17 research outputs found
The properties of the stellar populations in ULIRGs II: the star formation histories and evolution
This is the second of two papers presenting a detailed long-slit
spectroscopic study of the stellar populations in a sample of 36 ULIRGs. In the
previous paper we presented the sample, the data and the spectral synthesis
modelling while in this paper, we carry out a detailed analysis of the
modelling results. We find that the star formation histories of ULIRGs are
complex, with at least two epochs of star formation activity and that the
charcteristic timescale of the star formation acivity is <100Myr. These results
are consistent with models that predict an epoch of enhanced star formation
coinciding with the first pass of the merging nuclei, along with a further,
more intense, episode of star formation occurring as the nuclei finally merge
together. It is also found that the young stellar populations (YSPs) tend to be
younger and more reddened in the nuclear regions of the galaxies. This is in
good agreement with the merger simulations, which predict that the bulk of the
star formation activity in the final stages of mergers will occur in the
nuclear regions of the merging galaxies. In addition, our results show that
ULIRGs have total stellar masses that are similar to, or smaller than, the
break of the galaxy mass function (m* = 1.4 x 10^{11} Msolar). Finally, we find
no significant differences between the ages of the YSP in ULIRGs with and
without optically detected Seyfert nuclei, nor between those with warm and cool
mid- to far-IR colours. While this results do not entirely rule out the idea
that cool ULIRGs with HII/LINER spectra evolve into warm ULIRGs with
Seyfert-like spectra, it is clear that the AGN activity in local Seyfert-like
ULIRGs has not been triggered a substantial period (>=100 Myr) after the major
merger-induced starbursts in the nuclear regions.Comment: Accepted for publication in MNRAS. The paper contains 16 pages, 6
figures and 7 table
Starburst radio galaxies: General properties, evolutionary histories and triggering
In this paper we discuss the results of a programme of spectral synthesis modelling of a sample of starburst radio galaxies in the context of scenarios for the triggering of the activity and the evolution of the host galaxies. New optical spectra are also presented for a subset of the objects discussed. The starburst radio galaxies - comprising âŒ15-25 per cent of all powerful extragalactic radio sources - frequently show disturbed morphologies at optical wavelengths, and unusual radio structures, although their stellar masses are typical of radio galaxies as a class. In terms of the characteristic ages of their young stellar populations (YSPs), the objects can be divided into two groups: those with YSP ages tYSP†0.1 Gyr, in which the radio source has been triggered quasi-simultaneously with the main starburst episode, and those with older YSP in which the radio source has been triggered or re-triggered a significant period after the starburst episode. Most of the former group are associated with a large mid- to far-IR (MFIR) continuum and [Oiii] emission-line luminosities (LIR > 1011Lâ, W), while most of the latter have lower luminosities. Combining the information on the YSP with that on the optical morphologies of the host galaxies, we deduce that the majority of the starburst radio galaxies have been triggered in galaxy mergers in which at least one of the galaxies is gas rich. However, the triggering (or re-triggering) of the radio jets can occur immediately before, around or a significant period after the final coalescence of the merging nuclei, reflecting the complex gas infall histories of the merger events. Although âŒ25 per cent of starburst radio galaxies are sufficiently bright at MFIR wavelengths to be classified as ultraluminous infrared galaxies (ULIRGs), we show that only the most massive ULIRGs are capable of evolving into radio galaxies. Finally, for a small subset of starburst radio galaxies in rich clusters of galaxies, cooling flows associated with the hot X-ray haloes offer a viable alternative to mergers as a trigger for the radio jet activity. Overall, our results provide further evidence that a powerful radio jet activity can be triggered via a variety of mechanisms, including different evolutionary stages of major galaxy mergers; clearly, radio-loud AGN activity is not solely associated with a particular stage of a unique type of gas accretion event. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.This research has made use of the NASA/IPAC Extragalactic Data base (NED) which is operated by the Jet Propulsion Laboratory. Based on observations made with ESO Telescopes at the La Silla and Paranal observatories under programmes 70.B-0663(A), 71.B-0320(A), 078B-0660(A).Peer Reviewe
A census of young stellar populations in the warm ULIRG PKS1345+12
We present a detailed investigation of the young stellar populations(YSP) in
the radio-loud ultra luminous infrared galaxy (ULIRG) PKS1345+12, based on high
resolution HST imaging and long slit spectra taken with the WHT. While the
images clearly show bright knots suggestive of super star clusters(SSC), the
spectra reveal the presence of YSP in the diffuse light across the full extent
of the halo of the merging-double nucleus system. Spectral synthesis modelling
has been used to estimate the ages of the YSP for both the SSC and the diffuse
light sampled by the spectra. For the SSC we find ages t{SSC} < 6 Myr with
reddenings 0.2 < E(B-V) < 0.5 and masses 10e6 < M{SSC} < 10e7 M{solar}.
