Revealing a strongly reddened, faint active galactic nucleus population by stacking deep co-added images

József Varga, I. Csabai, László Dobos

Research output: Contribution to journalArticle

Abstract

More than half of the sources identified by recent radio sky surveys have not been detected by wide-field optical surveys. We present a study, based on our co-added image stacking technique, in which our aim is to detect the optical emission from unresolved, isolated radio sources of the Very Large Array (VLA) Faint Images of the Radio Sky at Twenty-cm (FIRST) survey that have no identified optical counterparts in the Sloan Digital Sky Survey (SDSS) Stripe 82 co-added data set. From the FIRST catalogue, 2116 such radio point sources were selected, and cut-out images, centred on the FIRST coordinates, were generated from the Stripe 82 images. The already co-added cut-outs were stacked once again to obtain images of high signal-to-noise ratio, in the hope that optical emission from the radio sources would become detectable. Multiple stacks were generated, based on the radio luminosity of the point sources. The resulting stacked images show central peaks similar to point sources. The peaks have very red colours with steep optical spectral energy distributions. We have found that the optical spectral index α ν falls in the range -2.9 ≤ α ν ≤ -2.2 (Sν∝ναν), depending only weakly on the radio flux. The total integration times of the stacks are between 270 and 300h, and the corresponding 5σ detection limit is estimated to be about m r ≃ 26.6 mag. We argue that the detected light is mainly from the central regions of dust-reddened Type 1 active galactic nuclei. Dust-reddened quasars might represent an early phase of quasar evolution, and thus they can also give us an insight into the formation of massive galaxies. The data used in the paper are available on-line at http://www.vo.elte.hu/doublestacking.

Original languageEnglish
Pages (from-to)833-850
Number of pages18
JournalMonthly Notices of the Royal Astronomical Society
Volume426
Issue number2
DOIs
Publication statusPublished - Oct 21 2012

Fingerprint

stacking
active galactic nuclei
radio
point sources
sky
point source
quasars
light emission
dust
Very Large Array (VLA)
spectral energy distribution
catalogs
signal-to-noise ratio
signal to noise ratios
luminosity
galaxies
color
energy

Keywords

  • Galaxies: active
  • Methods: data analysis
  • Quasars: general
  • Radio continuum: galaxies
  • Techniques: image processing

ASJC Scopus subject areas

  • Space and Planetary Science
  • Astronomy and Astrophysics

Cite this

Revealing a strongly reddened, faint active galactic nucleus population by stacking deep co-added images. / Varga, József; Csabai, I.; Dobos, László.

In: Monthly Notices of the Royal Astronomical Society, Vol. 426, No. 2, 21.10.2012, p. 833-850.

Research output: Contribution to journalArticle

@article{3f0a196cece2492fbc40b2aff7163de3,
title = "Revealing a strongly reddened, faint active galactic nucleus population by stacking deep co-added images",
abstract = "More than half of the sources identified by recent radio sky surveys have not been detected by wide-field optical surveys. We present a study, based on our co-added image stacking technique, in which our aim is to detect the optical emission from unresolved, isolated radio sources of the Very Large Array (VLA) Faint Images of the Radio Sky at Twenty-cm (FIRST) survey that have no identified optical counterparts in the Sloan Digital Sky Survey (SDSS) Stripe 82 co-added data set. From the FIRST catalogue, 2116 such radio point sources were selected, and cut-out images, centred on the FIRST coordinates, were generated from the Stripe 82 images. The already co-added cut-outs were stacked once again to obtain images of high signal-to-noise ratio, in the hope that optical emission from the radio sources would become detectable. Multiple stacks were generated, based on the radio luminosity of the point sources. The resulting stacked images show central peaks similar to point sources. The peaks have very red colours with steep optical spectral energy distributions. We have found that the optical spectral index α ν falls in the range -2.9 ≤ α ν ≤ -2.2 (Sν∝ναν), depending only weakly on the radio flux. The total integration times of the stacks are between 270 and 300h, and the corresponding 5σ detection limit is estimated to be about m r ≃ 26.6 mag. We argue that the detected light is mainly from the central regions of dust-reddened Type 1 active galactic nuclei. Dust-reddened quasars might represent an early phase of quasar evolution, and thus they can also give us an insight into the formation of massive galaxies. The data used in the paper are available on-line at http://www.vo.elte.hu/doublestacking.",
keywords = "Galaxies: active, Methods: data analysis, Quasars: general, Radio continuum: galaxies, Techniques: image processing",
author = "J{\'o}zsef Varga and I. Csabai and L{\'a}szl{\'o} Dobos",
year = "2012",
month = "10",
day = "21",
doi = "10.1111/j.1365-2966.2012.21560.x",
language = "English",
volume = "426",
pages = "833--850",
journal = "Monthly Notices of the Royal Astronomical Society",
issn = "0035-8711",
publisher = "Oxford University Press",
number = "2",

