Astronomy & Astrophysics manuscript no.
(will b e inserted by hand later)
UV observations of the galaxy cluster Ab ell 1795 with the
?
optical monitor on XMM-Newton
1 2 2 3 4 5 6
J.P.D.Mittaz , J.S.Kaastra ,T.Tamura , A.C.Fabian , R.F.Mushotzky , J.R.Peterson , Y.Ikeb e ,
7 5 8 9
D.H.Lumb ,F.Paerels , G.Stewart andS.Trudolyub ov
1
Department of Space and Climate Physics, University College London, Mullard Space Science Lab oratory,
Holmbury St. Mary,Surrey, U.K.
2
SRON Lab oratory for Space Research, Sorb onnelaan 2, 3584 CA, Utrecht, The Netherlands
3
Institute of Astronomy,UniversityofCambridge, Madingly Road, Cambridge, CB3 0HA, U.K.
4
NASA/GSFC, Co de 662, Greenb elt, MD20771, U.S.A.
5
Columbia Astrophysics Lab oratory and DepartmentofPhysics, Columbia University, 538 West 120th Street,
New York, NY 10027, U.S.A.
6
Max-Planck-Institut fur extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
7
Atrophysics Division, Europ ean Space Agency, ESTEC, Postbus 299, 2200AG No ordwijk, The Netherlands
8
DepartmentofPhysics and Astronomy, The University of Leicester, Leicester, LE1 7RH, U.K.
9
Los Alamos National Lab oratory, NIS-2, Los Alamos, NM 87545, U.S.A.
Received ;
Abstract. We present the results of an analysis of broad band UV observations of the central regions of Ab ell
1795 observed with the optical monitor on XMM-Newton. As have b een found with other UV observations of
the central regions of clusters of galaxies, we nd evidence for star formation. However, we also nd evidence for
absorption in the cD galaxy on a more extended scale than has b een seen with optical imaging. We also rep ort the
rst UV observation of part of the lamentary structure seen in H , X-rays and very deep U band imaging. The
part of the lamentwe see is very blue with UV colours consistentwithavery early (O/B) stellar p opulation.
This is the rst direct evidence of a dominant p opulation of early typ e stars at the centre of Ab ell 1795 and
implies very recent star formation at the centre of this cluster.
Key words. Galaxies: clusters: individual Ab ell 1795; Galaxies: stellar content; Ultraviolet: galaxies
1. Intro duction cores of co oling ow clusters, where the emission from
early typ e stars would b e strongest, are relatively sparse
The nal fate of material in the centre of co oling ow
(e.g. A1795 Smith et al. 1997, CL 0939+4713 Buson et al.
clusters remains a mystery.Perhaps the most obvious end
1998).
point for matter co oling out of the co oling ow is in stars,
The cD galaxy in Ab ell 1795 has b een extensively stud-
and it is indeed true that the cD galaxies in co oling ow
ied and shows a numb er of remarkable features, including
clusters often have anomalously blue optical colours (e.g.
extended nuclear emission line gas (Cowie et al 1983; Hu,
Allen 1995), implying higher rates of star formation than
Cowie & Wang 1985; Heckman et al. 1989), an unusual
normal. There is also a rep orted correlation b etween the
blue continuum (Hu 1992; Allen 1995) and lamentary
amplitude and radial extent of the colour anomalies and
structure in X-rays (Fabian et al. 2000). These prop erties
the inferred mass accretion rate from X-ray measurements
have made it one of the best candidates for star forma-
(McNamara & O'Connell 1992). However, much of the
tion arising from a co oling ow. Previous UV studies with
observations of star formation in clusters of galaxies is
UIT (Smith et al. 1997) haveshown the presence of emis-
based on optical measurements, and so crucial information
sion from early typ e stars in the cD galaxy with data that
ab out the emission from early typ e, and therefore young,
were consistent with either continuous star formation or a
stars is missing. Unfortunately, UV observations of the
recent (4 Myr) burst of star formation.
Here we rep ort on new multi- lter UV observations of
Send o print requests to : J. Mittaz
?
the central 8 arcminutes of the co oling ow cluster Ab ell
Based on observations obtained with XMM-Newton, an
1795 taken as part of an XMM-Newton PV observation.
ESA science mission with instruments and contributions di-
rectly funded by ESA Memb er States and the USA (NASA). Other asp ects of the XMM-Newton observations will be
2 J.P.D.Mittaz et al.: UV observations of the galaxy cluster Ab ell 1795 with the optical monitor
UVW2 - UVW1 UVM2 - UVW1
discussed elsewhere (Tamura et al. 2001, Arnaud et al.
