Transcript Elaine Sadler

HI absorption-line science:
exciting opportunities with ASKAP12
Elaine Sadler
University of Sydney / CAASTRO
on behalf of the ASKAP FLASH team
5 August 2013
Summary
Why is an HI absorption-line survey an ideal ASKAP Early
Science project?
• It opens up a completely new parameter space for HI spectral line
studies - no other radio interferometer has a wide-band spectral-line
capability at 700-1000 MHz (0.5 < z < 1 for the 21cm HI line).
• It can deliver unique and important new science results in a modest
amount of observing time (days to weeks) with ASKAP-12.
• It will showcase the exceptional radio-quiet qualities of the ASKAP
site at frequencies below 1 GHz.
• Our proposed survey is very flexible, can use any configuration of
ASKAP antennas, and could be carried out commensally with the
proposed EMU/POSSUM Early Science continuum survey.
2
Observing the 21cm HI line
Technique
Redshift
range
Measures
Notes
HI emission-line
surveys
0 < z < 0.2
Individual
galaxies
Detection rate drops with
redshift
HI emission-line
stacking
0 < z < 0.4
‘Average’ HI
properties
Detection rate still drops
with redshift, but depends
on the amount and quality of
optical redshift data
HI absorption-line
survey
0<z<1
with ASKAP
Individual
galaxies
Detection rate independent
of redshift
A key advantage of absorption surveys is that they tell us what kinds of
galaxies (uv-bright? dusty?) dominate in an HI-selected sample at high
redshift. Important for designing/interpreting stacking surveys.
Science goal: gas and galaxy evolution
The changing cosmic star-formation rate:
The rate at which new
stars form in galaxies
has decreased by
about a factor of 20
over the past 7-8 billion
years (from redshift
z~1 to 0).
(Hopkins & Beacom 2006)
What caused this?
A decline in the supply
of cold neutral gas in
galaxies?
We don’t know!
The cosmic HI mass density
Neutral hydrogen is the missing
link in our current models of
galaxy evolution.
• We know almost nothing about
the HI content of individual
galaxies in the distant universe
• A wide range of models and
simulations exist, and make
diverse predictions about the
cosmic HI mass density at z>0.
Better data are needed to
constrain them.
Optical: Damped Lyman- absorbers
Lyman  forest
DLA
(Ellison et al. 2001)
DLAs: Intervening absorbers with high HI column density (NHI > 2 x 1020 cm-2) can be used
to detect and study neutral hydrogen in the very distant universe. Ground-based
observations of the Lyman- line are only possible at redshift z > 1.7
Are optical DLA surveys biased?
Zwaan et al. (2005)
Optical QSO DLA surveys do not detect the highest column-density
absorbers expected on ~0.1% of sightlines, and “do not trace the majority of
star-forming gas in the universe” (Ledoux et al. 2003). Dust obscuration?
Radio: Intervening HI absorption
Radio 21cm measurements are
particularly sensitive to cold HI
(spin temp. TS < 200K) .
for observed
optical depth t, line width DV
Probability of intercepting a DLA
system (NHI > 2 x 1020 cm-2) on a
random sightline:
Darling et al. (2004)
dN/dZ=0.055 (1+z)1.11
(Storrie-Lombardi & Wolfe 2000)
Unlike optical, no redshift limit for detecting
radio 21cm absorption lines.
But do need many targets, wide bandwidth
e.g. ~6% for z=0.7, 300 MHz
Unique discovery space for ASKAP12
The only radio interferometer with a wide-band
capability at 700-1000 MHz – provides unique coverage of
the HI line at 0.5 < z <1 Radio-quiet site!
Telescope
Frequency range
Notes
JVLA (USA)
1 – 50 GHz
NRL providing a low-band
system at 50-500 MHz
WSRT Apertif (NL)
1.0 – 1.75 GHz
1.0 GHz limit set by RFI
GMRT (India)
1.0 – 1.45 GHz
Low-frequency band at 590630 MHz
Meerkat (S Africa) 0.9 – 1.67 GHz
[Phase 1, 2016-18]
GBT (USA)
No capability below 900 MHz
until at least 2018, possibly
later
290 MHz – 100 GHz Single dish, affected by RFI
What’s been done so far?
