### Abstract

Ultraviolet spectroscopy has been obtained with the faint object spectrograph of the Hubble Space Telescope in the Lyα forest of the quasar pair, LB 9605 (z_{em} = 1.834) and LB 9612 (z_{em} = 1.898), in order to measure the size of the Lyα absorbers. The quasars are separated by 1′.65 on the sky corresponding to a proper separation of 412 h^{-1} kpc at z = 1.83 (where h ≡ H_{0}/100 km s^{-1} Mpc^{-1}; q_{0} = 0.5). We detect five Lyα absorption lines common to both spectra within a velocity difference of 400 km s^{-1} in the redshift range 1.06 < z < 1.69, and 20 lines which are seen in the spectrum of one quasar but not the other. The number of coincidences expected for randomly distributed absorbers in this redshift interval is 3.2 ± 1.8, implying a less than 2 σ significance for the observed coincidences. If none of the observed coincidences are real, then we can place an upper limit on the absorber radius with 95% confidence of ∼285 h^{-1} kpc for redshifts 1.06 < z < 1.69. If, on the other hand, all of the observed coincidences are real, then a maximum likelihood estimate of the characteristic absorber radius in the context of identical, spherical clouds give a most probable radius of 380 h^{-1} kpc with 95% confidence that the characteristic radius lies in the range 305 < R < 595 h^{-1} kpc. Taken together with a low redshift estimate and new ground-based estimates, the upper limit on the absorber size provides tantalizing evidence for evolution in the radius of the Lyα absorbers with cosmic time, in the sense that the characteristic size of the Lyα absorbers increases with decreasing redshift.

Original language | English (US) |
---|---|

Pages (from-to) | 567-580 |

Number of pages | 14 |

Journal | Astrophysical Journal |

Volume | 494 |

Issue number | 2 PART I |

DOIs | |

State | Published - 1998 |

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### Keywords

- Intergalactic medium
- Quasars: absorption lines
- Quasars: individual (LB 9605, LB 9612)
- Ultraviolet: galaxies

### ASJC Scopus subject areas

- Space and Planetary Science

### Cite this

*Astrophysical Journal*,

*494*(2 PART I), 567-580. https://doi.org/10.1086/305215

**Ultraviolet spectroscopy of the quasar pair LB 9605, LB 9612 with the Hubble Space Telescope : Evolution in the size of the Lyα absorbers?** / Dinshaw, Nadine; Foltz, Craig B.; Impey, Christopher D; Weymann, Ray J.

Research output: Contribution to journal › Article

*Astrophysical Journal*, vol. 494, no. 2 PART I, pp. 567-580. https://doi.org/10.1086/305215

}

TY - JOUR

T1 - Ultraviolet spectroscopy of the quasar pair LB 9605, LB 9612 with the Hubble Space Telescope

T2 - Evolution in the size of the Lyα absorbers?

AU - Dinshaw, Nadine

AU - Foltz, Craig B.

AU - Impey, Christopher D

AU - Weymann, Ray J.

PY - 1998

Y1 - 1998

N2 - Ultraviolet spectroscopy has been obtained with the faint object spectrograph of the Hubble Space Telescope in the Lyα forest of the quasar pair, LB 9605 (zem = 1.834) and LB 9612 (zem = 1.898), in order to measure the size of the Lyα absorbers. The quasars are separated by 1′.65 on the sky corresponding to a proper separation of 412 h-1 kpc at z = 1.83 (where h ≡ H0/100 km s-1 Mpc-1; q0 = 0.5). We detect five Lyα absorption lines common to both spectra within a velocity difference of 400 km s-1 in the redshift range 1.06 < z < 1.69, and 20 lines which are seen in the spectrum of one quasar but not the other. The number of coincidences expected for randomly distributed absorbers in this redshift interval is 3.2 ± 1.8, implying a less than 2 σ significance for the observed coincidences. If none of the observed coincidences are real, then we can place an upper limit on the absorber radius with 95% confidence of ∼285 h-1 kpc for redshifts 1.06 < z < 1.69. If, on the other hand, all of the observed coincidences are real, then a maximum likelihood estimate of the characteristic absorber radius in the context of identical, spherical clouds give a most probable radius of 380 h-1 kpc with 95% confidence that the characteristic radius lies in the range 305 < R < 595 h-1 kpc. Taken together with a low redshift estimate and new ground-based estimates, the upper limit on the absorber size provides tantalizing evidence for evolution in the radius of the Lyα absorbers with cosmic time, in the sense that the characteristic size of the Lyα absorbers increases with decreasing redshift.

AB - Ultraviolet spectroscopy has been obtained with the faint object spectrograph of the Hubble Space Telescope in the Lyα forest of the quasar pair, LB 9605 (zem = 1.834) and LB 9612 (zem = 1.898), in order to measure the size of the Lyα absorbers. The quasars are separated by 1′.65 on the sky corresponding to a proper separation of 412 h-1 kpc at z = 1.83 (where h ≡ H0/100 km s-1 Mpc-1; q0 = 0.5). We detect five Lyα absorption lines common to both spectra within a velocity difference of 400 km s-1 in the redshift range 1.06 < z < 1.69, and 20 lines which are seen in the spectrum of one quasar but not the other. The number of coincidences expected for randomly distributed absorbers in this redshift interval is 3.2 ± 1.8, implying a less than 2 σ significance for the observed coincidences. If none of the observed coincidences are real, then we can place an upper limit on the absorber radius with 95% confidence of ∼285 h-1 kpc for redshifts 1.06 < z < 1.69. If, on the other hand, all of the observed coincidences are real, then a maximum likelihood estimate of the characteristic absorber radius in the context of identical, spherical clouds give a most probable radius of 380 h-1 kpc with 95% confidence that the characteristic radius lies in the range 305 < R < 595 h-1 kpc. Taken together with a low redshift estimate and new ground-based estimates, the upper limit on the absorber size provides tantalizing evidence for evolution in the radius of the Lyα absorbers with cosmic time, in the sense that the characteristic size of the Lyα absorbers increases with decreasing redshift.

KW - Intergalactic medium

KW - Quasars: absorption lines

KW - Quasars: individual (LB 9605, LB 9612)

KW - Ultraviolet: galaxies

UR - http://www.scopus.com/inward/record.url?scp=0004404525&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=0004404525&partnerID=8YFLogxK

U2 - 10.1086/305215

DO - 10.1086/305215

M3 - Article

VL - 494

SP - 567

EP - 580

JO - Astrophysical Journal

JF - Astrophysical Journal

SN - 0004-637X

IS - 2 PART I

ER -