O5 0.192 0.133
2001)
B0 0:454 0:628
A0 0:141 0:409
F0 0:405 0:018
2. OM data reduction
G0 2.170 1.294
Ab ell 1795 was observed in the U (3000A- 4000A), UVW1
K0 3.592 2.674
(2400A- 3600A), UVM2 (2000A- 2600A) and UVW2
(1800A- 2400A) lters (for more details of the optical
Table 1. The OM far UV colours of di erent stellar typ es
monitor see Mason et al. 2001). The observations used
the default con guration where the whole eld of view
is sampled using ve individual exp osures. Most of the
eld of view was sampled with 1 arcsecond pixels, but a
small section (110 arcseconds by 110 arcseconds) of the
central region of the eld was continuously monitored at
of the ma jor axis of the derived source ellipse. Since the
higher resolution (0.5 arcsecond pixels). This means di er-
measured FWHM in the UV is approximately 3 arcsec-
ent regions of the eld of view have di erent total exp osure
onds anything with a FWHM greater than 4 arcseconds
times. The outer regions of the detector havethelower ex-
is considered extended. Point like sources are shown as
p osure times of U: 1.5 ksec, UVW1: 2.5 ksec, UVM2: 3.0
crosses and extended sources are shown with a diamond.
ksec, UVW2: 3.98 ksec. The central p ortions of the detec-
Two ob jects of sp ecial interest are shown by other sym-
tor have longer exp osure times of U: 7.5 ksec, UVW1: 12.5
b ols. The cD galaxy is represented by the lled diamond,
ksec, UVM2: 15.0 ksec, UVW1: 19.9 ksec.
and the emission feature 20 arcseconds south of the cD
The data were pro cessed using the SAS (v4.1) tasks
galaxy is represented by a square (see section 3.2 and 3.3).
written to analyse optical monitor data. To maximise the
The derived detection limits determined from the back-
signal-to-noise we have summed all the data in a given
ground in each observation for each lter were U = 21.67,
detector window (area on the sky). This can be imp or-
UVW1 = 21.74, UVM2 = 20.89 and UVW2 = 20.22. On
tant in the UV where an individual exp osure do es not
the magnitude-colour plots the derived detection limits
have enough counts to enable removal of mo d-8 noise
are shown as dashed lines and there is good agreement
(see Mason et al. 2001). The sources were then de-
between the observed detection limits of the sources and
tected on each individual window (using the SAS task
the derived detection limits.
OMDETECT). and the output sources lists where then
The colour-colour plots show the range of UV colours
edited by hand to remove sources caused by stray lightin
exhibited by the sources. The ma jority of the sources lie
the OM (for details of instrumental artifacts see Mason et
on well de ned lines and the lower three panels show
al. 2001). Further, sources whose background was strongly
the p osition in the colour-colour plots of di erent stellar
a ected by stray light were re-extracted using an annu-
typ es (taken from Pickles (1998)) as well as a typical eld
lar background estimate as a more robust metho d. This
galaxy sp ectrum. For clarity, the far UV colours for di er-
mainly a ects the central few arcminutes of the eld.
ent stellar typ es are also listed in table 1. The apparently
The observed count rates were then converted into mag-
anomalous p osition of the O5 star is b ecause the UV lters
nitudes using the derived OM calibration. In the case of
bracket the strong absorption dip at 2200Acaused by the
the U lter, the instrumental magnitude was converted
presence of a wind. All the sp ectra have b een redshifted
into Johnson U using a colour correction. Since there is
to the cluster redshift but do not include any correction
no corresp onding standard for the far UV lters, UVW1,
for extinction. Marked on the plots are twocurves show-
UVM2 and UVW2 were kept in instrumental magnitudes.
ing the e ect of reddening by a Galactic (solid) and a
The estimated errors on the magnitude conversion are less
dusty galaxy (dotted) line (Witt et al. 1992). Most of the
than 0.05, derived from the latest OM calibration les.
ob jects lie on or near the lo ci de ned by the change in
In total, 284 sources were detected in the full 17
stellar sp ectral typ e. In general the p oint sources have the
17 arcminute eld of view and the count rates were then
colours of F and G stars, and while it is unlikely that the
converted to instrumental magnitudes. The identi cation
extended sources will have single stellar like sp ectra, it at
of the sources via correlation with catalogs and detailed
least allows an estimation of the dominant stellar typ e in
galaxy data will b e presented in a follow-up pap er.
