4 Optical properties

4.1 Optical-to-near-IR colours

A deep broad-band $K^\prime$ (1.92-2.29 $\mu$m) survey of the Lockman Hole region was carried out with the Omega-Prime camera (Bizenberger et al. 1998) at the Calar Alto 3.5 m telescope in 1997 and 1998. This survey covers approximately half the Lockman Hole field.

Several X-ray surveys (Hasinger et al. 1999; Giacconi et al. 2001; Lehmann et al. 2001a; Alexander et al. 2001a) have shown that the $R-K^\prime$ colour of the optical counterparts of X-ray sources increases with the optical faintness, and this in a more pronounced way than for the field sources (Rosati et al. 2002). This trend is also evident in the colour-magnitude diagram $R-K^\prime$ versus R shown in Fig. 4a. We note that the still unidentified objects are significantly redder than the bulk of the identified sources. Using the X-ray information on $N_{\rm H}$, we also find that there is a strong correlation between the X-ray absorption and optical colour. For comparison, we also plot the evolutionary tracks of an early, late and irregular galaxy type using the template library of Coleman et al. (1980), whose spectral energy distributions (SEDs) were extended to the near-IR and far UV using Bruzual & Charlot (1993) models as updated in 2000 (private communication). Magnitudes are normalized to the measured local value $K^\ast=10.8$, and no dust extinction is assumed. The QSO evolutionary track is derived from the empirical template from the Sloan Digital Sky Survey (Vanden Berk et al. 2001), together with the models of Granato et al. (1997), normalized to $M_{\rm B}^\ast=-22.4$ (for brighter objects the curve should be shifted to the left), for the extension in the near IR.

In the $R-K^\prime$ versus $K^\prime$ diagram, shown in Fig. 4b, there is no evident trend between $R-K^\prime$ colour and near-IR flux. Moreover, the range of $K^\prime$ magnitudes covered by the Type-2 AGN and unidentified sources is almost the same as that of the Type-1 AGN population. This is likely due to a combination of a less pronounced absorption effect in the $K^\prime$ band, a different K-correction for AGN-type spectra (small) and star-like galaxy spectra (large), as well as an increased contribution of the host galaxy light in the $K^\prime$ band relative to that of the AGN. Consequently the difference in the observed magnitudes between absorbed and unabsorbed sources is smaller than in the R band.

Figure 5: Optical/near-infrared colours as a function of redshift for a sample of ROSAT and XMM-Newton sources in the Lockman Hole with optical spectroscopy. Symbols are as in Fig. 3. The evolutionary tracks shown are the same as in Fig. 4. Type-1 AGN have the typical blue colour of a QSO and are unabsorbed, Type-2 AGN follow much redder optical colour tracks expected for their host galaxy - because the optical nucleus is obscured - and are intrinsically absorbed. There are, however, two high-redshift Type-1 QSOs whith strong X-ray absorption (see Sect.  7.4).

The $R-K^\prime$ versus redshift diagram is shown in Fig. 5 for the subsample with optical identification (redshift and AGN type) and X-ray spectral fit (see also Fig. 7 in Lehmann et al. 2001a). The correlation between optical classification, optical/near IR colour and X-ray absorption is even clearer than in Fig. 4. Most of Type-2 AGN, whose optical colours are dominated by the host galaxy, are also significantly absorbed (log $N{\rm _H} >21.5$), whereas Type-1 AGN are unobscured and the emission from the central AGN is contributing significantly to their optical colours. There are two exceptions, high redshift Type-1 QSOs, which are optically unobscured but absorbed in the X-ray band (see Sect. 7.4): this could indicate a variation in the gas-to-dust ratio (Granato et al. 1997; Maiolino et al. 2001a; Maiolino et al. 2001b). The colours of the three sources with photometric redshifts appear to be dominated by the light from their host galaxies.

