Looking Forward

Chapter IV: Circumgalactic Ly 𝛼 Emission and Its Host Galaxies

4.4 Looking Forward

Using preliminary results, we have demonstrated the potential of analyzing the composite CP2D spectra in the KBSS-KCWI survey, and the connection between Ly𝛼 emission and their host galaxies. We plan to expand our analyses in the following aspects.

We demonstrated that certain Ly𝛼emission does not change proportionally to the host galaxy properties through CP2D spectra. This will be further quantified by measuring the change in Ly𝛼spatial profile as shown in §4.2, which will demonstrate how the host galaxy affects Ly𝛼emission in different parts of the CP2D spectrum.

Meanwhile, radiative transfer simulations have predicted that the spectral profile of Ly𝛼emission reflects the H i gas properties in the CGM. For example, Gronke et al.

(2016) showed that in an outflowing clumpy H i medium, 𝐹_blue(Ly𝛼)/𝐹_red(Ly𝛼) correlates with outflow velocity and covering factor. Since outflow velocity and covering factor can also affect other spectral properties of Ly𝛼, e.g., peak separation, Figure 4.13 provides a testing ground for similar theoretical predictions. In the future, we will compare them quantitatively.

0 1 2 3 4 5

tran

(a rc se c)

Med = 0.050 N = 29

E(B-V)

0 1 2 3 4 5

tran

(a rc se c)

Med = 0.090 N = 30

1000 500 0 500 1000

v (km s

¹

) 0

1 2 3 4 5

tran

(a rc se c)

Med = 0.125 N = 29

0 10 20 30 40

D

tran

(p kp c)

10

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10

⁰

0 10 20 30 40

D

tran

(p kp c)

10

¹

10

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0 10 20 30 40

D

tran

(p kp c)

10

¹

10

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Figure 4.6: The composite CP2D spectra for three sets of galaxies divided into bins of𝐸(𝐵−𝑉). The top left corner of each panel shows the median value of𝐸(𝐵−𝑉), and the number of galaxies that went into each bin. The colormap is in log space, while the white contours are linear. Both are shown in the colorbars on the right side. The unit of the colorbars is normalized SB units.

0 1 2 3 4 5

tran

(a rc se c)

Med = 9.0 N = 29

log(M

*

/ M )

0 1 2 3 4 5

tran

(a rc se c)

Med = 9.6 N = 30

1000 500 0 500 1000

v (km s

¹

) 0

1 2 3 4 5

tran

(a rc se c)

Med = 10.3 N = 29

0 10 20 30 40

D

tran

(p kp c)

10

¹

10

⁰

0 10 20 30 40

D

tran

(p kp c)

10

¹

10

⁰

0 10 20 30 40

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tran

(p kp c)

10

¹

10

⁰

Figure 4.7: Same as in Figure 4.6, for galaxies binned in log(𝑀_∗/𝑀).

0 1 2 3 4 5

tran

(a rc se c)

Med = 0.60 N = 29

log(SFR / M yr

¹

)

0 1 2 3 4 5

tran

(a rc se c)

Med = 1.04 N = 30

1000 500 0 500 1000

v (km s

¹

) 0

1 2 3 4 5

tran

(a rc se c)

Med = 1.43 N = 29

0 10 20 30 40

D

tran

(p kp c)

10

¹

10

⁰

0 10 20 30 40

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(p kp c)

10

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Figure 4.8: Same as in Figure 4.6, for galaxies binned in log(SFR), where SFR is based on best-fit SED models.

0 1 2 3 4 5

tran

(a rc se c)

Med = -9.3 N = 29

log(sSFR / yr

¹

)

0 1 2 3 4 5

tran

(a rc se c)

Med = -8.6 N = 30

1000 500 0 500 1000

v (km s

¹

) 0

1 2 3 4 5

tran

(a rc se c)

Med = -7.9 N = 29

0 10 20 30 40

D

tran

(p kp c)

10

¹

10

⁰

0 10 20 30 40

D

tran

(p kp c)

10

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0 10 20 30 40

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Figure 4.9: Same as in Figure 4.6, for galaxies binned in log(sSFR).

0 1 2 3 4 5

tran

(a rc se c)

Med = 1.58 N = 14

F

H

(10

¹⁷

erg s

¹

cm

²

)

0 1 2 3 4 5

tran

(a rc se c)

Med = 5.26 N = 14

1000 500 0 500 1000

v (km s

¹

) 0

1 2 3 4 5

tran

(a rc se c)

Med = 8.22 N = 14

0 10 20 30 40

D

tran

(p kp c)

10

¹

10

⁰

0 10 20 30 40

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10

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0 10 20 30 40

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Figure 4.10: Same as in Figure 4.6, for galaxies binned in observed𝐹_H𝛼from 1D extracted slit-spectroscopy.