the galaxies. As might b e exp ected with the galaxies at the
centre of Ab ell 1795, many of the extended sources have
3. Results
`red' UV colours consistent with red galaxies. There are,
however, a number of very blue p oints including a number
3.1. Colour-colour plots
of extended sources. The extended source seen at the ex-
With our nal sample of 284 sources wehave constructed treme right in the UVW1-UVW2 and UVW1-UVM2 plot
magnitude-colour and colour-colour plots (see Figure 1). is a very bright galaxy a ected by coincidence losses (see
In the plots wehave discriminated b etween p ointlikeand Mason et al. 2001). The cD galaxy and its asso ciated UV
extended sources by lo oking at the distribution of the size blob also lie to the extreme right of the colour-colour plots.
J.P.D.Mittaz et al.: UV observations of the galaxy cluster Ab ell 1795 with the optical monitor 3
Fig. 1. The magnitude-colour plots and colour-colour plots for the Ab ell 1795 eld. The top three panels show magnitude-colour
plots together with the detection limits (dashed lines). The middle three panels show the colour-colour plots where p oint-like
sources are represented by crosses, extended sources are represented by op en diamonds, and the cD galaxy and UV blob are
showed as a lled diamond and square resp ectively. The lower three panels show the p osition of di erent stellar typ es in the
colour-colour plane. The solid lines show the e ect of Galactic extinction and dotted lines show the e ect of a dusty galaxy
extinction, b oth up to an E of 0.22 (Allen 1995).
B V
Fig. 2. The U and UVW2 images of the central regions of Ab ell 1795. The size of each image is 35 by 48 arcseconds with the
cD galaxy lo cated near the middle of each image and the UV blob approximately 20 arcseconds to the south of the cD. The cD
galaxy in the two lters app ears to showdi erent morphologies.
4 J.P.D.Mittaz et al.: UV observations of the galaxy cluster Ab ell 1795 with the optical monitor
3.2. The cD galaxy
Because we have colour data on the UV emission from
the cD we can set limits on the number of young stars. If
we make the assumption that all or most of the UVW2
emission (the bluest lter) arises from O stars (as was
done by Smith et al. (1997)), then the estimated number
of stars required to give the observed ux is approximately
12000. This estimate is unlikely to b e a ected by emission
from an old stellar p opulation since the e ect is small, less
that 2%. This is in approximate agreement with 18000 O
stars estimated bySmith et al. (1997), where their data
was taken in a far UV lter centred at 1520Amuchfurther
in the UV than UVW2, our bluest lter. However, b oth
estimates are likely to b e upp er limits, since from gure 1
Fig. 3. Colour-colour plot showing the p osition of the UV blob
wewould estimate that the cD galaxy has a UV sp ectrum
( lled square) and the cD galaxy ( lled diamond) together with
redder than that of an early B or late typ e O star.
the track of a single burst of star formation (op en diamonds),
or constant star formation rates (op en squares) evolving with
Figure 2 shows an image in sky co ordinates of the cD
time. The arrows show the e ect of extinction (b oth Galactic
galaxy taken in the U and UVW2 lters. In the U band
and a dusty galaxy) up to an E of 0.22 (Allen 1995). The
B V
the source shap e is consistent with the data of McNamara
arrow asso ciated with the cD galaxy shows the e ect of remov-
& O'Connell (1993), with an ellipticity of 0.6 and p osition
ing an old stellar p opulation sp ectrum.
angle of 17 degrees. However, in detail there are likely
to be problems with the scattered light in OM which
make the background subtraction a non trivial exercise.
mo del (Witt et al. 1992), and the arrow asso ciated with
Fortunately, the e ect of this scattered light is strongly
the cD galaxy shows the e ect of removing an old stel-
lter dep endent and there are no serious background sub-
lar p opulation on the p osition of the cD galaxy. The end
traction problems with the far UV lters. Lo oking in the
point of this track is where all the U band emission is
far UV, the UVW2 image shows a relatively sharp discon-
from the old stellar p opulation, an overestimate since the
tinuity running north-source to the right of the cD galaxy
cD galaxy is known to be bluer than a standard ellipti-
centre. The only other UV image published on the central
cal galaxy.Unlike Smith et al. (1997), our far UV colours
galaxy (Smith et al. 1997) also show a similar feature, al-
only seem to b e consistent with a burst of star formation
though the spatial resolution of the data taken with the
sometime between 50 and 200 Myrs ago (dep ending on
00
UIT was of the order of 6:8 , ab out twice the nominal
extinction and removal of an old stellar p opulation), and
00
FWHM of OM in the UV lters ( 3 ). Smith et al. ex-
not with continuous star formation. However, all our UV
plained this feature as due to the dust absorption seen in
lters are not as blue as the UV lter used by Smith et
the HST data (Pinkney et al. 1996) but gure 2 shows
al., so it is p ossible that the UV sp ectrum is more complex
that it app ears on a larger scale than the dust lanes seen
than the simple mo dels used here.
in the HST data.
