1,025 research outputs found
The redshifts of bright sub-mm sources
One of the key goals in observational cosmology over the next few years will
be to establish the redshift distribution of the recently-discovered sub-mm
source population. In this brief review I discuss and summarize the redshift
information which has been gleaned to date for the ~ 50 bright sub-mm sources
which have been uncovered via the six main classes of survey performed with
SCUBA on the JCMT over the last 2-3 years. Despite the biases inherent in some
of these surveys, and the crudeness of the redshift information available in
others, I conclude that all current information suggests that only 10-15 % of
luminous sub-mm sources lie at z < 2, and that the median redshift of this
population is z ~ 3. I suggest that such a high median redshift is arguably not
unexpected given current theories designed to explain the correlation between
black-hole mass and spheroid mass found at low redshift. In such scenarios,
peak AGN emission is expected to correspond to, or even to cause termination of
major star-formation activity in the host spheroid. In contrast, maximum dust
emission is expected to occur roughly half-way through the star-formation
process. Given that optical emission from bright quasars peaks at z = 2.5,
dust-emission from massive ellipticals might be reasonably expected to peak at
some point in the preceding ~ 1 Gyr, at z ~ 3. Confirmation or refutation of
this picture requires significantly-improved redshift information on bright
samples of SCUBA sources.Comment: 11 pages, 5 figures, FIRSED2000 conference proceedings, eds. I.M. van
Bemmel, B. Wilkes, & P. Barthel Elsevier New Astronomy Review
Triple trouble for XZ Tau : deep imaging with the Jansky Very Large Array
DF gratefully acknowledges support from STFC grant ST/J001422/1. RJI acknowledges support in the form of ERC Advanced Investigator programme, cosmicism. EI acknowledges funding from CONICYT/FONDECYT postdoctoral project no.: 3130504.We present new observations of the XZ Tau system made at high angular resolution (55 mas) with the Karl G. Jansky Very Large Array (VLA) at a wavelength of 7 mm. Observations of XZ Tau made with the VLA in 2004 appeared to show a triple-star system, with XZ Tau A resolved into two sources, XZ Tau A and XZ Tau C. The angular separation of XZ Tau A and C (0.09 arcsec) suggested a projected orbital separation of around 13 au with a possible orbital period of around 40 yr. Our follow-up observations were obtained approximately 8 yr later, a fifth of this putative orbital period, and should therefore allow us to constrain the orbital parameters of XZ Tau C, and evaluate the possibility that a recent periastron passage of C coincided with the launch of extended optical outflows from XZ Tau A. Despite improved sensitivity and resolution, as compared with the 2004 observations, we find no evidence of XZ Tau C in our data. Components A and B are detected with a signal-to-noise ratio greater than 10; their orbital motions are consistent with previous studies of the system, although the emission from XZ Tau A appears to be weaker. Three possible interpretations are offered: either XZ Tau C is transiting XZ Tau A, which is broadly consistent with the periastron passage hypothesis, or the emission seen in 2004 was that of a transient, or XZ Tau C does not exist. A fourth interpretation, that XZ Tau C was ejected from the system, is dismissed due to the lack of angular momentum redistribution in the orbits of XZ Tau A and XZ Tau B that would result from such an event. Transients are rare but cannot be ruled out in a T Tauri system known to exhibit variable behaviour. Our observations are insufficient to distinguish between the remaining possibilities, at least not until we obtain further VLA observations at a sufficiently later time. A further non-detection would allow us to reject the transit hypothesis, and the periastron passage of XZ Tau C as agent of XZ Tau A's outflows.Publisher PDFPeer reviewe
A submillimetre survey of the star-formation history of radio galaxies
We present the results of the first major systematic submillimetre survey of
radio galaxies spanning the redshift range 1 < z < 5. The primary aim of this
work is to elucidate the star-formation history of this sub-class of elliptical
galaxies by tracing the cosmological evolution of dust mass. Using SCUBA on the
JCMT we have obtained 850-micron photometry of 47 radio galaxies to a
consistent rms depth of 1 mJy, and have detected dust emission in 14 cases. The
radio galaxy targets have been selected from a series of low-frequency radio
surveys of increasing depth (3CRR, 6CE, etc), in order to allow us to separate
the effects of increasing redshift and increasing radio power on submillimetre
luminosity. Although the dynamic range of our study is inevitably small, we
find clear evidence that the typical submillimetre luminosity (and hence dust
mass) of a powerful radio galaxy is a strongly increasing function of redshift;
the detection rate rises from 15 per cent at z 2.5,
and the average submillimetre luminosity rises as (1+z)^3 out to z~4. Moreover
our extensive sample allows us to argue that this behaviour is not driven by
underlying correlations with other radio galaxy properties such as radio power,
radio spectral index, or radio source size/age. Although radio selection may
introduce other more subtle biases, the redshift distribution of our detected
objects is in fact consistent with the most recent estimates of the redshift
distribution of comparably bright submillimetre sources discovered in blank
field surveys. The evolution of submillimetre luminosity found here for radio
galaxies may thus be representative of massive ellipticals in general.Comment: 31 pages - 10 figures in main text, 3 pages of figures in appendix.
