96 research outputs found
VIMOS-VLT spectroscopy of the giant Ly-alpha nebulae associated with three z~2.5 radio galaxies
The morphological and spectroscopic properties of the giant (>60 kpc)
Ly-alpha nebulae associated with three radio galaxies at z~2.5 (MRC 1558-003,
MRC 2025-218 and MRC 0140-257) have been investigated using integral field
spectroscopic data obtained with VIMOS on VLT.
The morphologies are varied. The nebula of one source has a centrally peaked,
rounded appearance. In the other two objects, it consists of two spatial
components. The three nebulae are aligned with the radio axis within <30 deg.
The total Ly-alpha luminosities are in the range (0.3-3.4) x 1e44 erg s-1. The
Ly-alpha spectral profile shows strong variation through the nebulae, with FWHM
values in the range ~400-1500 km s-1 and velocity shifts V~120-600 km s-1.
We present an infall model which can explain successfully most Ly-alpha
morphological and spectroscopic properties of the nebula associated with MRC
1558-003. This adds further support to our previous conclusion that the
_quiescent_ giant nebulae associated with this and other high redshift powerful
radio galaxies are in infall. A problem for this model is the difficulty to
reproduce the large Ly-alpha FWHM values.
We have discovered a giant (~85 kpc) Ly-alpha nebula associated with the
radio galaxy MRC 0140-257 at z=2.64. It shows strikingly relaxed kinematics
(FWHM2) radio galaxies.Comment: 14 pages, 13 figures. Accepted for publication in MNRA
H-ATLAS/GAMA and HeViCS – dusty early-type galaxies in different environments
The Herschel Space Observatory has had a tremendous impact on the study of extragalactic dust. Specifically, early-type galaxies (ETG) have been the focus of several studies. In this paper, we combine results from two Herschel studies – a Virgo cluster study Herschel Virgo Cluster Survey (HeViCS) and a broader, low-redshift Herschel-Astrophysical Terahertz Large Area Survey (H-ATLAS)/Galaxy and Mass Assembly (GAMA) study – and contrast the dust and associated properties for similar mass galaxies. This comparison is motivated by differences in results exhibited between multiple Herschel studies of ETG. A comparison between consistent modified blackbody derived dust mass is carried out, revealing strong differences between the two samples in both dust mass and dust-to-stellar mass ratio. In particular, the HeViCS sample lacks massive ETG with as high a specific dust content as found in H-ATLAS. This is most likely connected with the difference in environment for the two samples. We calculate nearest neighbour environment densities in a consistent way, showing that H-ATLAS ETG occupy sparser regions of the local Universe, whereas HeViCS ETG occupy dense regions. This is also true for ETG that are not Herschel-detected but are in the Virgo and GAMA parent samples. Spectral energy distributions are fit to the panchromatic data. From these, we find that in H-ATLAS the specific star formation rate anticorrelates with stellar mass and reaches values as high as in our Galaxy. On the other hand HeViCS ETG appear to have little star formation. Based on the trends found here, H-ATLAS ETG are thought to have more extended star formation histories and a younger stellar population than HeViCS ETG
Metal enrichment in near-IR luminous galaxies at z~2: signatures of proto-ellipticals?
We present the analysis of the coadded rest-frame UV spectrum (1200<z<2000 A)
of five K-luminous galaxies at z~2 from the K20 survey. The composite spectrum
is characterized by strong absorption lines over the UV continuum from C, N, O,
Al, Si, and Fe in various ionization stages. While some of these lines are
interstellar, several among the strongest absorptions are identified with
stellar photospheric lines. Most of the photospheric and interstellar features
are stronger in the K-luminous composite spectrum than in LBGs at z~3. This
suggests higher metallicity and possibly also larger interstellar velocity
dispersion caused by macroscopic motions. The absorption lines and the slope of
the UV continuum is well matched by the spectrum of the nearby luminous
infrared galaxy NGC 6090, which is in the process of merging. A metallicity
higher than solar is suggested by comparing the pure photospheric lines (SiIII,
CIII, FeV) with starburst models. The evidence of high metallicity, together
with the high masses, high star-formation rates, and possibly strong
clustering, well qualify these galaxies as progenitors of local massive
ellipticals.Comment: 4 pages, 3 figures, Accepted ApJ Letter
Radio galaxies at z~2.5: results from Keck spectropolarimetry
In classifying the ensemble of powerful extragalactic radio sources,
considerable evidence has accumulated that radio galaxies and quasars are
orientation-dependent manifestations of the same parent population: massive
spheroidal galaxies containing correspondingly massive black holes. [...] In
this study, we exploit the opportunity to eliminate the quasar glare by
performing sensitive spectropolarimetry with the Keck II telescope of a sample
of radio galaxies with redshifts around 2.5. [...] We show that dust-reflected
quasar light generally dominates the restframe ultraviolet continuum of these
