News from MY Universe



2002 June 20: holidays, till september
 

2002 June 6: the largest CCD mosaic ever built by me (4o x 4o)
 

2002 May 23: a lot of clusters identified by the color-magnitude relation and the X-ray emitting gas
 

2002 May 9: reorganisation of the web pages!
 

2002 Apr 26: galaxies difficult to be seen
 

2002 Apr 11: mass segregation at z=0.3
 

2002 Mar 28: a very distant  (z>0.8) cluster of galaxy
 

2002 Mar 14: distances using artificial intelligence
 

2002 Feb 28: how faint can be a galaxy ?
 

2002 Feb 14: optical+X-ray images to pick-up clusters at high redshift
 
 
 
 
 
 

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2002 June 6: the largest CCD mosaic ever built by me (4o x 4o)
 


 

The image shows the KPNO/Deeprange field. By using 253 images reduced by Postman et al. (2002), I build the mosaic. It is 4o x 4o wide, 34k x 34k pixel wide,  and takes ~4 Gbyte. In order to produce an image that can be seen on the web, I resampled it putting 450 (original) pixels in one.  The vertical lines in the figure are due to  electrons escaped from the potential wells of the CCD pixels along the CCD columns. For the second one largest mosaic ever built by me looks here.
 
 
 

2002 Apr 26: galaxies difficult to be seen
 

In the near-infrared (lambda 1-2 µm)  the bright sky makes difficult to see galaxies, even the brightest ones, since their brightness is several times fainter that the sky brightness. It is much like to see something with a strong bulbe lamp pointed to your eyes. In spite of these observational difficulties several team started surveys of the sky in the near-infrared, such as DENIS, 2MASS , doing a great job.  However, one never should forget the data limits. After all, these two surveys exposed the sky for 1-3 sec, and for diffuse objects, like galaxies, this means that their outer envelopes are not detected. The figure shows the ratio between the flux measured by 2MASS and by deep pointed observations of nearby (famous and bright) early-type galaxies, many of which are taken from the NGC catalog, As it is obvious, the ratio is seldom equal to 1 and often the fraction of lost flux can be as high as 50 % or more.  These galaxies are quite bright (K<11 mag) and of normal-high surface brightness. Galaxies of lower surface brightness are even entirely missed by 2MASS.
 
 
 

2002 Apr 11: mass segregation at z=0.3


 

The near-infrared luminosity  is a proxy of stellar mass, since galaxies with different star formation histories and the same mass have similar near-infrared luminosities. Galaxies can be classified, therefore, in giant, if their mass exceeds some threshold value, and dwarfs. The figure shows how many of giants there are per dwarf. At the cluster center, this ratio is about 1, while outside the cluster core radius (250 Kpc for H0=50 km s-1 Mpc-1), the ratio is  0.3. This means that dwarf galaxies tend to avoid the cluster center, or in terms of masses, low mass galaxies avoid the cluster core. Such effect, called mass segregation, is a natural outcome of the standard hierarchical scenario of cluster formation, but was never been detected previously in such a direct way.
 
 
 
 

2002 Mar 28: a very distant  (z>0.8) cluster of galaxy

Clusters of galaxies are the largest virialized objects in the Universe. They are the most notable collection of galaxies and contain many early-type galaxies, that share similar colors. Hence, clusters can be recognized from other etherogeous collections of galaxies by the presence of objects having similar colors. Clusters of galaxies have large quantities of ionized gaz trapped in the potential well of the clusters, that emit in the X-ray. Given the size of the clusters (1 Mpc) and the resolution of the XMM-Netwon telescope, clusters look as extended sources in the X-ray.  The picture show a very distant cluster of galaxies (z>0.8, conservatively), previously unknown, as detected by means of two independent methods: as an extended X-ray source (observed by the satellite XMM-Newton by the XMM-LSS consortium, shown as blue contours), and as overdensity of galaxies having similar colors  (marked with circles).  The cyan square marks the cluster detection. North is up, East is to the left.
 
 
 

2002 Mar  14: distances using artificial intelligence

The Sloan Digital Survey provides fluxes and colors of millions of galaxies, and, for a subsample of them, also distances (called redshits by astronomers). By using artificial intelligence tools it is possible to estimate the redshift (distance) of the galaxies without direct measures of distances. In short, a training set, composed by  colors and luminosities of a random subsample of galaxies with redshift, is provided to a Multi-layer Perceptron, that learns from these examples how these features are related to the galaxy distance. The neural network is then validated on a independent validation set (need to avoid overfitting), and finally tested on another independent data set. The figure show the relation between the measured distance (zspec , unknown to the neural network) vs the estimated distance (zphot) for the test set. The dispersion around the one-to-one relation (i.e. zspec=zphot) is 0.020, or 6000 km s-1.
 
 
 
 
 
 
 

2002 Feb 28: how faint can be a galaxy ?


 
 

The figure  (Andreon & Cuillandre, ApJ, in press) shows the luminosity function, in three filters (wavelengths), of galaxies in the rich , famous and nearby Coma cluster.  The luminosity function is the number of galaxies of a given magnitude (luminosity). It is well known that individual galaxies can be very faint, but are these very faint galaxies rare examples or the rule? The figure shows that galaxies can be as faint as 3 bright globular clusters, and that there are a lot of such a galaxies, much more than brighter galaxies. These very low luminosity galaxies exceed in number even dwarfs, that are quite common, that in turn exceed in number giant galaxies. The hasded regions mark previous measurements of the luminosity function. The hashed region in the lower panel is the average over all literature luminosity functions preceding end of year 1998.
 
 

2002 Feb 14: optical+X-ray images to pick-up clusters at high redshift
 

The picture shows a cluster of galaxy, VMF40 (Vikhlinin et al. 1998, ApJ 502, 558), at z=0.6. The grayscale image is the R band image taken with the MOSAIC (35'x35') camera at the 4m CTIO, by the XMM-LSS consortium. The circles are clusters detected by using their red-sequence (the original method is described in Gladders & Yee 2000,  AJ 120, 1248), as modified by myself (Andreon 2002, in preparation). The contours is the X-ray emission measured by PSPC on Rosat. There is a good agreement between the "double" morphology of the cluster, pointed out by X-ray data, and the "double" or even "triple" optical morphology.  The color of the red sequence locates the cluster at z=0.55, quite near the literature estimated redshift of 0.6. The further circle in the bottom-left is a further cluster without X-ray counterpart in the shallow (3.5 Ks) PSPC exposure. The field of view is 11.5'x6.5'. North is up, East is to the left.
 
 
 
 
 
 


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