The spectroscopic identification is still in progress and, to date, we have 24 new XMM-Newton sources with measured redshift using LRIS at the Keck II telescope in March 2001 (PI: Maarten Schmidt). There is an increasing fraction of Type-2 AGN among these fainter X-ray sources, and almost all the identified Type-2 AGN are at z<1. The derived but still preliminary redshift distribution seems to be in clear disagreement with predictions from X-ray background models (e.g. Gilli et al. 2001) based on the integrated emission of Type-1 and Type-2 AGN and constrained by deep ROSAT surveys (see also Hasinger 2002; Rosati et al. 2002). This calls for a revision of the evolutionary parameters of these models for both the space density of Type-1 and Type-2 AGN and the obscuration fraction (Type-1/Type-2 ratio) as a function of the redshift. The latter is directly related to assumptions in the unified AGN scheme.

4.2 X-ray detected Extremely Red Objects

In recent years, much efforts have been devoted to understand the nature of Extremely Red Objects (EROs hereafter). We define EROs as objects with $R-K^\prime \geq 5$.

In a recent wide-area survey, Cimatti et al. (2002) have spectroscopically identified a sizable sample of field EROs with K<19.2 and found them to be almost equally divided between old passively evolving ellipticals and dusty star-forming galaxies at 0.7< z < 1.5. With XMM-Newton and Chandra observations (Alexander et al. 2001b; Brusa et al. 2002), the fraction of optical counterparts with extremely red colours has significantly increased when compared to the first examples of EROs found in ROSAT surveys (Lehmann et al. 2001a).

Figure 6: X-ray flux in the [0.5-2.0] a) and [2.0-10.] b) keV bands versus optical R magnitudes for those sources in the Lockman Hole with available R band photometry. Symbols are as in Fig. 3. The dashed lines are X-ray-to-optical flux ratio $\log (\frac{f_X}{f_R})$ of -1, 0and 1.

Figure 7: X-ray flux in the [0.5-2.0] a) and [2.0-10.] b) keV bands versus optical $K^\prime$ magnitudes for those sources in the Lockman Hole with available $K^\prime$ band photometry. Symbols are as in Fig. 3.

Figure 8: X-ray luminosity in the [0.5-2] a) and [0.5-10] b) keV rest-frame band versus the logarithm of the column density $N_{\rm H}$. Symbols are as in Fig. 3. Arrows show luminosities corrected for intrinsic absorption. The dashed lines in panel b) define the "Type-2 QSO region''.

 

 

Table 1: X-ray detected EROs.

	Ty-1a	Ty-2b	Unid.c	Absd
${R-K}^\prime \geq 3$	12 ($24\%$)	14 ($28\%$)	21 ($43\%$)	20 ($41\%$)
${R-K}^\prime \geq 4$	2 ($6\%$)	10 ($30\%$)	19 ($57\%$)	16 ($48\%$)
$R-K^\prime \geq 5$	0 ($0\%$)	2 ($11\%$)	14 ($78\%$)	12 ($67\%$)
${R-K}^\prime \geq 6$	0 ($0\%$)	0 ($0\%$)	6 ($100\%$)	4 ($67\%$)

^a Type-1 AGN.
^b Type-2 AGN.
^c Unidentified sources.
^d Sources with $\log~(N_{\rm H})>21.5$.

In the subsample of 66 X-ray sources with measured $R-K^\prime$colour, we find 18 (or 27$\%$) EROs. Two of them are Type-2 AGN, one is classified as a normal galaxy, one is an extended source and 14 are unidentified sources; no Type-1 AGN are found. From Table 1, we infer that the fraction of Type-1 AGN decreases with increasing values of $R-K^\prime$, whereas the fraction of unidentified sources and intrinsically absorbed ( $\log(N{\rm _H})>21.5$) sources increases. Moreover, all the X-ray detected EROs have an X-ray-to-optical flux ratio $\log (\frac{{f_X}[2-10~{\rm keV}]}{{f_R}})>1$ (see Fig. 6b) and they sample the hardest part of X-ray colour-colour diagrams (see Fig. 3). The X-ray luminosities in the [0.5-10] keV rest-frame energy band of the seven EROs with known spectroscopic and/or photometric redshift are in the range $2.6 \times 10^{42}{-}8 \times 10^{44}$ erg s^-1. We thus conclude that our X-ray selected sample of EROs is heavily dominated by sources with strong AGN activity and absorbed X-ray spectra (twelve, or 67$\%$, have $\log(N_{\rm H}) > 21.5$).

 

Table 2: X-ray catalogue.