However, in some regions of the galaxy we find that the spectra of the diffuse
light component can only be modelled with a relatively old post-starburst YSP
(0.04 - 1.0 Gyr) or with a disk galaxy template spectrum. The results
demonstrate the importance of accounting for reddening in photometric studies
of SSC, and highlight the dangers of focussing on the highest surface
brightness regions when trying to obtain a general impression of the star
formation activity in the host galaxies of ULIRGs. The case of PKS1345+12
provides clear evidence that the star formation histories of the YSP in ULIRGs
are complex. Intriguingly, our long-slit spectra show line splitting at the
locations of the SSC, indicating that they are moving at up to 450km s-1 with
respect to the local ambient gas. Given their kinematics, it is plausible that
the SSC have been formed either in fast moving gas streams/tidal tails that are
falling back into the nuclear regions as part of the merger process, or as a
consequence of jet-induced star formation linked to the extended, diffuse radio
emission detected in the halo of the galaxyComment: accepted for publication in MNRA
Optical spectroscopy of Arp220: the star formation history of the closest ULIRG
We present optical spectra of the merging system Arp 220,taken with the
William Herschel Telescope(WHT) on La Palma. These data were taken with the aim
of investigating the evolution and star formation history of this object.
Spectral synthesis modelling has been used to estimate the ages of the stellar
populations found in the diffuse light sampled by the spectra. The data show a
remarkable uniformity in the stellar populations across the full 65 arcsec
covered by our slit positions, sampling the measurable extent of the galaxy.
The results are consistent with a dominant intermediate-age stellar population
(ISP) with age 0.5 < t_{ISP} 0.9 Gyr that is present at all locations,
with varying contributions from a young ( 0.1 Gyr) stellar population
(YSP) component. However, it is notable that while the flux contribution of the
YSP component in the extended regions is relatively small ( 40%),
adequate fits in the nuclear region are only found for combinations with a
significant contribution of a YSP component (22 - 63%). Moreover, while a low
intrinsic reddening (E(B - V)\lsim 0.3) is found for the ISPs in the extended
regions, intrinsic reddening values as high as E(B - V) ~ 1.0 are required in
the galactic center. This clearly reflects the presence of a reddening
gradient, with higher concentrations of gas and dust towards the nuclear
regions, coinciding with dust lanes in the HST images. Overall, our results are
consistent with models that predict an epoch of enhanced star formation
coinciding with the first pass of the merging nuclei (represented by the ISP),
with a further episode of star formation occurring as the nuclei finally merge
together (represented by the YSP and ULIRG).Comment: 12 pages,3 Tables, 4 figures, accepted for publication in MNRA
Evolution of tidal disruption candidates discovered by XMM-Newton
It has been demonstrated that active galactic nuclei are powered by gas
accretion onto supermassive black holes located at their centres. The paradigm
that the nuclei of inactive galaxies are also occupied by black holes was
predicted long ago by theory. In the last decade, this conjecture was confirmed
by the discovery of giant-amplitude, non-recurrent X-ray flares from such
inactive galaxies and explained in terms of outburst radiation from stars
tidally disrupted by a dormant supermassive black hole at the nuclei of those
galaxies. Due to the scarcity of detected tidal disruption events, the
confirmation and follow-up of each new candidate is needed to strengthen the
theory through observational data, as well as to shed new light on the
characteristics of this type of events. Two tidal disruption candidates have
been detected with XMM-Newton during slew observations. Optical and X-ray
follow-up, post-outburst observations were performed on these highly variable
objects in order to further study their classification and temporal evolution.
