitude of a typical galaxy is roughly constant in the UV band. What is the UV continuum slope of this kind of galaxy? (Hint:  𝑓_λ∆𝜈=𝑓_𝜆∆𝜆 )

(D1.1.2) (12 points) Table 1 presents the observed IR photometry results for a 𝑧=6.60 galaxy called CR7. Plot the AB magnitude of CR7 versus the logarithm of the rest-frame wavelength on graph paper and labelled as Figure 1.

Table 1. (Observed Frame) IR Photometry of CR7 at z=6.60

Band	Y	J	H	K
Central Wavelength (μm）	1.05	1.25	1.65	2.15
AB Magnitude	24.71±0.11	24.63±0.13	25.08±0.14	25.15±0.15

(D1.1.3) (5 points) Calculate CR7’s UV slope, plot the best-fit UV continuum on Figure 1 and make a comparison with the results you obtained in (D1.1.1). Is it dustier than the typical galaxy in (D1.1.1)? Please answer with [YES] or [NO]. (Hint: Express m_ABas a function of 𝜆 and 𝑚₁₆₀₀ , where 𝑚₁₆₀₀ is the AB magnitude at 𝜆₀=160 nm (1600 Å))