}

TY - JOUR

T1 - Revealing a strongly reddened, faint active galactic nucleus population by stacking deep co-added images

AU - Varga, József

AU - Csabai, I.

AU - Dobos, László

PY - 2012/10/21

Y1 - 2012/10/21

N2 - More than half of the sources identified by recent radio sky surveys have not been detected by wide-field optical surveys. We present a study, based on our co-added image stacking technique, in which our aim is to detect the optical emission from unresolved, isolated radio sources of the Very Large Array (VLA) Faint Images of the Radio Sky at Twenty-cm (FIRST) survey that have no identified optical counterparts in the Sloan Digital Sky Survey (SDSS) Stripe 82 co-added data set. From the FIRST catalogue, 2116 such radio point sources were selected, and cut-out images, centred on the FIRST coordinates, were generated from the Stripe 82 images. The already co-added cut-outs were stacked once again to obtain images of high signal-to-noise ratio, in the hope that optical emission from the radio sources would become detectable. Multiple stacks were generated, based on the radio luminosity of the point sources. The resulting stacked images show central peaks similar to point sources. The peaks have very red colours with steep optical spectral energy distributions. We have found that the optical spectral index α ν falls in the range -2.9 ≤ α ν ≤ -2.2 (Sν∝ναν), depending only weakly on the radio flux. The total integration times of the stacks are between 270 and 300h, and the corresponding 5σ detection limit is estimated to be about m r ≃ 26.6 mag. We argue that the detected light is mainly from the central regions of dust-reddened Type 1 active galactic nuclei. Dust-reddened quasars might represent an early phase of quasar evolution, and thus they can also give us an insight into the formation of massive galaxies. The data used in the paper are available on-line at http://www.vo.elte.hu/doublestacking.

AB - More than half of the sources identified by recent radio sky surveys have not been detected by wide-field optical surveys. We present a study, based on our co-added image stacking technique, in which our aim is to detect the optical emission from unresolved, isolated radio sources of the Very Large Array (VLA) Faint Images of the Radio Sky at Twenty-cm (FIRST) survey that have no identified optical counterparts in the Sloan Digital Sky Survey (SDSS) Stripe 82 co-added data set. From the FIRST catalogue, 2116 such radio point sources were selected, and cut-out images, centred on the FIRST coordinates, were generated from the Stripe 82 images. The already co-added cut-outs were stacked once again to obtain images of high signal-to-noise ratio, in the hope that optical emission from the radio sources would become detectable. Multiple stacks were generated, based on the radio luminosity of the point sources. The resulting stacked images show central peaks similar to point sources. The peaks have very red colours with steep optical spectral energy distributions. We have found that the optical spectral index α ν falls in the range -2.9 ≤ α ν ≤ -2.2 (Sν∝ναν), depending only weakly on the radio flux. The total integration times of the stacks are between 270 and 300h, and the corresponding 5σ detection limit is estimated to be about m r ≃ 26.6 mag. We argue that the detected light is mainly from the central regions of dust-reddened Type 1 active galactic nuclei. Dust-reddened quasars might represent an early phase of quasar evolution, and thus they can also give us an insight into the formation of massive galaxies. The data used in the paper are available on-line at http://www.vo.elte.hu/doublestacking.

KW - Galaxies: active

KW - Methods: data analysis

KW - Quasars: general

KW - Radio continuum: galaxies

KW - Techniques: image processing

UR - http://www.scopus.com/inward/record.url?scp=84867058092&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=84867058092&partnerID=8YFLogxK

U2 - 10.1111/j.1365-2966.2012.21560.x

DO - 10.1111/j.1365-2966.2012.21560.x

M3 - Article

AN - SCOPUS:84867058092

VL - 426

SP - 833

EP - 850

JO - Monthly Notices of the Royal Astronomical Society

JF - Monthly Notices of the Royal Astronomical Society

SN - 0035-8711

IS - 2

ER -