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\title{Catch the Biggest Ones: 
Slitless Spectroscopy of Extragalactic Objects
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\titlemark{Slitless Spectroscopy of Extragalactic Objects}

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\author{Vito Francesco Polcaro, Roberto Francesco Viotti}
\affil{Istituto di Astrofisica Spaziale e Fisica Cosmica, Roma, Italy}

\author{Stefano Bernabei}
\affil{Bologna Astronomical Observatory, INAF, Bologna, Italy}

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\contact{Vito Francesco Polcaro}
\email{polcaro@rm.iasf.cnr.it}

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\paindex{Polcaro, V. F.}
\aindex{Viotti, R. F.}     
\aindex{Bernabei, S.}

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\authormark{Polcaro, Viotti \& Bernabei}

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\keywords{
spectroscopy: multiobject, 
spectroscopy: slitless, 
stars: extragalactic, 
surveys
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Despite the great progress made in the last decade, the 
study of very massive stars remains hampered by the lack 
of statistics and only a few hundred stars with 
M$_{init}$$\ge$30 M$_{\odot}$ are known in our Galaxy. 
Low resolution, slitless spectral images 
were obtained at the Loiano Observatory in the wavelength window 
3300--6420 \AA, in order to identify in extragalactic objects 
targets displaying the most prominent features 
that characterise the spectrum of bright hot stars, WR and 
Luminous Blue Variables (LBV), and of nebular regions as well.  
We present the results with the well studied galaxy M 101, where 
the previously known H {\,{\sc ii}} regions 
with WR star signatures are easily identified. 
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\caption{Unfiltered image of M 101 (top) and the related slitless spectral image (bottom). Wavelengths increase from top to bottom. 
The two bright H {\,{\sc ii}} regions NGC 5455 and NGC 5461 are 
clearly identified by the H$\beta$ and [O {\,{\sc iii}}] lines. 
} \label{P10-2:T1.10-fig-1}
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\caption{ 
Tracings of the spectrum of the H {\,{\sc ii}} regions in M 101: 
NGC 5455 (top) and NGC 5461 (bottom). 
The main spectral features are marked. 
} \label{P10-2:T1.10-fig-2}
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Polcaro \& Viotti (1998) proposed at the ADASS VII Conference a 
simple slitless spectroscopic method that was effectively employed in 
the search of very massive stars, as well as in the identification 
of the optical counterparts of galactic X--ray sources (see e.g. 
Bernabei \& Polcaro 2001a,b; Israel et al. 1999). 
In this regard, despite of the great  
progress made in the last decade, the study of the very massive 
stars remains hampered by the lack of statistics and 
only a few hundred stars with M$_{init}$$\ge$30 M$_{\odot}$ are known in our Galaxy. Actually, being short--living objects, the known very  massive stars are associated with regions of large interstellar 
extinction, hence they are not easy to identify in our Galaxy. 
In fact, many the known objects of this category have been found 
in external galaxies and in the Magellanic Clouds. 
For instance, only one half of the LBVs known to date are 
Galactic objects. On the other hand, 
the new generation of optical telescopes makes the spectroscopy 
of bright extragalactic stars a relatively easy job: therefore, 
we can hope to increase our knowledge of very massive 
objects from the study of this class of stars in nearby external 
galaxies seen `face--on', that is with a little interstellar 
matter interposed. 
However, the problem arises in the preliminary identification 
of the targets: very massive stars are difficult to identify 
on the base of photometric surveys alone, being their colours 
strongly affected by local and circumstellar reddening. 
Unfortunately, the method that we have employed to date 
for the search of galactic objects, is scarcely useful, 
being based on a combination of filters and grisms, 
suitable only for relatively bright objects (our sensitivity 
limits is of the order of V$\approx$14).


Because of these reasons, we developed a new slitless procedure, 
allowing the spectroscopic survey of a relatively wide sky region 
on a single image, using the current observatory's 
instrumental set--up. 
As a test of the procedure we have obtained a slitless image 
of a 13$\times$13 arcmin$^2$ region around the well studied 
galaxy M 101 using the Loiano Observatory 1.52 cm telescope 
equipped with the Bologna Faint Objects Spectrometer and Camera 
(BFOSC, Gualandi \& Merighi 2001), and 
an EEV D129915 CCD (1300$\times$1340 pixels). 

A grism (no.3, with a resolution of 5.5 \AA) selects a wavelength 
window (nominal bandpass: 3300--6420 \AA) centred near H$\beta$, where the 
most characteristic spectral features of bright hot stars, 
Wolf--Rayet stars and 
Luminous Blue Variables (LBV) are present, as well as those of 
nebular H {\,{\sc ii}} regions and AGN's.  
The BFOSC unfiltered image of M 101 is shown on the top of Fig. 1,
and on the bottom the corresponding 60 sec slitless spectral image, 
with the central wavelength (about 4900 \AA) coincident with 
the stellar image, so that the target identification of the 
`strips' in the spectral image is straightforward.
Notice that the actual recorded wavelength range depends on 
the y--position (i.e. declination) of the object in the field. 
Hence, the H$\alpha$ region could be present in the spectra 
of the objects sited in the upper part of the field (e.g. NGC 5455) 
but is absent in the more northern objects,
as it is the case of NGC 5461.  
The 60 sec spectral image of M 101 (bottom of Fig. 1) shows many 
`spectral strips' corresponding to different regions of M 101, 
the most intense ones are the central core of the galaxy, 
a compact region $\sim$7 arcmin to the SE, and two 
H {\,{\sc ii}} regions SW and SE of M 101: NGC 5455 and NCG 5461. 
The extracted spectra of the two H {\,{\sc ii}} regions are 
shown in Fig. 2 in order to illustrate the data quality that can 
be obtained from this procedure also with a short exposure time. 
They are characterized 
by strong nebular emission lines of hydrogen (H$\alpha$, H$\beta$, 
H$\gamma$), and of [O {\,{\sc iii}}] (5007, 4959 and 4363 \AA). 
The H$\beta$--[O {\,{\sc iii}}] signature of the two regions 
is easily seen in the spectral image. 
In principle, these lines together with the 4640--4686 \AA\ 
feature, can be used for a quick--look identification 
of emission--line objects in the field, 
such as Of, WR, LBV stars, and compact H {\,{\sc ii}} regions 
(e.g., D'Odorico et al. 1983).  
Diffuse emission is also present in the spectrum of NGC 5455, 
probably from extended diffuse nebulosity, but the very short 
exposure time does not allow a more detailed analysis of
this feature. 

Finally, we recall that this method not only permits the object 
identification in the field, but also an accurate 
spectrophotometric investigation of the many objects present 
in the image, by using photometric standards in the same 
or in a nearby field. This will be the next step of our work. 


%			      References
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\begin{references}
\reference 
Bernabei, S. \& Polcaro, V.\ F.,  2001a, \aap, 366, 817  
\reference 
Bernabei, S. \& Polcaro, V.\ F.\, 2001b, \aap, 371, 123
\reference 
Gualandi, R. \& Merighi, R.,  2001, 
BFOSC - Bologna Faint Object Spectrograph \& Camera, 
{\sc Manuale Utente} Rel 2.0  (in Italian) R.T.25-03-2001
\reference 
Israel, G. L, Covino, S., Polcaro, V. F. \& Stella, L.\ 
1999, \aap, 345, L1 
\reference 
D'Odorico, S., Rosa, M. \& Wampler, E.\ J.\ 1983, \aaps, 53, 97 
\reference 
    Polcaro, V. F. \& Viotti R.\ 1998, \adassvii, 145, 78

\end{references}

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