Largest existing survey at 0.5 < z < 1 (Darling et al. 2013).
‘Semi-blind’ survey for intervening HI absorption against a sample of
181 bright background radio sources with z > 1.1. Made ten redetections of known systems, no new detections.
“We attribute the lack of new detections in our large survey to severe
and persistent RFI… Optical selection bias also contributes”
RFI spectrum at the GBT site
(via NRAO web pages)
Roughly half the GBT band below 1 GHz is lost to RFI. 7001000 MHz is considered one of the better regions!
RFI spectrum at the ASKAP site
Measured Frequency Occupancy (plot from Aaron Chippendale)
What fraction of channels are RFI affected at high sensitivity?
(percentage of occupied 27.4 kHz channels in 10 MHz blocks in 2hr
spectra )
700-1000 MHz band is
extremely clean!
|
Measured Spectrum at MRO | A. Chippendale
What can ASKAP do?
Figure of merit:
Search path Dz set by
Number of sources
searched (to a given
column density limit)
multiplied by the Redshift
interval searched for each
source.
ASKAP-12 can easily
outperform any existing
telescope in searches
for high column-density
HI absorbers.
Huge multiplex advantage
from wide field of view!
Radio: associated HI absorption
Science goal: tracing gas
flows and AGN triggering in
powerful radio galaxies
Nearby galaxy NGC 6868, continuum flux
density ~120 mJy at 1.4 GHz.
ATCA:Associated
Oosterloo etHI
al.,absorption
targeted HI,atz or
= 0.01
near the redshift of radio galaxies and
quasars: seen in 10% to 30% of nearby radio galaxies, redshift evolution
unclear. Traces gas kinematics in the central regions,and can reveal jetdriven outflows of gas (Morganti et al. 2003, 2005).
Associated absorption in HIPASS
4 detections in 210 nearby radio galaxies (z < 0.04)
New absorption
(Allison et al. 2013, in prep.)
Strong associated HI absorption linked to
presence of OH/H20 megamasers?
HIPASS - ATCA comparison
15 arcmin resolution
New absorber
10 arcsec resolution
New
New absorber
Results show that
spatial resolution is not
critical for detecting
these strong HI
absorption lines.
Early science with ASKAP-12
A 2hr integration with ASKAP-12 will
find the strongest (intervening and
associated) HI absorption systems,
using continuum sources brighter
than ~100 mJy. The most effective
strategy is to maximize the survey
area, then build up sensitivity later as
ASKAP is extended.
HI
PKS 1814-637:
ATCA real-time
display, 270
seconds int. HI
line has z=0.064,
t = 0.19
Lines show the detection limit in HI optical depth t versus continuum flux density for
a 2-hr integration with 6, 12, 18 and 36 ASKAP antennas. Some known associated
and intervening HI absorbers are also shown as individual points.
Early Science plans for ASKAP-12
Minimum requirements for HI absorption-line Early Science:
• Observations in the 700-1000 MHz band – opens up important new
discovery space at 0.5 < z < 1 for the HI line (and is free of RFI).
A multi-band survey (700-1000, 1000-1300 and 1300-1600 MHz) could also be carried out if
feasible and better suited to commensal observations for EMU/POSSUM. This would increase
the observing time, but also broaden the redshift range (to 0 < z < 1 for HI).
• In ~1 week (60 hours, ~1500 sightlines) – first glimpse into a new
parameter space, outperforms all existing telescope in searches for high
column-density HI absorbers. [~1000 deg2 of sky, estimate ~5 intervening, 30 assoc. ]
• In ~1 month (200 hours, ~5000 sightlines) – first unbiased sample of
HI-selected galaxies at z > 0.5, answer question of whether QSO DLA
surveys are biased. [~3000 deg2 of sky, estimate ~15 intervening, 100 assoc. lines ]
• In ~3 months (600 hours, ~15,000 sightlines) – statistical samples of
intervening and associated absorbers, can start studying redshift evolution.
[~10,000 deg2 of sky, , estimate ~50 intervening, 300 assoc. lines ]