XID	Rosat	Typea	RA	Dec	Off-axis	counts	Fluxb	Fluxb	Fluxb	R	$K^\prime$	$R-K^\prime$	z	$\log(N_{\rm H})^c$	$\Gamma^d$	$L_{\rm X}$e	$L_{\rm X}$e
					anglef	[0.5-7]	[0.5-2]	[2-10]	[5-10]							[0.5-2]	[2-10]
1	28	2	10 54 21.3	+57 25 43	13.58	6071	11.06	20.31	9.30	...	...	...	0.205	21.29 21.2621.32	1.89 1.851.93	43.356	43.715
2	32	1	10 52 39.6	+57 24 32	4.28	4440	5.87	6.02	2.97	17.9	16.1	1.8	1.113	0.00	2.49 2.452.53	44.911	44.745
3	6	1	10 53 16.8	+57 35 52	8.40	3590	5.90	8.60	3.86	18.8	16.6	2.2	1.204	0.00	1.91 1.871.94	44.783	44.964
4	29	1	10 53 35.1	+57 25 43	7.65	3164	4.73	6.45	2.62	19.5	17.2	2.3	0.784	0.00	2.02 1.982.06	44.276	44.366
5	8	5	10 51 30.9	+57 34 39	11.30	1958	4.66	0.65	0.00	...	...	...	...	...	...	...	...
6	16	1	10 53 39.7	+57 31 05	7.95	1537	3.16	3.15	1.33	19.4	16.3	3.1	0.586	0.00	2.50 2.452.56	43.880	43.663
7	0	5	10 53 00.5	+57 42 10	13.58	1271	3.58	0.00	0.00	...	...	...	...	...	...	...	...
8	31	1	10 53 31.9	+57 24 56	7.64	1208	1.84	2.21	1.16	20.5	17.7	2.8	1.956	0.00	1.93 1.861.99	44.882	45.088
9	232	5	10 53 36.3	+57 38 01	11.67	962	2.42	0.00	0.00	...	...	...	...	...	...	...	...
10	25	1	10 53 44.9	+57 28 41	8.33	746	1.17	1.69	0.50	20.8	18.0	2.8	1.816	0.00	1.89 1.801.91	44.484	44.708
11	9	1	10 51 54.3	+57 34 36	8.74	744	1.24	1.60	0.74	21.2	18.3	2.9	0.877	20.20 19.7620.30	2.18 2.062.35	43.833	43.837
12	513	1	10 52 54.4	+57 23 42	5.32	681	0.86	1.33	0.62	22.3	18.3	4.0	0.761	0.00	1.75 1.651.88	43.365	43.631
13	37	1	10 52 48.1	+57 21 17	7.54	612	1.11	0.77	0.34	20.1	17.3	2.8	0.467	0.00	2.81 2.722.91	43.310	42.905
14	2	1	10 52 30.1	+57 39 13	10.57	599	1.14	1.55	0.48	...	...	...	1.437	21.03 20.3021.36	2.03 1.892.12	44.127	44.352
15	12	2	10 51 48.6	+57 32 47	8.32	570	0.41	3.29	2.17	22.9	18.0	4.9	0.990	23.50 23.2923.74	2.02 1.612.80	42.922	44.239
16	77	1	10 52 59.1	+57 30 29	2.75	516	0.62	1.15	0.70	22.1	18.8	3.3	1.676	0.00	1.56 1.451.71	44.100	44.528
17	0	9	10 54 07.1	+57 35 27	13.09	511	1.35	1.44	0.63	...	...	...	...	0.00	2.29 2.202.38	...	...
18	426	1	10 53 03.8	+57 29 24	2.87	443	0.76	0.81	0.00	22.0	17.5	4.5	0.788	20.79 19.7620.90	2.14 1.912.38	43.202	43.288
19	14	9	10 52 42.2	+57 31 58	3.18	427	0.46	1.28	0.64	25.0	19.6	5.4	1.94 phot.g	22.59 22.3922.77	1.67 1.411.97	43.825	44.644
20	120	1	10 53 09.3	+57 28 21	3.57	426	0.54	0.75	0.45	20.4	17.7	2.7	1.568	0.00	2.12 1.982.28	44.192	44.307
