# 4C +41.17

4C +41.17 is a radio galaxy located in the constellation Auriga. With the redshift of 3.79, it is located nearly 11.7 billion light-years from Earth. At the time of its discovery in 1988, it was one of the most distant galaxies ever seen.

## Characteristics

4C +41.17 is classified as one of the largest radio galaxies in the early universe with emission at K$_s$ spread nonuniformly over a 3 × 6 (42 × 84 kpc) area. A potentiator of a massive elliptical galaxy located in the center of low-redshift galaxy cluster, it hosts a powerful radio source with a high-powered (10$^{46}$ ergs/s) astrophysical jet that is producing shock-excited emission-line nebulosity through its 1000 km/s shocks. With a C IV luminosity emanating from the shock, this implies the preshock density in the line-emitting cloud is high as it contains a hydrogen density of 110 cm$^{−3}$. This causes shock-initiated star formation proceeding on a timescale (a few × 10$^6$ yr) within estimated dynamical age (3 × 10$^7$ yr) of the radio source.
4C +41.17 is a hyperluminous infrared galaxy, with a star formation rate of >103 Msolar yr-1, making it a strong candidate for being a primeval galaxy, in the process of a major episode of star formation. With radio luminosity of L$_{500MHz}$ > 1027 W Hz$^{−1}$, 4C +41.17 is an extremely rare object that has a number density of ~10−8 Mpc−3 in the redshift range 2 < z < 5.
With characteristics like having a steep radio spectrum of (α ~ −1.3) together with an extended optical continuum emission and large rest frame Lyα equivalent width of ~270 Å, this identifies 4C+41.17 as a high redshift radio galaxy. It has a high infrared luminosity of L$_{FIR}$ ~ 1013 L$_⊙$ with large dust mass located in the dust lane in the center of the galaxy and molecular gas reservoir, making it a site of star production.

## Observations of 4C +41.17

From the interferometer used at IRAM 30m telescope in Spain, researchers detected two carbon oxide (CO) systems in 4C +41.17. These systems are measured by M_dyn ~ 6 × 1010 M⊙ which is separated by 1\farcs8 (13 kpc), and 400\kms in velocity. The carbon oxide systems then coincide with two different dark lanes in a deep Lya image. One of the CO component is found to coincide with the cm-radio core of 4C +41.17, with a close redshift to the \HeII AGN line. The second component is located near the base of a cone-shaped region that is southwest of the nucleus, resembling emission-line cones seen in nearby active galactic nuclei and starburst galaxies. The characteristics of the CO sources and their mm/submm dust continuum are similar to those found in ultraluminous infrared galaxies and in some high-z radio galaxies and quasars. The fact that 4C 41.17 contains two CO systems is a sign that it might have gone a merger with another galaxy.
There is also a strong presence of strong X-ray emission originating from a point source that is coincident with the nucleus. According to researchers, an extended X-ray emission is found having a luminosity of ~10$^{45}$ ergs/s. The emission covers 100 kpc (15″) diameter region which it surrounds the radio galaxy, and follows the general distribution pattern of radio emission in the radio lobes of this source and the giant Lyα emission-line halo distribution. However, the spectrum of the X-ray emission on the other hand, is nonthermal with a power-law index consistent to a radio synchrotron. This signifies the X-ray emission is most likely an inverse Compton scattering of photons that are far-infrared from a relativistic electron population associate with past and current activity from the central object.
4C +41.17 was also observed by Herschel, which about 65% of the extracted sources at 70, 160, 250, 350 and 500 micron, are identified as mid-infrared sources that were observed through Spitzer Space Telescope at 3.6, 4.5, 5.8, 8 and 24 micron. From these observations, the Herschel sources are mostly foreground towards radio galaxy and therefore not belong to any structure associated with the galaxy.
Hubble Space Telescope did observe 4C +41.17 through detections using good signal-to-noise ratio with a spatially resolution of 0.1" (440 pc); this suggests 35% of this emission ends up in the form of a high brightness clumpy regions extending by about 0.5" (1.7 kpc). This morphology is remarkably similar to that of the radio components. A fainter diffuse region of optical emission is seen extending westward from the center of the nuclear complex about 1.2" (5.3 kpc) out along the radio axis, indicating the emission of stellar origin with an estimated mass of about 10^10^M_sun_ of stars in each <= 500 pc clump.

