Spitzer Observations of Young Red Quasars

Bit of a selfish post this week, but hey, it’s my blog and it’s not often this happens, so you won’t get these types of posts regularly…

Last week I was finally able to release to astro-ph my latest paper on red quasars to the public. I had hoped it would become a press release and for a long time it seemed quite possible, but it was not meant to be. However, it will become a web feature on the Spitzer website, so be on the lookout for it on September 20th or so. I’ll make sure I’ll link it somewhere on the webpage. Anyway, the paper had been submitted for a long time now and it was also accepted in July, but since the “press status” was not clear, I held it back. It itches in your fingers to yell to the world about it, but I guess everybody feels that way about their work.

So, the gist of the paper? Well, boys and girls, gather ’round and hear the tales of the quasars, how they grew to be so luminous in the centers of their galaxies… Last time we left you in the company of these 13 young lads (AGN or quasars as their fellows called them), they were seen to be discovering how they came to be. Their traced their birth back to the merger of two galaxies (seen in Hubble ACS observations), the instability of the two being able to funnel material to the center and ignite the black holes. Since the quasars were still obscured to some degree by that material it was easier to discern the hosts and their merger features such as tidal tails and such. Nevertheless, even accounting for that obscuration, the host galaxies seemed more disturbed than usual. In fact, there was a faint (!) correlation between the amount of obscuration/reddening and the disturbance of the hosts – a fact that we interpreted for them to be young quasars, not established like the unobscured quasars we know. There were other clues, like the X-ray slopes and high Fe complexes, implying large accretion rates or a large fraction of BALs and blueshifted OIII lines, implying winds emanating from the quasar.

So with those observations we were awarded Spitzer time. The main driver was to disentangle the different IR contributions, the cold-ish (30-200K) star formation from the host from the hot AGN contribution (3000K). The spectra would also allow us to see if we saw any PAH complexes (seen in regions of star formation) or Silicate in absorption (seen when the dust doing the absorption is cold).

So with all the spectra and photometric points, we were able to fit different models to them and gauge the contribution from each one. See the pic on the right for an example on how such a modeling works. The dots are all the photometry points we have along with their uncertainties (SDSS, 2MASS, WISE, MIPS) and the line is the IRS spectrum. The red thick line is the best fit model and the others just signify the different contributions from all, the light and dark blue lines, for example are tracers of star formation. Once we were able to disentangle the AGN from the host, we could finally get the true bolometric luminosity of the quasar. Correcting for obscuration, we were also able to get the black hole masses of the quasars, too.

Phew, lots of info, but with all that data (HST, Spitzer, IRTF, Chandra, Palomar and Keck), we now had all the ingredients to make the following assertions.

a) Our quasars are much more obscured by COLD dust than normal quasars. They are not Type 2s, which don’t show Silicate in absorption that much. This confirms the Hubble images and previous Chandra results showing much higher dust:gas ratios than normal. In fact there isn’t ANY of our quasars that shows Silicate in emission, something that is quite frequent for broad line AGN (which ours are).

b) The quasars affected by Silicate in absorption are also more likely to be accreting at a higher rate, with the most active slurpers having Super-Eddington accretion rates. These also show signs of blueshifted OIII lines, indicative of winds. These were also the systems showing high disturbances (though that is quite a weak correlation, as it was in the 2008 paper).

c) The highly accreting systems are below the M-L relation (mass of the black hole relates to the bulge luminosity – the so-called Magorrian relation). This is whether you count the total host after PSF subtraction or the fitted bulge – they are WAY below (see title picture!). Young quasars are undermassive in relation to their host luminosities, so in a sense they need to “catch-up”. The ignition of star formation precedes quasar activity.

d) Unfortunately, one thing we were not able to prove was any correlation with star formation rates. Most of the quasars had hosts with luminosities in the LIRG regime (10^11-10^12 solar luminosities), which signifies star formation rates of about 10-100 solar masses per year, but there was no rhyme or reason behind which galaxy got to form more stars. Oh well, not everything can fit into a nice little story.

So, what’s next? I’d probably want some ALMA time for these guys. Or better yet, I’d like to look more closely at systems that are showing the winds – those seem to be quite interesting, don’t you think? Stay tuned, I might have something up my sleeve. 😉

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