21	27	1	10 53 51.5	+57 27 04	9.37	412	0.65	0.89	0.56	20.8	18.6	2.2	1.720	0.00	1.66 1.561.76	44.258	44.634
22	5	1	10 53 02.5	+57 37 57	9.53	387	0.63	1.06	0.48	21.0	18.3	2.7	1.881	0.00	1.94 1.792.11	44.169	44.383
23	30	1	10 52 57.3	+57 25 06	4.16	368	0.61	0.72	0.00	21.1	18.0	3.1	1.527	0.00	2.35 2.212.50	44.239	44.185
24	0	2	10 52 37.8	+57 33 22	4.62	352	0.17	2.03	1.61	22.6	17.5	5.1	0.707	23.01 22.7023.18	1.63 1.401.87	42.362	43.711
25	84	9	10 52 17.0	+57 20 17	9.21	332	0.32	1.51	0.75	25.5	19.3	6.2	2.71 phot.g	23.51 23.3723.63	2.27 1.922.64	...	44.903
26	117	2	10 53 48.6	+57 30 35	8.99	321	0.34	1.27	1.02	22.9	19.5	3.4	0.780	22.27 22.0622.53	1.50 1.201.90	42.327	43.572
27	45	2	10 53 19.2	+57 18 52	11.06	287	0.47	0.52	0.00	21.2	...	...	0.711	20.32 19.7620.48	1.77 1.592.01	43.018	43.284
28	23	1	10 52 24.6	+57 30 10	2.83	286	0.35	0.64	0.00	22.4	18.0	4.4	1.009	20.94 19.7621.32	1.95 1.652.33	43.248	43.485
29	13	1	10 52 13.1	+57 32 22	5.36	277	0.39	0.50	0.00	22.0	19.5	2.5	1.872	20.73 19.7621.11	1.81 1.632.07	...	44.230
30	52	1	10 52 43.4	+57 15 45	13.04	250	0.57	0.46	0.00	...	...	...	2.144	0.00	2.29 2.152.48	44.555	44.552
31	104	2	10 52 41.3	+57 36 50	8.05	224	0.28	0.81	0.49	18.8	...	...	0.137	22.26 21.6022.29	1.95 1.312.25	41.209	41.862
32	491	9	10 52 25.3	+57 25 50	3.79	216	0.16	0.72	0.49	24.6	18.3	6.3	...	22.04 21.7322.30	2.17 1.522.89	...	...
33	123	9	10 51 28.3	+57 27 37	10.11	209	0.16	1.24	0.60	23.6	20.0	3.6	...	22.15 21.8322.40	1.90 1.302.34	...	...
34	80	1	10 51 44.6	+57 28 07	7.87	207	0.30	0.45	0.00	21.2	19.2	2.0	3.409	21.08 19.7622.11	1.75 1.532.06	...	44.958
35	116	2	10 52 37.5	+57 31 05	2.40	206	0.26	0.47	0.32	20.9	16.1	4.8	0.708	20.99 19.7621.41	1.50 1.271.75	42.789	43.304
36	814	1	10 52 45.2	+57 21 21	7.45	199	0.30	0.29	0.00	20.5	19.0	1.5	2.832	0.00	2.12 1.882.32	44.335	44.524
37	486	9	10 52 43.2	+57 28 00	0.79	191	0.19	0.44	0.34	24.4	19.1	5.3	1.21 phot.g	22.02 21.7222.29	1.49 1.161.59	42.855	43.879
38	0	2	10 52 06.6	+57 29 24	4.92	187	0.15	0.84	0.34	21.8	17.5	4.3	0.708	21.93 21.2322.31	1.47 0.971.83	...	43.072
39	0	9	10 54 04.5	+57 20 36	13.72	179	0.44	0.00	0.00	...	...	...	...	0.00	2.74 2.573.00	...	...
40	20	5	10 54 10.5	+57 30 40	11.90	178	0.43	0.00	0.00	...	...	...	...	...	...	...	...