This revised version has been re-structured, but the analysis and conclusions
have not changed. Accepted for publication in MNRA
Evidence for extended, obscured starbursts in submm galaxies
We compare high-resolution optical and radio imaging of 12 luminous submm
galaxies at z=2.2+/-0.2 observed with HST and the MERLIN and VLA at comparable
spatial resolution, 0.3" (2kpc). The radio emission traces the likely
far-infrared morphology of these dusty, luminous galaxies. In ~30% of the
sample the radio appears unresolved, suggesting that the emission is compact:
either an obscured AGN or nuclear starburst. However, in the majority, ~70%
(8/12), the radio emission is resolved by MERLIN/VLA on scales of ~1" (10 kpc).
For these galaxies the radio morphologies are broadly similar to their
restframe UV emission seen by HST. We discuss the probable mechanisms for the
extended emission and conclude that their luminous radio and submm emission
arises from a large, spatially-extended starburst. The median SFRs are
1700Mo/yr occuring within a ~40kpc^2 region, giving a star formation density of
45Mo/yr/kpc^2. Such vigorous and extended starbursts appear to be uniquely
associated with the submm population. A more detailed comparison of the
distribution of UV and radio emission shows that the broad similarities on
large scales are not carried through to smaller scales, where there is rarely a
one-to-one correspondance. We interpret this as resulting from highly
structured internal obscuration, suggesting that the vigorous activity is
producing wind-blown channels through the obscuration in these galaxies. If
correct this underlines the difficulty of using UV morphologies to understand
structural properties of this population and also may explain the surprising
frequency of Ly-alpha emission in their spectra. [Abridged]Comment: 7 pages, 3 figure
Six months of mass outflow and inclined rings in the ejecta of V1494 Aql
V1494 Aql was a very fast nova which reached a visual maximum of mv≃ 4.0 by the end of 1999 December 3. We report observations from 4 to 284 d after discovery, including submillimetre- and centimetre-band fluxes, a single MERLIN image and optical spectroscopy in the 410 to 700 nm range. The extent of the radio continuum emission is consistent with a recent lower distance estimate of 1.6 kpc. We conclude that the optical and radio emission arises from the same expanding ejecta. We show that these observations are not consistent with simple kinematical spherical shell models used in the past to explain the rise and fall of the radio flux density in these objects. The resolved remnant structure is consistent with an inclined ring of enhanced density within the ejecta. Optical spectroscopy indicates likely continued mass ejection for over 195 d, with the material becoming optically thin in the visible sometime between 195 and 285 d after outburst
The SCUBA Local Universe Galaxy Survey I: First Measurements of the Submillimetre Luminosity and Dust Mass Functions
We have used SCUBA to observe a complete sample of 104 galaxies selected at
60 microns from the IRAS BGS and we present here the 850 micron measurements.
Fitting the 60,100 and 850 micron fluxes with a single temperature dust model
gives the sample mean temperature T=36 K and beta = 1.3. We do not rule out the
possibility of dust which is colder than this, if a 20 K component was present
then our dust masses would increase by factor 1.5-3. We present the first
measurements of the luminosity and dust mass functions, which were well fitted
by Schechter functions (unlike those 60 microns). We have correlated many
global galaxy properties with the submillimetre and find that there is a
tendancy for less optically luminous galaxies to contain warmer dust and have
greater star formation efficiencies (cf. Young 1999). The average gas-to-dust
ratio for the sample is 581 +/- 43 (using both atomic and molecular hydrogen),
significantly higher than the Galactic value of 160. We believe this
discrepancy is due to a cold dust component at T < 20 K. There is a suprisingly
tight correlation between dust mass and the mass of molecular hydrogen as
estimated from CO measurements, with an intrinsic scatter of ~50%.Comment: 24 pages, 15 figures, 8 tables, accepted for publication in MNRA
The extended counterpart of submm source Lockman850.1
The IRAM Plateau de Bure mm interferometer and deep K-band imaging have been
used to identify the brightest submm source detected in the Lockman field of
the UK 8mJy SCUBA survey. The near infrared counterpart is an extended
(20-30kpc), clumpy, and extremely red object. The spectral energy distribution
suggests it to be a dusty star forming object at a redshift of about 3 (2-4).
Its star formation rate and near-infrared properties are consistent with
Lockman850.1 being a massive elliptical in formation.Comment: 4 ps/eps figures. To appear in A&
A massive reservoir of low-excitation molecular gas at high redshift
Molecular hydrogen is an important component of galaxies because it fuels
star formation and accretion onto AGN, the two processes that generate the
large infrared luminosities of gas-rich galaxies. Observations of spectral-line
emission from the tracer molecule CO are used to probe the properties of this
gas. But the lines that have been studied in the local Universe, mostly the
lower rotational transitions of J = 1-0 and J = 2-1, have hitherto been
unobservable in high-redshift galaxies. Instead, higher transitions have been
used, although the densities and temperatures required to excite these higher
transitions may not be reached by much of the gas. As a result, past
observations may have underestimated the total amount of molecular gas by a
substantial amount. Here we report the discovery of large amounts of
low-excitation molecular gas around the infrared-luminous quasar, APM
08279+5255 at z = 3.91, using the two lowest excitation lines of 12CO (J = 1-0
and J = 2-1). The maps confirm the presence of hot and dense gas near the
nucleus, and reveal an extended reservoir of molecular gas with low excitation
that is 10 to 100 times more massive than the gas traced by higher-excitation
observations. This raises the possibility that significant amounts of
low-excitation molecular gas may lurk in the environments of high-redshift (z >
3) galaxies.Comment: To appear as a Letter to Nature, 4th January 200
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