0Assigned ID: 172
----- Attempting Submission 1 (Thu Apr 10 11:20:56 GMT 2014) -----First revised submission, after including post flashes to mitigate CTE.908449048----- Attempting Submission 1 (Tue Jul 22 10:44:18 GMT 2014) -----
HST Phase II Proposal 13821 (centaurus_stage2.apt) successfully submitted.
----- Attempting Submission 2 (Tue Aug 26 09:07:47 GMT 2014) -----H-alpha Filaments and Feedback in NGC4696 at the centre of the Centaurus clusterOne third of Brightest Cluster Galaxies (BCG) have strong optical
emission lines originating in an extensive filamentary nebula. The
prototype is the spectacular nebula around NGC1275 in the Perseus
cluster which we are have imaged in H-alpha and [NII] with HST,
spatially resolving the filamentary structure. Here we propose deep
observations of the nebula in NGC4696, the BCG in the Centaurus cluster.
NGC4696, at approximately half the distance of NGC1275 is the nearest
BCG nebula with molecular filaments, thus affording the highest linear
spatial resolution in this type of source.
These observations will test the magnetic support model for the
filaments by resolving and measuring the width of the filaments, and
test the particle heating model for the excitation of the filaments
through measurement of their surface brightness distribution and
morphology.
The relation of the filaments to the dust lanes, and the complex Faraday
Rotation maps of this source will provide further diagnostics not
available in NGC1275. The results will have a wide relevance to similar
nebulae found around distant radio galaxies and young galaxies.We propose to image the BCG (NGC~4696) in the Centaurus cluster in
H$\alpha$+[NII] with WFC3/UVIS and the F665N narrow band filter.
HST is required for this project in order to obtain
the high spatial resolution. WFC3 has a comprehensive filter set well
matched for the H$\alpha$ imaging.
NGC~4696 ground-based imaging (Crawford et al 2005),
with seeing of about one arcsec, shows a bright extended filament 22 arcsec
to the South East of the nucleus with an (observed) H$\alpha$ surface
brightness of
$\sim 1\times10^{-16}$~erg~s$^{-1}$~cm$^{-2}$~arcsec$^{-2}$ (the value quoted in
the paper is $1.4\times10^{-16}$~erg~s$^{-1}$~cm$^{-2}$~arcsec$^{-2}$ after
correction for Galactic reddening by a factor of 33\%). In this proposal we
aim to construct a good map with a signal-to-noise ratio of ~10 per 2$\times$2
pixel resolution element down to one third of this surface brightness.
Our NGC~1275 HST data (Fig.~1b,c) show detailed
structures down to 0.1-0.2 arcsec scales, indicating that the
intrinsic surface brightness is much higher than observed in the ground-based
data. Assuming the filaments
have a spatial size of 0.3 arcsec would imply that their surface
brightness is $\sim$10 times higher than observed in the ground-based optical
data. We note also that in NGC~4696 the NII$\lambda6584$ line is twice a
bright as H$\alpha$, and that this will also be transmitted by our selected
narrow-band filter.
We model the extended emission (using the WFC3 ETC) as arising from
two emission lines (H$\alpha$ and [NII]$\lambda 6584$) of surface brightness
3 and 6 $\times 10^{-16}$~erg~s$^{-1}$~cm$^{-2}$~arcsec$^{-2}$ (a line width
of 200 km/s is used) plus an underlying elliptical galaxy
continuum. The normalization for the galaxy taken from the work of
Schombert (1987) shows that the surface brightness of NGC4696
is 21 Vmag arcsec$^{-2}$ at a radial distance of 28 arcsec from the nucleus,
appropriate to the fainter outermost filaments. However, more modern references suggest R~21 at this radius (Penereiro et al 1994; Sparks et al 1989).
For the simulation we use a Galactic extinction value normalized
to E(B-V)=0.12mag, standard background values and choose a 4-point box dither pattern
for the full orbit exposures and a 3-point line dither pattern for the half-orbit exposures.
The APT indicates that after overheads there is time for ~2700s of on-source exposure
on NGC~4696 in a single orbit after spacecraft and instrument overheads are taken into account.
We require an off-band image in a line-free spectral region
to enable the galaxy continuum to be subtracted. This can be obtained
efficiently using the F814W filter for NGC~4696.
A further image, taken in a bluer continuum band, will allow dust
extinction maps to be made and any young star clusters to be seen. The
F467M filter is ideal, as it transmits a relatively blue and,
crucially, emission-line free part of the spectrum with good system
throughput.
In Table 1 we provide a summary of the target, its properties,
selected filters, exposure times and the expected signal-to-noise
ratio based on ETC calculations. A major contributor to background
counts (and hence noise) in the emission line image (F665N) is the
underlying galaxy. This is included in the ETC calculation with the
surface brightness given in column 6. We assumed each full orbit to be
split into four exposures by a 4-point box dither and calculated
signal-to-noise ratios for a 2x2 pixel (0.08x0.08 arcsec) spatial
region. Half-orbit exposures use the 3-point line dither.
Although the underlying galaxy emission gives sufficient counts to ensure good charge transfer near the centre
of the galaxy, near the outermost visible filaments, this is not the case. We therefore choose to post-flash the chip to ensure a minimum of 12e- even far away from the galaxy to enable possible detection of very remote filaments. The required post-flash was included in the ETC calculations.
A total 10.5 orbits will give us a signal-to-noise ratio of 8.6 in
the narrow-band F665N filter in a 2$\times$2 pixel resolution element.
A further 0.5 orbit in the broad-band F814W filter will give a
signal-to-noise ratio of 25.9 (in the same spatial region), sufficient
to subtract the underlying galaxy without significantly degrading the
signal-to-noise ratio of the narrow-band observations. Two orbits of
exposure taken with the F467M filter will allow a signal-to-noise
ratio of 8.3 to be reached.
\begin{table}
\centerline{Table 1. Proposed observations}
\begin{tabular}{lccccccccc}
\hline
Target& Redshift & Band & Line SB & E$_{B-V}$ & Gal & Inst & Filter & Exp & S/N\\
\hline
NGC~4696 & 0.00987& H$\alpha$+[NII] & 3,6e-16 & 0.12 & $\mu_R$=21.0 & WFC3 & F665N &10.5 & 8.6 \\
NGC~4696 & 0.00987& Red cont & - & 0.12 & $\mu_V$=21.0 & WFC3 & F814W &0.5 & 25.9 \\
NGC~4696 & 0.00987& Blue cont & - & 0.12 & $\mu_R$=21.0 & WFC3 & F467M &2.0 & 8.3 \\
\hline
\end{tabular}
%\caption[]{Targets and proposed observations.}
Column 4 gives the line surface brightnesses in erg~s$^{-1}$~cm$^{-2}$~arcsec$^{-2}$ of H$\alpha$ and [NII]$\lambda 6584$.
Column 6 gives the galaxy continuum surface brightness in mags arcsec$^{-2}$.
Column 9 gives the exposure time in orbits to reach the signal-to-noise ratio given in column 10
in a 2x2 pixel spatial region.
\label{tab:calratio}
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