 

Table 2: continued.

XID	Rosat	Typea	RA	Dec	Off-axis	counts	Fluxb	Fluxb	Fluxb	R	$K^\prime$	$R-K^\prime$	z	$\log(N_{\rm H})^c$	$\Gamma^d$	$L_{\rm X}$e	$L_{\rm X}$e
					anglef	[0.5-7]	[0.5-2]	[2-10]	[5-10]							[0.5-2]	[2-10]
41	0	2	10 53 05.4	+57 28 10	3.08	176	0.00	0.88	1.15	23.1	17.8	5.3	0.792	23.55 23.3123.80	2.00 1.332.85	42.413	43.613
42	821	1	10 53 22.1	+57 28 52	5.26	170	0.25	0.26	0.00	22.7	18.7	4.0	2.340	21.43 19.7621.75	1.89 1.592.24	44.004	44.478
43	75	1	10 51 25.3	+57 30 48	10.62	161	0.27	0.33	0.57	19.3	17.6	1.7	3.410	21.84 19.7622.32	1.84 1.552.15	...	45.000
44	477	1	10 53 05.6	+57 34 25	6.39	159	0.22	0.26	0.00	20.3	18.4	1.9	2.949	0.00	2.21 1.912.55	44.439	44.651
45	430	9	10 53 15.3	+57 26 30	4.91	158	0.14	0.64	0.00	24.3	19.0	5.3	...	21.95 21.9522.83	1.75 0.969.76	...	...
46	0	1	10 52 36.7	+57 34 02	5.31	149	0.14	0.52	0.42	24.2	...	...	0.164	21.85 21.2822.55	1.51 1.002.74	41.020	41.831
47	0	9	10 53 48.1	+57 28 16	8.77	148	0.18	0.42	0.00	25.5	18.5	7.0	...	21.88 21.5622.16	1.83 1.172.82	...	...
48	607	9	10 52 20.2	+57 23 06	6.46	147	0.19	0.28	0.00	24.1	19.8	4.3	...	20.52 19.7621.00	1.91 1.632.34	...	...
49	0	9	10 51 11.8	+57 26 35	12.47	142	0.25	0.36	0.00	...	...	...	...	19.90 19.7620.85	1.52 1.261.90	...	...
50	901	2	10 52 53.0	+57 28 59	1.36	138	0.05	0.33	1.25	15.3	11.9	3.4	0.204	23.64 23.3723.82	2.97 1.934.02	41.031	42.219
51	18	1	10 52 28.2	+57 31 05	3.03	136	0.17	0.26	0.00	22.6	19.5	3.1	0.993	0.00	1.78 1.412.21	42.952	43.198
52	428	1	10 53 24.6	+57 28 19	5.62	132	0.24	0.41	0.00	22.4	18.8	3.6	1.598	21.20 19.7621.73	1.77 1.492.08	43.626	44.061
53	38	1	10 53 29.6	+57 21 04	9.96	130	0.27	0.33	0.00	21.3	...	...	1.145	20.34 19.7620.60	2.18 1.932.58	43.514	43.539
54	804	1	10 53 12.4	+57 34 25	6.88	128	0.19	0.28	0.00	22.7	19.1	3.6	1.213	0.00	2.24 1.922.45	43.108	43.256
55	0	9	10 54 10.9	+57 30 57	12.00	127	0.31	0.37	0.00	...	...	...	...	19.92 19.7620.00	2.34 2.142.58	...	...
56	0	2	10 52 51.6	+57 32 00	3.41	125	0.16	0.28	0.00	21.3	16.8	4.5	0.664	21.41 20.0021.84	1.81 1.352.21	42.538	42.996
57	36	1	10 52 25.7	+57 23 02	6.22	125	0.22	0.27	0.00	22.5	18.6	3.9	1.524	21.34 19.7621.65	2.56 2.062.82	43.604	43.640
58	802	5	10 52 21.9	+57 37 34	9.22	123	0.28	0.00	0.00	15.9	...	...	...	...	...	...	...