The e ect of dust a ects our conclusions regarding the
3.3. The UV blob
star formation history of the cD galaxy.Hu (1992) esti-
On the colour-colour plots (Figure 1) the bluest extended mated the intrinsic extinction to the lamentasE =
B V
source in the whole eld lies just south of the cD galaxy. 0:14 but with a wide range of p ossible values (0:02 <
This ob ject can be seen in gure 2 as a blob of very E < 0:22). Allen (1995) has suggested that the value
B V
blue emission approximately 20 arcseconds south of the is closer to 0.22. Smith et al. (1997) concluded that after
cD galaxy. Its approximate co ordinates are R.A.: 13 48 taking into account the e ect of dust their UV data was
52.8 DEC: +26 35 33 which is accurate to 2-3 arcseconds. consistent with either an old stellar p opulation together
This blob is almost certainly the same feature as blob E with a single burst of star formation o ccurring approxi-
seen in McNamara et al (1996) and it is therefore likely mately 2-5 Myr ago, or constant star formation over the
that it is part of the same lamentary structures seen last 5-10 Gyr. Figure 3 shows the p osition of the cD galaxy
in the deep U band imaging of McNamara et al (1996). in the UVW2-UVW1, UVM2-UVW1 plane together with
However, with the OM data we can, for the rst time, the p osition of di erent star formation mo dels taken from
get estimates of its UV colours. Unlike the case for the Bruzual & Charlot (1993). Wehaveconcentrated on the
cD galaxy, the colours of the blob (the lled square) are farUVdatatoavoid any problem with the scattered light.
much more consistent with an early star sp ectrum (early For both mo dels the di erent p oints on the plot corre-
B or late O star), implying that star formation is more sp onding to di erent ages after the burst of star forma-
recent in this ob ject. Figure 3 shows the far UV colours of tion with the relevant time shown next to the symbol.
the UV blob compared with di erent star formation mo d- Also plotted on gure 3 are the lines of extinction from
els. Given the relatively large error bars the UV colours two di erent extinction laws, Galactic and a dustygalaxy
J.P.D.Mittaz et al.: UV observations of the galaxy cluster Ab ell 1795 with the optical monitor 5
young stars. The OM data is consistent with their result are consistent with b oth typ es of extinction law together
of a single burst of star formation, although the estimated with a burst of star formation that started b efore 100 Myr
timescale for the o ccurrence of this burst is longer than ago, or continuous star formation. If we use the extinction
that derived by Smith at al. (1997). However, the OM l- assumed by Smith et al (1997) (E = 0.14), then the
B V
ters and the one used by Smith et al. cover di erentwave- maximum time scale for a single burst of star formation
bands, so any di erence may b e due to the ob ject having drops to within 10 Myrs, or to 100 Myr for continuous
a more complex sp ectrum than the simple mo dels used for star formation. Since the blob is very blue in all colours,
comparison. The cD galaxy also app ears to have quite a lot the implication is that star formation is very recent, with
of structure with an apparent colour gradient across the little evidence for an evolved stellar p opulation, and must
galaxy together with a discontinuity running north south b e related to some recent o ccurrence or phenomena at the
seen in the far UV lters. A p ossible explanation for the centreofAbell1795.
discontinuity is obscuration by dust or gas blo cking out
Again we can estimate the number of O or B stars
the UV emission in that area. An estimate of the E
required to give the observed ux, which is estimated as
B V
required to removethe UVW2 emission is 0:16, twice
1130 O stars or 4980 B stars. The implied UV luminosity
43 1
the measured E for the central p ortions of Ab ell 1795
from the O stars is then approximately 1:2 10 ergs s .
