1 Introduction

The IRAS sky survey was the first observation in which many quasars in the far-infrared were detected, and Neugebauer et al. (1986) presented IRAS measurements of 179 quasars from 12 to 100 $\mu$m. Combining IRAS observations with data taken in other wavelengths, Sanders et al. (1989) have given SEDs from $\sim$0.3 nm to 6 cm of 109 quasars in the Palomar-Green (PG) survey (Green et al. 1986). Compiling Einstein, IUE, and IRAS data with a supplement of ground-based observations, Elvis et al. (1994) have presented SEDs for a sample of 47 normal quasars, and derived the SEDs for radio-quiet and radio-loud quasars. Recently, Andreani et al. (1999) have presented SEDs for 120 optically selected quasars at low- and high-redshift, combining sub-mm and mm observations with optical, near-infrared, IRAS, and radio observations.

The gross shape of quasars SEDs is characterized by two major features; the big blue bump shortward of 0.3 $\mu$m and the infrared bump between 2 and 200 $\mu$m (Sanders et al. 1989; Elvis et al. 1994). It is generally considered that the blue bump is dominated by thermal emission from an accretion disk. The infrared bump is ubiquitous in both radio-quiet and radio-loud quasars. The infrared emission in radio-loud quasars has generally been taken to be dominated by non-thermal emission (e.g., Impey & Neugebauer 1988; Bloom et al. 1994; Neugebauer & Matthews 1999).

The origin of the infrared emission in radio quiet quasars is generally attributed to thermal emission from heated dust. The rise from 1 $\mu$m minimum toward 3 $\mu$m, which is universally present in quasars, is naturally explained by the sublimation of dust grains at $\sim$1500 K (e.g., Kobayashi et al. 1993). This is expected from the current unified models of active galactic nuclei, which have a dusty obscuring torus around a central source. Pier & Krolik (1993) computed the infrared properties predicted for the dust obscuring tori surrounding central sources by modeling free parameters of the inner radius of the torus to its thickness, the Thomson depths constraining the outer radius of the torus, and the flux of the nuclear radiation. The infrared emission in 1-10 $\mu$m of PG quasars is well explained by their models, while the emission in the far-infrared is much greater than that predicted. Several models were proposed to account for the far-infrared emission of quasars; these are (1) warm dust in a distorted disk extending from 0.1 kpc to more than 1 kpc, which is heated directly by radiation from central source (Sanders et al. 1989), (2) dust clouds in the narrow line region heated by the radiation from the central sources and star-forming regions (Rowan-Robinson 1995), and (3) dust in the obscuring tori extending to 200-3 kpc (Andreani et al. 1999). However, it should be noted that temporal variations of 10 $\mu$m brightness of radio-quiet quasar PG 1535+547 found by Neugebauer & Matthews (1999) casts doubt on the thermal origin of the infrared emission in radio-quiet quasars.

High-redshift quasars have been widely recognized to provide unique probes of high-redshift star formation and galaxy evolution. Metallic abundance in high-redshift quasars is solar or higher metallicities out to z > 4. Comparing the UV-to-optical spectra of 186 quasars with 0 < z < 3.8, Osmer et al. (1994) concluded that there was no evidence for redshift-dependent spectral changes. Hamann & Ferland (1993) analyzed N $_{{\rm V}}$/C $_{{\rm IV}}$ and N $_{{\rm V}}$/He $_{{\rm II}}$ broad emission ratios, and found that metallicities in the broad line gas at high redshift are 1-10 times solar. The ratio of UV Fe $_{{\rm II}}$/Mg $_{{\rm II}}$ of B 1422+231 at z = 3.6 shows that the host galaxy was already in the late-evolutionary phase of the Fe enrichment by SNe Ia at z = 3.6 (Kawara et al. 1996; Yoshii et al. 1998). These all suggest that the central part of host galaxies formed rapidly at very high-redshift $z \geq 10$, and metallicity enrichment in the central part was already completed at z = 4-5.

Dust emission from high-redshift quasars provide another probe to study star formation and evolution of host galaxies, especially in outer regions. Up to date, many quasars at z > 3 have been detected in the region from 350 $\mu$m to 1.3 mm (e.g., Andreani et al. 1993; Chini & Krügel 1994; Dunlop et al. 1994; Isaak et al. 1994; McMahon et al. 1994; Ivison 1995; Omont et al. 1996a; Hughes et al. 1997; Benford et al. 1999; Carilli et al. 2000). An extensive 240 GHz survey by Chini et al. (1989) revealed that the majority of z < 1 quasars have dust masses about a few times 10 $^7~M_{\odot}$, comparable to normal spiral galaxies, thus suggesting that dust is heated by radiation from the central sources. On the other hand, dust masses $\geq$ 10 $^8~M_{\odot}$ have been found in six of 16 quasars at z > 4 (Omont et al. 1996a), and CO emission was found in three of these (Ohta et al. 1996; Omont et al. 1996b; Guilloteau et al. 1997, 1999; Carilli et al. 1999). This may imply that dust emission in these host galaxies may be dominated by radiation from star-forming regions at high-redshift.

To study the distribution of dust in high-redshift quasars, mid- and far-infrared observations are indispensable. The emission from sublimated dust could be observed in the mid-infrared, thus probing that the innermost part of the obscuring tori, and the distribution of dust from obscuring tori to disks (or outer star-forming regions) could be obtained from far-infrared observations. The IRAS sky survey has only detected several quasars at z > 3 with exceptionally high luminosity in the far-infrared (Neugebauer et al. 1986; Bechtold et al. 1994; Irwin et al. 1998); F08279+5255 (APM 08279+5255) at z = 3.87, 2126-158 (PKS 2126-15) at z = 3.275, and 0320-388 at z = 3.12. In hoping to detect more quasars, several groups have carried out mid- and far-infrared photometry using the Infrared Space Observatory (ISO; Kessler et al. 1996). The first report of the ISO European Central Quasar Programme which observed 70 quasars between 4.8 and 200 $\mu$m including high-redshift quasars has been published by Haas et al. (1998), and the ISO/NASA AGN Key Project has also observed 72 quasars and AGNs covering a range of redshift up to 4.7 (Wilkes et al. 1999). As an ISO open time program, we have executed mid- and far-infrared observations of eight quasars at 1.4 < z < 3.7 using the raster mapping mode. This paper presents the results of the photometry of these quasars supplemented with optical and near-infrared data taken on the ground. In the course of this work, mid- and far-infrared sources have serendipitously been discovered. These sources will be described in the forthcoming paper (Oyabu & Kawara 2000). Throughout this paper, H₀ = 75 km s^-1 Mpc^-1 with q₀ = 0 is assumed.