59	54	1	10 53 07.7	+57 15 05	14.11	122	0.21	0.28	0.00	...	...	...	2.416	0.00	1.66 1.491.74	44.139	44.546
60	0	3	10 52 47.6	+57 36 22	7.60	121	0.00	0.72	1.12	17.9	...	...	0.118	23.35 22.8223.71	2.69 2.183.18	40.228	41.566
61	0	9	10 52 48.5	+57 41 28	12.69	118	0.25	0.26	0.00	...	...	...	...	20.51 19.7621.15	2.08 1.662.73	...	...
62	0	9	10 53 01.8	+57 15 00	14.03	113	0.18	0.28	0.00	...	...	...	...	20.75 19.7621.34	1.65 1.212.25	...	...
63	0	2	10 52 52.1	+57 31 33	3.03	106	0.07	0.45	0.00	19.1	16.0	3.1	...	21.97 21.4022.39	1.74 0.952.60	...	...
64	0	9	10 53 30.5	+57 25 16	7.31	104	0.15	0.00	0.00	25.9	19.7	6.2	...	0.00	1.84 1.771.91	...	...
65	0	9	10 52 11.1	+57 32 04	5.38	102	0.06	0.53	0.00	25.3	19.5	5.8	...	22.04 21.6522.42	1.74 1.402.60	...	...
66	870	1	10 52 25.4	+57 22 51	6.40	101	0.18	0.00	0.00	21.3	17.8	3.5	0.807	0.00	2.25 1.892.69	42.701	42.663
67	34	4	10 52 58.2	+57 23 56	5.27	100	0.17	0.25	0.00	26.6	...	...	0.340	...	...	42.173	42.546
68	128	4	10 53 50.8	+57 25 13	9.81	98	0.21	0.27	0.00	19.9	...	...	0.033	...	...	39.754	39.902
69	634	1	10 53 11.6	+57 23 08	6.86	97	0.13	0.23	0.00	23.2	...	...	1.544	0.00	1.43 1.161.73	...	43.715
70	0	9	10 53 15.5	+57 24 51	5.90	95	0.04	0.53	0.00	24.5	19.4	5.1	...	22.12 21.6322.54	1.77 1.023.09	...	...
71	82	1	10 53 12.3	+57 25 06	5.41	94	0.14	0.00	0.00	22.8	19.0	3.8	0.960	0.00	1.64 1.381.93	43.022	43.471
72	0	9	10 52 31.8	+57 24 30	4.55	94	0.00	0.49	0.34	24.1	19.6	4.5	...	...	...	...	...
73	19	1	10 51 37.3	+57 30 42	9.02	92	0.13	0.34	0.00	22.2	19.0	3.2	0.894	20.20 19.7621.64	1.68 1.332.30	42.521	42.843
74	0	9	10 52 43.4	+57 35 46	6.97	91	0.14	0.00	0.00	...	...	...	...	0.00	1.66 1.282.64	...	...
75	0	9	10 51 46.6	+57 30 35	7.78	91	0.18	0.00	0.00	25.6	19.2	6.4	...	0.00	1.96 1.382.38	...	...
76	0	9	10 51 55.3	+57 29 34	6.45	90	0.12	0.00	0.00	24.7	20.4	4.3	...	0.00	1.63 1.222.30	...	...
77	0	9	10 53 47.1	+57 33 51	9.98	90	0.21	0.00	0.00	...	...	...	...	0.00	1.91 1.642.27	...	...
78	861	1	10 53 58.3	+57 29 23	10.14	89	0.15	0.00	0.00	22.5	...	...	1.843	0.00	1.84 1.592.05	43.725	44.007
79	228	4	10 53 39.8	+57 35 18	10.01	88	0.20	0.42	0.00	22.8	17.2	5.6	1.250	...	...	43.843	44.022
80	0	9	10 52 07.2	+57 34 12	7.23	87	0.12	0.27	0.00	23.1	...	...	...	...	...	...	...