B V
as a whole. We therefore maybe seeing obscuring mate-
Of course again this is an upp er limit to the UV luminos-
rial on much large scales than the rep orted dust lanes seen
ity since it assumes only O stars contribute to the ux.
with HST data (Pinkney et al. 1996). While the resolu-
From these numbers we can then estimate a star forma-
1
tion of OM is insucient to make a direct link between
tion rate of 10M yr (assuming the IMF of Miller &
the HST data and OM data, it is p ossible that the UV
Scalo (1979)). This is still much less than the mass dep osi-
obscuration seen with OM is related to these dust lanes,
tion rate from analysis of previous X-raydataof 130M
1
since both the dust lanes and the UV absorption lie on
yr (Fabian et al. 1994, Tamura et al. 2001). Again, ex-
the same side of the cD galaxy. The OM data may there-
tinction is very imp ortant in the UV and based on the
fore be tracking dust or gas out to a larger radius than
extinction estimates (see section 3.2) we can increase the
was p ossible with the HST data b ecause the UV lters
observed luminosities and therefore inferred star forma-
are uniquely sensitive to extinction.
tion rates by a factor of 3, but still less than the X-ray
rate.
There is a well known H lament to the south of
4.2. The UV blob
the cD galaxy (e.g. Cowie, Hu, Jenkins and York 1983)
which is aligned in a north south direction. This blob lies
The UV blob is potentially the most interesting ob ject
close to the peak of the H emission along the lament
seen in the data. For a start it gives us the p ossibility
and so may be asso ciated with it. However, comparing
of studying the star formation at the centre of a co oling
the relative uxes makes it unlikely that these stars are
ow cluster without the central cD galaxy getting in the
solely resp onsible for the H emission. At the peak of
16
way. The UV colours imply not only the presence of young
the H emission the estimated ux was 2 10 ergs
2 1 2
stars, but there is little evidence of an evolved stellar p op-
cm s arcsec for the H +[NII]6568A complex.
2
ulation exp ected from an elliptical galaxy. It is therefore
The approximate total size of the UV blob is 20 arcsec
possible that the blob is the consequence of recent star
implying that the total H + [NI I] emission asso ciated
15 2 1
formation alone, without a pre-existing stellar p opulation.
with the blob is 4 10 ergs cm s or a luminosity
40 1
It is also lo cated very close to, or within the lamentary
of 7 10 ergs s . Using the numb ers quoted in Allen
structure seen in b oth H and deep U band imaging, al-
(1995) for the amountofH emission from a single O5 star
36 2 1 39 1
though since the p ositions obtainable from OM at the
( 6 10 ergs cm s )gives 5 10 ergs s for all
present moment are somewhat uncertain an exact posi-
the stars in the blob. If we include the e ect of extinction
tional match is not p ossible. From the measurements dis-
wewould increase the observed UV ux and therefore the
40
cussed ab ove it seem unlikely that all of the H emission
estimated luminosityofH , whichthenbecomes1:5 10
2 1
is p owered from the UV light seen in the OM data, with at
ergs cm s .Thisis still lower than the observed ux
least a factor of 4-5 discrepancy b etween the observed ux
by a factor of four or so. Coupled with the fact that the
and that required for the H emission. However, some of
estimation of the H emission can b e considered to b e only
the H emission may arise from sho cked gas (e.g. Anton
an upp er limit, it seems unlikely that all the H emission
1993) and it has b een suggested that co oling material from
can b e p owered by the observed UV ux seen in the OM
the co oling ow could contribute to the ionising ux (e.g.
data.
Voit & Donahue 1990). It is also not clear from the OM
data whether there is further UV emission along the tail
4. Discussion
since there is not sucient signal-to-noise. The emission
at the peak of the blob in the UVW2 lter is at ab out
4.1. The cD galaxy
3 1 1
1:8 10 counts s arcsec or 7 ab ove background
so a factor of 2 decrease in ux would make it hard to see As has b een rep orted by Smith et al. (1997), UV data
any residual emission. In the other lters, such as UVW1, on the cD galaxy indicate that there is a p opulation of
6 J.P.D.Mittaz et al.: UV observations of the galaxy cluster Ab ell 1795 with the optical monitor
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611
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5. Conclusions
The XMM-Newton OM data have illustrated the useful-
ness of UV observations in studying p opulations of early
typ e stars in clusters of galaxies. Wehave set constraints
on mo dels of star formation at the centre of Ab ell 1795
and have also detected the presence of extra extinction in
the cD galaxy.Wehave also presented the rst UV obser-
vations of emission from the lament seen in deep U band
images and in H to the south of the cD galaxy. In partic-
ular the observations of the central regions of Ab ell 1795
have highlighted a p opulation of early typ e stars lo cated
at or near the centre of the gravitational potential that
is most likely the consequence of recent star formation.
While the exact signi cance of this in terms of detailed
mo dels of material co oling out of the intercluster medium
is not yet clear, these data should, in combination with
optical/IR data, provide constraints on the timescale and
rate of star formation from the intercluster medium itself.