sources and that a highly clumped scattering medium results in almost grey
scattering of the active galactic nucleus photons. The observations, however,
do not exclude a substantial star formation rate averaged over a Gyr of
evolution. The sub-mm reradiation from the scattering dust is likely to
represent only a small fraction (~10%) of the total far infrared luminosity. An
analysis of the emission lines excited in the interstellar medium of the host
galaxy by the hard quasar radiation field reveals evidence of a dramatic
chemical evolution within the spheroid during this epoch. Secondary nitrogen
production in intermediate mass stars produces a characteristic signature in
the NV/CIV and NV/HeII line ratios which has been seen previously in the broad
line region of quasars at similar redshifts. We find intriguing correlations
between the strengths of the Ly alpha and NV emission lines and the degree of
ultraviolet continuum polarization which may represent the dispersal of dust
associated with the chemical enrichment of the spheroid.Comment: 24 pages, 12 figures, accepted for publication in Astronomy &
Astrophysics. Properly formatted for A4 and Letter paper formats ps files
available at http://www.eso.org/~jvernet/research.htm
The Herschel Virgo Cluster Survey XVI: a cluster inventory
Herschel FIR observations are used to construct Virgo cluster galaxy
luminosity functions and to show that the cluster lacks the very bright and the
numerous faint sources detected in field galaxy surveys. The far-infrared SEDs
are fitted to obtain dust masses and temperatures and the dust mass function.
The cluster is over dense in dust by about a factor of 100 compared to the
field. The same emissivity (beta) temperature relation applies for different
galaxies as that found for different regions of M31. We use optical and HI data
to show that Virgo is over dense in stars and atomic gas by about a factor of
100 and 20 respectively. Metallicity values are used to measure the mass of
metals in the gas phase. The mean metallicity is about 0.7 solar and 50% of the
metals are in the dust. For the cluster as a whole the mass density of stars in
galaxies is 8 times that of the gas and the gas mass density is 130 times that
of the metals. We use our data to consider the chemical evolution of the
individual galaxies, inferring that the measured variations in effective yield
are due to galaxies having different ages, being affected to varying degrees by
gas loss. Four galaxy scaling relations are considered: mass-metallicity,
mass-velocity, mass-star formation rate and mass-radius - we suggest that
initial galaxy mass is the prime driver of a galaxy's ultimate destiny.
Finally, we use X-ray observations and galaxy dynamics to assess the dark and
baryonic matter content compared to the cosmological model
The identification of dust heating mechanisms in nearby galaxies using Herschel 160/250 and 250/350 micron surface brightness ratios
We examined variations in the 160/250 and 250/350 micron surface brightness
ratios within 24 nearby (<30 Mpc) face-on spiral galaxies observed with the
Herschel Space Observatory to identify the heating mechanisms for dust emitting
at these wavelengths. The analysis consisted of both qualitative and
quantitative comparisons of the 160/250 and 250/350 micron ratios to H alpha
and 24 micron surface brightnesses, which trace the light from star forming
regions, and 3.6 micron emission, which traces the light from the older stellar
populations of the galaxies. We find broad variations in the heating mechanisms
for the dust. In one subset of galaxies, we found evidence that emission at
<=160 microns (and in rare cases potentially at <=350 microns) originates from
dust heated by star forming regions. In another subset, we found that the
emission at >=250 microns (and sometimes at >=160 microns) originates from dust
heated by the older stellar population. In the rest of the sample, either the
results are indeterminate or both of these stellar populations may contribute
equally to the global dust heating. The observed variations in dust heating
mechanisms does not necessarily match what has been predicted by dust emission
and radiative transfer models, which could lead to overestimated dust
temperatures, underestimated dust masses, false detections of variability in
dust emissivity, and inaccurate star formation rate measurements.Comment: Accepted for publication in MNRA
H-ATLAS/GAMA and HeViCS - dusty early-type galaxies in different environments