 

Table 2: continued.

XID	Rosat	Typea	RA	Dec	Off-axis	counts	Fluxb	Fluxb	Fluxb	R	$K^\prime$	$R-K^\prime$	z	$\log(N_{\rm H})^c$	$\Gamma^d$	$L_{\rm X}^e$	$L_{\rm X}^e$
					anglef	[0.5-7]	[0.5-2]	[2-10]	[5-10]							[0.5-2]	[2-10]
81	0	9	10 52 54.8	+57 31 51	3.44	87	0.08	0.28	0.33	22.9	...	...	...	21.71 20.0022.24	1.22 0.391.98	...	...
82	0	9	10 52 37.0	+57 16 03	12.77	86	0.15	0.00	0.00	...	...	...	...	0.00	1.98 1.712.24	...	...
83	0	9	10 53 30.9	+57 39 23	12.39	81	0.00	0.64	0.61	...	...	...	...	22.04 21.9522.70	1.10 0.401.80	...	...
84	0	2	10 51 50.1	+57 25 21	7.90	80	0.07	0.34	0.00	21.8	17.4	4.4	0.676	21.97 21.1822.37	1.42 0.832.23	41.890	42.901
85	0	9	10 53 21.4	+57 31 48	5.98	78	0.10	0.27	0.00	25.5	19.3	6.2	...	21.53 20.9021.53	2.42 1.474.39	...	...
86	0	9	10 53 46.9	+57 26 06	9.01	75	0.10	0.00	0.39	23.2	19.6	3.6	...	0.00	2.00 1.523.03	...	...
87	0	9	10 52 23.8	+57 25 32	4.16	75	0.09	0.00	0.00	24.7	19.6	5.1	...	0.00	1.63 1.262.37	...	...
88	41	4	10 53 19.0	+57 20 48	9.36	73	0.15	0.00	0.00	17.5	...	...	0.340	...	...	42.336	42.279
89	0	9	10 52 42.3	+57 29 11	0.41	73	0.08	0.20	0.00	24.0	19.8	4.2	...	19.60 19.7621.57	1.06 0.681.65	...	...
90	33	1	10 52 00.3	+57 24 20	7.27	73	0.07	0.20	0.32	22.2	19.4	2.8	0.974	0.00	0.67 0.201.37	42.309	43.115
91	0	9	10 51 20.6	+57 26 59	11.22	73	0.12	0.00	0.00	...	...	...	...	...	...	...	...
92	0	3	10 52 58.6	+57 33 35	5.24	72	0.10	0.00	0.00	24.3	19.0	5.3	0.417	20.97 20.9022.52	1.61 1.102.53	41.853	42.282
93	21	9	10 51 55.1	+57 30 43	6.70	72	0.09	0.00	0.00	24.2	20.1	4.1	...	0.00	1.89 1.302.90	...	...
94	434	2	10 52 58.4	+57 22 51	6.30	71	0.13	0.00	0.00	22.2	18.0	4.2	0.762	21.36 20.7821.66	2.12 1.692.56	42.862	43.123
95	0	9	10 52 31.5	+57 25 03	4.05	71	0.00	0.23	0.38	24.8	19.8	5.0	...	22.28 19.7622.85	1.13 -0.062.31	...	...
96	39	1	10 52 09.7	+57 21 04	8.94	71	0.14	0.00	0.00	21.7	20.6	1.1	3.281	22.69 22.3023.00	2.55 2.003.20	...	44.472
97	108	2	10 52 27.8	+57 33 30	5.14	71	0.09	0.00	0.00	22.1	18.4	3.7	0.696	21.08 19.7622.01	1.36 0.782.33	42.097	42.719
98	0	9	10 52 41.5	+57 30 39	1.88	70	0.07	0.00	0.00	25.5	20.3	5.2	...	0.00	1.46 1.211.51	...	...

^a Optical classification: 1 = Type1 AGN, 2 = Type2 AGN, 3 = galaxy, 4 = group/cluster, 5 = star, 9 = unidentified source.

^b In units of 10^-14 erg s^-1 cm^-2.

^c Intrinsic absorption in excess to the Galactic ones ($\sim$ $5.7\times 10^{19}$ cm^-2) and 90$\%$ confidence range.

^d Spectral Index and $90\%$ confidence range.

^e Log of observed luminosity in the rest-frame band, in units of erg s^-1.

^f In arcmin.

^g Photometric redshifts (see Lehmann et al. 2001a).

In the 1 Msec observation of the Chandra Deep Field South (Tozzi et al. 2001; Rosati et al. 2002) about 5$\%$ of the optically selected EROs are detected at X-ray energies, and their stacked spectrum is consistent with absorbed objects. In that field 19$\%$ of the X-ray sources are EROs, down only to the flux limits of our complete sample in the Lockman Hole.