We show that the detected low-state X-ray emission for these two candidates has
properties such that it must still be related to the flare. The X-ray
luminosity of the objects decreases according to theoretical predictions for
tidal disruption events. At present, optical spectra of the sources do not
present any evident signature of the disruption event. In addition, the tidal
disruption rate as derived from the XMM-Newton slew survey has been computed
and agrees with previous studies.Comment: 12 pages, 8 figures, A&A accepte
Galaxy Pairs in the Sloan Digital Sky Survey - III: Evidence of Induced Star Formation from Optical Colours
We have assembled a large, high quality catalogue of galaxy colours from the
Sloan Digital Sky Survey Data Release 7, and have identified 21,347 galaxies in
pairs spanning a range of projected separations (r_p < 80 h_{70}^{-1} kpc),
relative velocities (\Delta v < 10,000 km/s, which includes projected pairs
that are essential for quality control), and stellar mass ratios (from 1:10 to
10:1). We find that the red fraction of galaxies in pairs is higher than that
of a control sample matched in stellar mass and redshift, and demonstrate that
this difference is likely due to the fact that galaxy pairs reside in higher
density environments than non-paired galaxies. We detect clear signs of
interaction-induced star formation within the blue galaxies in pairs, as
evidenced by a higher fraction of extremely blue galaxies, along with blueward
offsets between the colours of paired versus control galaxies. These signs are
strongest in close pairs (r_p < 30 h_{70}^{-1} kpc and \Delta v < 200 km/s),
diminish for more widely separated pairs (r_p > 60 h_{70}^{-1} kpc and \Delta v
< 200 km/s) and disappear for close projected pairs (r_p < 30 h_{70}^{-1} kpc
and \Delta v > 3000 km/s). These effects are also stronger in central (fibre)
colours than in global colours, and are found primarily in low- to
medium-density environments. Conversely, no such trends are seen in red
galaxies, apart from a small reddening at small separations which may result
from residual errors with photometry in crowded fields. When interpreted in
conjunction with a simple model of induced starbursts, these results are
consistent with a scenario in which close peri-centre passages trigger induced
star formation in the centres of galaxies which are sufficiently gas rich,
after which time the galaxies gradually redden as they separate and their
starbursts age.Comment: 17 pages. Accepted for publication in MNRA
PKS 0347+05: a radio-loud/radio-quiet double active galactic nucleus system triggered in a major galaxy merger
We present optical, infrared and radio observations of the powerful FRII
radio source PKS0347+05 (z=0.3390), and demonstrate that it is a rare example
of a radio-loud/radio-quiet double AGN system, comprising a weak line radio
galaxy (WLRG) separated by 25 kpc (in projection) from a Seyfert 1 nucleus at
the same redshift. Our deep Gemini optical images show a highly disturbed
morphology, with a warped dust lane crossing through the halo and nuclear
regions of the radio galaxy host, tidal tails, and a bridge connecting the
radio galaxy to the Seyfert 1 nucleus. Spectral synthesis modelling of our
Gemini optical spectrum of the radio galaxy shows evidence for a reddened young
stellar population of age <100 Myr. Further evidence for recent star formation
activity in this source is provided by the detection of strong PAH features in
mid-IR Spitzer/IRS spectra. Together, these observations support a model in
which both AGN have been triggered simultaneously in a major galaxy merger.
However, despite the presence of a powerful FRII radio source, and the
apparently plentiful supply of fuel provided by the merger, the nucleus of the
radio galaxy shows only weak, low ionization emission line activity. We
speculate that the fuel supply to nuclear regions of the radio galaxy has
recently switched off (within the last ~10^6 yr), but the information about the
resulting decrease in nuclear AGN activity has yet to reach the extended lobes
and hotspots of the FRII radio source. Based on this scenario, we estimate that
powerful, intermediate redshift FRII radio sources have lifetimes of t\sim
5x10^6 yr. Overall, our observations emphasise that the fuelling of AGN
activity in major galaxy mergers is likely to be highly intermittent.Comment: 12 pages, 10 figures, accepted for publication in MNRA