## Star formation rate

Deep spectropolarimetric observations via the W. M. Keck Telescope conducted by researchers in Hawaii, finds out that 4C +41.17 is unpolarized between ʎ $_{rest}$ ~ 1400 Å2000 Å. This indicates scattered light has no dominance over the aligned ultraviolet continuum. Instead, they found that 4C +41.17 show absorption lines and features of P Cygni similar to those seen in z ≈ 2–3 star-forming galaxies and nearby starburst systems containing Wolf-Rayets. It is possible that galactic outflow partially contributes to absorption-line profiles but unlikely for the high-velocity wings of the high ionization lines being dominated by through galactic winds since there is large outflow mass implied by the absorption line strengths.
Through the detection of S V λ1502 stellar photospheric absorption line, the shape of blue wing of the Si IV profile, unpolarized continuum emission, the inability of any active galactic nuclei-related processes accounted for the ultraviolet continuum flux, and similarity of the UV continuum spectra of 4C 41.17 overall as well as the nearby star-forming region NGC 1741B1, these characteristics strongly suggest that ultraviolet light from 4C 41.17 is dominated by young, hot stars, in which its star formation rate is roughly 140-1100 h-250 M⊙ yr-1.

## Cloud interaction in Ly-alpha halo with radio jets

4C +41.17 is located in the center of a large Lyman-alpha halo. Such penetrations of jets through its galactic halo causes substantial jet to cloud interactions. In this case of the jet-cloud interaction evident near the radio knot B2 in 4C +41.17, researchers suggested a glancing incidence of the jet causes a partial bow shock, that is driven into towards the cloud. This is manifest through associated shock-excited line emission and associated star formation in the bifurcated structure. The jet then deposits much of its momentum at the site, before continuing onward to the next knot B3 where decelerated jet plasma is accumulated as radio lobes.
A deeper observation shows there are three distinct components in 4C +41.17. The emission-line gas is made up of made up of two components which are associated with the other components B1, B2, and B3 of the inner radio source. They are relatively narrow lines containing Lyman-alpha, silicon and helium with FWHM of 500,650 km/s, and a broad Lyman-alpha and chromium with FWHM 11,001,400 km/s. The third component consists of narrow absorption lines often associated with the narrow emission lines in P Cygni. This lines have FWHM 400,800 km/s.
Most observed narrow emission lines are usually produced either in the jet bow shock or in the photoionized winds from newly formed stars, but with an exception of carbon group IV elements like carbon, silicon, germanium, tin, and lead. Such of these elements are found weaker in older stars than 3 × 10$^6$ yr. The C IV emission is narrower indicating most of the flux originates in the precursor material that is ahead of bow shock. This is consistent with the velocity of 1000 km/s that researchers adopted for the normal component of the shock. For velocity shocks, almost all of the C IV emission originates from the precursor.

## Cloud mass in the halo of 4C +41.17

The cloud mass in 4C +41.17 is found to be 3.4 kpc wide. It has an area equal to the entire Ly-alpha bright region adjacent to B2 region measuring 65 kpc$^2$. This means a mass of ≈ 8 × 10$^{10}$f(C IV) M○. This cloud mass indicates that star formation occurs within the galaxy as a result of gravitational collapse. The free-fall time for the cloud density is estimated to be t$_{ff}$ ≈ 2100$^{-1/2}$ ≈ 1.4 × 10$^7$ ($_{nh}$/10 cm$^{−3}$) $^{-1/2}$ that is comparable to the dynamical timescale of the radio source in 4C +41.17 itself.