0522-AGN Two-season ACTPol Extragalactic Point Sources and their Polarization properties

20240522

CROW

Active Galaxies and AGN

Active Galactic Nucleus (AGN): An AGN is a compact region at the center of an active galaxy. It is powered by a supermassive black hole that is millions to billions of times the mass of the Sun. The black hole's immense gravitational pull attracts surrounding gas and dust, forming an accretion disk. As material in the accretion disk spirals inward, it heats up and emits vast amounts of energy.

Key Features of AGN:

Relativistic Jets: Some AGNs produce powerful jets of charged particles that travel at nearly the speed of light. These jets can extend thousands of light-years into space. Synchrotron Radiation: Electrons in the jets emit synchrotron radiation as they spiral around magnetic field lines. This radiation is a key feature of AGNs and can be observed across various wavelengths. Broad and Narrow Emission Lines: The spectra of AGNs often show broad and narrow emission lines, which are produced by gas moving at high velocities near the black hole. Variability: AGNs can vary in brightness over timescales ranging from hours to years, reflecting changes in the accretion rate or other dynamic processes near the black hole. Types of AGNs:

Quasars: Extremely luminous AGNs that can outshine their host galaxies. Seyfert Galaxies: A class of active galaxies with bright nuclei and strong emission lines. Blazars: A type of AGN with jets pointed almost directly at Earth, making them appear exceptionally bright and variable. Understanding AGNs is crucial for studying the evolution of galaxies and the role of supermassive black holes in the universe.

Type of AGN

Active Galactic Nuclei (AGN) are categorized based on their observational characteristics, which are influenced by factors such as the orientation of the AGN relative to the observer, the presence of jets, and the properties of the surrounding material. Here are the main types of AGNs:

1. Quasars Definition: Quasars (quasi-stellar objects) are the most luminous type of AGN. They can outshine their host galaxies and are visible across vast distances. Characteristics: Extremely high luminosity. Broad emission lines in their spectra. Strong radio emissions in some cases (radio-loud quasars). Significant redshift, indicating they are very distant and thus seen as they were in the early universe.

2. Seyfert Galaxies Definition: Seyfert galaxies are a class of active galaxies with bright nuclei and strong emission lines. They are less luminous than quasars but still much brighter than normal galaxies. Characteristics: Divided into two types: Seyfert 1 and Seyfert 2. Seyfert 1: Show both broad and narrow emission lines, indicating high-velocity gas close to the black hole. Seyfert 2: Show only narrow emission lines, suggesting that the broad-line region is obscured by dust and gas. Often found in spiral galaxies.

3. Blazars Definition: Blazars are a type of AGN with jets pointed almost directly at Earth, making them appear exceptionally bright and variable. Characteristics: Divided into two subtypes: BL Lacertae objects (BL Lacs) and Optically Violent Variables (OVVs). BL Lacs: Feature weak or no emission lines and are dominated by non-thermal emission. OVVs: Show strong and variable optical emission lines. Exhibit rapid variability in brightness. Strong polarization and high-energy emissions (X-rays and gamma rays).

4. Radio Galaxies Definition: Radio galaxies are AGNs that emit large amounts of radio waves, often associated with large-scale jets and lobes. Characteristics: Divided into two types based on their radio morphology: Fanaroff-Riley type I (FR I) and Fanaroff-Riley type II (FR II). FR I: Have jets that fade with distance from the core. FR II: Have bright hotspots at the ends of their jets. Often found in elliptical galaxies.

5. LINERs (Low-Ionization Nuclear Emission-line Regions) Definition: LINERs are a type of AGN with spectra dominated by low-ionization emission lines. Characteristics: Less luminous than Seyfert galaxies and quasars. Emission lines suggest lower ionization states of gas. Common in both spiral and elliptical galaxies. Summary AGNs are diverse and can be classified into several types based on their luminosity, spectral characteristics, and the orientation of their jets. Understanding these types helps astronomers study the different mechanisms powering these energetic phenomena and their impact on galaxy evolution.

PAPER

The Atacama Cosmology Telescope: Two-season ACTPol Extragalactic Point Sources and their Polarization properties

尘埃星系（DSFGs）的特征是由尘埃发出的热辐射，覆盖从毫米到远红外波长。这些尘埃被星系中大质量恒星的紫外和光学辐射加热。尘埃的遮蔽使得DSFGs无法被光学和紫外观测设备探测到。在毫米和亚毫米波长的观测可以提供更完整的恒星形成历史（Casey et al. 2014）。在这些波长下，最亮的高红移DSFGs主要通过引力透镜效应被探测到（Vieira et al. 2013）。在毫米/亚毫米波段，射电辐射的频谱随频率下降，而尘埃辐射的频谱随频率急剧上升。因此，在波长超过1.5毫米的天空中，亮源主要是射电源，而尘埃星系则主导了亚毫米天空，特别是在低流量密度下。第二版普朗克紧凑源目录（Planck Collaboration et al. 2016）涵盖了九个频率通道，捕捉到了从射电源到尘埃星系的过渡。他们发现，主导源种群的变化发生在217到353 GHz的频率之间，即1.382到0.850毫米的波长。