NKA acknowledges the support of the Science and Technology Facilities Council. LD, RJI and SJM acknowledge support from the European Research Council Advanced Grant COSMICISM. IDL gratefully acknowledges the support of the Flemish Fund for Scientific Research (FWO-Vlaanderen). KR acknowledges support from the European Research Council Starting Grant SEDmorph (P.I. V. Wild). Date of acceptance: 22/05/2015The Herschel Space Observatory has had a tremendous impact on the study of extragalactic dust. Specifically, early-type galaxies (ETG) have been the focus of several studies. In this paper, we combine results from two Herschel studies -a Virgo cluster study Herschel Virgo Cluster Survey (HeViCS) and a broader, low-redshift Herschel-Astrophysical Terahertz Large Area Survey (H-ATLAS)/Galaxy and Mass Assembly (GAMA) study -and contrast the dust and associated properties for similar mass galaxies. This comparison is motivated by differences in results exhibited between multiple Herschel studies of ETG. A comparison between consistent modified blackbody derived dust mass is carried out, revealing strong differences between the two samples in both dust mass and dust-to-stellar mass ratio. In particular, the HeViCS sample lacks massive ETG with as high a specific dust content as found in H-ATLAS. This is most likely connected with the difference in environment for the two samples. We calculate nearest neighbour environment densities in a consistent way, showing that H-ATLAS ETG occupy sparser regions of the local Universe, whereas HeViCS ETG occupy dense regions. This is also true for ETG that are not Herschel-detected but are in the Virgo and GAMA parent samples. Spectral energy distributions are fit to the panchromatic data. From these, we find that in H-ATLAS the specific star formation rate anticorrelates with stellar mass and reaches values as high as in our Galaxy. On the other hand HeViCS ETG appear to have little star formation. Based on the trends found here, H-ATLAS ETG are thought to have more extended star formation histories and a younger stellar population than HeViCS ETG.Publisher PDFPeer reviewe
The Herschel Virgo Cluster Survey: VII. Dust in cluster dwarf elliptical galaxies
We use the Science Demonstration Phase data of the Herschel Virgo Cluster
Survey to search for dust emission of early-type dwarf galaxies in the central
regions of the Virgo Cluster as an alternative way of identifying the
interstellar medium.We present the first possible far-infrared detection of
cluster early-type dwarf galaxies: VCC781 and VCC951 are detected at the 10
sigma level in the SPIRE 250 micron image. Both detected galaxies have dust
masses of the order of 10^5 Msun and average dust temperatures ~20K. The
detection rate (less than 1%) is quite high compared to the 1.7% detection rate
for Hi emission, considering that dwarfs in the central regions are more Hi
deficient. We conclude that the removal of interstellar dust from dwarf
galaxies resulting from ram pressure stripping, harassment, or tidal effects
must be as efficient as the removal of interstellar gas.Comment: Letter accepted for publication in A&A (Herschel special issue
PACS photometry of the Herschel Reference Survey - Far-infrared/sub-millimeter colours as tracers of dust properties in nearby galaxies
We present Herschel/PACS 100 and 160 micron integrated photometry for the 323
galaxies in the Herschel Reference Survey (HRS), a K-band-, volume-limited
sample of galaxies in the local Universe. Once combined with the Herschel/SPIRE
observations already available, these data make the HRS the largest
representative sample of nearby galaxies with homogeneous coverage across the
100-500 micron wavelength range. In this paper, we take advantage of this
unique dataset to investigate the properties and shape of the
far-infrared/sub-millimeter spectral energy distribution in nearby galaxies. We
show that, in the stellar mass range covered by the HRS (8<log(M*/Msun)<12),
the far-infrared/sub-millimeter colours are inconsistent with a single modified
black-body having the same dust emissivity index beta for all galaxies. In
particular, either beta decreases, or multiple temperature components are
needed, when moving from metal-rich/gas-poor to metal-poor/gas-rich galaxies.
We thus investigate how the dust temperature and mass obtained from a single
modified black-body depend on the assumptions made on beta. We show that, while
the correlations between dust temperature, galaxy structure and star formation
rate are strongly model dependent, the dust mass scaling relations are much
more reliable, and variations of beta only change the strength of the observed
trends.Comment: 26 pages, 8 figures, 2 tables. Accepted for publication in MNRA
GPS radio sources: new optical observations and an updated master list
* Aims. Identify optical counterparts, address uncertain identifications and
measure previously unknown redshifts of the host galaxies of candidate GPS
radio sources, and study their stellar populations. * Methods. Long slit
spectroscopy and deep optical imaging in the B, V and R bands, obtained with
the Very Large Telescope. * Results. We obtain new redshifts for B0316+161,
B0407-658, B0904+039, B1433-040, and identify the optical counterparts of
B0008-421 and B0742+103. We confirm the previous identification for B0316+161,
B0407-658, B0554-026, and B0904+039, and find that the previous identification
for B0914+114 is incorrect. Using updated published radio spectral information
we classify as non GPS the following sources: B0407-658, B0437-454, B1648+015.
The optical colors of typical GPS sources are consistent with single
instantaneous burst stellar population models but do not yield useful
information on age or metallicity. A new master list of GPS sources is
presented.Comment: 10 pages + GPS master list. Accepeted for publication by A&
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