0 1 2 3 4 5

tran

(a rc se c)

Med = 0.24 N = 17

log([OIII] / H )

0 1 2 3 4 5

tran

(a rc se c)

Med = 0.63 N = 18

1000 500 0 500 1000

v (km s

¹

) 0

1 2 3 4 5

tran

(a rc se c)

Med = 0.83 N = 17

0 10 20 30 40

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tran

(p kp c)

10

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0 10 20 30 40

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Figure 4.11: Same as in Figure 4.6, for galaxies binned in log( [OIII]/H𝛽).

0 1 2 3 4 5

tran

(a rc se c)

Med = -8.75 N = 36

W (Ly ) (Å)

0 1 2 3 4 5

tran

(a rc se c)

Med = 9.00 N = 37

1000 500 0 500 1000

v (km s

¹

) 0

1 2 3 4 5

tran

(a rc se c)

Med = 43.75 N = 36

0 10 20 30 40

D

tran

(p kp c)

10

¹

10

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0 10 20 30 40

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Figure 4.12: Same as in Figure 4.6, for galaxies binned in the DTB𝑊_𝜆(Ly𝛼).

0 1 2 3 4 5

tran

(a rc se c)

Med = 0.14 N = 37

F

blue

(tot) / F

red

(tot)

0 1 2 3 4 5

tran

(a rc se c)

Med = 0.30 N = 36

1000 500 0 500 1000

v (km s

¹

) 0

1 2 3 4 5

tran

(a rc se c)

Med = 0.57 N = 37

0 10 20 30 40

D

tran

(p kp c)

10

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0 10 20 30 40

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tran

(p kp c)

10

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Figure 4.13: Same as in Figure 4.6, for galaxies binned in𝐹_blue(tot)/𝐹_red(tot).

Furthermore, low-ionization metal lines in the 1D spectra of the host galaxies are indicators of the cold gas covering fraction. The rest-frame equivalent width of the combined metal absorption (𝑊_LIS) was found to be correlated with the Ly𝛼emission in DTB spectra (Shapley et al., 2003; Trainor et al., 2019). We will measure𝑊_LIS in our sample, and measure its effect on the composite CP2D spectra.

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C h a p t e r 5

CONCLUSION

In this thesis, we have presented novel results from observations of the Ly𝛼line – in both absorption and emission – in the circumgalactic medium surrounding galaxies in the Keck Baryonic Structure Survey at𝑧' 2−3. Figure 5.1 provides a schematic illustration of what has been discovered in this thesis.

We first constructed the KGPS sample with > 200,000 independent sightlines that have projected distances . 3 pMpc from the foreground galaxies. The average Ly𝛼 absorption shows distinctive spatial and spectral distributions. The apparent Ly𝛼optical depth exhibits a “core” component at 𝐷_tran . 50 pkpc and|Δ𝑣| < 500 km s⁻¹. At 𝐷_tran & 100 pkpc, the absorption gradually transitions to a diffuse component whose spectral profile broadens as 𝐷_tran increases. By comparing the observed 𝜏_ap map with the projected 𝑁_HI distribution in the FIRE simulation, we found qualitative agreement. After fitting the observed absorption map to a simple analytic model consistsing of outflow, inflow, and Hubble expansion, we found that outflow dominates the H i kinematics at 𝐷_tran . 50 pkpc. Accretion flows and Hubble expansion gradually takes over as we move outward, with Hubble expansion becoming dominant at𝐷_tran & 150 pkpc. Between'50−150 pkpc, Ly𝛼absorption exhibits a narrow spectral profile that is consistent with the spectral resolution, where the confluence of outflow, accretion, and Hubble expansion create a caustic-like distortion in the 𝑣_LOS. By comparing the characteriztic outflow velocity with the escape velocity, we also confirmed that the majority of the outflowing H i does not carry enough kinetic energy to escape from the gravitational potential of an NFW halo. Furthermore, by comparing the Ly𝛼absorption profiles as functions of𝐷_LOS (assuming pure Hubble expansion) and 𝐷_tran, we found that the peculiar velocity from inflow and outflow results in significant reshift-space distortion at𝐷_tran . 500 pkpc.

The physical scale of outflows in the CGM is well reflected by the Ly𝛼 emission profile in the average CP2D spectrum from the KBSS-KCWI survey. The averaged CP2D spectrum shows a dominant redshifted component withΔ𝑣∼300 km s⁻¹ and a weaker blueshifted component atΔ𝑣 ∼ −300 km s⁻¹, matching the Ly𝛼spectral profile expected from resonant scattering through a radially outflowing medium.

Figure 5.1: A schematic diagram for H i in the CGM around a typical star-forming galaxy at 𝑧 = 2−3, as a qualitative summary of the major results of this thesis.

Outflows originating from regions of rapid star formation inside galaxies from star- forming regions inside galaxies dominates the H i kinematics in all directions at 𝐷_tran . 50 pkpc. At 𝐷_tran & 100 pkpc, accretion flows and/or ambient CGM that carries H i from> 1 pMpc, likely in the form of “cold accretion” streams based on recent cosmological simulations, gradually takes over.