对河外射电源极化特性的测量本身就为研究这些源的天体物理学开辟了一条有趣的途径。在高达约200 GHz的频率下探测到的极化源预计主要是射电源，而随着频率的增加，被探测到的尘埃星系数量也会增加。对于射电响亮的活动星系核（AGN），毫米/亚毫米波长的极化数据揭示了其相对论喷流未分辨区域的磁场细节（Nartallo et al. 1998）。同步辐射的线性极化度本质上可以高达60-80%（Saikia & Salter 1988）。然而，紧凑型河外射电源的观测极化分数通常远低于10%，这被认为是视线方向上的矢量平均结果。然而，至少有一些源的极化分数可以更高，个别源的极化分数可达20%。

此外，人们对尘埃星系（DSFGs）的极化程度知之甚少，但由于星系磁场的复杂结构在视线方向上的反转以及尘埃颗粒的无序排列，综合整个星系的极化通量可能较低。Greaves & Holland (2002) 和 Matthews et al. (2009) 分别在850 μm波长下对两个星系M82和M87进行了极化测量。他们发现M82的整体净极化度仅为0.4%。Seiffert et al. (2007) 在850 μm波长下对超亮红外星系（ULIRG）Arp 220的极化辐射设定了1.54%的上限。Trombetti et al. (2018) 分析了普朗克地图中河外源的极化分数，并在353 GHz和217 GHz频率下对尘埃源的极化分数设定了90%置信上限，分别为2.2%和3.9%。

How to choose which polarization point sources for catalogue

Combining the detections in the D56, D5 and D6 maps, 169 sources brighter than 30 mJy in total intensity are selected for the polarization analysis presented in Section 4. This threshold of 30 mJy is chosen because the measurement of polarized flux for dim sources is heavily biased by noise as discussed in Section 4.

CROW

beam solid angle

Calculating Beam Solid Angle in CMB Data Analysis

In Cosmic Microwave Background (CMB) data analysis, the beam solid angle is a crucial parameter that describes the angular extent of the telescope's beam. It is essential for converting observed intensities to flux densities and for understanding the effective resolution of the telescope. Here’s how to calculate the beam solid angle:

Step-by-Step Calculation of Beam Solid Angle

1. Understanding the Beam Profile: The beam profile of a telescope can often be approximated by a Gaussian function. The power pattern of the beam is typically described by:

   $$B(\theta, \phi) = B_0 \exp\left(-\frac{\theta^2}{2\sigma^2}\right)$$

   where (\theta) and (\phi) are the angular coordinates, (B_0) is the peak value of the beam, and (\sigma) is the standard deviation of the Gaussian beam in radians.

2. Beam Solid Angle Definition: The beam solid angle (\Omega_{\text{beam}}) is defined as the integral of the beam pattern over the entire solid angle:

   $$\Omega_{\text{beam}} = \int_{4\pi} B(\theta, \phi) d\Omega$$
3. Integration for a Gaussian Beam: For a symmetric Gaussian beam, the integral simplifies due to symmetry, and it can be shown that the beam solid angle is:

   $$\Omega_{\text{beam}} = 2\pi \int_{0}^{\pi} \sin\theta \, B(\theta) \, d\theta$$

   Converting the integral into a more manageable form by using the Gaussian beam profile:

   $$\Omega_{\text{beam}} = 2\pi \int_{0}^{\pi} \sin\theta \exp\left(-\frac{\theta^2}{2\sigma^2}\right) d\theta$$
4. Approximation for Narrow Beams: For narrow beams (i.e., (\theta \ll 1) radian), (\sin\theta \approx \theta), and the limits of integration can be extended to (\infty):

   $$\Omega_{\text{beam}} \approx 2\pi \int_{0}^{\infty} \theta \exp\left(-\frac{\theta^2}{2\sigma^2}\right) d\theta$$
5. Solving the Integral: The integral is a standard form and can be evaluated by substitution (u = \frac{\theta^2}{2\sigma^2}), (du = \frac{\theta}{\sigma^2} d\theta):

   $$\Omega_{\text{beam}} = 2\pi \sigma^2 \int_{0}^{\infty} \exp(-u) \, du = 2\pi \sigma^2$$
6. Expressing in Terms of FWHM: Often, the beam width is given in terms of the Full Width at Half Maximum (FWHM). The relationship between (\sigma) and FWHM for a Gaussian is:

   $$\text{FWHM} = \sqrt{8 \ln 2} \, \sigma \approx 2.355 \, \sigma$$

   Therefore, the beam solid angle can also be expressed as:

   $$\Omega_{\text{beam}} = 2\pi \left( \frac{\text{FWHM}}{2.355} \right)^2 = \frac{\pi \, \text{FWHM}^2}{4 \ln 2}$$

Practical Example

If the FWHM of the telescope's beam is 1 arcminute (1/60 degrees), convert this to radians:

$$\text{FWHM} = \frac{1}{60} \times \frac{\pi}{180} \approx 2.90888 \times 10^{-4} \, \text{radians}$$

Now calculate the beam solid angle:

$$\Omega_{\text{beam}} = \frac{\pi \, (2.90888 \times 10^{-4})^2}{4 \ln 2} \approx 6.707 \times 10^{-8} \, \text{steradians}$$

Summary

The beam solid angle for a Gaussian beam is primarily dependent on the beam width, often described by its FWHM. By understanding the beam profile and using appropriate mathematical approximations, you can calculate the beam solid angle, which is vital for accurate CMB data analysis.