如何使用Python Astropy将角秒转换为Mpc？

Question

我需要将以下量从角秒转换为百万秒差距：

a = 737.28 # arcsec
z = 0.3 # redshift
d = ? # Mpc

I am using flat lambdaCDM using parameters
H0 = 67.8
omega_m = 0.308

使用的宇宙学模型：Ade等人2016年 https://arxiv.org/pdf/1502.01589.pdf 表1 2013F(DS)

到目前为止，我已经尝试了这个。

from astropy.cosmology import FlatLambdaCDM
import astropy.units as u

cosmo = FlatLambdaCDM(H0=70, Om0=0.3)
cosmo.luminosity_distance(z=0.3)

# I am not sure how to convert arcsec to Mpc here.

替代方案： http://arcsec2parsec.joseonorbe.com/index.html

该网站提供了可行的解决方法，并给出了3.38 Mpc的结果，但我不能简单地引用一个网站，因此需要使用Python来再现该结果。

- BhishanPoudel

Answer 1

要计算距离，需要将角直径距离乘以角度大小。 l = D_A(z) × θ

参考文献：http://arcsec2parsec.joseonorbe.com/about.html

from astropy.cosmology import FlatLambdaCDM
import numpy as np
cosmo = FlatLambdaCDM(H0=67.8, Om0=0.308)

# angular diameter distance in Mpc
d_A = cosmo.angular_diameter_distance(z=0.3)
theta = 737.28 # arcsec

# pi radian = 180 degree ==> 1deg = pi/180 ==> 1arcsec = pi/180/3600
theta_radian = theta * np.pi / 180 / 3600

# arc length = radius * angle
distance_Mpc = d_A * theta_radian

print(distance_Mpc) # 3.3846475 Mpc

更新
如评论中建议的那样，我们也可以使用astropy单位。

from astropy.cosmology import FlatLambdaCDM
import numpy as np
from astropy import units as u
cosmo = FlatLambdaCDM(H0=67.8, Om0=0.308)

d_A = cosmo.angular_diameter_distance(z=0.3)
print(d_A) # 946.9318492873492 Mpc

theta = 737.28*u.arcsec
distance_Mpc = (theta * d_A).to(u.Mpc, u.dimensionless_angles()) # unit is Mpc only now

print(distance_Mpc) # 3.384745689510495 Mpc

- BhishanPoudel

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或者，您可以使用 theta = 737.28*u.arcsec; distance_Mpc = (theta * d_A).to(u.Mpc, u.dimensionless_angles())，它使用了一个等效性，更加通用。 - keflavich